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Gain and uncooled cameras / dither


Terrierist
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Folks, your advice, please.

Current set-up - William Optics ZenithStar 71, 9x50 Skywatcher with QHY5ii for guiding EQ5 (tuned) ZWO ASI178MC un-cooled with IDAS LP2 2" inline, EQMod/APT/PHD2.

I've seen some pretty stunning images produced with an un-cooled camera and wonder what I am doing wrong?

I'm running the camera at Gain-0 and Offset-25 as I'm applying old ISO logic lower the gain, less noisy the image. Does upping the gain a little have such an adverse effect with a CMOS camera? Being under city skies, what sort of integration time should I be looking at for something like, say, Pleiades or M13?  Would dither help, if so, how the heck do I set it up as for the life of me I can't sort it out! :D

I'm starting to build a pier and the base for this will be going down in the next couple of weeks, It really looks like astronomy is here to stay with me and especially imaging as I find it such a bloomin' interesting challenge!

Image attached, this was about 2 hours of lights with flats, bias and darks processed in AstroPixel Processor.1547320780_Andromeda06102018.thumb.jpg.31647e6c0968b00c9de6f9e9a0431b57.jpg

Edited by Terrierist
forgot to put my bit about dither in
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That offset seems a bit low, but don't take my word for it, you need to check your bias file and see what values you get.

Just take couple of bias frames and look for smallest pixel values across the image - no pixel should have as low value as 4ADU. Ideally you want your lowest values to be >40-50ADU. If you have some sort of stats tool - just look at min in each bias file, and if each of them has min pixel value larger than said ~50 then your offset is good. If not - look to increase it.

ASI178 is 14bit camera, and going lower gain just increases full well capacity, but actually raises read noise slightly.

Since you have version without active cooling, my guess is that you are using shorter exposures - in this case you don't really need all that full well capacity, so increase gain. Go for gain 51 or 111. Offset will depend on it, so adjust your offset once you determine which gain value will you work with. Gain 0 has read noise of 2.2e, 51 will have about 1.9e while 111 will have ~1.7e. With gain 51 you will have 13bit FW, and gain 111 about 12bit (or same as most other CMOS cameras at unity gain).

That will not give you significant boost, since read noise is already small, and thermal noise and LP noise are probably dominating.

Dithering can only help, so if you manage to get the hang of it - go for it.

Also good thing to do is to use a lot of calibration frames. And I mean a lot. So at least 60+ of each if you can manage, and I would not say that even over 100 is too much (I tend to use 256 of each, and only 64 of darks for 4 minute and over per single sub - but this is because I have set point cooling and build dark lib, you probably shoot darks each session, so you will need to balance with number of subs).

You will probably need lot's of total exposure, so try to get as much as you can - do single night session (you can get good 4h per night or even more), and process that. Then do another night on same target and add data to stack to see how much improvement there is. Of course, all regular guidelines apply - adjust your expectations based on conditions - transparency, moon phase, ... aim for target high in altitude (I usually set midpoint in time for session and look for targets that are close to meridian or crossing it around midpoint, so I shoot before meridian and after - same amount of time - that way I get target when it's highest in alt).

It is capable little camera, and you should be able to do some interesting targets - it has narrow FOV due to small size, so for some targets (like Pleiades) you will need to do mosaic to frame properly. As for expected exposure time, not sure if I can help much - it really depends on many factors - LP is the worst offender for me for example, but camera is capable, here is example of what it can do:

(80mm F/4.8 scope, and cooled version of ASI178, also OSC, total exposure time around 8h from red zone, so heavy LP at mag 18.5 - I think I used LPS P2 as well back then, but can't be 100% sure).

m101v2-optimized.png

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Sir, thank you for an amazingly comprehensive reply.

Could you help me with the following as I do not know how to achieve what you mention - "Just take couple of bias frames and look for smallest pixel values across the image - no pixel should have as low value as 4ADU. Ideally you want your lowest values to be >40-50ADU. If you have some sort of stats tool - just look at min in each bias file, and if each of them has min pixel value larger than said ~50 then your offset is good. If not - look to increase it."

How do I find smallest pixel values and ADU from the Bias images?

Thanks in advance.

 

 

Kev

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What software do you use to process your images?

Usually such software will have some sort of stats panel and show you histogram. Load bias sub and just take stats (min, max, average, median, ....) - look at min value.

If your software of choice does not have such functionality - you can download ImageJ (java based, free/open source software).

With it, it is the simple matter of:

File / open (select your fits file)

Analyze / Measure - and look at the results window, there should be Min column - inspect min value there.

Since you use APT (I'm not familiar with it), there might be info window on your current sub - it might list min pixel value in current sub - that would be easiest way to do it - just take bias at certain gain / offset and examine info. I use SGP to capture and it has info window listing all main stats for sub - like this:

image.png.132e4683905c7eaab453ccad4d442950.png

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