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ZWO ASI1600 Mono Raw Format: 8bit or 16bit

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I just bought an ASI1600 Mono camera and i'm puzzled by which RAW format to choose.
The camera has a 12bit ADU, so saving images at 8bit (16MB files) some information should be lost. On the other hand 16bit (32MB) gives room for all information contained in the image. Which option are you guys using?
I would go for the 16bit just to be sure but I have to transfer the files over the internet so size is an issue.
Thanks
4 years 5 months ago #44973

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I am using 16 bit RAW. When I use 8 bit the results are horrible.


HTH, Wouter
The following user(s) said Thank You: Ferrante Enriques
4 years 5 months ago #44974

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hi Wouter,
I was only taking darks so far so had no chance to check a visual result.
I will stick with the 16bit option as you suggested, thanks.
4 years 5 months ago #44975

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Hi,

Some more advise for you if I may: for image processing it is essential that all frames (bias, dark, flat, light) have the same bit depth otherwise you'll enter a nightmare. Also, bias, dark and light should be taken at the same gain, offset and temperature. It is less important for bias but I'd advise to do that anyway. Dark and light should have the same exposure time to optimise reducing amp glow. Fortunately the ASI1600MM has active cooling so you can shoot the darks during day time. I basically created a dark library for 30, 60, 90, 120, 180, 300, 600 and 1200 seconds. for L, R, G and B I typically use 180 or 300 seconds and for Ha 600 or 1200 seconds. The library takes about a full day to shoot but it can be used for a year or even longer.


Wouter
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4 years 5 months ago #44979

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hi Wouter
thanks again for your support.
I already had a CMOS camera (ASI 071) so the amp glow issue and related impact on darks and flats is something I was able to manage.
First time with a mono, so the input about the duration is very helpful. Btw at what gain / offset you image? for LRGB and NB
Thanks
4 years 5 months ago #44994

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For simplicity I decided to use the same gain and offset for all my images. There is a big discussion on gain and offset for the ASI1600 cameras on CloudyNights, see

www.cloudynights.com/topic/570020-gain-s...-for-asi1600mm-cool/

In short: the lower the gain the bigger the dynamic range as can be seen in comment #5. After discussing this with some fellow-Dutch amateurs I decided to stick to Gain 75 and Offset 12. That seems to work very well for me.
4 years 5 months ago #44995

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I hope to be getting one of these soon... maybe Christmas? But I've been looking around for anything I can find on this camera and that link is a keeper! Thanks
4 years 5 months ago #45005

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That's OK for your location, Wouter, with pretty dark skies. If you are living in a light-polluted city, you are gaining nothing from gain 75 and you are losing valuable exposure time instead.
I am imaging under Bortle 8/9 conditions and for those conditions gain 240, offset 40 and exposure times of 8s for L, 20s for R, G, B, 120s for Ha, 60s for O3, 120s (gain 270!) for S2 seem to work best for me.

Subject to further optimization, as always.

For any Bortle skies in between, I would interpolate.
You will have to experiment and optimize for your own sky conditions.

Gravity sucks and light pollution is a bitch!

Cheers

Jo
4 years 5 months ago #45009

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Hi Jo,
above gain 200 the read noise doesn't decrease any further while the full well capacity and thus dynamic range are dropping linearly. I do not see the advantage* of going at such high gains given the fact that moreover you are forced to do shorter frames that in the overall integration increase the read noise.
For my first tries (Bortle 8/9) I'm settling at 75/12 for LRGB and 200/50 for SHO adjust time according to saturation.

(*) Unless you are using lucky imaging or want better guiding.

ferrante
4 years 5 months ago #45014

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Hi Jo, Ferrante,

Yes, different light pollution requires different gain and offset settings. Luckily CloudyNights comes to the rescue here as well. See this post

www.cloudynights.com/topic/573886-sub-ex...00-and-maybe-qhy163/

and particularly the tables in the first and second entry there. Those should allow you to interpolate for your own location, gain, offset and focal ratio. Please note that the tables state the shortest exposure time possible to still be shot noise limited so perhaps best not to take shorter exposures. Longer are possible though that may lead to more saturated stars. The downside of using short exposure times is that you need to process many more subs which takes longer. So, as with chalices, choose wisely ;)


Wouter
4 years 5 months ago #45016

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Thanks, Ferrante. That is all correct. But let me briefly explain why I arrived at the values I have stated, which work for my location, but may have to be optimized for other locations.

