Re: HD 145913

[Arne Henden <aah@nofs.navy.mil>]

As Michael R. mentioned, the important qualifying word in my
reply was "bright".  MK was talking about the measurement
of a 7th magnitude object, bright for just about any telescope.
   The goal is to increase signal/noise and to remove
systematics when you are trying to obtain 0.01mag
or better photometry.  The other key word is "experiment."
Try different approaches and see what works best for you
for this specific project.
Arne


Stupendous Man wrote:
>   Arne Henden wrote:
> 
> 
>> I usualy also recommend apertures around 5x fwhm diameter
>> when trying to do accurate photometry of bright objects, so you
>> might experiment with aperture size and see what happens.
> 
> 
>   Michael Koppelman replied:
> 
> 
>>There is nothing going on (that I can see) after 4 or 5 pixels. You are 
>>suggesting a radius (in this case) of 7 pixels or so. Seems like I 
>>would get a lot more noise and a lot less signal in that case.
> 
> 
>   There are two reasons Arne (and others) suggest using such large
> apertures.
> 
>       a) if you are trying to calibrate your photometry of stars in 
>          the field of a variable against, say, Landolt standards
>          in a different set of images, then the PSF might change
>          significantly from the target star images to the standard
>          star images (especially at large airmass, as Michael K is
>          observing).  In this case, a different fraction of light
>          might fall within a "small" aperture, and so one would
>          not make a fair comparison between target and standard
>          frames.
> 
>       b) even if you are doing purely differential photometry of stars
>          in a single field, there can be small differences in the PSF
>          across the field.  Again, using large apertures protects you
>          from this effect.  
> 
>   Note that Arne qualified his statement "photometry of bright objects"
> with the word "bright".  For bright stars, the majority of the noise
> in the measurement is due to variations in the photon count from the
> star; noise from the sky is negligible.  In this case, using a large
> aperture doesn't hurt.
> 
>   If you are working near the plate limit, trying to measure the 
> faint stars, then, yes, using a small aperture may be necessary 
> to avoid the extra noise from the sky.
> 
>                                 Michael Richmond
> 
> 
> 
>