CCD v CMOS

[Stuart1971]

2 hours ago, Lee_P said:

I use an ASI2600MC Pro -- same sensor as your camera I believe. I always shoot 120s subs, regardless of target or whether I'm using an L-eXtreme or no filter at all. Always 120s. Bortle 8 skies, to make things even harder. Check my gallery and you'll see it can work well

 

Superb results there, very impressive for 2 min subs……

[teoria_del_big_bang]

9 minutes ago, Stuart1971 said:

So excuse my ignorance, but does that mean upping the gain will help, with shorter exposures and so using mode 1 and gain 56 would be a good idea….?

You have told me before that you use these settings to good effect, so will give them a try, I tried mode 1 gain 0 last time out with 5 min subs, but still noisy, but I guess that maybe somewhat down to the almost full moon….🤔🤔

That.s how I read it but this and other threads you have also seen lead me to think I may not know as much as I though I did.
Unfortunately my biggest enemy is the weather, especially since getting the new camera as good imaging time was very limited last year and so just did not want to waste time experimenting with different modes, gains exposure times, just wanted some data to play with.
Given endless clear dark nights I would love to do all sorts of trials to fine all these optimal values, but I read a few other threads, on SGL and other web sites when the camera first came out and many were using mode 1 gain 56 or mode 0 gain 28 (or there abouts) both where the traces step down on the noise to gain graphs and so basically stuck to those values, they seemed to work well so didn't want to fix what seemed to be working, so ne real science from my side anyway.

Steve

Edited January 17, 2022 by teoria_del_big_bang

[Stuart1971]

1 hour ago, Lee_P said:

Gain 100, always. Controlled using an ASIAIR Plus. 

Just been looking a the graph of the ASI2600c and the QHY2600c and it seems that your gain 100 is equivalent to Mode 1 gain 60 on the QHY, so that encourages me even more to try this setting….👍🏼

[Stuart1971]

1 minute ago, teoria_del_big_bang said:

That.s how I read it but this and other threads you have also seen lead me to think I may not know as much as I though I did.
Unfortunately my biggest enemy is the weather, especially since getting the new camera as good imaging time was very limited last year and so just did not want to waste time experimenting with different modes, gains exposure times, just wanted some data to play with.
Given endless clear dark nights I would love to do all sorts of trials to fine all these optimal values, but I read a few other threads, on SGL and other web sites when the camera first came out and many were using mode 1 gain 56 or mode 0 gain 28 (or there abouts) both where the traces step down on the noise to gain graphs and so basically stuck to those values, they seemed to work well so didn't want to fix what seemed to be working, so ne real science from my side anyway.

Steve

Would you say gain 56 or 60, (in mode 1) as the start of the drop is 56, so would it not be best at 60 where it’s the lowest read noise….or don’t you think it matters….?

[centroid]

1 hour ago, MartinB said:

Robin Glover explains that there is minimum exposure time required to neutralise the effect of read noise.  He also points out that there is no benefit in increasing sub exposures beyond this for achieving a desired ultimate SNR of the final stacked image.  I have no argument with this at all.  When I got my QSI 532 CCD back in 2010 I used the CCDware sub exposure calculator to work out my minimum sub exposure, it was invaluable.  However, Robin Glover isn't saying that longer exposures are detrimental.  Craig Stark has done a lot of work with CCD cameras examining exposure times and SNR in the real world.   SNR continues to increase beyond minimum optimal sub exposure time.  The limiting factor is the point where pixel response becomes significantly non linear of brighter parts of the image such as bright stars are becoming saturated.  Yes, gusts of wind and rogue clouds give a bigger hit when they ruin a longer sub.  Satellites less of an issue for me since I rarely have less than 30 subs per channel.  One other issue with short subs is time lost through dithering.  You certainly need to be avoiding dithering after every sub otherwise the loss of exposure time becomes substantial.  

