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Need some help, if possible.


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I am new to using a OSC CMOS camera and I have got the QHY268c

Now a couple of nights ago, I took 104 x 2min subs (3 hours 28 mins) at low gain mode and 0 gain, through the Idas LP filter with my Tak FSQ85, bortle 6 skies.

My guiding was not great but this was really just a test, I expected great things, but when I processed in APP with Darks, flats and dark flats, I got a really noisy image, well I hope that’s what it is, the detail is there…I think if stretched hard, but looks awful like loads of noise…but wondered if someone could check this link to see the stack from APP and it has a small stretch and even background, and cropped to remove stacking artefacts,  I would like to see what anyone could get from it, and tell me if I am correct about the noise, and what it needs to correct…I’m guessing much more data….

I thought with 104 x 2min subs it would be better, especially with a low gain used, in fact the lowest the camera will do….…🤔🤔

link to APP stack

https://www.dropbox.com/s/gwbkotzahgb04ie/Iris_Nebula-RGB-cropped-LP-st.tiff?dl=0

 

And a jpg 

 

A31D1CD8-5A1F-4FF2-8B5C-29D70B15F79D.jpeg

Edited by Stuart1971
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Is this data already stretched?

How about posting linear data instead? Also - use 32bit float point precision data instead of 16bit, especially if you have 16bit camera and stacked 100+ subs.

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Stuart, in the last thread about the offset I was curious to why are you using gain 0 ?

Isn't the readout noise 3X that at Gain 30 in mode 0 ? 
 

image.png.f61677f1e4f1df9edb90d9839df5a9f5.png

Also the fulwell capacity is at its greatest at gain 0 so wouldn't the signal to noise ratio also be greater than at gain 30 ? 
image.thumb.png.79068f96a21ac5c0b7a2ab61783eb19e.png

I admit I may well be wrong as not so familiar with all this , that's how I was thinking it would be.

Steve

Edited by teoria_del_big_bang
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I'd use #1 mode (blue line) and gain of 0. With that - I'd use longer exposures, like at least 5-6 minutes and possibly even longer.

- Impact of read noise one controls with single exposure length versus local LP

- Full well capacity and dynamic range are simply not important when stacking. If you fear that you'll loose star cores due to saturation - do several short exposures that you'll blend in later when stacking to replace saturated pixels.

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36 minutes ago, vlaiv said:

I'd use #1 mode (blue line) and gain of 0. With that - I'd use longer exposures, like at least 5-6 minutes and possibly even longer.

- Impact of read noise one controls with single exposure length versus local LP

- Full well capacity and dynamic range are simply not important when stacking. If you fear that you'll loose star cores due to saturation - do several short exposures that you'll blend in later when stacking to replace saturated pixels.

What is the reason though for using Gain 0 rather than say something slightly above 30 where the readout noise drops dramatically ?

Steve

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2 hours ago, teoria_del_big_bang said:

Stuart, in the last thread about the offset I was curious to why are you using gain 0 ?

Isn't the readout noise 3X that at Gain 30 in mode 0 ? 
 

image.png.f61677f1e4f1df9edb90d9839df5a9f5.png

Also the fulwell capacity is at its greatest at gain 0 so wouldn't the signal to noise ratio also be greater than at gain 30 ? 
image.thumb.png.79068f96a21ac5c0b7a2ab61783eb19e.png

I admit I may well be wrong as not so familiar with all this , that's how I was thinking it would be.

Steve

Exactly, I know nothing of how to use CMOS cameras and this gain and offset mullarky…so I need all the help I can get, so what is a good gain to use with this camera…??

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2 hours ago, vlaiv said:

In the meantime:

iris.thumb.jpeg.7ec1a3a6ef95df73d0fa16b92c328a83.jpeg

Even this prestretched 16bit version has a lot to offer ...

Thanks, Do you really think so, I really expected it to look more detailed….but maybe I’m asking a bit too much….

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2 hours ago, vlaiv said:

Is this data already stretched?

