Multi-band transmitter and monitoring system for Eclipse monitoring (Part 1)

It should not have escaped your attention - at least if you live in North America - there there have been/will be two significant solar eclipses occurring in recent/near times:  One that occurred on October 14, 2023 and another eclipse that will happen during April, 2024.  The path of "totality" of the October eclipse happened to pass through Utah (where I live) so it is no surprise that I went out of my way to see it - just as I did back in 2012:  You can read my blog entry about that here.

 Figure 1:
The eclipse in progress - a few minutes
before "annularity".
(Photo by C. L. Turner)

I will shortly produce a blog entry related to my activities around the October 14, 2023 eclipse as well.

The October eclipse was of the "annular" type meaning that the moon is near-ish apogee meaning that the subtended angle of its disk is insufficient to completely block the sun owing to the moon's greater-than-average distance from Earth:  Unlike a solar eclipse, there is no time during the eclipse where it is safe to look at the sun/moon directly, without eye protection.

The sun will be mostly blocked, however, meaning that those in the path of "totality" experienced a rather eerie local twilight with shadows casting images of the solar disk:  Around the periphery of the moon it was be possible to make out the outline of lunar mountains - and those unfortunate to stare at the sun during this time will receive a ring-shaped burn to their retina.

From the aspect of a radio amateur, however, the effects of a total and annular solar eclipse are largely identical:  The diminution of the "D" layer and partial recombination of the "F" layers of the ionosphere causing what are essentially nighttime propagation conditions during the daytime - geographically limited to those areas under the lunar shadow.

In an effort to help study these sort of effects - and to (hopefully) better-understand the propagation effects, a number of amateurs went (and are) going out into the field - in or near the path of "totality" - and setting up simultaneous, multi-band transmitters.

Producing usable data

Having "Eclipse QSO Parties" where amateur radio operators make contacts during the eclipse likely goes back nearly a century - the rarity of a solar eclipse making the event even more enigmatic.  In more recent years amateurs have been involved in "citizen science" where they make observations by monitoring signals - or facilitate the making of observations by transmitting them - and this happened during the October eclipse and should also happen during the April event as well.

While doing this sort of thing is just plain "fun", a subset of this group is of the metrological sort (that's "metrology", no "meteorology"!) and endeavor to impart on their transmissions - and observations of received signals - additional constraints that are intended to make this data useful in a scientific sense - specifically:

• Stable transmit frequencies.  During the event, the perturbations of the ionosphere will impart on propagated signals Doppler shift and spread:  Being able to measure this with accuracy and precision (which are NOT the same thing!) adds another layer of extractable information to the observations.
  

• Stable receivers.  As with the transmitters, having a stable receiver is imperative to allow accurate measurement of the Doppler shift and spread.  Additionally, being able to monitor the amplitude of a received signal can provide clues as to the nature of the changing conditions.
  

• Monitoring/transmitting at multiple frequencies.  As the ionospheric conditions change, its effects at different frequencies also changes.  In general, the loss of ionization (caused by darkness) reduces propagation at higher frequencies (e.g. >10 MHz) and with lessened "D" layer absorption lower frequencies (<10 MHz) the propagation at those frequencies is enhanced.  With the different effects at different frequencies, being able to simultaneously monitor multiple signals across the HF spectrum can provide additional insight as to the effects.
  

To this end, the transmission and monitoring of signals by this informal group have established the following:

• GPS-referenced transmitters.  The transmitters will be "locked" to GPS-referenced oscillators or atomic standards to keep the transmitted frequencies both stable, accurate - and known to within milliHertz.
  

• GPS referenced receivers.  As with the transmitters, the receivers will also be GPS-referenced or atomic-referenced to provide milliHertz accuracy and stability.

With this level of accuracy and precision the frequency uncertainties related to the receiver and transmitter can be removed from the Doppler data.  For generation of stable frequencies, a "GPS Disciplined Oscillator" is often used - but very good Rubidium-based references are also available, although unlike a GPS-based reference, the time-of-day cannot be obtained from them.

Why this is important:

Not to demean previous efforts in monitoring propagation - including that which occurs during an eclipse - but unless appropriate measures are taken, their contribution to "real" scientific analysis can be unwittingly diminished.  Here are a few points to consider:

• Receiver frequency stability.  One aspect of propagation on HF is that the signal paths between the receiver and transmitter change as the ionosphere itself changes.  These changes can be on the order of Hertz in some cases, but these changes are often measured in 10s of milliHertz.  Very few receivers have that sort of stability and the drift of such a receiver can make detection of these Doppler shifts impossible.
  

• Signal amplitude measurement.  HF signals change in amplitude constantly - and this can tell us something about the path.  Pretty much all modern receivers have some form of AGC (Automatic Gain Control) whose job it is to make sure that the speaker output is constant.  If you are trying to infer signal strength, however, making a recording with AGC active renders meaningful measurements of signal strength pretty much impossible.  Not often considered is the fact that such changes in propagation also affect the background noise - which is also important to be able to measure - and this, too, is impossible with AGC active.
  

• Time-stamping recordings.  Knowing when a recording starts and stops with precision allows correlation with other's efforts.  Fortunately this is likely the easiest aspect to manage as a computer with an accurate clock can automatically do so (provided that one takes care to preserve the time stamps of the file, or has file names that contain such information) - and it is particularly easy if one happens to be recording a time station like WWV, WWVH, WWVB or CHU.

