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Old 20-11-2020, 10:48 AM
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12, 14, or 16 bit

I've had a gut full of my QHY9 and the dark streaks next to bright stars. The camera I am using is about 8 months old and it is doing it as well as the last one. I am in discussion with someone who may be able to fix it, if so, well and good.


The real crux of my thoughts are if I decide to purchase a new camera, and I go down the CMOS route which seems to be a popular choice, do I really need a 16 bit camera or will 12 or 14 bit suffice? If computers can only display at 8 bits(if I have that right), then your dynamic range is compressed already, is it not?


Any insight would be helpful. I tried to read up about bit depth and I think I get it.



Stay safe.
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Old 20-11-2020, 12:48 PM
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Camelopardalis (Dunk)
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Bart, regardless of which camera you go for, you'll want to read up on how to get the best from it. This means not using gain that is so high that it overly crushes the dynamic range.

Whether it's 12-bit or 14-bit or whatever isn't so relevant. What happens when you stack a number of images is that the precision of the end result ends up (more or less) the same. I'm oversimplifying, but that's the bottom line.
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Old 20-11-2020, 01:32 PM
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While it is harder to compare as there is a chip generation between them, I find my ASI2600 (16 bit and approx 50Ke full well at lowest gain) to be a far nicer camera that produces much "smoother" images than my ASI294 which has 64Ke well depth at the lowest gain but at the most equivalent gain conversion wise to the 2600 has only about 16Ke effective full well due to the 14 bit architecture, that 16K ADU is then multiplied to fill the FITS 16 bit frame. There was plenty of debate about it while I was waiting for the ASI2600 to arrive, but the 2600 is unarguably just a better camera than the 294 was.

Even when read noise differences are taken in to account, at their most comparable and useful gain settings (120 for the ASI294 and 0 for the 2600, the closest each camera has to 1:1 conversion) you see about 1.8e read noise for the ASI294 and 3.6 for the 2600, the new camera has almost twice the dynamic range as it can encode up to approx 50Ke where the ASI294 can only run to 16Ke at that gain. The practical difference is that on dimmer targets the 2600 responds well to 600 second subs where the 14 bit 294 just turned into a mess with increased saturated stars, and while the 2600 at that lowest gain has approx double the read noise of the 294 at the equivalent gain conversion wise, it does not look it.

Each to their own and budget might be what answers the question, but I don't plan to buy another camera where the bit depth does not allow for near or actual 1:1 conversion at a truly useful gain setting, not for deep sky imaging anyway.

To touch on the point Dunk has made about learning to get the best of it. That is one of the surprises with the ASI2600, getting really good results out of that has been a matter of plugging it in, shooting darks and then just using it, even flats are really easy to get and apply. I am sure it is not the only camera like that on the market but my ASI294 as a comparison takes a lot more work in terms of getting flats to actually work well, and even than only up to a point. I stopped trying to go after targets on a dark background as it was just too much work to try to get them looking nice where the ASI2600 on the witches head for instance was just "wow, look at that" after 21 subs, with the ASI294 (I am still using it) I aim for 100+ for a similar looking result but even then the flat framing issue would rear it's head.
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Old 20-11-2020, 03:43 PM
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Yeah the 2600 looks really nice

I have a 183 with its tiny pixels and relatively shallow wells, but it's pretty straightforward to get it in its stride (ZWO: gain 50-111, depending on scope).

Then it's just the age-old method of exposing for long enough to get over the read noise.
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Old 20-11-2020, 03:52 PM
Startrek (Martin)
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Iíve only had my 2600MC for 4 months and the performance in both Bortle 8 Sydney skyglow and my semi dark site Bortle 3 skies is tremendous
Iíve processed with and without darks and depending on conditions , the target object , sub exposure length and location, I could not see any significant improvement using darks ( although I capture them in any case )
I set my camera through Ascom and usually select Highest Dynamic Range ( Gain 0 offset 50, this camera does not have a ď Unity GainĒ as such ) to achieve full well depth while sacrificing only a small amount of additional read noise
Dark current is extremely low even at my cooling setting of -15C
If your after a Cooled OSC camera with an AP-C sensor size then I can strongly recommend the ZWOASI2600MC
ZWO also make a full frame version the 6200MC ( 3.76uM pixel ) and a 2400MC (5.94 uM pixel size ) both extremely good specs on these cameras
Cheers
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Old 20-11-2020, 08:12 PM
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Bart,
The difference the bit depth will make is in terms of limiting how many photons you can count in each pixel. A 12 bit sensor can count no more than 4096 (at unit gain), regardless of the theoretical well depth of the sensor, before it "saturates".

But that said, you can simply take more shorter shots instead to avoid saturating the counter/well in each sub in order to get the same total exposure time.

The downside of taking more shorter shots is that you get more read noise for the same total exposure time. But if your exposures are long enough to be sky limited, all is good. And the CMOS sensors tend to have such low read noise that the time to get sky limited is quite short - at least under light pollution!! There is plenty of discussion on IIS regarding finding sky limited exposure times (or gimme a call!).

I use a 12 bit camera (ZWO 1600MM-C) and seems to work fine. Obviously higher bit width would give you more flexibility if you wanted to do longer subs for some reason, but bottom line is the 12 bit one works fine IMHO.

