As we know all ASI cameras enable us small settings to adjust through ASCOM camera driver such as Gain and Offset. In this short article, we will show you a few test images of how cameras setting affects your captured image. For this test, we used ASI1600MM-Cool and ASI071MC-Cool, two very popular ZWO DSO cameras.
ASI ASCOM camera driver interface is very simple. In this case, we tested only predefined settings values for Gain and Offset. We have three options; highest dynamic range (G: 0 O: 10), unit gain (G: 139 O: 21) and lowest read noise (G: 300 O: 50). Which setting is the best for DSO imaging?
ASI1600 camera setting test
The first test we made with a 30-cm f4 telescope on G53F Friction Drive Mount was used, ASI1600MM-Cool and L filter. We point telescope on very popular target M1, the Crab Nebula in the constellation of Taurus. We captured three images with the highest dynamic range, unit gain, and lowest read noise settings. All three images were exposed 200-seconds at -20°C and bias, dark and flat not used. As we see lowest read noise settings give us very saturated images (stars and nebula). The reason for this is very low full well (bellow 1K) at this gain and offset values. Unit gain gives us a much better result. We have saturated only brighter/bigger stars. From our point of view, the best result gives us the highest dynamic range. Gain and offset values for this preset give us full well 20K. We got not saturated stars, good background patterns and also very good SNR. If we exposure too long the sky glow will cover the read noise and there is no big difference with low read noise (LRN) and high read noise (HDR) camera settings.
Short exposures with HDR and LRN camera settings on same objects give us much more interesting results. Both images were exposed 10 x 10-seconds at -20°C and bias, dark and flat not used. On this case we see LRN image have less noise and higher SNR.
Another test images were exposed 20 x 5-seconds at -20°C and bias, dark and flat not used. LRN image has less noise and higher SNR.
For the final test, we compared 1 x 1-seconds exposure image at -20°C and bias, dark and flat not used. The result is very similar. LRN image has less noise and higher SNR.
ASI071 camera setting test
We also made a test with a 135-mm f2.8 lens, ASI071MC-Cool and L filter (2″ IR-UV CUT ). We point telescope on a very popular Virgo Cluster (cluster of galaxies). Also in this test, we captured three images with the highest dynamic range, unit gain, and lowest read noise settings. All three images were exposed at 200-seconds at -20°C and bias, dark and flat not used. As we see lowest read noise settings give us very saturated stars, especially in R in G channel. At these values we have full well the only 3K. Unit gain has similar issues, but yes with lees saturated stars. Also in this test, we think the best result gives us the highest dynamic range with full well only 46K.
Our visually conclusions are, highest HDR settings give better result at longer exposures. We have got not saturated stars, good SNR and also very good or same read noise pattern like images with LRN values. But if you preferred shorter exposures due your “lucky imaging technique” or mount track limit we prefer LRN settings.