[Aavso-photometry] In defense of the ST-9 & CCD choice Considerations

[arne]

mlfleenor at charter.net wrote:
> 
> Re: 2 arc-second/pix sky coverage  check out CCD Univeristy @ apogee's website.
> http://www.ccd.com/ccd113.html
> 
> In re-reading Tim’s post I see that he  said 2-3  pixel sky coverage not 2. I was wanting to illustrate that as long as you are not running under-sampled that this criterion was not something that has to be explicitly followed. 
> 
> I use my SCT with a compressor and often run binned 2x2 yielding 1.24 arc-sec/pix resolution or ~ 2.4 pixel sky coverage- however many times I am running 1x1 @ 0.62 arc-sec/pix  resulting in 5-6 pixel sky coverage with my typical seeing. Clearly outside of the 2-3 pixel criterion yet my results are excellent. ( I often achieve 1mmag precision with R-band 5 minute averages with extra-solar planet candidates during out of transit baselines. ) 
> 
> My frustration with the ST-9 was due in part to some of the images having blocky stars- I was under-sampling! I also was running @ 2260mm focal length. I could’ve run the SCT @ f/10 but here again the fov would be much too small for me. 
>  
>  I do take a slight sensitivity hit but also need the resolution to resolve close stars. 
> While the ST-9 may better meet the  magic sampling requirement there are other considerations.
> 
>  I bet there are a lot more folks doing photometry with SCT’s using ST8’s instead of ST-9’s .
> 
The Apogee site says:
"A good rule of thumb to avoid undersampling is to divide your seeing in half
and choose a pixel size that provides that amount of sky coverage. For example,
if your seeing conditions are generally 4 arcseconds, you should achieve a sky
coverage of 2 arcseconds per pixel. If your seeing conditions are often
arcsecond, you'll want a pixel size that yields 0.5 arcseconds per pixel."

This is perfectly correct and not "silly".  The guideline is to not use
*fewer* pixels than two per fwhm, and your description above indicates that
you often use 5-6 pixels per fwhm.  This is fine and not in violation
of any guideline that I know of.  In the past, I've seen sites that
just said "use 2arcsec/pixel", which might be fine for some folks, but
wrong for others - a single number cannot work for everyone.  That is what
I thought you had found somewhere and what we've been trying to avoid.
Again, these are guidelines - I've seen good photometry with less than
two pixels per fwhm, but if you have a choice, it is best to sample
properly.

The under-sampling is why I suggested the ST8 for David.  For others,
who might be using longer focal length telescopes, or might be in
significantly worse seeing conditions, the ST9 is a perfectly
adequate solution and does not undersample.

Whether more photometry is being done with ST8's or ST9's depends a lot
on what set of statistics you choose.
Arne