Paul
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Old 20-11-2020, 08:29 PM
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multiweb (Marc)
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Quote:
Originally Posted by Bart View Post
I've had a gut full of my QHY9 and the dark streaks next to bright stars.
My QHY9 mono is 10 years old and I occasionally get dark streaks on bright stars. Not all the time. There is a clamping setting in nebulosity that is supposed to take care of this. I can't explain why it does it sometimes and not others. It seems to do it when the stars are fatter as in long FL imaging. Never had any issues with widefield or the FSQ but did with the C11 or CN-212. What do you think it might be and what fix are you talking about?
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Old 21-11-2020, 09:40 AM
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Quote:
Originally Posted by multiweb View Post
My QHY9 mono is 10 years old and I occasionally get dark streaks on bright stars. Not all the time. There is a clamping setting in nebulosity that is supposed to take care of this. I can't explain why it does it sometimes and not others. It seems to do it when the stars are fatter as in long FL imaging. Never had any issues with widefield or the FSQ but did with the C11 or CN-212. What do you think it might be and what fix are you talking about?



I managed to fix my camera as per here:
http://www.iceinspace.com.au/forum/s...d.php?t=130615


although there was still a faint line as per here:
"It's much better - I had to really stretch it with HDR toning to see any black line.
See the pic of Alpha Centauri."

http://www.iceinspace.com.au/forum/a...e.php?a=179281


If I was prepared to make super fine adjustments with the external potentiometer
that I installed to replace R27
then I'm sure I could get it perfect.
As per this pic:
Attached Thumbnails
Click for full-size image (QHY-9M  with external adjustment.jpg)
184.8 KB21 views
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Old 21-11-2020, 01:40 PM
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Thanks everyone, I appreciate the thoughts. I really like the qhy9, it has served me well. The bright streaks are becoming more obvious as the camera ages. It really ruins a good image. Allan has explained the fix well.
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Old 21-11-2020, 03:20 PM
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Quote:
Originally Posted by Bart View Post
Thanks everyone, I appreciate the thoughts. I really like the qhy9, it has served me well. The bright streaks are becoming more obvious as the camera ages. It really ruins a good image. Allan has explained the fix well.

Hi Bart,
the QHY9m has a KAF8300 sensor with a Hydrogen Alpha 656nm
quantum efficiency of about 48%.
see here:
https://www.baader-planetarium.com/e...ccd_camera.pdf

I'm finding it difficult to get a graph showing the quantum efficiencies
of all the latest CMOS chip cameras.
Can anyone find one?
You have to be careful with QE as sometimes the vertical axis on their graphs
refers to relative QE where they make the highest value 100%
usually on the peak of Green.
That can be very misleading and I think it's a sales trick.

The new CMOS cameras are supposed to be a lot better than that although many
of the CMOS sensor chips from just a few years ago are not much better at 656 nm - the Hydrogen Alpha wavelength.


It's nice to have 16 bits to play with on the QHY9m.
It could give an advantage when stretching the data.
I use NASA FITS liberator to do that.
Apart from the dark line problem - I really like the QHY9m.
I think I've solved it using the QHY9 instructions from the factory.
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Old 22-11-2020, 04:45 PM
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Quote:
Originally Posted by alpal View Post
I'm finding it difficult to get a graph showing the quantum efficiencies
of all the latest CMOS chip cameras.
Can anyone find one?
LOL. Yes. For the 1600 Pro, ZWO say "We suppose the QE peak value is more than 60%."

https://astronomy-imaging-camera.com...asi1600mm-cool
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Old 22-11-2020, 05:20 PM
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Originally Posted by DiscoDuck View Post
LOL. Yes. For the 1600 Pro, ZWO say "We suppose the QE peak value is more than 60%."

https://astronomy-imaging-camera.com...asi1600mm-cool

Thanks - that's funny -
there is a graph that compares 2 CCD cameras with 2 CMOS cameras here:
http://www.astrosurf.com/buil/CMOSvs...s/tab3-843.jpg


from this web page:
http://www.astrosurf.com/buil/CMOSvsCCD/index.html




cheers
Allan
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Old 22-11-2020, 05:20 PM
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It's not exactly pure speculation... when the 1600 first came out, there were several comparisons made against comparable sensors such as Sony's ICX694.

Edit: alpal beat me to it
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Old 22-11-2020, 05:23 PM
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Quote:
Originally Posted by alpal View Post
I managed to fix my camera as per here:
http://www.iceinspace.com.au/forum/s...d.php?t=130615


although there was still a faint line as per here:
"It's much better - I had to really stretch it with HDR toning to see any black line.
See the pic of Alpha Centauri."

http://www.iceinspace.com.au/forum/a...e.php?a=179281


If I was prepared to make super fine adjustments with the external potentiometer
that I installed to replace R27
then I'm sure I could get it perfect.
As per this pic:
ha yes... I remember now. It's been a while.

Quote:
Originally Posted by Bart View Post
Thanks everyone, I appreciate the thoughts. I really like the qhy9, it has served me well. The bright streaks are becoming more obvious as the camera ages. It really ruins a good image. Allan has explained the fix well.
Yes I do get column defects as it ages. I have one obvious and one in the making. They seem to start from hot pixels. I guess cosmic rays get the better of the sensor after a while. It's not a show stopper though. You can use a bad pixel map and dither. I remember an old STL11k peppered in bad columns and dark subtraction took care of all that.
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Old 22-11-2020, 05:40 PM
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Quote:
Originally Posted by multiweb View Post
ha yes... I remember now. It's been a while.

Yes I do get column defects as it ages. I have one obvious and one in the making. They seem to start from hot pixels. I guess cosmic rays get the better of the sensor after a while. It's not a show stopper though. You can use a bad pixel map and dither. I remember an old STL11k peppered in bad columns and dark subtraction took care of all that.



I've got a bad column but it only shows when there is no cooling used.
Dither should get rid of all defects.
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Old 24-11-2020, 07:14 PM
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Here's a good graph -
we need one like this showing all the latest CMOS cameras
and their QE against common filters.


http://www.astrosurf.com/zeubeu/images/capteurs.png
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