|
Exposure time is always going to be the ultimate factor and this is the same in both landscape photography just as it is in astrophotography. Taking an early sunrise with 1/200s exposure at F/16, whether you’re at ISO100 or ISO12800, is going to be seriously lacking enough photons for a clean image.
Changing the gain in the ASI1600 changes the read noise and the dynamic range. In AP, total integration is the key determining factor because there is no way of compressing 20 hours of exposure into a SINGLE exposure.
With the ASI1600, as you increase the Gain you decrease the read noise. This has less of an affect on total integration as it does on each exposure required. At Gain 76 you have a read noise of about 1.9e- while at Gain 200 you will be at about 1.3e-.
With narrowband you may need 600s exposures at Gain 76 to get sufficiently about the read noise. At Gain 200 that may drop to 280s.
This does NOT mean that Gain 200 is as good as Gain 76 with half the exposure time. It only relates to the exposure time that they both become read noise limited in the darkest parts of the image.
Where all of this becomes more important is between your ASI1600 and a KAF-8300 with its 9e- read noise. If you need 300s at Unity (~1.6e- read noise) you will need around 2 hours (that’s a single exposure mind you) to get the same darkest parts of the image to the same level above the read noise.
The reason you need to change the exposure time for your flats when changing the Gain is that the Full Well Capacity gets smaller as the Gain is increased. At Gain 76 you need 4000e- to get to 50% capacity. At Unity 4000e- fills the wells to basically saturation so you need to halve the exposure time. By Gain 200 you need to halve the exposure time again (from memory) to keep it at 50% we’ll capacity.
|