(This post is from an email conversation with a friend. I thought the content might be useful for others.) [Updated 12/31/19 – Typos and clarifications.]
For the Mercury transit, I didn’t use any special equipment except the solar filter: Z7 body with Nikkor 200-500 @500 w/TC20E III, for a focal length of 1000mm. That’s way too short to get close to filling the frame for a full frame body, so I put the body in crop mode so the files wouldn’t be unnecessarily big with lots of black space around the sun. (I think you have to be around 2600mm on a full frame body before you can’t fit the solar or lunar disk in the frame.) My final images are only 2500×2500 pixels. 🙁 I had to be on business in Phoenix over the weekend, and decided I just didn’t have the energy to take my 800/5.6 out there. But that would have been much better, since I could have gotten much more focal length and thus image size on a lot more pixels, and that’s always better.
The lens / camera were just on a regular tripod, so the sun traveled across the field of view and had to be re-centered every three or four minutes.
As mentioned earlier, the one CRITICAL special equipment is a certified good solar filter. And the solar filter MUST be put on the object end of whatever optical instrument you are using to image / view the sun, not the eyepiece / sensor / eye end. E.g., if you were wearing your eclipse glasses and viewed the sun through a telescope / camera / binoculars you would cook your eyes nearly immediately!!! And you would probably cook the instrument, too. You have to filter the sun BEFORE it gets concentrated by the optical instrument.
Anyway, I used the Baader AstroSolar Film OD 5.0 taped to cardboard and then taped to the end of my lens. I used the same stuff for the eclipse in 2017, but for that I carefully trimmed the cardboard to snugly fit each of the lenses I was using.
Since the Z7 has a built in intervalometer and silent mode (electronic shutter, vibrationless) I just set the camera for 1/500 @ f/16 and ISO 500 in manual mode and set the camera off taking pictures every half second for a few minutes at a time. I ended up with a couple of 180 frame or so sequences, one of which was set at f/11 @ ISO 250, so I could see which was worse, the lens with TC wide open or diffraction at f/16. (Seemed to be a wash.)
As to your stacking question, there are at least three types of stacking I am familiar with. One is HDR, which is a stacked set of photos of the same subject at essentially the same time taken with a range of shutter speeds so that software can discern and combine the best exposure for each part of the photo and combine them to give much greater dynamic range than can be achieved in a single exposure. A second is focus stacking, which is a sequence of photos racking through the focus range from the closest to farthest parts of the subject so that software can discern and combine the sharpest focus for each part of the photo and combine them to produce a photo where all parts of the subject are in focus. The third is image stacking, which is a stack of photos of the same subject with the same exposure settings so that software can combine all the nearly identical images into a single image where every pixel is the average, mean, or some other calculated value based on that pixel in all the images contributing to the final product, which will have much less noise, eliminate transient objects (like airplanes or people), and potentially combine for much greater light gathering for deep sky objects.
I’m learning about astro software, and stacking (the third type listed above) is a fundamental method for most astro photos. There are a lot of astro stacking apps out there, though the best free ones are old and Windows only (AutoStakkert3! and Registax 6). I tried those on macOS via Crossover and a number of native Mac apps and finally found one that would work on solar disk images: Siril. Most of the apps have a few common features. Stacking is one, of course, along with aligning all the images, but another is image analysis. “Lucky imaging” is the term used to describe taking a lot of photos in sequence and then using an app to analyze the quality of each image, and then using just the best for the final processing. This allows you to benefit from the rare and brief moments of excellent atmospheric conditions between your lens and the object you’re shooting. The difference between normal, excellent, and bad “seeing” is dramatic, so having an app that can quickly identify which photos in the sequence are excellent makes a huge difference in your final image.
In the three shots I shared today, the first one is a single image identified by the software as the single best one of them in the sequence. The second one is a combined stack of the best images from the sequence. (I can’t remember exactly how many were picked.) You’ll notice how much smoother it is. But you’ll also notice that the silhouette of Mercury is elongated since images over a couple of minutes were combined into one. After the pre-processing, I added some color to both images for a less black and white look.
And the final image is a single frame from the last Venus transit in 2012 taken with a D800 through my 8” SCT telescope. Its focal length is 2000mm, so the solar image was larger and gave me about 4500×4500 pixels. The silhouette of Venus is much larger than that of Mercury, too, both because Venus is quite a bit larger than Mercury, and because it is a lot closer to Earth when it transits. I added color then, too, but did it a little differently. You can readily see how much more active the sun was then with all the sunspots. Last Monday the surface of the sun was completely clear.