 

The "law of diminishing returns" Martin 👍

[iantaylor2uk]

2 hours ago, MartinB said:

Robin Glover explains that there is minimum exposure time required to neutralise the effect of read noise.  He also points out that there is no benefit in increasing sub exposures beyond this for achieving a desired ultimate SNR of the final stacked image.  I have no argument with this at all.  When I got my QSI 532 CCD back in 2010 I used the CCDware sub exposure calculator to work out my minimum sub exposure, it was invaluable.  However, Robin Glover isn't saying that longer exposures are detrimental.  Craig Stark has done a lot of work with CCD cameras examining exposure times and SNR in the real world.   SNR continues to increase beyond minimum optimal sub exposure time.  The limiting factor is the point where pixel response becomes significantly non linear of brighter parts of the image such as bright stars are becoming saturated.  Yes, gusts of wind and rogue clouds give a bigger hit when they ruin a longer sub.  Satellites less of an issue for me since I rarely have less than 30 subs per channel.  One other issue with short subs is time lost through dithering.  You certainly need to be avoiding dithering after every sub otherwise the loss of exposure time becomes substantial.  

 

I wouldn't bother dithering at all until you start seeing good images - you can worry about that later

[ONIKKINEN]

1 hour ago, iantaylor2uk said:

I wouldn't bother dithering at all until you start seeing good images - you can worry about that later

I have to disagree, dithering is one of the most important things to do. Perhaps even more important than guiding IMO. Walking noise is something you cant really get rid of if it gets too bad due to not dithering enough.

[Lee_P]

6 hours ago, ONIKKINEN said:

I have to disagree, dithering is one of the most important things to do. Perhaps even more important than guiding IMO. Walking noise is something you cant really get rid of if it gets too bad due to not dithering enough.

 @ONIKKINEN is spot-on in my opinion. 

Edited January 18, 2022 by Lee_P

[teoria_del_big_bang]

9 hours ago, Stuart1971 said:

Would you say gain 56 or 60, (in mode 1) as the start of the drop is 56, so would it not be best at 60 where it’s the lowest read noise….or don’t you think it matters….?

Sorry I left the conversation, had early night.
I admit since revisiting all this and looking at the graphs I would say yes but I think even QHY say 56 is the sweet spot that's why I chose it, whether its just the graph that looks that way I am not sure.

I am afraid the thread has drifted a little bit from the OP's original post - sorry @centroid

Steve
 

[teoria_del_big_bang]

11 hours ago, Stuart1971 said:

Would you say gain 56 or 60, (in mode 1) as the start of the drop is 56, so would it not be best at 60 where it’s the lowest read noise….or don’t you think it matters….?

This is what it says on the QHY website which is why I think I went for Gain 56.

Multiple Readout Modes is a new function for newer QHY Cameras.  Different readout modes have different driver timing, etc., and result in different performance. The QHY268 currently has four readout modes, and more modes will be added in the future. These readout modes are currently supported in the QHY ASCOM Camera Driver, SharpCAP software and the N.I.N.A software.

Readout Mode #0 (Photographic Mode). In this mode there is a drop in the noise between Gain 25 and Gain 26.  We recommend setting the Gain to 26 to begin.  At this setting the full well is 27ke- and readout noise is 2.7e-.  For every long exposures you can lower the gain from this point to increase the full well capacity.

Readout Mode #1 (High Gain Mode).  Please note there is a HGC/LGC switch point at gain55 to gain56. Gain0-55 uses LGC and Gain55-100 uses HGC.

Also as it has sort of taken this tread off topic I have tried to start a new thread for QHY268 users to share information, whether it yields anything of interest I will have to see 🙂 

QHY268M & QHY268C Users Thread (Please share your knowledge and tips)

Steve

[Stuart1971]

14 minutes ago, teoria_del_big_bang said:

This is what it says on the QHY website which is why I think I went for Gain 56.

Multiple Readout Modes is a new function for newer QHY Cameras.  Different readout modes have different driver timing, etc., and result in different performance. The QHY268 currently has four readout modes, and more modes will be added in the future. These readout modes are currently supported in the QHY ASCOM Camera Driver, SharpCAP software and the N.I.N.A software.

