Michael A. Covington    Michael A. Covington, Ph.D.
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How many lights, darks, and flats?
Solar eclipse
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After the eclipse

Cloudy: The distinctive thing about this solar eclipse, in contrast to 2017, is that most of the path had cloudy weather — high, thin clouds if nothing else — meaning pictures don't show the faint outer corona. The weather changed at the last minute, and many of the best eclipse photographers were completely or nearly clouded out. The best views were had by people who thought they were going to second-rate sites, such as my grandchildren in Scottsburg, Indiana (with only one minute of totality).

Fake pictures: The lack of good pictures created a vacuum in which some people were able to share fakes. Unlike last time, I haven't seen obvious fakes with the sun in front of clouds, or anything like that. But a famous painting by Cathryn Machin, based on a spectacular high-dynamic-range photograph by Sebastian Voltmer, is being circulated on Facebook as if by "NASA's James Webb Space Telescope." (The James Webb Telescope is far too far from the earth to see a solar eclipse, and it would be destroyed if it were aimed anywhere near the sun.) Apparently to elude image-search engines, the fake that I saw had been mirror-imaged and rotated.

Update: The fake pictures have started. Many are dead giveaways, such as eclipse in front of clouds, or eclipse with scenery not in the eclipse path (e.g., the Seattle Space Needle). Don't be taken in.

Eye injuries: Because I write about eclipses, I keep track of eye injuries caused by improper viewing (see also here) in the hope of fine-tuning the warnings given to the public. So far I have not heard of a 2024 eclipse eye injury (if you know of one, with confirmation, not just gossip, please tell me). But there is already a hoax circulating about how a boy was blinded — actually a video "skit" from 2017.

There is also an optometrist somewhere saying "viewing the eclipse is not worth the risk," which is not the usual recommendation. You might as well tell people not to go to the Grand Canyon because of the risk of falling in. For recommendations by an optometrist whose specialty is eclipse safety, click here. Eclipse glasses and pinhole projectors are safe and injuries have become rare. They happen by deliberate defiance, not by accident.

[Update:] There are starting to be scattered reports, all about people who viewed the eclipse with no protection at all, or in one case "through a phone" whatever that means. It will be some time before we find out whether the injuries were severe. There are also reports of eclipse glasses that did not meet safety standards, but no injuries are attributed to them, and it's not clear how far out of spec they were; if they had even half the specified density (log scale) they would probably prevent injuries but be difficult to use. I await more information.

[Update:] I have heard of one politician "blaming the eclipse on climate change," which makes me wonder what someone thinks an eclipse is. I wonder if this is a garbled report of someone blaming the inaccurate weather forecasts on climate change, which is plausible.

Besieged: Naturally, I posted about the eclipse on Facebook, and I can't keep up with the number of people wanting to communicate with me about it. It's nice to hear from everybody all at once, but I'm not on Facebook full-time and can't commit to even seeing, much less answering, everything that is sent to me. It has been wild!

No more for a long time: The sad thing about this eclipse for me is that it is the last one for a long time. In the eastern United States, we have to wait until 2045 for a total eclipse (at which time I will be almost 88 years old) and 2029 for even a good look at a partial one (there are partial eclipses less conveniently observable in 2026 and 2028).

That means this year was probably the last time I will have been involved in educating the American public about an eclipse — something I first did as a middle-schooler in 1970.

The other side of the world doesn't have such slim pickings. If I can get away and fund the trip, I would be glad to go to Spain in 2026, Gibraltar in 2027, or Australia in 2028. Maybe...


Solar eclipse

Today, a surprisingly large part of the U.S. population was in the path of a total solar eclipse, which passed over Dallas, Little Rock, Indianapolis, Cleveland, Buffalo, and Rochester, and then grazed Toronto and Montreal.

Unfortunately, I was not able to travel to see it. Many who did so were disappointed, because unpredictable weather took its toll, and many sites were unexpectedly cloudy.

My grandchildren and their parents, however, had an excellent view of the eclipse from Scottsburg, Indiana. At least I helped make their trip possible, even though I could not be there with them.

Here in Athens, Georgia, we had a deep partial eclipse. This picture is a stack of 12 exposures, aligned and sharpened.


Here's the equipment I used — a vintage-1980 Celestron 5 with a vintage-2012 Canon 60Da camera and a relatively new Thousand Oaks solar filter.


I also did a lot of amusing experiments with crescent-shaped spots of light. You have probably noticed that when sunlight goes through a small hole in a wall, the resulting spot of light is larger than the hole, and is round. It is in fact an image of the sun. During an eclipse, those spots are images of the eclipsed sun, which is crescent-shaped.

So here I am, making a small hole with my thumb and forefinger, and letting the eclipsed sun shine through it. You'll see that the same thing also happened accidentally with my collar, making a second, less distinct, crescent-shaped spot of light.


Here are crescents of light in the shadow of a tree:


And for the pièce de résistance, here's a colander and its shadow:



I first noticed this phenomenon when observing an eclipse with my father and sister in 1963 at our house in Moultrie. Dad drilled holes of different sizes in a scrap of countertop and found that they all worked; the smaller ones worked better. I also noticed a similar, though less distinct, effect with light coming through the wall of the carport, which had bricks spaced apart (see this picture).


How many calibration frames (darks and flats)?

Stand by for further coverage of this later, but here are some preliminary results.

