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

[Michael Newberry]

Michael,

I've done good photometry using lousy seeing and also with seeing better 
than 2 pixels FWHM. It depends on many factors, including how good the data 
are otherwise---the sky, the quality of the detector, the software, the 
technique, etc., and and your knowledge. But there are some general 
guidelines that will lead you to better results, in general.

A better criterion for the best photometry is around 2.3 to 2.5 pixels FWHM 
under the best seeing you will "typically" have. I don;t mean the 1 night 
per year case, but more like the seeing you will have on, say, 10--20% of 
your photometry nights. On nights of poorer seeing, you can always get near 
this criterion by binning the CCD.

There was a paper in PASP back in the mid 90-'s (A Canadian 
astronomer---give me a while to get the reference) in which repeated tests 
of aperture photometry precision were done using different FWHM on synthetic 
images. The best results came out with sampling around 2.2 to 2.5 pixels 
FWHM, which is a little below the Nyquist frequency (which is 2 pixels per 
FWHM).

Michael Newberry


----- Original Message ----- 
From: "Michael Koppelman" <lolife at bitstream.net>
To: "Michael Newberry" <mnewberry at mirametrics.com>
Cc: <mlfleenor at charter.net>; <aavso-photometry at aavso.org>
Sent: Tuesday, June 12, 2007 9:14 AM
Subject: Re: [Aavso-photometry] In defense of the ST-9 & CCD choice 
Considerations


> (Mike cleared this up while I was writing this, too, it seems...)
>
> It looks like what they are saying is:
>
> "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 1 arcsecond, you'll want a  pixel size that yields 
> 0.5 arcseconds per pixel."
>
> This seems to be just the Rayleigh criterion and is probably not  horrible 
> advice.
>
> http://en.wikipedia.org/wiki/Rayleigh_criterion
>
> M.
>
>
> On Jun 12, 2007, at 11:03 AM, Michael Newberry wrote:
>
>> If CCDU makes such statements, then I would kindly suggest that  Apogee's
>> website is not giving the best advice. To my knowledge, no one at  Apogee 
>> is
>> an astronomer. Arne and I (and others) have "been there and done  that" 
>> for a
>> lot of years. Take Arne's advice!
>>
>> Michael Newberry
>
>