The Belt and the Sword
4 months ago
Years ago I did a fair amount of visual obs, a little AP, just recently I snagged an old orange C8 and Meade LXD75 GEM and finally got a chance to get to some dark skies. But for a first time out I didn't put the camera on the scope, just the mount. Here's the Belt and Sword of Orion from that night:
Total exposure: 12 min (6 x 2 min exposures). Some random observations and a question for you AP pros.
Camera: The Nex7 does better than I expected in the H-alpha (far red) region. The Horsehead is clearly visible below the Flame nebula -- shadows have been boosted about 10% or so. Could use more exposure on both the Horsehead and M78 (upper left) though. The flip-out screen and magnified focus are very handy, much better than peering through a VF. But the lack of intervalometer function is not.
Lens: The OM100/2.8 is a great little terrestrial lens but evidently not so great at AP. There's a fair bit of purple fringing, and the diffraction patterns are kind of funky. Still, not a bad lens, and I'd use it again for this purpose if I need the FOV. Pretty even illumination at f/4 -- at least on the N7 -- so I didn't use flats. This is slightly cropped (about 10%).
Processing: Each 2 min exposure is actually a sum of 4 30 sec exposures. Even so the heart of the nebula is blown out, and I had to crank hard on the curves before summing to retain detail in the sum. I can get more out of the Horsehead and Flame by using more exposures, but then M42 washes out. Next time I do M42 I'll turn down the exposure and use more frames... No darks, no flats (would probably need flats wide open, though, maybe in less dark skies as well). Not really sure I know what I'm doing here (actually, I'm pretty sure I don't).
Question: How do you assess color? After working on an image for so long every color cast looks equally right and equally wrong. For this particular image I suspect the cyan is too strong, toning it down makes every unnaturally(?) magenta (the Horsehead comes out more though). Partly that's the PF, which is a lens issue (or rather, lens interacting with CFA and sensor issue), but even the background stars pick up a pink cast.