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

[Michael Newberry]

Tom,

Although the ideal is to have optimal sampling always, there are often times 
then it just works out that you have 2 pixels or less in the FWHM of the 
photometry aperture. When you are doing aperture photometry and the data are 
poorly sampled or you nust use a "small" aperture for some other reason, 
such as crowding, then the quality of the result boils down to 2 things:

1) partial pixel handling
2) centroiding accuracy.

Michael Newberry

----- Original Message ----- 
From: "Tom Krajci" <tom_krajci at tularosa.net>
To: <aavso-photometry at mira.aavso.org>
Sent: Wednesday, June 13, 2007 9:35 AM
Subject: Re: [Aavso-photometry] In defense of the ST-9 & CCD 
choiceConsiderations


> >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.
>
> -------------------------------------------
> 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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