C90 Mak First Light: the Moon!

 

From my first session with my brand-new Celestron C90 Mak. f.l. 1250mm, f/14, so it's not bright. And yet, with my EOS 7D set at ISO 800, this is 1/400th of a second, and even half that might have produced good results.

It was also cloudy; this was snapped between the clouds, and actually with a little haze still present. Kenko Sky Memo S equatorial mount and clock drive, with alignment only guessed at using a compass, and yet the tracking worked reasonably well -- about as well as I ever used to manage with my old Edmund Scientific reflector, but not nearly as good as I managed last week using the polar scope.

If I'm doing the math right, a 90mm objective observing 600nm (orange) light has a diffraction limit of 8 microradians, or 1.65 arc-seconds. The moon here is about 2700 pixels across, smallest features are 5 pixels or less, call it 1/500th of the moon's diameter. 1/500th of 1800 arc-seconds is about 3.6 arc-seconds, so I might actually be close to the maximum theoretical resolution? Off by less than a factor of two would be pretty good. At first glance, I didn't think this scope was anywhere near that good. There does seem to be some scattering I can see in some of my test images, which definitely adds noise/reduces contrast.

Definitely still a lot of learning to do to get the best out of this new tool, but not a bad first effort.

Beginner's Guide to Working with Nebulosity 4

 

I've been getting back into both photography and astronomy, so, astrophotography. The picture above is the Orion Nebula, shot on Nov. 22, 2020, and processed with Nebulosity 4 software as well as Gimp. It represents my best effort so far, in optics (improved focus and exposure), mechanics (polar alignment, tracking and vibration), and image processing. I think the raw data I took that night still has more to give, so I'll keep working on it. Click on the image for an expanded view!

Nebulosity 4 is astrophoto software that can control some digital cameras and provides a great many editing features for the resulting images. I acquired it because I liked its image stacking features the best of three or four tools I tried on the Mac, but I'm gradually using more of the features. I don't have any real plans to shift to full laptop/software control in the field right now, but you never know.

It turns out that, despite the more-expensive-than-a-game-but-ridiculously-low-for-professionals price of US$95, documentation is a little sparse, so it has taken me a while to kind of grasp the expected workflow, including learning about darks and flats and biases. Most frustratingly, after learning about those concepts, I spent quite a while trying to understand the basic mechanics of working with Canon CR2 raw image files. So, I'm collecting what I've learned so far, in this blog post.

Resources:

• Really, the first thing to look at is the Nebulosity 3 manual, since there is (AFAICT, as of 2020/11/26) no separate Nebulosity 4 manual yet. (Frustratingly, I kept the link directly to that PDF, but not the page that linked to it, and now I can't find the web page -- an indication that the website needs some love? Ah, found it again -- it's under "Downloads", but you have to scroll down.) In that document, on p. 29, you'll find a nine-step formula for processing images -- exactly what I was looking for, and which took me hours to find. It's the basis for what I write below.
• This YouTube video by Alex Cardenas is fantastic. It's a near-perfect tutorial on how to do the stacking in Nebulosity, once you have your set of frames ready. However, Alex was working from separate sets of R, G, and B files, whereas I'm working first from JPEGs and then from Canon raw files.
  
• This info on Canon CR2 raw image files was a big help in learning about what's going on in the raw files themselves, and what needs to happen to turn them into color images. In particular, Section 4 of that shows how pixels are laid out on the sensor chip, which helps you understand what you're seeing if you are looking at the whole raw file in black and white. Armed with this info, I was able to figure out what operations needed to happen, then the next step was to learn how to make them happen in Nebulosity (see the first bullet point in this list).
  
• This presentation by Craig Stark from 2014 is good enough to be useful, but it's long, almost two hours. The best part I've seen so far (I'm about halfway through) starts at 22:10, discussing the basic linear math of what he calls "Classic dark subtraction". I'm sure there are other good sources on the particular topic on the web.

I'll put more about my process into another post.

The image at the top:

Orion Nebulae M42, M43, in the sword hanging down from Orion's belt. The image is turned sideways; the two bright stars to the left are two-thirds of the belt.  Around the lower one (left as we usually think of it), Alnitak (zeta Orionis), there is visible also a bit of nebula.

• Camera: EOS 7D body, APS-C sensor
• Lens: Canon EF70-200 f/2.8L USM lens, set at 145mm f.l.
• Settings: ISO 3200, f/2.8, 30 second exposures
• Mount: Kenko SkyMemo S equatorial drive and leveled tripod
• Images: high-quality JPEG
• 47 light frames
• 15 dark frames (camera is old, with a lot of stuck pixels, so the darks really help!)

The focus may not have been perfect, but was pretty good; achieved using simulated exposure and digital zoom to set focus, and at max zoom could see quite a bit of vibration from the mount.

I'd like to reshoot with zoom set at 200mm, and possibly stopped down to see if that reduces some of the coma, but fundamentally I think this is pretty close to the capability of this lens, sensor and mount.  Raw images are also pretty noisy, should try ISO 1600 or even 800.

Shot around 3am local time, about 3.5 hours before sunrise.  Shot at Arakine Dam, Chiba Prefecture; perhaps the best dark within two hours' drive of our house, but you can still see quite a bit of sky glow from Chiba city and Tokyo.  The Himalayas it ain't.

Stacking done with Nebulosity, which seems to be excellent for this task.  A little more flexibility in adjusting the light curves would be nice, but that's easy in Gimp once the alignment and stacking are done.  For this image, I simply set a black level floor of around the sky glow, no other light curve tweaking; cropped in Gimp.