How about posting linear data instead? Also - use 32bit float point precision data instead of 16bit, especially if you have 16bit camera and stacked 100+ subs.

It had a small stretch in APP…yes..

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2 hours ago, vlaiv said:

In the meantime:

iris.thumb.jpeg.7ec1a3a6ef95df73d0fa16b92c328a83.jpeg

Even this prestretched 16bit version has a lot to offer ...

Is this my Dropbox data as is, or have you done anything to it, if so what and in what software, as the noise does not look as bad..

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1 hour ago, teoria_del_big_bang said:

What is the reason though for using Gain 0 rather than say something slightly above 30 where the readout noise drops dramatically ?

Steve

Only benefit of going with higher gain for this mode is to have lower read noise.

I like unity gain for several reasons and main is that it makes it easy to do ADU to e conversion - they are the same with unity gain. Other reasons include no rounding errors introduced.

With #1 mode - you have unity gain at gain 0 - or so it seems from the published graphs.

Sure it has higher read noise of 3.5e, but that is not much worse than best read noise this camera provides - which is around 1.5e.

Read noise should be only considered when selecting exposure length. If sub length is chosen properly - there will be minimal difference between 1.5e read noise and 3.5e read noise stack of same total duration. If you choose sub duration properly for 1.5e - then it is very easy to select proper sub duration for 3.5e - just multiply with their ratio 3.5e / 1.5e = x2.334 - so you need about x2.34 longer subs with 3.5e read noise to get virtually the same result.

Given that original image was taken with 2 minute subs - I just multiplied that with 2.5-3 (I did mental math not actual calculation) and ended up with 5-6 minute exposures. Actual exposure length really depends on setup and LP conditions.

 

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18 minutes ago, Stuart1971 said:

Exactly, I know nothing of how to use CMOS cameras and this gain and offset mullarky…so I need all the help I can get, so what is a good gain to use with this camera…??

Offset is easy. You determine that once you have your gain selected and you figure it out using bias. You want none of your pixel in bias to be at 0, so as long as you have at least one being equal to zero - raise your offset.

Selecting gain:

- you want gain setting that gives you low enough read noise. How low? That depends on your shooting conditions. Fast optics + strong LP favor shorter subs and one can get away with higher read noise. Here is general rule of thumb: read noise higher than 7-8e - think 10 minute or more subs. Read noise around 4e - think 5-6 minute subs. Read noise around 1.5e - think ~2 minute subs (or even 1 minute if you have strong LP like Bortle 7-8).

- next factor is full well at given gain. This is really not critical as you can take filler subs / short subs at the end in case you have stars that saturate (and you almost always will).

- I like to have e/ADU value that is easy to work with and does not introduce much rounding errors. Say 1 e/ADU or unity is my first choice if available. Next are 2e/ADU or 0.5e/ADU and then 0.25 e/ADU. Going with 3 or 4 e/ADU usually means high read noise because designers of sensor need to mask off that much rounding errors with some other noise source and they don't really bother to optimize read noise at these settings.

There you go - not much of science to it. Just pick a value and work with it.

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7 minutes ago, vlaiv said:

Only benefit of going with higher gain for this mode is to have lower read noise.

I like unity gain for several reasons and main is that it makes it easy to do ADU to e conversion - they are the same with unity gain. Other reasons include no rounding errors introduced.

With #1 mode - you have unity gain at gain 0 - or so it seems from the published graphs.

Sure it has higher read noise of 3.5e, but that is not much worse than best read noise this camera provides - which is around 1.5e.

Read noise should be only considered when selecting exposure length. If sub length is chosen properly - there will be minimal difference between 1.5e read noise and 3.5e read noise stack of same total duration. If you choose sub duration properly for 1.5e - then it is very easy to select proper sub duration for 3.5e - just multiply with their ratio 3.5e / 1.5e = x2.334 - so you need about x2.34 longer subs with 3.5e read noise to get virtually the same result.

Given that original image was taken with 2 minute subs - I just multiplied that with 2.5-3 (I did mental math not actual calculation) and ended up with 5-6 minute exposures. Actual exposure length really depends on setup and LP conditions.

