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In the previous section, we used a formula to calculate the exposure time for a particular gain setting. Since I don't want to do that calculation every time I image, I've chosen some default exposures. I used a spreadsheet to calculate exposure times for several different gain settings and picked the ones I preferred. Now, I have one setting I'll always use for Lum, another for color, and a third for narrowband. This makes planning easier.
What makes one gain setting better or worse than another? To answer that, we start by understanding dynamic range.
Dynamic Range
Dynamic Range
Dynamic range expresses how many different levels of brightness a camera can capture; the more the better. High dynamic range allows for smooth transitions between light and dark. It also gives us more room to expose stars without blowing them out and losing color.
DR is expressed by the photographic term stops. More stops equal more dynamic range.
A camera loses DR as the gain increases, as seen in this chart for my 294 MM Pro. Gain is along the X-axis. As the gain increases, the DR drops. Around gain=120, DR jumps back up due to this camera's high gain mode. HGM restores DR after a certain gain, but the DR resumes dropping as the gain increases further. Not all cameras have this feature.
The second big tick on the X-axis scale is gain 50, where the DR is 12.5 stops. At 100 the DR is 11.8, and so on.
A high dynamic range is important for me, so I only use gain settings where the DR is fairly close to the maximum. However, there are other considerations, as well:
Trade-offs Of Gain Settings
Advantages of a lower gain setting:
Higher DR and longer exposures. Longer exposures reduce the number of subframes, requiring less storage space and speeding up pre-processing.
Disadvantages of a lower gain setting:
Higher read noise. Fewer subframes may make it difficult to reject satellite trails when stacking, and drizzle requires a minimum number of frames to work. Exposures may be longer than a mount can successfully track.
Advantages of a higher gain setting:
Less read noise and more, shorter exposures, Shorter exposures can help if your mount doesn't track well. It is useful to have many exposures for effective rejection and drizzle.
Disadvantages of a higher gain setting:
Lower DR and more subframes, which take up more storage space and require longer to pre-process.
So Finally, Let's Choose Some Default Gain Settings
I made this spreadsheet for my 294 MM Pro at Starfront Observatories in Texas. The skies are too dark for a swamp setting of 10, so I used 5.
I chose a few gain settings just to see what exposure times would look like. From this chart, i feel the best choices are Gain=50/166s for Lum, Gain=50/498 for RGB, and Gain=121/492s for narrowband.
Gain=50 gives me almost full dynamic range, but uses shorter exposures than gain=0. Higher gain values reduce the DR and don't make the exposures much shorter, until gain=121, when they become so short I'd have hundreds of subframes to store and process.
Any narrowband exposure below gain=121 is ridiculously long. I'm not going to try tracking for 9000 seconds! Since I don't a significantly shorter exposure time at gain=200, I settled on gain=121.
I'll round these exposures to 160s, 480s, and 500s.
Here is another chart using a swamp factor of 10. The Lum filter exposure time is just too short and would lead to hundreds or even thousands of subframes. The narrowband exposure time is too long to track and wouldn't provide enough subframes for rejection or drizzle.
However, RGB is 70s, which isn't too bad. I'll round it up to 90s to keep things easy to remember. I've tried both Gain=50/480s and the Gain=121/90s. I tend to prefer the 90s exposures.
So I now have default exposures of:
LUM - Gain 50 for 160 seconds.
RGB - Gain 121 for 90 seconds.
Narrowband- Gain 121 for 500 seconds.
Trial/error showed that Offset values of 5for low gain mode and 10 for high gain mode work well.
I use these exposures for almost every project. This keeps my sequences simple, or at least more simple than they would be, and reduces the number of different master dark frames I need.
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