Quote:
Originally Posted by HeavyT
I'm imaging at f7.6, using an ASI1600mm cooled to -15C.
Taking 900s subs, I routinely clip at least the end of the histogram capturing Sii and Oiii. On Eta Carina I could almost properly expose Ha at 420s.
I like to try to capture faint nebulosity and I know I have the option of increasing gain from 139/15. But I haven't, because I don't want to lose dynamic range.
Perhaps it's desperation, but the thought occurred to me: dynamic range would be the exposure range between highest and lowest exposure. If these values are governed by the speed of your scope, then boosting all signal would increase both highest and lowest values - bringing my "lowest" value into the histogram properly.
So would lowest and highest be increased by the same amount in this situation, or do they get compressed closer together? Given that the scope is slower to begin with, so the photons have a harder time building up on the sensor.
Thinking about say increasing gain to 200 only. I know if I went to extremes with the gain, I'd lose range.
I know the best option would be to collect 3600s subs (I've seen tables with this recommendation for this imaging speed and gain setting), but gathering 75hrs on one target over shorter nights is not something I'm keen to do.
Anyone have thoughts or experiences to share on this?
Thanks and clear skies
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Suggest that offset 15 is too low and a little bit of the faint nebulosity is probably being thrown away. Maybe set it at 50 and leave it there for everything. If you change offset, you will need a complete new calibration set though - sorry.
Other than that, there is not much change in final dynamic range after stacking with this camera by going to higher gain - up to about gain 100. Over gain 100 you start to lose a bit of final dynamic range at higher settings, but in compensation, the final read noise drops, so you get better results on faint nebulosity. Gain 200 would seem to be a good compromise for faint narrowband. Use lower gain and shorter subs if you want to retain detail in the very brightest stuff (eg the homunculus), but you will reduce the signal-to-noise on the faint stuff by doing so (in your terms, maximum signal and noise get "compressed ..together" at high gain). There will always be this type of compromise to be made when trying to image something with wide variations in signal level and it depends on what you value in the final image. Modern stacking software allows high dynamic range stacking, where combinations of exposures taken at different sub lengths are combined to fill in lost detail in the brightest bits - never tried it, but might be useful if you value bright detail.
With a slow scope in narrowband dark sky conditions, longer subs will always be better. The final faint signal after stacking will be the same for a given total exposure time, regardless of sub length (eg signal(10x60) = signal(1x600). However, the total read noise will be higher if you use more/shorter subs (eg RN(10x60) = 3.3xRN(1x600). ie, in the example above, the (faint stuff) signal to noise ratio will be 3.3x better with 1x600 than with 10x60 (ignoring shot noise). Agree that it is not practical for numerous reasons to use extreme sub lengths - maybe settle on 10-20 minutes at gain 200 (depending on your mount).
cheers Ray