Information about mscpixarea
in the mscred package

TWO email messages from Frank Valdes, the second has just a bit more information that the first.

-------- Original Message --------
Subject: Re: 2 mosaic processing questions
Date: Thu, 16 Sep 99 11:46:56 MST
From: valdes@noao.edu (Frank Valdes)
To: schommer@ctiow7

Hi Bob,

1) The task for the pixel area correction is MSCPIXAREA.  The parameters
are

PACKAGE = mscred
   TASK = mscpixarea

input   =                       List of input MEF files
output  =                       List of output MEF files
(outtype=             multiply) Output type
(norm   =                INDEF) Area normalization
(verbose=                   no) Verbose output?

The outtype can be "area" to output the pixel area factors, "multiply"
to multiply the factors into the input image, or "divide" to divide the
factors into the input image.  The distinction between multiply and
divide is whether you want to add the correction to a flat to then
be applied to the data or apply it to flat fielded data.  The "norm"
parameter is what changes the pixel area in sq arcsec to a correction
factor.  If you leave it as INDEF then it will normalize by the area
of the pixel at the tangent point.

Cheers,
Frank

> >From ata@urania.astro.wesleyan.edu Thu Dec  2 14:12:39 1999
> Date: Thu, 2 Dec 1999 16:11:57 -0500 (EST)
> From: Ata Sarajedini 
> To: valdes@noao.edu
> Subject: 0.9m mosaic
> 
> Hi Frank:
> 
> I've gotten to the point of getting flattened images. If I want to do
> stellar photometry on each individual CCD separately, what else do I need
> to do to the images?
> 
> -Ata
> 
> 
> Date: Thu, 2 Dec 99 15:42:04 MST
> From: valdes (Frank Valdes)
> To: ata@urania.astro.wesleyan.edu
> Subject: Re: 0.9m mosaic
> 
> Hi Ata,
> 
> You need to correct for the pixel scale varying across the field.  Flat
> fielding forced the sky to be constant in each pixel but it really
> should be lower when the pixel scale is smaller in sq. arcsec / pixel.
> There is a task in MSCRED called MSCPIXAREA to do this:
> 
>         ms> mscpixarea  

> 
> where the input and output are the MEF files.  It uses the astrometric
> solution in the header to get the pixel areas in arc sec.  The
> "multiply" option means to multiply each pixel by a normalized area.
> The normalization can be selected by the task (norm=INDEF) or by you.
> The normalization must be the same for all data to avoid a zero point
> error.  FYI, the "divide" option would be used to correct a flat field
> or sky flat so that you would not need to do this to every image,
> however, you probably have done this and it is not a big deal.
> 
> When the pixel scale varies another factor to consider, when comparing
> photometry at different points, is to make sure the apertures or PSF
> is adjusted appropriately.  Of course if you correct to a very large
> aperture this should go away.
> 
> The effect is large at the 4m and small at the 0.9m.
> 
> Let me know if you have questions.  You might also ask Lindsey Davis
> who is the IRAF digital photometry expert in our group.
> 
> Cheers,
> Frank
>