How are counts per frame calculated in CCD for spectrometry?

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Discussion Overview

The discussion revolves around the calculation of counts per frame in CCD (Charge-Coupled Device) for spectrometry, particularly in the context of visible and infrared applications. Participants explore the relationship between exposure time, the number of frames, and the resulting signal counts, as well as the implications for image quality and noise reduction techniques such as stacking.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question how counts per frame are calculated, suggesting a potential confusion between exposure time and the number of frames.
  • It is proposed that the number of counts is determined by the measured signal level in electrons, which correlates to the number of photons detected by each pixel.
  • One participant raises a hypothetical scenario regarding dark matter detection, questioning if the counts per frame would remain constant regardless of exposure time.
  • Another participant explains that longer exposures can lead to blurred images due to atmospheric turbulence, while shorter exposures can be stacked to create sharper images.
  • There is a discussion about whether stacking techniques are applicable in spectral capture for spectrometers, with differing opinions on its relevance.
  • A participant introduces the idea that noise can be reduced as the square root of the number of frames, referencing a supplier's explanation related to Poisson noise.
  • Some participants discuss the effectiveness of frame averaging versus longer exposure times in improving signal-to-noise ratios, with varying interpretations of their implications.
  • One participant shares a personal experience of using stacking to enhance images of the Milky Way, suggesting that stacking can reveal faint objects obscured by noise.
  • Another participant agrees that multiple exposures can provide finer differentiation of signals, though they note this may not fundamentally improve signal-to-noise ratios.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between exposure time, frame count, and noise reduction. There is no consensus on the applicability of stacking techniques in spectrometry, and the discussion remains unresolved regarding the best practices for optimizing counts per frame.

Contextual Notes

Some participants highlight limitations in understanding the relationship between exposure time and counts, as well as the potential for confusion regarding the effects of stacking and averaging on noise reduction.

Who May Find This Useful

This discussion may be of interest to those involved in CCD spectrometry, astrophotography, or anyone exploring the technical aspects of image processing and noise reduction in scientific imaging.

  • #61
Hmmm. I thought readout noise was centered at zero, but I could be mistaken.
 
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  • #62
I think it depends upon the CCD but I have never worried about it much. Certainly the background can be subtracted off as required.
 
  • #63
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.
 
  • #64
new6ton said:
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.

new6ton said:
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.
 
  • #65
ccd sensitivity.gif


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?
 
  • #66
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.
 
  • #67
new6ton said:
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).

new6ton said:
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.
 
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  • #68
new6ton said:
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.
 
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  • #69
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
 

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