[Aavso-photometry] New Guy...starting out with ILAqr

[Shawn Dvorak (RHO)]

Mike,

Welcome to the world of photometry!  Focusing isn't critical, up to a point. 
Most people use aperture photometry, where star brightnesses are measured by 
summing up all the pixels within a set area centered on the star.  As long 
as poor focus doesn't spread a significant amount of the star's light 
outside of this area then there shouldn't be any problems.  However, when 
doing point-spread function (PSF) photometry it is more important.  With PSF 
the photometry software measures the profile of each star and calculates the 
star's brightness based on the parameters of that fit.  PSF generally 
expects a nice Gaussian distribution, something you might not have with a 
star that's out of focus.  This is much more of a problem with reflectors 
with their central obstruction than with a refractor like you have.

Being slightly out of focus can be an advantage.  By spreading out the light 
a bit you reduce the brightness of the central core, which lets you take 
longer images without running into linearity problems.  This helps when you 
have comp stars that differ greatly in brightness from the target object.

Shawn


----- Original Message ----- 
From: <mlfleenor at charter.net>
To: <aavso-photometry at mira.aavso.org>
Sent: Sunday, October 17, 2004 1:10 AM
Subject: [Aavso-photometry] New Guy...starting out with ILAqr


> Hello,
>
> I am a brand new AAVSO member and thought I'd start out with the ILAqr 
> campaign. I am currently using an ST10XME/Tak FSQ106 and a Schuler "V" 
> filter for my photometric setup. I have been shooting pretty pictures for 
> several years now from my often cloudy suburban location in Knoxville,TN. 
> I have very limited experience with photometry doing some unfiltered CV 
> work in the past but I am interested in transit observations of potential 
> exoplanet harboring stars.
>
> I have a question as I am in the middle of a run right now. Is it 
> recommended to re-check focus during an exposure sequence? I am setup with 
> robo-focus and with CCD Autopilot I usually re-check focus automatically 
> every hour to ensure thermal contraction of the ota hasn't ruined my 
> focus.
>
> Thanks,
>
> Mike
>
>
>
>
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