How are counts per frame calculated in CCD for spectrometry?

[hutchphd]

Can you convince yourself that the cold is improving the dark current on the CCD? Might improve your stoicism...

 

[new6ton]

A stack of 100 one-second exposures has the exact same amount of signal as a single 100-second exposure. The difference is that there are sources of noise that are not time dependent, and these can overwhelm a very short exposure with a very small signal. But, if the signal is very high, such as filming video outside during the daytime, you're just fine. This noise is negligible compared to the signal and the difference between the stack of short exposures and the single long exposure is minimal.

Can you give some actual examples of applictions with sources of noise that are not time-dependent?

 

[Drakkith]

Can you give some actual examples of applictions with sources of noise that are not time-dependent?

I already gave one in an earlier post. The readout noise from the CCD is not time dependent.

 

[Drakkith]

That's true but, without a significant 'light bucket' scope, you would never be using 1s.

Sure, but the principle is the same for any time frame. There are plenty of people who don't use autoguiding, so they have to set their exposures for perhaps 15-30 seconds at most to avoid accumulating tracking errors in each frame.

 

[new6ton]

I already gave one in an earlier post. The readout noise from the CCD is not time dependent.

But is it not all digital cameras use CCD? You said "But, if the signal is very high, such as filming video outside during the daytime, you're just fine. This noise is negligible compared to the signal and the difference between the stack of short exposures and the single long exposure is minimal.". Filming video uses CCD. and it is time dependent. How do you reconcile it with the readout noise from CCD which is not time dependent? Filming video using CCD engages the readout noise too.

 

[sophiecentaur]

But is it not all digital cameras use CCD?

This is a good point. Many modern cameras use CMOS sensors. I was at a presentation at E2V (leading sensor manufacturers) and was told that the future is probably CMOS but they are fairly committed to CCD for the near future, at least. CMOS performance is improving all the time.
My Pentax k2s and my ZWO camera are both CMOS.

The readout noise from the CCD is not time dependent.

We need to re-examine that statement because I'm not sure what you mean exactly. Are you saying that the error in value that's read out will be the same for any exposure length? If there is a source of random 'error' which is once-per-exposure then more exposures will reduce the effect of that error. If there's merely a constant offset from pixel to pixel then darks and flats would allow that to be eliminated. The readout noise will appear as a variation from pixel to pixel, over the image and that will be less with stacking (no?). The spectrum of the noise is relevant, I imagine but I don't know enough about the detail to say which way it would go.

 

[sophiecentaur]

There are plenty of people who don't use autoguiding, so they have to set their exposures for perhaps 15-30 seconds at most to avoid accumulating tracking errors in each frame.

Yes - that was me, until recently but I would say that the optimum method could easily be different and getting details out of the bottom few quantisation levels is a forlorn hope. (I was actually responding initially to your 1s exposure example which I would say would greatly limit the choice of objects.)
I have recently joined Havering Astronomical Society (Essex, UK), in which a group of enthusiastic AP'ers have been very active, lately. I feel put to shame by their willingness to spend so much time and money. But they are collecting an impressive set of images of Messier Objects. Here. Worth looking at, I would say - especially as nearly all of their sites are not far from Central London.

 

[Drakkith]

But is it not all digital cameras use CCD? You said "But, if the signal is very high, such as filming video outside during the daytime, you're just fine. This noise is negligible compared to the signal and the difference between the stack of short exposures and the single long exposure is minimal.". Filming video uses CCD. and it is time dependent. How do you reconcile it with the readout noise from CCD which is not time dependent? Filming video using CCD engages the readout noise too.

All digital sensors have some readout noise as far as I know. And I don't know what you mean by about the video. The readout noise is the same regardless of your exposure time. That's what I mean by it not being time dependent.

We need to re-examine that statement because I'm not sure what you mean exactly. Are you saying that the error in value that's read out will be the same for any exposure length? If there is a source of random 'error' which is once-per-exposure then more exposures will reduce the effect of that error. If there's merely a constant offset from pixel to pixel then darks and flats would allow that to be eliminated. The readout noise will appear as a variation from pixel to pixel, over the image and that will be less with stacking (no?). The spectrum of the noise is relevant, I imagine but I don't know enough about the detail to say which way it would go.