In other words, the act of "holding a microphone up to a speaker" or simply recording the output of a receiver to a .wav file with little/no additional context makes for a curious keepsake, but it makes the challenge of gleaning useful data from it more difficult.

One of our challenges as "citizen scientists" is to make the data as useful as possible to us and others - and this task has been made far easier with inexpensive and very good hardware than it ever has been - provided we take care to do so.  What follows in this article - and subsequent parts - are my reflections on some possible ways to do this:  These are certainly not the only ways - or even the best ways - and even those considerations will change over time as more/different resources and gear become available to the average citizen scientist. 

* * *

How this is done - Receiver:

The frequency stability and accuracy of MOST amateur transceivers is nowhere near good enough to provide usable observations of Doppler shift on such signals - even if the transceiver is equipped with a TCXO or other high-stability oscillator:  Of the few radios that can do this "out of the box" are some of the Flex transceivers equipped with a GPS disciplined oscillator.

To a certain degree, an out-of-the-box KiwiSDR can do this if properly set-up:  With a good, reliable GPS signals and when placed within a temperature-stable environment (e.g. temperature change of 1 degree C or so during the time of the observation) they can be stable enough to provide useful data - but there is no guarantee of such.

To remove such uncertainty a GPS-based frequency reference is often applied to the KiwiSDR - often in the form of the Leo Bodnar GPS reference, producing a frequency of precisely 66.660 MHz.  This combination produces both stable and accurate results.  Unfortunately, if you don't already have a KiwiSDR, you probably aren't going to get one as the original version was discontinued in 2022:  A "KiwiSDR 2" is in the works, but there' no guarantee that it will make it into production, let alone be available in time for the April, 2024 eclipse. 

Figure 2:
The RX-888 (Mk2) - a simple and relatively inexpensive
box that is capable of "inhaling" all of HF at once.
Click on the image for a larger version.

The RX-888 (Mk2)

A suitable work-around has been found to be the RX-888 (Mk2) - a simple direct-sampling SDR - available for about $160 shipped (if you look around).  This device has the capability of accepting an external 27 MHz clock (if you add an external cable/connector to the internal U.FL connector provided for this purpose) in which it can become as stable and accurate as the external reference.

This SDR - unlike the KiwiSDR, the Red Pitaya and others - has no onboard processing capability as it is simply an analog-to-digital coupled with a USB3 interface so it takes a fairly powerful computer and special processing software to be able to handle a full-spectrum acquisition of HF frequencies.

Software that is particularly well-suited to this task is KA9Q-Radio (link).  Using the "overlap and save" technique, it is extraordinarily efficient in processing the 65 Megasamples-per-second of data needed to "inhale" the entire HF spectrum.  This software is efficient enough that a modest quad-core Intel i5 or i7 is more than up to the task - and such PCs can be had for well under $200 on the used market.

KA9Q-Radio can produce hundreds of simultaneous virtual receivers of arbitrary modes and bandwidths which means that one such virtual receiver can be produced for each WSPR frequency band:  Similar virtual receivers could be established for FT-8, FT-4, WWV/H and CHU frequencies.  The outputs of these receivers - which could be a simple, single-channel stream or a pair of audio in I/Q configuration - can be recorded for later analysis and/or sent to another program (such as the WSJT-X suite) for analysis.

Additionally, using the WSPRDaemon software, the multi-frequency capability of KA9Q-Radio can be further-leveraged to produce not only decodes of WSPR and FST4W data, but also make rotating, archival I/Q recordings around the WSPR frequency segments - or any other frequency segments (such as WWV, CHU, Mediumwave or Shortwave broadcast, etc.) that you wish.

Comment:  I have written about the RX-888 in previous blog posts:

• Improving the thermal management of the RX-888 (Mk 2) - link 
• Measuring signal dynamics of the RX-888 (Mk 2) - link
  

Full-Spectrum recording

Yet another capability possible with the RX-888 (Mk2) is the ability to make a "full spectrum" recording - that is, write the full sample rate (typically 64.8 Msps) to a storage device.  The result are files of about 7.7 gigabytes per minute of recording that contain everything that was received by the RX-888, with the same frequency accuracy and precision as the GPS reference used to clock the sample rate of the '888.  

What this means is that there is the potential that these recordings can be analyzed later to further divine aspects of the propagation changes that occurred during, before and after the eclipse - especially by observing signals or aspects of the RF environment itself that one may not have initially thought to consider:  This also can allow the monitoring of the overall background noise across the HF spectrum to see what changes during the eclipse, potentially filling in details that might have been missed on the narrowband recordings.

Because such a recording contains the recordings of time stations (WWV, WWVH, CHU and even WWVB) it may be possible to divine changes in propagation delay between those transmit sites and the receive sites.  If a similar GPS-based signal is injected locally, this, too, can form another data point - not only for the purposes of comparison of off-air signals, but also to help synchronize and validate the recording itself.

By observing such a local signal it would be possible to time the recording to within a few 10s of nanoseconds of GPS time - and it would also be practical to determine if the recording itself was "damaged" in some way (e.g. missed samples from the receiver):  Even if a recording is "flawed" in some way, knowing the precise location an duration of the missing data allows this to be taken into account and to a large extent, permit the data "around" it to still be useful.

Actually doing it:

Up to this point there has been a lot of "it's possible to" and "we have the capability of" mentioned - but pretty much everything mentioned so far was used during the October, 2023 eclipse.  To a degree, this eclipse is considered to be a rehearsal for the April 2024 event in that we would be using the same techniques - refined, of course, based on our experiences.