 

But with 2 min subs wouldn’t mode 1 and a gain of 30 be better due to the lower read  noise, which seems to have plagued my image above…

all I really want is a good starting point, which I don’t have currently, as you can see, also what did you do the image above in your version, as it had a brighter, albeit greenish background, but the dust seems to stand out more…?

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20 minutes ago, Stuart1971 said:

Is this my Dropbox data as is, or have you done anything to it, if so what and in what software, as the noise does not look as bad..

Yes that is what has been posted above.

Here is my workflow (it would give better results with linear data saved as 32bit float point):

- Loaded image in Gimp and split channels into R, G and B

- Saved each as fits file

- Loaded in ImageJ, converted to 32bit float point, binned each sub x4 and saved

- Loaded in Gimp, did RGB compose to get RGB image again from individual channels and did stretch with levels.

- Created copy of image as a layer and did wavelet denoise. Added mask to that layer to apply denoised version only in dark parts of the image (otherwise it blurs too much of parts where signal is strong as is and that do not need denoising).

- saved as Jpeg.

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2 minutes ago, Stuart1971 said:

But with 2 min subs wouldn’t mode 1 and a gain of 30 be better due to the lower read  noise, which seems to have plagued my image above…

all I really want is a good starting point, which I don’t have currently, as you can see, also what did you do the image above in your version, as it had a brighter, albeit greenish background, but the dust seems to stand out more…?

Is 2 minutes what you can comfortably do with your mount and going longer would be an issue?

In that case - Go with gain 75 again mode #1 (blue line). That will give you e/ADU of 0.25 and read noise of ~1.55e.

You'll be effectively working with 14bit data and your FWC will be 16K, but that is ok.

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5 minutes ago, Stuart1971 said:

albeit greenish background,

Greenish background?

I did not perform background wipe. Maybe I should do it and see what I get?

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7 minutes ago, vlaiv said:

Offset is easy. You determine that once you have your gain selected and you figure it out using bias. You want none of your pixel in bias to be at 0, so as long as you have at least one being equal to zero - raise your offset.

Selecting gain:

- you want gain setting that gives you low enough read noise. How low? That depends on your shooting conditions. Fast optics + strong LP favor shorter subs and one can get away with higher read noise. Here is general rule of thumb: read noise higher than 7-8e - think 10 minute or more subs. Read noise around 4e - think 5-6 minute subs. Read noise around 1.5e - think ~2 minute subs (or even 1 minute if you have strong LP like Bortle 7-8).

- next factor is full well at given gain. This is really not critical as you can take filler subs / short subs at the end in case you have stars that saturate (and you almost always will).

- I like to have e/ADU value that is easy to work with and does not introduce much rounding errors. Say 1 e/ADU or unity is my first choice if available. Next are 2e/ADU or 0.5e/ADU and then 0.25 e/ADU. Going with 3 or 4 e/ADU usually means high read noise because designers of sensor need to mask off that much rounding errors with some other noise source and they don't really bother to optimize read noise at these settings.

There you go - not much of science to it. Just pick a value and work with it.

My optics are 450mm f5.3 tak FSQ85, so fairly fast and medium LP, hence using 2 min subs, and you say to use shorted subs in those conditions, yet the read noise is a plenty and shows in the image, so should I just take many more subs at 2 min, or up the gain…?

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3 minutes ago, vlaiv said:

Is 2 minutes what you can comfortably do with your mount and going longer would be an issue?

In that case - Go with gain 75 again mode #1 (blue line). That will give you e/ADU of 0.25 and read noise of ~1.55e.

You'll be effectively working with 14bit data and your FWC will be 16K, but that is ok.

No, I can go much longer, I have had 10 min unguided subs on this mount, I was just under the impression that with CMOS shorter subs but more of them was better….am I very wrong with this thinking, as it seems so….?

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Just now, Stuart1971 said:

My optics are 450mm f5.3 tak FSQ85, so fairly fast and medium LP, hence using 2 min subs, and you say to use shorted subs in those conditions, yet the read noise is a plenty and shows in the image, so should I just take many more subs at 2 min, or up the gain…?

