Advice please on sub length and gain for ZWO 1600mono

[Spookyer]

Hi all, I am pretty new to using my ZWO ASI1600 mono cooled camera (my first cmos camera) and I am currently trying to wrap my head around sub length and gain in relation to particular targets.

I have done my research on forums and watched quite a few videos. I found the work from the Sharpcap creator Dr Robin Glover the most comprehensive.

For my FSQ106 f5 refractor the numbers are telling me with my estimated 4.2 bortle skies that I should be around here these durations for each filter at 100gain:
Lum 26 seconds
RGB 79 seconds (3x Lum)
NB (3nm) 44mins (100x Lum)

My thinking at the moment is that 100gain would work for LRGB but that I should maybe increase the gain to 150 for NB work and bring the optimal sub length for NB down to 28mins.

Does this seem a reasonable start point?

Cheers
Brett

[Drac0]

Did you use his calculator (http://http://tools.sharpcap.co.uk/)? According to my calc there, @ F5/100 gain under bortle 4.2 skies your exposure lengths could be shorter, depending on the acceptable noise percentage (5% or 2%).
(Formula = %acceptable noise multiplier x read noise squared / sky electron rate).

Lum: 22 to 54 sec
RGB: 65 to 162 sec
NB: 2000 to 5000 sec (30 minutes plus, getting past the limits of the camera exposure times)

This is based on calculations by Dr Glover, using Bortle 4.2 skies, F5 scope, 100 gain giving 2.0 read noise, 3.8 pixel size, 3nm NB & LRGB filters.

Increasing the gain up to 150 brings the read noise down to ~1.6, reducing the NB time to about 20 minutes. While these may be 'optimum' times, you do have to consider your mount capabilities and other factors that may affect guiding or the quality of the final results - it's no good using these figures if you end up throwing out half your data. Personally I would stick to 10 min subs and just take more subs.

Just note that the calculations are greatly affected by the bortle level used, I underestimate mine for the calculations on my spreadsheet. My skies are supposed to be ~4.3 but I use bortle 5 to allow for ever increasing light pollution. Using Bortle 5 for you brings the 'optimal' times for 3nm NB down to ~10 minutes.

Cheers,
Mark

[Spookyer]

Mark, appreciate you looking at this, I think I am in the ball park here. Agree the long NB may not always be practical. Just trying to get the theory correct.

Is the idea of going to the higher gain correct for NB targets? Is 150 about where I should be aiming?

If a target is quite high in NB signal should I be leaving the gain at 100?

cheers

[Drac0]

With the 1600MM the only advantage of higher gain is a slight reduction in read noise & shorter exposures. Unlike newer cameras there is no point where HCG mode kicks in to get back full well depth, dynamic range or a large drop in read noise. Maybe someone using a 1600MM can chip in here, but I see no real problem either way you go - the read noise reduction starts levelling out after 100 gain & the changes are insignificant compared to other factors. I use the same gain for both LRGB & NB, because of the camera I use, but I do know others that use 0 gain for LRGB & 120 gain (HGC) for narrowband with the same camera (294MM).

The 1600MM has a 'unity gain' of 139, so perhaps try that? I hope someone using a 1600MM does drop by as this is all based on calculations & real world results could be different.

Cheers,
Mark

[oska]

I have one and similar skies, using Antlia 3nm.
You have your back of the envelope calcs done, it's probably time to run some tests.
Gains of 0, 75, 139, 200, 250, 300
Exp 30, 60, 90, 120, 180, 300, 600
and evaluate from there.

I lazily chose 139 & 200 mainly, with exp of 60 (L), 180 (RGB) and 300 (SHO).

In practice, I confirm each target with NINA's exp calc just to be sure(ish)

caveat:
1: nightly seeing will mess with the numbers more than any one thing.
2: I'm lazy and didn't do a rigorous analysis of the results

[g__day]

I am just getting back to astro imaging after a long time off - I have two imaging scopes side by side using ZWO cameras (about 13km NW of Sydney centre).

I am using a ZWO 1600MM-c + 7 filter wheel and ZWO EAF into Williams Optics 132mm refractor and a ZWO 071MC-c Pro into a Moonlight focuser into a Williams Optics 115mm refractor.

Last few nights I am doing test runs on NGC 5139 - mostly I shoot 2 min subs for my testing purposes. I control the 71MC-c using APT and the 1600MM-c using SGP - all at 1x1 binning.

From SGP the 1600MM-c colour filters RGB recieved a gain of 240 - don't ask why - I know how to set the gain and offset (to 50) now per imaging event's sub. The shots were all usable though a tad overexposed. The Luminance shots were totally overblown and useless - I will try a gain of 100 or lower next clear night.

On the 115mm scope with the 71MC-c the results were:
1. 180 sec shots with 240 Gain = all shots fine - around 2,200 stars detected by Deep Sky Stacker (DSS)
2. 240 sec shots with 100 Gain = all shots fine - a bit dim, DSS detects 650 - 750 stars
3. 30 sec shots 240 Gain = 50 stars detected by DSS
4. 60 sec shots 240 Gain = 130 stars detected by DSS
5. 120 sec shots Gain 240 = 3,400 stars detected by DSS
6. 300 sec shots Gain 240 = 787 stars detects - image looks way to overblown at the core

So on the 71MC-c / 115mm refractor I would guess 2 min subs at 240 Gain is ideal - going past this longer subs at lower Gain detected far less stars (I guess the core starts to blow out and you loose individual stars). To me the goal with a OSC camera would be I imagine on this target - find the settings that give you the most detected stars and highest score in DSS. I must admit I never thought of using DSS in this way until now - to do a test scoring run to figure out what camera exposure and Gain settings will land you the best subs before you go for a real imaging run for your local night skies - but now I realise this it seems illusive obivious!

For the 1600MM-c I am going to try Luminance shots of 2 - 3 mins between 0 - 140 Gain and see what I get; for the colour filters 240 gain was usable but I will vary it down and bit and see what the results look like according to DSS #stars, FWHM and Score results!

Hope this helps - happy to share data as we are all learning some new things here!

[Camelopardalis]

Brett, take a few dark shots at your desired gain and offset 50 (default) and note the median value. You can get this value by loading the dark frame into almost any imaging software, including SGP, PixInsight and FITS liberator (or many others).

Ray (Shiraz) calculated a bunch of tables for target ADU values while imaging. I’ll see if I can dig the thread up, but it makes everything easy.

https://www.iceinspace.com.au/forum/showthread.php?t=155908

IIRC, for gain 139 add about 400 to the median value obtained above. This is your target ADU. Now aim the scope high up at an uninteresting patch of sky on a moon-free night. Run test exposures at a spread of exposure times until the median value in the test shot approximates your target ADU. Job done!

This technique should be repeated for different filters…for example, narrowband filters will cut out more light and it will take longer to hit the target.

[Spookyer]

Thanks Duncan, I will give that a try when the weather lifts.

brett

[Drac0]

Glad a few 1600MM users have chimed in. "D