While this blog will mostly refer to my efforts (because I was there!) there were a number of similarly-equipped parties out in the fields and at home/fixed stations transmitting and receiving and it is the cumulative effort - and especially the discussions of what worked and what did not - that will be valuable in preparation for the April event.  Not to be overlooked, this also gives us valuable experience with propagation monitoring overall - an ongoing effort using WSPRDaemon - where we have been looking for/using other hardware/software to augment/improve our capabilities.

In Part 2 I'll talk about the receive hardware and techniques in more detail.

Stolen from ka7oei.blogspot.com

[END]

Monitoring the "CT" MedFER beacon from "Eclipse land"

Figure 1:
The MedFER beacon and vertical, tophatted
antenna on the metal roof of my house, attached
to an evaporative ("swamp") cooler.
Click on the image for a larger version.

I must admit that I was "part of the problem" - that is, one of the hordes of people that went north to view the August 21, 2017 eclipse along its line of totality.  In my case I left my home near Salt Lake City, Utah on the Friday before at about 4AM, arriving 4 hours and 10 minutes later - this, after a couple of rest and fuel stops.  On the return trip I waited until 9:30 AM on the Wednesday after, a trip that also took almost exactly 4 hours and 10 minutes, including a stop or two - and I had no traffic in either case.

This post isn't about my eclipse experiences, though, but rather the receiving of my "MedFER" beacon at a distance of about 230 miles (approx. 370km) as a crow flies.

What's a MedFER beacon?

In a previous post I described a stand-alone PSK31 beacon operating just below 1705 kHz at the very top of the AM broadcast ("Mediumwave") band under FCC Part 15 §219 (read those rules here).  This portion of the FCC rules allow the operation of a transmitter on any frequency (barring interference) between 510 and 1705 kHz with an input power of 100 milliwatts using an antenna that is no longer than 3 meters, "including ground lead."  By operating just below the very top of the allowed frequency range I could maximize my antenna's efficiency and place my signal as far away from the sidebands and splatter of the few stations (seven in the U.S. and Mexico) that operate on 1700 kHz.

Figure 2:
Inside the loading coil, showing the variometer, used to fine-
tune the inductance to bring the antenna system to
resonance.  This coil is mounted in a plastic 5-gallon
bucket, inverted, to protect it from weather.

As described in the article linked above, this beacon uses a Class-E output amplifier which allows more than 90% of its DC input power to be delivered as RF, making the most of the 100 milliwatt restriction of the input power.  To maximize the efficiency of the antenna system a large loading coil with a variometer is used, wound using copper tubing, to counteract the reactance of the antenna.  The antenna itself is two pieces:  A section, 1 meter long, mounted to the evaporative cooler sitting on and connected to the metal roof of my house and above that, isolated from the bottom section is an additional 2-meter long section that is tophatted to increase the capacitance and reduce the required amount of loading inductance to improve overall efficiency.

As it happens, the antenna is mounted in almost exactly the center of the metal roof of my house so one of the main sources of loss - the ground - is significantly reduced, but even with all of this effort the measured feedpoint resistance is between 13 and 17 ohms implying an overall antenna efficiency of just a few percent at most.

Figure 3:
The tophatted vertical antenna, loading coil and transmitter, looking up
from the base.  In the extreme foreground along the left side of the
picture can be part of the weather-resistant metal box that
contains the transmitter.
Click on the image for a larger version.

Originally intended only as a PSK31 beacon, I later added the capability of operating on 1700 kHz using AM and being able to do on/off keying of the carrier at the original "1705" kHz PSK31 frequency, permitting the transmission of Morse code messages.  For the purpose of maximizing the likelihood of the signal being detected, this last mode - Morse - I operate using "QRSS3", a "Slow" Morse sending speed where the "dit" length of the characters is being transmitted is 3 seconds - as is the space between character elements - and a "dah" and the space between characters themselves is 9 seconds.

Sending Morse code at such a low speed allows sub-Hz detection bandwidths to be used, greatly improving the rejection of other signals and increasing the probability that the possibly-minute amount of energy reaching the receive antenna may be detected.

Detecting it from afar:

Even though this beacon had been "received" as far away as Vancouver, BC (about 800 miles, or 1300 km) using QRSS during deep, winter nights, I was curious if I could hear it during a summer night near Moore, ID at that 230 mile (370km) distance.  Because we were "camping" in a friend's yard, we (Ron, K7RJ and I) had to put up an antenna to receive the signal.

The first first antenna that we put up received strong AC mains-related noise - likely because it paralleled the power line along the road.  Re-stringing the same 125-ish feet (about 37 meters) of antenna wire at a right angle to the power line and stretching out a counterpoise along the ground got better results:  Somewhat less power line noise.  It was quickly discovered that I needed to run both the receiver and the laptop on battery as any connection to the power line seemed to conduct noise into the receiver - probably a combination of noise already on the power line as well as the low-level harmonics of the computer's switching power supply.

I'd originally tried using my SDR-14 receiver, but I soon realized that between the rather low signal levels being intercepted by the wire - which was only about 10 feet (3 meters) off the ground - and the relative insensitivity of this device, I wasn't able to "drive" its A/D converter very hard, resulting in considerable "dilution" of the received signals due to quantization noise.  In other words, it was probably only using 2-4  bits of the device's 14 bit A/D converter!

