[Aavso-photometry] Flat Fields

[Wolfgang Renz]

Hi Jeff

Flats are especially good to get rid of the high frequency (pixel to
pixel) sensitivity differences. If the stars of interest fall on weaker/
dark pixel they will influence the photometry according to their
deviation and their part of the total flux.

Flats are good to get rid of most of the low frequency differences.
This is not just dust donuts. If you have vignetting of 20%, the abs
mag of the stars at the edges/corners will be of course off by ~ 0.2
mag.

For differential photometry with good guiding and polar alignment
(keeping the stars of interest at the very same pixel) and when the
stars of interest fall on well behaving pixel (not dead and not hot
and therefore still in the linear range) you can skip the flats. Then
you will get just a more or less significant offset in the abs mag.

Without good guiding and polar alignment or when just tracking or
taking images in Alt/Az mode, the stars of interest will move across
the chip. If the movement of the stars is just short, differences due to
vignetting might not be a major issue, but dust donuts might and
severe pixel to pixel sensitivity differences will. It has happend more
than once that a star was suspected to be variable just because
it moved across an anormal pixel during a TS.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Jeff Hopkins" <phxjeff at hposoft.com>
To: <aavso-photometry at mira.aavso.org>
Sent: Thursday, September 13, 2007 12:33 AM
Subject: [Aavso-photometry] Flat Fields


> I have been experimenting with flat fields. Everyone has said to get 
> the best data you must flat field your image.  Has anyone actually 
> done experiments to show how important this is?
> 
> In my experiments I have found that there is less than 0.01 magnitude 
> difference between photometric data taken from original images
> versus those calibrated with flat fields.
> 
> Taking flat fields is a major effort and extra work in processing. 
> While it is nice to get rid of dust donuts, just how much does it 
> affect the photometric data to flat field the image?
> 
> The theory sounds fine, but in reality I have not found that it makes 
> much difference. Taking three 10 second images in a given filter with 
> darks subtracted produces 3 original images. Flat fielding them and 
> comparing original image 1 to image 1 flat fielded show magnitude 
> differences of less than 0.01.
> 
> Comparing image 1 to image 2 and image 3, I see greater than 0.01 
> magnitude variations in both the original set and the flat fielded 
> set. That makes me think that perhaps the flat fielding benefit is in 
> the noise.
> 
> I am still experimenting and am not please with the flat fielding 
> I've done. I have tried light box, dome and twilight flats. Either I 
> am doing something wrong or this seems to be a wasted effort.
> 
> I'm curious if anyone else has experimented and determined empirical
> data on the value of the flats.
> 
> Thanks.
> 
> Jeff
> -- 
> Jeff Hopkins
> HPO SOFT
> Counting Photons
> http://www.hposoft.com/Astro/astro.html
> Hopkins Phoenix Observatory
> 7812 West Clayton Drive
> Phoenix, Arizona 85033-2439 U.S.A.
> (623)849-5889
> (623) 247-1190 (Fax)
> www.hposoft.com