Re: HD 145913

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

With 10.3arcsec fwhm, I assume you are using really big apertures.
One common problem is the partial inclusion of a faint neighbor in the
aperture; this will give you pseudo-flickering as more or less
of the starlight is included.  When you are dealing with 0.01mag
photometry, a star 5 magnitudes fainter becomes important.  You
should look at the object in an image that goes relatively deep,
good resolution, but not saturated, and see what surrounds your
object.  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.
Arne

Michael Koppelman wrote:
> On Friday, May 9, 2003, at 10:07 AM, Arne Henden wrote:
> 
>> fwhm = 3.1, but what units?  arcsec? pixels?  If arcsec, how
>> many pixels is that with your system?
> 
> 
> Sorry, that's pixels and I'm at 3.4 arcsec/pixel so about 10.5 arcsec is 
> my FWHM.
> 
>>   The 0.01mag error on the comp star is suggestive that the
>> 0.04mag flicker is real, but you just have to check everything.
>> If you are using multiple comp stars, then I assume the "comp"
>> you list on your plot is just one of those stars?  What is the
>> typical error between the different comp stars?  You did BVRI,
>> so how did the color of the object compare with other stars in
>> the field, especially at R,I?  Sometimes binaries will show the
>> blue star in the blue filters and the red star in the red filters,
>> so you can tell that you have a composite system.
>>
> 
> I will check this stuff when I get back to my data. Thanks, Arne.
> 
> Michael
> 
> 
> 
>