I then switched to my FT-817 (with a TCXO known to be accurate to better than one part-per-million) which had no troubling "hearing" the background noise.  Feeding the output of the '817 into an external 24 bit USB sound card (the sound card input of my fairly high-end laptop - as with most laptops - is really "sucky") I did a "sanity check" of the frequency calibration of the FT-817 and the sound card's sample rate using the 10 MHz WWV signal and found it to be within a Hertz of the correct frequency and then re-tuned the receiver to 1704.00 kHz using upper-sideband.  It had been several years since I'd measured the precise frequency of my MedFER beacon's carrier, last being observed at 1704.966 kHz, so I knew that it would be "pretty close" to that value - but I wasn't sure how much its crystal might have drifted over time.

For the signal analysis I used both "Spectrum Lab" by DL4YHF (link here) and the "Argo" program by I2PHD (link here).  Spectrum Lab is a general-purpose spectral analysis program with a lot of configurability which means that there are a lot of "knobs" to tweak, but Argo is purposely designed for modes like QRSS using optimized, built-in presets and it was via Argo that I first spotted some suspiciously coherent signals at an audio frequency of between 978 and 980 Hz, corresponding to an RF carrier frequency of 1704.978 to 1704.980 kHz - a bit higher than I'd expected.

As we watched the screen we could see a line appear and disappear with the QSB (fading) and we finally got a segment that was strong enough to discern the callsign that I was sending - my initials "CT".

Figure 4
An annotated screen capture of a brief reception, about 45 minutes after local sunset, of the "CT" beacon using QRSS3 with the "oldest" signals at the left.  As can be seen, the signal fades in so that the "T" of a previous ID, a complete "CT" and a partial "C" and a final "T" can be seen on the far right.  Along the top of the screen we see that ARGO is reporting the peak signals to be at an audio frequency of 978.82 Hz which, assuming that the FT-817 is accurately tuned to 1704.00 kHz indicates an actual transmit frequency of about 1704.979 kHz.

As we continued to watch the ARGO display now and again we could see the signal fade in and out and be occasionally clobbered by the sidebands of an AM radio station on 1700 kHz - at least until something was turned on in a nearby house that put interference everywhere around the receive frequency.

The original plan:

The main reason for leaving the MedFER beacon on the air during the eclipse and going through the trouble of setting up an antenna was to see if, during the depth of the eclipse, its signal popped up, out of the noise - the idea being that the ionospheric "D" layer would disassociate in the temporary darkness along the path between my home where the eclipse would attain about 91% totality and the receive location within the path of totality, hoping that its signal would emerge.  In preparation for this I set up the receiver and the ARGO program to automatically capture - and then re-checked it about 5 minutes before totality.

Unfortunately, while I'd properly set up ARGO to capture, I'd not noticed that I'd failed to click on the "Start Capturing" button in ARGO and the computer happily ran unattended until, perhaps, 20 minutes after totality, so I have no way of knowing if the signal did pop up during that time.  I do know that when I'd checked on it a few minutes before totality there was no sign of the "CT" beacon on the display.

In retrospect, I should have done several things differently:

• Brought a shielded "H" loop that would offer a bit of receive signal directionality and the ability to reject some of the locally-generated noise and would have saved us the hassle of stringing hundreds of feet of wire through trees.  Some amplification with this loop would also have helped the SDR-14 work properly.  Alternatively, a simple active whip (such as a PA0RDT "mini-whip") could have been built and used, its location chosen for lowest noise pick-up.
• Actually checked to make certain that the screen capture was activated!
• Record the entire event to an uncompressed audio (e.g. ".WAV") file so that it could be re-analyzed later.

 Oh well, you live and learn!

P.S.  After I returned I measured the carrier frequency of the MedFER beacon using a GPS-locked frequency reference and found it to be 1704.979 kHz - just what was measured from afar!

[End]

This information stolen from ka7oei.blogspot.com

Analyzing "fake" solar eclipse viewing glasses - how good/bad are they?

Note:  Please read and heed the warnings in this article.

About a month and a half ago I ordered some "Eclipse Viewing Glasses" from Amazon - these being those cardboard things with plastic filters.  When I got them, I looked through them and saw that they were very dark - and in looking briefly at the sun through them they seemed OK.

Figure 1:
The suspect eclipse viewing glasses.
These are the typical cardboard frame glasses with very dark plastic lenses.
Click on the image for a slightly larger version.

I was surprised and chagrined when, a few days ago, I got an email from Amazon saying that they were unable to verify to their satisfaction that the supplier of these glasses had, in fact, used proper ISO rated filters and were refunding the purchase price. This didn't mean that they were defective - it's just that they couldn't "guarantee" that they weren't.

I was somewhat annoyed, of course, that this had happened too soon prior to the event to be able to get some "proper" glasses, but I then started thinking:  These glasses look dark - how good - or bad - are they?

I decided to analyze them.

WARNING - PLEASE READ!

What follows is my own, personal analysis of "potentially defective" products that, even when used properly, may result in permanent eye damage.  This analysis was done using equipment at hand and should not considered to be scientifically rigorous or precise.

DO NOT take what follows as a recommendation - or even an inference - that the glasses that I tested are safe, or that if you have similar-looking glasses, that they, too, are safe to use!

Figure 2:
The 60 watt LED light used for testing.  This "flashlight" consists of
a 60 watt Luminus white LED with a "secondary" lens placed in front of it.
The "primary" lens (a 7" diameter Fresnel) used to collimate the beam
was removed for this testing.
Click on the image for a larger version.

