Thanks, I’m familiar with “seeing”. The unwanted, different noise at different ISO’s in your stacking makes sense. Like I said, I was just trying to make sense of why the bright stars aren’t more blown out than they are, in your final image.
Well, as far as stars go....they are, stars.
When it comes to the cutting edge of "low light" imaging, most of the more recent technological innovations that you can find over on sites like ImageSensorsWorld, are rated on their ability to see "in starlight". Starlight is in the "Fraction of a Lux" range as far as total illumination goes...they are exceptionally dim, and pretty much anything, including a half moon, can blot them out of existence.
At ISO 400, under 0.01 Lux, your sensor "speed" is actually quite slow...relatively speaking. In normal photography terms, ISO 400 is moderately fast. ISO 1600 is quite fast. ISO 6400 is ultra fast. In imaging, ISO 400 is probably more akin to "normal" ISO 50, and ISO 1600 is more like ISO 200 or 400. You actually have a lot of time to expose before your pixels saturate, even with a sensor like the 7D's, which only has ~20ke- at max saturation @ ISO 100 (and only around 6ke- at ISO 400!).
It should be noted that the very central part of most stars, the "centroid", is often clipped. When I image galaxies, which I usually do at ISO 800 because they are so dim, most of the larger and brighter stars are a little clipped in the center. But that usually isn't a big deal, because of the way you process. Processing is all about bringing up the shadows without modifying the highlights at all...your compressing a MASSIVE amount of dynamic range into 8 stops...so its lift, lift, lift, lift! In astrophotography terms, it's called stretching, but that is only because we usually start out with 32-bit floating point TIFF images, which are capable of storing dozens of stops of dynamic range...so we "stretch" our images initially, then convert down to 16 bit integer TIFF, then do some more stretching, pushing, and pulling with levels and curves to bring out the nebula detail.
Obviously I don’t know much of anything about the technique. Can you explain “dynamic dark scaling”? You don’t have to use over 300 words, I don’t want to take up all your shooting time!
Dark frames are intended to remove fixed pattern and dark current noise from light frames. In order for that to be possible, the dark frames must be created at the same shutter speed, ISO setting AND temperature as the light frames...since all three of those factors affect how dark current (which doubles/halves every 6°C) and fixed pattern noise present. Dynamic dark scaling is a newer technique in applying dark frames, that allows you to use darks taken within an acceptable relative temperature range to your lights. There are a few different specific algorithms, but the general idea is to scale up or down the noise levels in a master dark to match the temperature of each light frame as the dark is subtracted from them one by one.
It goes without saying that it's extremely difficult to create a library of dark frames that matches every potential light frame. You have a number of standard exposure times...30s, 60s, 90s, 120s, 180s, 210s, 240s, 300s, 360s, 420s, 480s, 540s, 600s. On Canon sensors, the two most acceptable ISO settings are usually 400 and 800...go below that, and read noise is too high, go above that and read noise doesn't shrink appreciably relative to the drop in max saturation. Some people use ISO 1600 anyway, and sometimes it's just plain necessary. That right there is a factorial of 13 shutter speeds and three ISO speeds. Throw in the range of temperatures from -10°C to 80°C, with a tolerance of 2°C, and you have a factor of 13 shutter speeds, three ISO speeds, and 45 distinct temperatures. It's practically impossible to create a dark frame library complete enough that you could match the right darks to any given light frame.
Hence the benefit of dynamic dark scaling. It'll let you create a library of darks spanning far fewer specific temperature levels. You might stick the camera in the freezer for -10°C and 0°C, in the refrigerator for 10°C, and maybe get some darks for 20°C and 40°C. Dynamic dark scaling can take care of the rest. Bitch of it is, you still need to get darks for these five temperatures for the same 13 shutter speeds and three ISO settings, but a least it's only five temperatures, rather than 45 temperatures!