In the preceding post I mentioned how difficult it was focusing for sunspot detail on the disk of the Sun. What I’d been doing was using the Canon CameraConnect app in Live View mode on my phone with a large hooded loupe over the screen so I could see it while standing in the Sun. I manually adjusted the focusing ring on my Canon EF 400mm f/5.6L USM trying to get better contrast/definition on the few sunspots that were visible. There were a few problems with this:
Touching the lens to adjust the focus caused everything to shake around violently (with a 400mm x 1.4x teleconverter, even small angular perturbations translate to a lot of pixels on the sensor).
The focusing mechanism has a not-insignificant amount of slop/backlash between the external focusing ring motion in the optics.
My inability to make microscopic adjustments to the focus (even if there were no backlash and I had the most gentle touch).
Common advice online is that you should “just pick something really far away and focus on that, then you’ll be in focus for the Sun [or Moon, etc.].” Based on my past experience mostly using standard and wide-angle lenses, I would have agreed, but my testing with the aforementioned 400mm + teleconverter showed there was a bigger difference than I expected between “terrestrial and far away” and “astronomical distances away.”
The true way to well-focused astro photos, as it it turns out, is to let the camera lens make its own focusing adjustments. The USM mechanism in such lenses can make such small nudges to the focusing assembly that you may wonder if it’s even working.
Aside from Canon nerfing WiFi support for the 6D in the newer versions of the Canon EOS Utility, it is a really neat program and worked just fine using a USB-A to mini-USB cable (can’t remember the last time I used a mini-USB cable). Hard to believe this camera is already 11 years old, controlling the optics from my laptop makes it feel as if the camera took a step forward at least half as many years. Here’s a screen recording of focusing on the Moon using the program’s Live View controls.
At this focal length (560mm equivalent) and tight digital crop, the severity of the seeing is really bad. “Seeing” is the constant random boiling motion affecting every part of the image. Air has a refraction index (how much it bends the path of light) that depends a lot on the density and thus temperature and pressure (and also on humidity and gas composition). All these factors can vary to lesser or greater degrees as you move through the atmosphere, so all the air along the line of sight to the Moon can have a big effect on the sharpness and stability of the image. This is why stars twinkle. With their minuscule angular diameters and our usually turbulent atmosphere it’s a wonder any of their ancient photons ever reach our eyes at all.
Trying to “see” through the seeing, I leaned toward shorter shutter speeds compensating with higher ISO, but there’s only so much sharpness that will buy me. Some jerkiness is also from my star tracking mount. It’s a Vixen Polarie, which I’d wholeheartedly recommend for an affordable portable DSLR astronomy kit. This perhaps isn’t the greatest advertisement for it, but it’s doing its best considering I’m exceeding the “do not exceed” weight limit by about 25%. I’m just glad it keeps the Moon in the general neighborhood.
I took a handful of photos and a few of them came out reasonably sharp once I’d dialed the focus in on my laptop. Afterward, without touching the focus I slewed the camera over to a building I could see from the park. After increasing the exposure some number of full stops, I took the picture on the right. Measuring in Google Earth, the pictured “WEW” sign is nearly 400m away — a little over 1-millionth the distance to the Moon. Small compared to astronomical distances, but still seems like “near-as-makes-no-difference infinity” for any camera lens. And yet:
Each photo is a 1:1 crop from my Canon 6D’s sensor. I am very impressed at how out-of-focus the sign was at ~0.4 km vs. the in-focus Moon at 400,000 km. If I’d continued to follow the online “focus on a distant object” advice I’d be missing out on a lot of sharpness at astronomical distances.
Anyway, that’s all for today. I’m not sure why I’ve been in a posting mood lately, I’ve hardly thought about this blog in the past 6 years. At some point I’ll just stop posting again, maybe for the last time. Who knows. I doubt there are many readers out there in this day and age of social media and the Google “site:reddit.com” SEO-spam band-aid fix.
Quick addendum; found a non-free workaround for Canon EOS Utility’s removal of support for ad hoc WiFi networks (as the hosted WiFi AP in the 6D apparently is). I already had a cheap 2.4GHz travel router I bought for a cruise ship vacation some years ago, on the ship it let us share a single paid cruise ship internet WiFi plan across all our devices. I dusted it off, booted it up, and connected both laptop and 6D to the internetless router. As expected I was able to connect via EOS Utility. Of course I could have done this with my home WiFi network the whole time, but that’s not available wherever I am in the great outdoors and the travel router only sips a few hundred milliamps from whatever USB power source you happen to have around. I will say that the performance connecting to the 6D directly via USB cable is much better. Over WiFi the Live View latency was very high and the framerate low — I estimate 1.5-2 fps at most, slideshow territory. Over the USB cable it felt more like 10-15 fps which, while not great, was not terrible. One remaining annoyance is that EOS Utility saves images to my laptop during Live View shooting and does not leave a copy on the SD card, which is annoying because my photo post-processing workflow does not involve my laptop at all.