This analysis is relevant only the glasses that I have and there is no guarantee that glasses that you have may be similar.  If you choose to use similar glasses that you might have, you are doing so at your own risk and I cannot be held liable for your actions!


YOU HAVE BEEN WARNED!

White Light transmission test:

I happen to have on hand a homemade flashlight that uses a 60 watt white LED that, when viewed up close, would certainly be capable of causing eye damage when operating at full power - and this seemed to be a good, repeatable candidate for testing.  For measuring the brightness I used a PIN photodiode (a Hammatsu S1223-01) and relative measurements in intensity could be ascertained by measuring the photon-induced currents by measuring that current with and without the filter in place.

Using my trusty Fluke 87V multimeter, when placed 1/4" (about 6mm) away from the light's secondary lens I consistently measured a current of about 53 milliamps - a significantly higher current than I can get from exposing this same photodiode to the noonday sun.  In the darkened room I then had the challenge of measuring far smaller current.

Switching the Fluke to its "Hi Resolution" mode, I had, at the lowest range, a resolution of 10 nanoamps - but I was getting a consistent reading of several hundred nanoamps even when I covered the photodiode completely.  It finally occurred to me that the photodiode - being a diode - might be picking up stray RF from radio and TV stations as well as the ever-present electromagnetic field from the wires within our houses so I placed a 0.0022uF capacitor across it and now had a reading of -30 nanoamps, or -0.03 microamps.  Reversing the leads on the meter did not change this reading so I figured that this was due to an offset in the meter itself so I "zeroed" it out using the meter's "relative reading" function.  Just to make sure that the all of the current that I was measuring was from the front of the photodiode I covered the back side with black electrical tape.

Figure 3:
A close up of the S1223-01 photodiode and capacitor in front of the LED.
The bypass capacitor was added to minimize rectification of stray RF
and EM fields which caused a slight "bias" in the low-current readings.
Click on the image for a lager version.

I then placed the plastic film lens of the glasses in front of the LED, atop the flashlights secondary lens - and...

It melted.

Drat!

Moving to a still-intact "unmelted" portion of the lens I held it against the photodiode this time, placing it about 1/4" away from the LED as well and got a consistent reading of 0.03-0.04 microamps, or 30-40 nanoamps.  Re-doing this measurement several times, I verified the consistency of these numbers.

Because the intensity of the light is proportional to the photodiode current, we can be reasonably assured that the ratio of the "with glasses" and "without glasses" currents are indicative of the amount of attenuation afforded by these glasses, so:

53mA = 5.3*10E-2 amps - direct LED, no glasses
40nA = 4.0*10E-8 amps - through the glasses

The ratio is therefore:

5.3*10E-2 / 4.8*10E-8 = 1325000

What this implies is that there is a 1.325 million-fold reduction in the brightness of the light. Compare this with #12 welding glass which has about a 30000 (30k)-fold reduction of visible light and the absolute minimum that is considered to be "safe" for direct viewing while #14 offers about a 300000 (300k)-fold reduction.  According to various sources (NASA, etc.) a reduction of 100000 (100k)-fold will yield safe direct viewing.  The commonly available #10 welding glass offers only "about" a 10000 (10k)-fold reduction at best and is not considered to be safe for direct solar viewing.

Figure 4:
The typical spectral output of a "white" LED (blue line) and
a typical silicon PIN photiode (black line.)  The distinct peak
is from the internal blue LED while the "yellow" Ce:YAG
phosphors emit longer wavelengths to produce a "white" light.
As can be seen, the sensitivity of the photodiode increases
with longer wavelengths while the spectral output of a white
LED drops.
Click on the image for a larger version.

This reading can't be taken entirely at face value as this assumes that the solar glasses have an even color response over the visible range - but in looking through them, they are distinctly red-orange.  What this means is that the spectrum of the white LED - which is mostly red-yellow and some blue (because white LEDs use blue LEDs and a phosphor to produce the rest of the spectrum) and very little infrared - means that we are doing a bit of apples-oranges comparison.

In addition to this, the response of the photodiode itself is not "flat" over the visible spectrum, peaking in the near-infrared and trailing off with shorter wavelengths - that is, toward the blue end.  Figure 4, above, shows the relative peak light outputs of a typical "white" LED overlaid with the response of the photodiode and once can see that they are somewhat complimentary.

To a limited degree, these two different curves will negate each other in that the sensitivity of the photodiode is a tilted toward the "red" end of the spectrum.  With the inference being that these glasses may be "dark enough", I wanted to make some more measurements.

Photographing the sun:

As it happens I have a Baader ND 5.0 solar film filter for my 8" telescope to allow direct, safe viewing of the sun via the telescope.  Because I'd melted a pair of glasses in front of the LED, I wasn't willing to make the same measurement with this (expensive!) filter so I decided to place each filter in front of the camera lens and photograph the sun using identical exposure settings as seen in Figure 5, below.

Figure 5:
The Baader filter on the left and the suspect glasses on the right.
These pictures were taken through a 200mm zoom lens using a Sigma SD-1 camera set to ISO 200 at F8 and 1/320th of a second.  Both use identical, fixed "Daylight" white balance.
Click on the image for a lager version.