First, I need to make a correction. I am not using 240 gain for LRGB, I am using unitary gain, i.e. 139 and 21 offset with the ASI1600MM-Pro, according to the tables that come with that camera.
I am using 240 gain for Ha and O3, 270 gain for S2. 120 s exposure for Ha and S2, 60 s for O3 in the city.

Obviously, all images should be recorded at 16bit depth.

Now to the reasons why.

I found the cloudy nights tables for which Wouter provided the link very helpful, but a bit too complicated. So I simplified them for practical purposes. The final result is nearly the same.
The cloudy nights tables recommend that the mean ADU should be 800 - 1100 ADUs above the dark ADUs, which in my case with the ASI1600MM are ~690 ADUs at 120s exposure time and -10 C sensor temperature. Given that I am imaging from the upper end of light pollution from the middle of a white zone, I aim at a mean ADU value of 1200 above the dark ADUs, so around 1900-2000. Read noise does not come into play under conditions of extreme light pollution, so one can safely neglect that, I think. It is extremely low for the ASI1600MM anyway.

The next consideration is that I want to avoid taking exposures longer than 120s. First, short exposures, as you point out, minimize guiding issues and result in fewer wasted frames. Second, where I live I have to contend with lots or airplane traffic. I have trails on approximately 10% of my 120 s exposures, so if I go longer, that % will only increase.

With these two constraints, my critical parameters are set. The remaining one, i.e. how to arrive at 2000 mean ADU with 120s exposure time max, is gain adjustment. That needs to be calibrated experimentally for the particular conditions and no specific gain can be given here, except for stating that the lower the better.

In other words, if you want to shoot longer LRGB exposures, you certainly can, I can just say that with all the noise coming from Bortle 8/9 light polluted skies, long exposure times are a waste. The dynamic range one wins by lowering the gain is more than offset by the light pollution, up to 240 gain in my case (the reduced dynamic range and increased noise becomes noticeable at 300 gain and gets rapidly worse above, so I definitely want to stay below that).

Aside from that, LRGB from a light-polluted city really gives acceptable results only on the brightest objects (like M42). I have almost given up on LRGB imaging from the city, I am almost exclusively using narrowband, i.e. Ha, O3 and S2. That still works, but even so, I definitely recommend filters of 5 nm or less bandwidth. The narrower the band, the less the effects of light pollution (obviously).

I hope this helps. Please let me know if the reasoning makes sense to you and if you see any room for improvement.

Cheers

Jo

PS: Check out the images I have posted under my profile. You can clearly see the progression in quality over time as I adjusted the values as stated above, especially when you compare the images of the Trifid with OSC and with narrowband. The noise in the OSC image is horrific, greatly mitigated by the narrowband imaging, even for the reflection nebula part.

And before I forget it, DEFINITELY get the cooled version!!!! Cooling the sensor is absolutely critical for reducing noise. Do not skimp on the $300 difference in price tag. I discovered how HUGE the difference is when I forgot to turn on the cooler one night last summer. The difference was between a good image and NO image, just noise and a wisp of haziness on top of it. Buying the uncooled ASI1600 only makes sense if you are living north of the Arctic circle and even there, global warming will render it useless within the next few years.

To illustrate the point: This is an image taken with a DSLR, though, from my area. ISO 400 30s, resulting mean ADUs 75! (on a scale from 0-255, so 8 bit). 500 exposures stacked and stretched to optimal range in PixInsight, but not processed further.
And this is probably the brightest LRGB object in the sky.

The following user(s) said Thank You: Ferrante Enriques
Last edit: 4 years 5 months ago by Jose Corazon.
4 years 5 months ago #45028
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hi Jo,
thank you for sharing your experience. And I understand that given the constraint you have this is an optimal solution.
Luckily I will leave the camera soon under a Bortle 2/3 sky and do not have issue guiding more than 5 min.

When you say that CN recommends "mean ADU should be 800 - 1100 ADUs above the dark ADUs" you mean a sky background area of your light frame not the dark frame mean ADU, right? else you are not taking in account light pollution.
Ferrante
4 years 5 months ago #45039

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