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

[Tom Krajci]

>Tomas L. Gomez wrote:
> This interesting discussion about pixel size came at a very appropriate
> time for me: I was about to buy an ST9 camera for my new
> LX200R 8-inch, but thanks to this mails I am now pondering about
> going for the ST8!. This will be my "travelling telescope", meaning
> that I will take it to dark places in the countryside, so I guess that
> the seeing will be good (before I buy, I will check this).

A few points:

- don't forget about 'local seeing'.  If you use your scope on a concrete
pad...you may have to spray water on that sun-warmed pad to get rid of the
warm air rising from it in the early evening hours.  Or, you may have to add
a fan system to cool your telescope to ambient air temperature to get the
best seeing.  Or use fans to cool your dome to prevent warm air from rising
through the slit (chimney effect).  Local seeing is something you can
control.

- At what point does a certain FWHM/PSF/star profile get so small for your
particular camera that undersampling gets to be a problem that spoils your
data?  You can determine this empirically.  Take about 30 frames of a given
star field (of known constant stars) with sharp focus on a good seeing
night.  Measure PSF and measure the standard deviation of some differential
photometry.  Now repeat this for various amounts of defocus.  Yes, there are
other variables at play here, but you can at get a decent idea if your best
seeing is undersampled or not.  Different CCD's will have different points
at which you encounter undersampling problems.

- Don't forget about undersampling issues with your photometry software.  I
have found (when chasing faint stars on good seeing nights) that if I use
too small a measurement aperture...I get quasi-undersampling effects in my
output data.  Again, different software will handle this 'small aperture
fever' scenario differently.  (I can send interested folks some graphics
that show this effect.  Heck, maybe we want to add these graphics to the
AAVSO CCD observing manual?)

I hope this helps.

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Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/

Center for Backyard Astrophysics (CBA)
http://cba.phys.columbia.edu CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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