What is very apparent is that the Baader filter is pretty much neutral in tone while the glasses are quite red.  To get a more meaningful measurement, I used an image manipulation program to determine the relative brightness of the R, G and B channels with their values rescaled to 8 bits:  Because the camera that I used - a Sigma SD-1 actually has RGB channels with its Foveon sensor rather than the more typical Bayer CMY matrix, these levels are reasonably accurate.  Note that the numbers below do not take "gamma" (discussed later) into account.

For the Baader filter:

• Red = 163
• Green = 167
• Blue = 162

For the glasses:

• Red = 211
• Green = 67
• Blue = 0 

Again, this seems to confirm that the glasses are quite red - with a bit of yellow and thrown in, which explains the orange-ish color.  Clearly, the glasses let in more red than the Baader, but the visible energy overall would appear to be roughly comparable using this method.

What the eye cannot see:

It is not just the visible light that can damage the eye's retina, but also ultraviolet and infrared and these wavelengths are a problem because their invisibility will not trigger the normal, protective pupilary response.  I have no easy way to measure the attenuation of ultraviolet of these glasses, but the complete lack of blue - and the fact that many plastics do a pretty good job of blocking UV - I wasn't particularly worried about it.  If one was worried, ordinary glasses or a piece of polycarbonate plastic would likely block much of the UV that managed to get through.

Infrared is another concern - and the sun puts out a lot of it!  What's more is that many plastics - even strongly tinted - will transmit near infrared quite easily even though they may block visible light.  An example of this are "theater gels" that are used to color stage lighting:  These gels can have a deep hue, but most are nearly transparent to infrared - and this also helps prevent them from instantly  bursting into flame when placed in front of hot lights.

Because of this I decided to include near-infrared in my measurements.  In addition to my Sigma SD-1, I also have an older SD-14 and a property of both of these cameras is that they have easily-removable "hot mirrors" which double as dust protectors.  What this means is that in a matter of seconds, one can adapt the camera to "see" infrared.  Using my SD-14 (that camera is mostly retired, and I didn't want to get dust on the SD-1's sensor) I repeated the same test with the hot mirror removed as can be seen in Figure 6.

Figure 6:
The Baader filter on the left and the glasses on the right showing the relative brightness when photographed in visible light + near infrared.
This camera, a Sigma SD-14, was set to ISO 100 at F25 and 1/400th of a second using the same 200mm lens as Figure 5.
Click on the image for a larger version.

According to published specifications (see this link) the response of the red channel of the Foveon sensor is fairly flat from about 575 to 775 nanometers and useful out a bit past 900 nanometers while the other channels - particularly the blue - have a bit of overlapping response while the hot mirror itself very strongly attenuates wavelengths longer than 675 nanometers.  What this means is that by analyzing the pictures in Figure 5, we can get an idea as to how much infrared the respective filters pass by noting the 8-bit converted RGB levels:

For the Baader filter:

• Red = 111
• Green = 0
• Blue = 62

For the glasses:

• Red = 224
• Green = 0
• Blue = 84 

While the camera used for figures 5 and 6 aren't the same, they use the same technology of imager which is known to have the same spectral response.  Taking into account the ISO differences, there is an approximate 3-4 F-stop difference between the two exposures (some of this is due to the fact that the morning sun was higher when the infrared pictures were taken) indicating that there is a significant amount of infrared energy - particularly manifest by the fact that the exposure had to be reduced such that the green channel no longer shows any readings when using the Baader filter.   

(Follow this link for a comparison of the transmission spectra of common filter media and follow this link for a discussion about the Baader filter in particular.)

What is clear is that the glasses let in a significant amount more infrared than the Baader filter within the response curve of the sensor - but by how much?

The data indicates that the pixel brightness of the "Red+IR" channel of the glasses is twice that of that of the Baader filter, but if one accounts for the gamma correction applied to photographic images (read about that here - link) - and presume this gamma value to be 2 - we can determine that the actual differences between the two is closer to 4:1.

What does all of this mean?

In terms of visible light, these particular "fake" glasses appear to transmit about the same amount of visible light as the known-safe Baader filter - although the glasses aren't offering true color rendition, putting a distinct red-orange cast on the solar disk.  In the infrared range - likely between 675 and 950nM - the glasses seem to permit about 4 times the light of the Baader filter.

At this point is is worth reminding the reader that this Baader filter is considered to be "safe" when placed over a telescope - in this case, my 8" telescope, as the various glass/plastic lenses along the optical path (e.g. corrector lens, eyepiece, etc.) will adequately block any stray UV.  What this means is that despite the tremendous light-gathering advantage of this telescope over the naked eye, the Baader filter still has a generous safety margin.  (It should be noted that this Baader film is not advertised to be "safe for direct viewing".  Their direct-viewing film has a stronger blue/UV and IR blocking.)

What may be inferred from this is that, based solely on the measurements that obtained with these glasses it would seem that they may let in about 4 times the amount of infrared (e.g. >675nm) light as the Baader filter.

Again, I did not have the facility to determine if these glasses adequately block UVA/B radiation - but the combination of these glasses and good-quality sunglasses will block UV A/B - and provide additional light reduction overall.

Will I use them?

Based on my testing, these particular glasses seem to be reasonably safe in most of the way that matter, but whatever "direct viewing" method that I choose (e.g. these glasses or other alternatives) I will be conservative:  Taking only occasional glances.

(I will acquire some "bona-fide" glasses and analyze them when I get a chance.)

* * *

Once again:

WARNING - PLEASE READ!
 