You should adjust your sub length according background level. You want background / LP level to swamp read noise.

Once you do that, then all you have to do when making an image is to put enough time into it - do 2-4h of imaging and if you are not happy - do more imaging. There is very wide range of times needed for different targets. Some targets are dim and some are bright.

Interstellar dust is very dim. And you really need to put in time to make it show.

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Just now, Stuart1971 said:

No, I can go much longer, I have had 10 min unguided subs on this mount, I was just under the impression that with CMOS shorter subs but more of them was better….am I very wrong with this thinking, as it seems so….?

Ok, as far as sub duration.

Fewer longer subs that add up to same total imaging time as many short will always produce better result.

Having said that - there is a point at which there is no difference as extending sub duration brings in diminishing returns.

So yes - single 4h sub is the best - it will give you best result. It will be measurable but will it be seen by eye?

If you stack 4x1h - you won't see difference.

If you stack 20 x 12 minute - you probably won't see difference either.

In fact - you can go down to certain sub duration like 48 x 5 minute where you will not be able to see the difference in the finished image. But at some point, there will be difference. Trick is to select that sub duration where you still don't see difference but subs are short enough that you have enough subs to work with and you don't loose too much data if you drop a sub or something happens.

Only way to figure out that sub duration for your setup and LP is to measure background LP levels.

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5 minutes ago, vlaiv said:

You should adjust your sub length according background level. You want background / LP level to swamp read noise.

Once you do that, then all you have to do when making an image is to put enough time into it - do 2-4h of imaging and if you are not happy - do more imaging. There is very wide range of times needed for different targets. Some targets are dim and some are bright.

Interstellar dust is very dim. And you really need to put in time to make it show.

So basically with shorter 2 min subs, I should really use a much higher gain, and with longer subs I can use lower gains, so in my case I had it all wrong using short subs and low gains, hence ,loads of read noise…yes…?

I am in a bortle 6 zone, and used to do 5 min subs with a CCD camera and got good results…

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Just now, Stuart1971 said:

So basically with shorter 2 min subs, I should really use a much higher gain, and with longer subs I can use lower gains, so in my case I had it all wrong using short subs and low gains, hence ,loads of read noise…yes…?

I can't really answer that question properly. I can only guess.

You can measure your background levels and then figure it out.

What CCD camera did you have and what was mode and gain setting that you used for this image?

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I just saw so many great images taken with CMOS cameras, that consisted of a couple of hundred 60 second subs, so thought short sums was the way to go, but I was not taking into account that they were probably using a night gain…which I did not….

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Just now, vlaiv said:

I can't really answer that question properly. I can only guess.

You can measure your background levels and then figure it out.

What CCD camera did you have and what was mode and gain setting that you used for this image?

This image was taken on mode 0 and gain 0….

My old CCD was the SXVR M25c

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50 minutes ago, Stuart1971 said:

Exactly, I know nothing of how to use CMOS cameras and this gain and offset mullarky…so I need all the help I can get, so what is a good gain to use with this camera…??

To be honest I too am not that sure either, I too seem to think I understand it all and then later down the line an not sure why I did think I understood it at all (but that could be my age 🙂 )  but when I first got the camera I did some searching and read several articles about this camera (don;t ask me which - I would have to search again which I might well do bit not tonight) and people were suggesting to use mode 1 and a gain of 56 to take advantage of that big drop in read noise, or some suggested 30 in mode 0 for the same reason.

image.thumb.png.c56d69a1d5f4c68b71038016176079bb.png

But when I saw you were using gain 0 I have started to look again now there are more of these cameras out there and looks like a few are now saying Gain 0 mode 1, and they all say offset is not really important but seem to use between 10 and 40.
And  if @vlaiv says use 0 gain that good enough for me so may try this next time, just need to take some more darks for the library if I do but that not an issue plenty of cloudy wet noghts to get those done 🙂 

Steve

Edited by teoria_del_big_bang
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