With the help of some other astrophotography experts on the Cloudy Nights forum, I've been doing mathematical analysis of the way deep-sky images are calibrated and stacked. We normally take many exposures of the object ("lights"), matching dark frames to counteract fixed-pattern noise in the sensor ("darks"), images of a flat field to counteract pixel non-uniformity and dust specks ("flats"), and "flat dark" or "bias" frames to subtract out the nonzero baseline in the flats.

How many of each should we take? Preliminary results:

Lights: As many as possible. My current practice is to take 20 to 120 2-minute exposures. The biggest source of noise in the finished image is shot noise (Poisson noise) in the lights, and if you take N lights instead of one, you have 1/sqrt(N) as much of this noise.

Darks: Need not be as numerous as the lights, because they do not have shot noise from the image, although they still have read noise. Their purpose is to correct hot pixels and fixed-pattern leakage. Some of the newest sensors may not need them. My practice will be to take 10 to 20.

Flats: Need not be numerous because any noise in them is small compared to the signal. Expose them as generously as you can without actually bumping into the sensor's maximum value, so that their read noise will be as small a fraction as possible of the total output. Generous exposure helps more than taking a lot of flats. My practice is to take 10, which may be more than are needed, but they're quick to take.

Flat darks or bias frames: A few are sufficient; I currently take 10.

The big discovery is that dithering is VERY important. Dithering means not aiming the camera at exactly the same point in the sky for all the exposures. That way, when frames are combined, lined up on the stars, any noise that is in fixed positions on the frame will be spread around and averaged out.

And where does this noise come from? Residual random noise in the master dark and master flat. Each of those has a small amount of read noise that couldn't be eliminated, and it's in a fixed position because the same master dark and flat are applied to all frames. So the frames need to be dithered on the stars, to spread this residual noise around. The benefit can be dramatic.

Even a little dithering helps a lot. It can be sufficient to spread the whole session across just four or five positions. So when I'm using a star tracker that can't dither under computer control, I'm going to simply stop it and re-aim it periodically, shifting randomly a small fraction of the field of view.

Some time back, Robin Glover demonstrated that if the exposures are reasonably generous (so that the sky background is appreciably lighter than black), the read noise (random noise) in the sensor is surprisingly unimportant; we can put the brakes on our headlong rush toward lower-noise sensors, although I still want one because they tend to be better in other ways too. That conclusion holds up through all my analyses.

Note however that his recommended exposures pertain to faint objects and assume that, in the finished picture, the sky background will be medium-dark gray. It is quite legitimate to use shorter exposures if you are photographing a relatively bright object (such as M42) and are going to render the sky darker.

News updates

Sadly, for several reasons, we're not going to get to go to Indiana to see Monday's solar eclipse, as we had planned. I will observe it from here, where it will be a deep partial eclipse, and hope that the grandchildren have a good trip to totality, which passes near their home.

Not only is Melody not quite up to a long road trip yet, yesterday I had some urgent dental work done. I was sent to Tucker, Georgia (Melody's childhood home town) when the dental surgeon in Athens suddenly had to cancel. So I was right across the street from where she went to 8th grade, and down the street from where she went to church, and even got to see (momentarily) the public library. And today I'm eating applesauce for breakfast. Hoping to be back to normal soon, but glad not to be driving to Indiana today.

Meanwhile, the State of Georgia has stopped publishing the blue COVID graphs that I've quoted so often. I'd say, "The war is over," except that I had COVID just two months ago.

"Thou shalt not bear false witness against thy neighbor"
Another aspect of weaponized stupidity

I am finding that many of my fellow Christians are very poorly instructed about this one of the Ten Commandments. In fact, they often feel they are doing very righteous things when they violate it.

The commandment says not to make false accusations. I take that to include not spreading accusations of wrongdoing without good evidence and a good reason to spread them.

Even when the one accused is an organization, or a person known to have other serious faults.

If you're saying someone did wrong, this commandment applies to you.

I know that sometimes, someone else's wrongdoing is a matter of public concern. If so, I think you still have a duty to be honest about the evidence, about whether the wrong is proven or just suspected.

"I heard" or "somebody said" is not good enough. Nor is "my best friend shared this on Facebook" where "this" is something from a perfect stranger.

I have had to rebuke some of my fellow Christians rather strongly for spreading the hoax that "Facebook has banned the Lord's Prayer." Facebook has not done that, and if you check, you'll find no evidence that it has. There is no controversy. Just gossip.

But people spread it the moment they hear it from a total stranger. And they feel so righteous when they spread this malicious lie!

Maybe because they're blaming an organization rather than an individual; maybe because they think they're supposed to hate secular organizations, even when relying on them; or maybe because they're just foolish.

There's a deeper layer here. What I am realizing is that some people don't even know what truth is.

I take it for granted that truth is facts; truth exists in the real world; truth is what you can confirm by looking at the real world.

For some, though, truth is just words running around in your head, things you're supposed to say.

For some, truth is just emotions. If you "feel" like believing or saying something, you believe it and say it. To such people, logic and evidence don't matter. It is sad, but such people are surprisingly numerous.

And for some, truth is a matter of taking sides, like choosing to cheer for the Gators or the Bulldogs; and then you're supposed to say what your side says, or what seems to show loyalty to your side.

I think that with the Lord's Prayer Hoax we're seeing the last of these. People think that to be loyal Christians, they are supposed to believe and spread any accusation of anybody persecuting Christians, whether or not it is true.

If the Book of Proverbs is still in your Bible — if you have any idea what godly character is — this isn't it.


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