What preceded was my own, personal analysis of potentially defective products that, even when used properly, may result in permanent eye damage.  This analysis was done using equipment at hand and should not considered to be scientifically rigorous or precise.

DO NOT take what follows as a recommendation - or even an inference - that the glasses that I tested are safe, or that if you have similar-looking glasses, that they, too, are safe to use!

This analysis is relevant only the glasses that I have and there no guarantee that glasses that you have may be similar.  If you choose to use similar glasses that you might have, you are doing so at your own risk and I cannot be held liable for your actions!

YOU HAVE BEEN WARNED! 




After the eclipse:

As it turns out, I did use these glasses - at least part time.  There turned out to be an excess of eclipse glasses on-hand, many of them being verifiably "genuine".  With these on-hand I'll do an update and make a comparison between the two.


     [End]

This page stolen from "ka7oei.blogspot.com".

Stared at sun, had fun...

As promised, my dad, younger brother and I wandered down to southwestern Utah, placing ourselves in line with the center of the eclipse - and waited.

Rather than contend with hoards of people descending on well-publicized towns like Kanarraville - almost exactly in line with the eclipse but conveniently located just off Interstate 15 - we headed toward a small almost/former town of Modena located a few miles east of the Utah-Nevada border.  Driving west from Cedar City, Utah on highway 56 we noted that pretty much all of the passenger car accessible side roads and turnouts were occupied by people who seemed to be readying themselves for the event that was to occur just 3 hours later.

Upon reaching Modena, we drove through the town and considered stopping there, taking advantage of the trees and desolate, picturesque landscape and partial ghost town and were somewhat surprised that we didn't see anyone else obviously set up to witness the event.  We decided, however, to continue on the highway and a few miles down the road we left the pavement and followed a well-graded dirt road into the surrounding, low mountains.  After a few miles - and passing one or two other places where people were setting up to stare at the sun - we found a wide area that was excellent both for viewing and throwing our sleeping bags on the ground for the evening.

Figure 1:
Setting up the 8" telescope with sun filter.
Click on the image for a larger version.

With about 2 hours to spare before the first bite was to be taken out of the sun, we lugged our gear about a hundred feet further south to clear a nearby brush-covered ridge and I began setting up my old 8" Celestron reflector telescope, outfitting it with a Baader filter in a homebrew, cardboard mount to enable safe viewing.  Soon, the eyepiece revealed a boiling, bespeckled sun and we then proceeded to set up the rest of our gear.  Soon after this, Gordon, K7HFV arrived having driven down under separate cover.

I had with me an adapter that would convert the 1-1/4" eyepiece mount to a standard Pentax "K" mount and my brother had the foresight to order an adapter that would permit mounting of his Sony DSLR.  With this arrangement we were soon projecting a live image of the sun directly onto his camera's sensor, using the "live view" function to focus the resulting image as precisely as possible.  As expected, only about 80% of the sun's disk would actually fit onto the imager so any "full-disk" pictures had to be taken in two, overlapping parts, adjusting the telescope slightly for each, the pair of images to be pasted together later.

Figure 2:
A solar projecting telescope used to monitor the eclipse.
Click on the image for a larger version.

I had with me a small power inverter to run the telescope's AC tracking motor (I said that it was an old telescope!) and not having set its mount up perfectly, we had to occasionally bump the right ascension to re-center the image, but this was very easy to do - especially considering that we were constantly moving things back-and-forth to get full-disk images, anyway.

One thing that was apparent from our drive in was that there was quite a bit of airborne dust without very much wind, a phenomenon attributed to wildfires burning a few hundred miles south in Arizona.  As the sun set in the west the sky became brighter as the sunlight was scattered by the dust and in our telescope, the image of the sun was slightly fuzzier and less stable than we'd hoped with the sunspots popping in and out of sharp focus randomly. In the images seen through the eyepiece - and on the camera - we could also see a bit of fluctuating colored fringe - something we attributed to atmospheric refraction as the reflector telescope itself should not have imparted a visible degree of chromatic aberration on the visible image by itself:  This effect seemed to increase slightly as the sun's angle dropped toward the horizon.

Within a few seconds of the time predicted, we could see the first bit of the moon's encroachment onto the sun's disk and at about that time (6:26 PM local) I measured the solar intensity as being 84200 lux - a value approximately one third of what one might see during local noon on a clear, summer day.

Figure 3:
The back end of the solar projecting telescope with a solar crescent.
There is a green filter in the optical path to reduce chromatic
aberration.

On the day before the eclipse (Saturday, 5/19) I'd put together a few simple devices for the safe viewing of the eclipse.  One of these was a "solar projecting telescope" seen in the picture above.  This device consists of a double-convex lens at the far end of the tube immediately followed by a pair of "Kelly Green" theatrical gel filters followed by one "Fire" red gel element.  About 1/4 of the way from the "Sun End" of the tube was a strong, plano-concave lens that then spread the image out a bit before hitting a ground glass screen that I'd installed about 1/5 of the way up from the "Eyeball" end in which I'd installed another double-convex lens to permit either direct viewing of the image on the ground glass screen, or a magnified view of that image when viewed from a few inches away.  As can be seen from the picture, a shroud was placed over the tube both to shield the viewer from the direct sun and to aid in aiming by adjusting the device on the tripod so that it cast no shadow on the backside of the shroud itself.  As you can see from the picture, it was all assembled into a stiff, cardboard tube with the lens, filter and ground-glass mounts being constructed of scraps of cardboard and black posterboard held together with thermoset ("hot-melt") and yellow wood glue!

