[Aavso-photometry] CCDs vs. Single-channel Photometry

[Richard Miles]

Jeff Hopkins wrote:
> One thing I wonder about with CCD photometry is the small dynamic range. 
> Even 16 bit devices allow only 64K steps. With photon counting you have 
> over a million steps and approaching 10 million. Does stacking somehow 
> help the CCD data? If so, wouldn't that be essentially like just averaging 
> the data?
>

===> Excuse me for picking up on this point 'cos you are right - it is an 
important consideration especially for brighter objects when comparing CCDs 
and single-channel systems.

Michael K. has previously noted that he typically takes 8 images to 
determine an accurate magnitude PLUS an accurate measure of the error 
involved from the variation in the measured mags of other stars in each of 
the 8 frames.  I do the same usually using a set of 10 images.  There is a 
second reason for doing this and that is to combat the small dynamic range 
of CCDs: some models have smaller effective dynamic ranges than others and 
these in particular benefit if you base your magnitude on a set of 8 or 10 
decent images, sometimes many more.  So yes, stacking helps but you usually 
reduce each frame separately then calculate the mean of the data, associated 
errors, etc.  With CCDs, you only stack to measure the faintest objects 
where you need to achieve a reasonable SNR for photometry to be meaningful 
(e.g. to reach SNR=6 or more if you want your photometric precision to be 
better than +/-0.15 mag or so).

So, for example, my cut-off limit for photometry is V=18 using a 28-cm 
scope.  When I used the same scope for PMT-based photometry, the equivalent 
cut-off was V=12.  So I reach 6 magnitudes fainter in switching to a CCD 
camera.  In principle I could do single-channel photometry at V=18 just the 
same but I would need a 5-meter telescope in my back garden!

On the other hand, I agree that if someone wants to work on stars brighter 
than say V=5, and their skies are generally very good then the 
single-channel photometer route is a good choice.

Richard M.