I mean that at readout the sensor adds a small amount of noise that is always the same (the RMS value). I don't have my reference book handy, but I think it's just Poisson noise, so stacking will reduce this noise just like it does other sources of noise.

 

[sophiecentaur]

@Drakkith I get you about the noise and the sample will be a single value +/- to the electron count, according to the instantaneous (memoryless ?
) noise value at sampling instant. I think it would be Poisson or even 1/f ?

 

[hutchphd]

The readout noise will appear as a variation from pixel to pixel, over the image and that will be less with stacking (no?)

In my understanding that is not the best way to think about it. Read noise is random for every "read" event: this is true for reading successive pixels in a frame or multiple reads of a particular pixel in different frames. So it produces both spatial RMS and temporal RMS of the same size. The total RMS noise for N reads will grow as √N. So if N frames are stacked (and averaged)

1. This then adds additionally to the overall temporal RMS (relative to a single long exposure where N=1) as @Drakkith pointed out for each pixel
2. This allows the spatial variability to be reduced by √N in the stack average

The spatial variability reduction is usually worth the small price of additional read noise (particularly for images...less so for low level quantification )
Note that the read noise is likely not centered at zero (maybe Poisson maybe electric truncation) so an additional background subtraction of a flat field average can be useful.

 

[Drakkith]

Hmmm. I thought readout noise was centered at zero, but I could be mistaken.

 

[hutchphd]

I think it depends upon the CCD but I have never worried about it much. Certainly the background can be subtracted off as required.

 

[new6ton]

When the signal is comparable or close to the read out noise that more exposure can spell a difference? Remember I wrote this thread about Counts per Frame in CCD in visible as well as IR Spectroscopy. So please share your experience in IR spectroscopy now. With these feature soon available in ordinary smartphone, and we can scan for say date rape drugs in drinks or how sweet is the apple at grocery with our smartphone. It would be handy knowledge.

 

[Drakkith]

When the signal is comparable or close to the read out noise that more exposure can spell a difference?

More exposures always improves SNR. It's just that when your signal is very strong the SNR is already very, very high and you just don't need to improve it.

Remember I wrote this thread about Counts per Frame in CCD in visible as well as IR Spectroscopy. So please share your experience in IR spectroscopy now.

There's no difference between the two in terms of noise and how sensors work. Everything that's been discussed here applies to both IR and visible.

 

[new6ton]

Is readout noise dependent on wavelength?

Anyway. I just have one more question and don't want to post in a new thread to save pages.

Can you give example of an actual photo where the sensitivity response of the CCD is flat (uniform from red to violet) instead of curve? And how an image would look like if your eyes have flat response too able to see visible light with equal sensitivity?

 

[sophiecentaur]

Red and blue wavelengths human sensitivity is lower than greens. So a flat response would boost reds and blues, giving a strong magenta tint to all images.
But that’s an unrealistic answer to an unrealistic question, I think.

 

[Drakkith]

Is readout noise dependent on wavelength?

No, because readout noise has nothing to do with wavelengths. It has to do with the sensor's ability to accurately count electrons shuffled in from each photosite (pixel).

Can you give example of an actual photo where the sensitivity response of the CCD is flat (uniform from red to violet) instead of curve?

No such CCD's exist. All have some sort of curve in their response sensitivity.

 

[Drakkith]

And how an image would look like if your eyes have flat response too able to see visible light with equal sensitivity?

Well, we don't, so we can't say what it would look like if we did because our brains would also interpret this differently. To throw out a random guess, I'd say that everything would look very much the same as it does now.

 

[hutchphd]

The quantum efficiency of all both CCD and CMOS detectors peaks slightly above 900nm. Any other wavelength will be intrinsically noisier. Whether that noise is significant depends upon a host of factors including details of fabrication and signal strength. etc etc.etc