CMOS/CCD - ADC

[Guest]

At present I use a Canon 450d DSLR and I'm looking at the possibility of buying a dedicated CCD or CMOS mono camera. My interest is in Photometry not imaging. Browsing the catalogues I noticed that most of the CCD cameras have 16 bit ADC's whereas the CMOS cameras have mainly 12 or 14 bit ADCs. It seemed to me that for photometry I would be losing resolution with a 12 bit ADC CMOS camera but I have a feeling that might be an over simplification. I wondered if anyone had any comments.

Cheers

Steve

 

[CraigT82]

I think that CCDs are also preffered for photometry due to their greater range of linear response.

@JeremyS is your man in the know for this one. 

[JeremyS]

12 minutes ago, CraigT82 said:

I think that CCDs are also preffered for photometry due to their greater range of linear response.

@JeremyS is your man in the know for this one. 

CMOS is becoming much more common for photometry these days. Once the linearity has been quantified. I’ve not switched from CCD but I think @andrew s has

There’s a discussion on the BAA Forum here: https://britastro.org/node/22497

Edited June 25, 2021 by JeremyS

[Guest]

Thank you both. Is that difference in ADC range between 12 and 16 bit significant? I was thinking that the overall resolution will probably depend on other things as well as ADC range and that in practice there is no difference. I'm thinking that I'd probably go for CMOS since CCD seems to be on the decline. I'm already doing photometry successfully with my DSLR on variable stars but I'm interested in exoplanets as well.

 

 

[andrew s]

Yes I use CMOS.  The best modern backed thinned CMOS camera are as good as and often better than the equivalent CCD cameras. The only remaining issues is that due to the mass market the pixels are small. This can be used to advantage in removing the telegraph noise CMOS produces. 

Have a look at C Buil's site http://www.astrosurf.com/buil/index.html for some reviews. 

Regards Andrew 

The higher read noise in CCD cameras reduces the effective but depth so not a lot of difference but newer CMOS are 14 or 16 bit now.

Edited June 25, 2021 by andrew s

[rsarwar]

On 25/06/2021 at 23:17, andrew s said:

The higher read noise in CCD cameras reduces the effective but depth so not a lot of difference but newer CMOS are 14 or 16 bit now.

indeed, the 16-bit CMOS in QHY268M can outporm almost any 16-bit CCD .

[newbie alert]

1 hour ago, rsarwar said:

indeed, the 16-bit CMOS in QHY268M can outporm almost any 16-bit CCD .

Cough,  cough

[david_taurus83]

Fightin' talk!

[DaveS]

Seconds out, round one *ding ding*

[rsarwar]

4 hours ago, newbie alert said:

Cough,  cough

 

I agree that CCD is better for photometry, but for imaging, cmos now have the same QE with lower read noise and well depth and same bit depth.

Unfortunately I don't have any CCD so I can't compare the two using same scope and light pollution. So I cannot prove my statement with data. 

 

EDIT : did you decide if you wanna modd you cgem? 

Edited July 5, 2021 by rsarwar

[newbie alert]

Sorry, not going to get dragged into the cmos/ ccd debate yet again , but that kind of comment visually  comes from someone that's never used a ccd.. yes cmos will overtake ccd, and id say it's nearly there in doing so, but there's no need to try and force the issue.. really annoys me... I'm really impressed with my inferior ccd, and looking to buy another one once I've saved some more pennies..

[andrew s]

We are blessed with wide choice of excellent CCD and CMOS cameras. Think yourselves luck we have moved on from trying to hypersensitise film to a fraction of the performance of even the worst of  those available today.

Regards  Andrew 

[Whirlwind]

On 25/06/2021 at 20:58, woodblock said:

At present I use a Canon 450d DSLR and I'm looking at the possibility of buying a dedicated CCD or CMOS mono camera. My interest is in Photometry not imaging. Browsing the catalogues I noticed that most of the CCD cameras have 16 bit ADC's whereas the CMOS cameras have mainly 12 or 14 bit ADCs. It seemed to me that for photometry I would be losing resolution with a 12 bit ADC CMOS camera but I have a feeling that might be an over simplification. I wondered if anyone had any comments.

Cheers

Steve

 

The ADC isn't that important between CCD and CMOS overall.  For bright photometric objects either should be fine especially if it is for 'fun'.  The bigger issue (which comes more prevalent with faint targets/where changes are small (e.g. exoplanets)) is that you want a consistent noise on a frame by frame basis.  Here CCD currently still have the advantage.  CMOS have ADCs on their individual pixels whereas CCDs just have one.  It means that the readout from a CCD is consistent so when you create calibration frames the noise represented in them is the same as the actual science image.  In CMOS you can get variations on a pixel level (the proverbial walking noise effect that can be seen sometimes when stacked).  You can remove this by dithering in imaging but for photometry you don't really want to change pixels (ideally you want the target sitting on the same pixel(s) all the time as it keeps your noise values as close as to the same as possible).