Figure 4:
The moon, encroaching on the sun's disk.
Click on the image for a larger version.

This telescope wasn't intended to provide a crystal-clear image - and it didn't, as can be seen from the above picture - but it was a safe, convenient way to get a quick, safe glance as the current phase of the eclipse.


As the event progressed, it started to get darker as evidenced by the lower lux readings on my light meter.  While it still seemed to be plenty bright, there was an eerie aspect to the illumination of the surrounding landscape that defied description:  Was it the increasing sharpness of the shadows as the sun's disk was reduced in size, or was it the gradual loss of contrast between the highlights and the shadows or was it largely psychological, with the increasing incongruity between the brightness of the landscape and our expectation of what it should look like at that time of day?

Working the camera and telescope, my brother continued to snap pictures every minute or two as the moon made its way across the face of the sun, gradually blocking the visible sunspot groups as it did so.  The rest of us occasionally snapped pictures of each other, the surrounding landscape, and made various attempts to take our own pictures, sans-telescope.

Figure 6:
The "ring of fire" observed during the peak of the eclipse.
Click on the image for a larger version.

At about 7:33 pm local time, the entirety of the moon was contained within the solar disk and the landscape was eerily drab, the colors seemingly muted and the air suddenly feeling cooler than before.  At this point my meter indicated a reading of 3570 lux - a brightness of about 4% of what it had been just prior to the start of the eclipse and roughly comparable to what might be seen in a well-lit room:  To be sure, at least some of this decrease would be due to the lower angle of the sun in the dusty atmosphere.

Resisting the temptation to do so, we avoided sneaking a peek at the sun during the "ring of fire" phase of the eclipse knowing full well that within the ring of sunlight, the light was just as bright as it normally would be - the difference being that a ring would be burned onto the retina rather than a circle!  It was during this period of maximum occlusion that by brother was furiously snapping pictures - one of these being visible to the right.  As can be seen, our location put us almost exactly (within a couple of miles) in the center of the path forming a nearly perfect ring of sunlight around the moon that was visibly changing by the second!  Impressively, in many of these pictures picture one can make out "jaggies" along the edges  from the lunar mountains.

Figure 7:
The last seconds of the annular eclipse.
Click on the image for a larger version.

Too soon, the moon finished its traverse across the sun's face, extinguishing a narrow ribbon of sun along the edge as it did so - a scene captured in the image to the left.

Gradually, the eerie darkness of the landscape was replaced by the warm glow of sunset as the moon moved away from the sun's disk while setting on the western horizon.

In the gloaming, we relocated the telescope to the immediate vicinity of our cars and ate the snacks that we'd brought with us.  Later that night, I pointed the telescope at familiar night-sky objects such as the thin crescent of Venus and the rings of Saturn accompanied by several of its moons.  After putting the equipment away for the night, we bedded down under the stars, occasionally waking to glimpse the rise of the Milky Way as we spun our way toward the sunrise.

In the morning we got up, packed our gear back in the cars and headed off toward Cedar City where he had breakfast before heading back north to Salt Lake.

* * *

Figure 8:
The happy Eclipse God!

Later, I heard from my friend Ron, K7RJ who had been several miles to the east of us, also along the center line of the eclipse, just off highway 56 short of Modena.  There, he had also witnessed the spectacular sight with his wife Elaine, N7BDZ.  Unlike us, they both had plans for the following day (Monday) and they started back to Salt Lake a bit after the eclipse.  What would normally have been a 4 hour drive turned into a 6 hour drive as hoards of eclipse glimpsers - all with the same idea - clogged the 2-lane northbound interstate on their return trips, but he arrived home safely, albeit a bit later than expected.

Clearly, the Eclipse God was smiling upon us, granting us clear skies and safe travels!


* * *

Credits:  All of the telescope-based pictures of the sun/moon were taken by my brother while the picture of us at the telescope was taken by Gordon.  Those of the solar projecting telescope were taken by me while Ron provided a picture of the happy eclipse god.

[End]

This page stolen from ka7oei.blogspot.com

Staring at the sun - for fun!

Like many people in the southwestern/western U.S., I've been making preparations to (safely) stare at the sun for a few minutes during the upcoming weekend.

I am, of course, referring to the Annular Eclipse of May 20, 2012 that will cut a brief, darkened swath across much of the south to west of the U.S. on that day, mostly blocking out the sun from Texas to the Pacific coast.

Like many, I plan to place myself in the middle of the maximum portion of the eclipse where the moon's disk is predicted to be entirely within that of the sun's for a bit over 4 minutes while we stare through "sun-safe" filters, look at the images on the screens of cameras and makeshift camera obscuras and amuse ourselves with the varying shapes of shadows being cast on the ground during its various phases.

Here in Utah, only the south-west corner of the state will experience "totality" of the annular eclipse but some very good and remote locations are just a few hours away by car:  I'll throw a few pictures up here after it's done.

It should go without saying that unless one has purpose-specific lenses designed for direct viewing of the sun, NEVER look at the sun - or even a portion of the sun with the naked eye - or even really dark sunglasses.  Even though the sun will be "darker" during the eclipse, remember that instead of the normal "sun-shaped" burn that would appear on your retina, you'll get an "eclipse-shaped" burn instead, which isn't any good, either!

[End]

This page stolen from ka7oei.blogspot.com