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Hi guys. Recently I upgraded my remote rig with an ASI 2600MM Duo, mounted on a reduced FRA400 ([email protected]). I found the sweet spot for broadband targets with 300-360s exposures at gain 100, but I can't find the right exposure time for NB (Antlia EDGE 4.5nm Ha). Looking at the histogram in NINA, I would assume an exposure of 1200-1800s, and even the dedicated plugin suggests something similar (1600s). However, I am doubtful, as a search here on AB shows that no one with this camera shoots for so long, not even with much slower focal ratios. The average is around 300s, with some exceptions at 600s. So I wonder if with this camera it is not crucial to overcome the readout noise, or if it is appropriate to modify the gain to have shorter exposure times. The setup is installed in Manciano under a sky with an average SQM of 21. Thanks in advance to anyone who wants to share some info with me.
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Not exactly apples to apples, but I have a Poseidon-M which has the same sensor (minus the duo part). I also have Antlia 3nm filters that I use with a reduced FRA600 shooting 420mm @F3.92. My skies are not quite as dark being about a bortle 4.5 or roughly 20.5 sqm. For my setup, I always use 0 gain for broadband and HCG for Narrowband. I do this to get the most dynamic range possible. Exposures times vary based on the target, but are usually 180 seconds for broadband and 600 seconds for Narrowband. Maybe this can help, but take it with a grain of salt as every setup is a little different.
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Unless it's an extremely bright target, my go to with a 571 sensor has been 20 minutes for narrowband with HCG. Bortle won't matter as much for narrowband, but for what it's worth, I'm around 20.7 on average for SQMs. Theres really no limit to your exposures with narrowband though. I've even shot 40 minute, but the actual gain feels alot less dramatic, plus you still need to plan out enough subs for rejection purposes.
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I shot regularly at gain 200 with NB with good results on my 2600mm. There are exceptions, but I find most targets don't require higher dynamic range.
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Imo, but maybe some will disagree or can correct me if I've been wrong all along: it doesn't matter. As I see it, at 100 gain you get almost no loss in dynamic range but definitive drop in read noise, so "Set it and never change it" is my go-to. I personally think the exposure time depends: I like to just keep it at 300/600s for broad/narrowband, regardless of my telescopes (f6.3, f10) but for casual RGB shots of stars to just add into a primarily narrowband image started playing with shorter 120s now. For me personally it comes down to mostly general conditions, rather than "the perfect technical exposure time". Imaging in a windy area and are prone to gusts, losing entire 10 minute subs will hurt more and be more likely than when just doing 5 minutes instead, so maybe taking a step down isn't a bad idea. OTOH, stacking and data storage is a breeze if you have only half the frames. I would totally do half an hour exposures in narrowband if I knew for certain that clouds, wind and guiding are always gonna play along perfectly ;) |
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Right exposure time should be calculated, but I think that 600-1200s would be okey with your specs. 300s is example too short for you to swamp read noise. It's totally different in cities but your sky is dark as mine so exposure time would be more important if you want to keep read noise level low. I have also FRA400/F3.9 and this with Antlia 3nm narrowbands and ASI294MM camera (sky 21-21.5). I use 900s (15 min) exposures and that is prefect for me. |