Readout Mode #0 (Photographic Mode). In this mode there is a drop in the noise between Gain 25 and Gain 26.  We recommend setting the Gain to 26 to begin.  At this setting the full well is 27ke- and readout noise is 2.7e-.  For every long exposures you can lower the gain from this point to increase the full well capacity.

Readout Mode #1 (High Gain Mode).  Please note there is a HGC/LGC switch point at gain55 to gain56. Gain0-55 uses LGC and Gain55-100 uses HGC.

Also as it has sort of taken this tread off topic I have tried to start a new thread for QHY268 users to share information, whether it yields anything of interest I will have to see 🙂 

QHY268M & QHY268C Users Thread (Please share your knowledge and tips)

Steve

It does make you wonder why they even bother with the mode 0 if the mode 1 will be better all round, which it seems to show on the graph, who wants to use a lower gain with much higher read noise, under what situation would you want or need that…???

[Stuart1971]

2 hours ago, Lee_P said:

 @ONIKKINEN is spot-on in my opinion. 

 

8 hours ago, ONIKKINEN said:

I have to disagree, dithering is one of the most important things to do. Perhaps even more important than guiding IMO. Walking noise is something you cant really get rid of if it gets too bad due to not dithering enough.

This is something I don’t do….how much impact would this have on my noisy images…??

[centroid]

2 hours ago, teoria_del_big_bang said:

I'm afraid the thread has drifted a little bit from the OP's original post - sorry @centroid

Steve
 

No problem Steve. Its all interesting stuff, and I think my original post had a good 'airing', and had 'run its course' anyway 🙂

[Xsubmariner]

 Fortunate to gain experience in both camps at an early stage, I quickly realised their modus operandi were quite different. As my imaging setups increased I ended up with a mix of CCD and CMOs camera’s across my imaging rigs and mobile system. Today I have only one CCD SX 36 and several latest generation CMOs camera’s, QHY and ZWO.

 A review of images across various forums would suggest CCD and CMOs (latest Gen) are comparable in ability to deliver quality images.

The main driver for me migrating to CMOs is the U.K. weather and general lack of quality skies for imaging. My CCD camera’s with 10-30 min frames produced too many incomplete data sets. In believe CMOs camera’s are just better suited to U.K. imaging.

[teoria_del_big_bang]

4 minutes ago, Xsubmariner said:

The main driver for me migrating to CMOs is the U.K. weather and general lack of quality skies for imaging. My CCD camera’s with 10-30 min frames produced too many incomplete data sets. In believe CMOs camera’s are just better suited to U.K. imaging.

If its like 2021 I think the only scope suited to UK imaging is a space telescope (is Hubble up for sale ?).

This has been an interesting thread, for myself who is pretty new I have no experience of CCD as both my cameras have been CMOS after a brief start with DSLR.

Steve

[iantaylor2uk]

9 hours ago, ONIKKINEN said:

I have to disagree, dithering is one of the most important things to do. Perhaps even more important than guiding IMO. Walking noise is something you cant really get rid of if it gets too bad due to not dithering enough.

What I meant was that in view of the original poster not being able to see any decent images, dithering is lower down on the list of priorities in my opinion. Clearly the most important things to do are: (1) frame the object properly, (2) get the focus right, (3) get the gain settings and sub length right so you can preview the image to check it looks OK, (4) get the autoguiding working, and then take a series of images, stack them to see how they come out. If there is walking noise, you can add dithering later on. 

[Phillyo]

On 18/01/2022 at 10:07, Stuart1971 said:

 

This is something I don’t do….how much impact would this have on my noisy images…??

If you are guiding using PHD2 (or similar) then I can't imagine why you wouldn't want to dither. It makes a huge difference to image quality by removing walking noise which is a nightmare to deal with in Post. Plus it's as simple as just clicking a box and selecting a dither amount in pixels. That's it.