iraf.net

From: Brendan Sheehan <bsheehan_nuig@yahoo.co.uk>
Date: July 12, 2005 11:02:56 AM MST
To: iraf@noao.edu
Subject: exposure time for addstar
Hello,I am trying to add stars to a background only image.
My PSF model was made using phot-pstselect-psf and
tested using allstar against the image which the psf
model was created. This initial image is for 1 sec,
but would like to scale my PSF model so that the added
stars in the background image will be the equivalent
of 300 sec exposure. Have I set the datapars.itime =
1 (sec) for the generation of the psf model. Do I
reset this to 300 just before the addstar task? or is
there another way. (would expect the counts/pixel to
increase for a longer exposure, but I am not getting
that)Any advice or help would be great
Thanks
Brendan
From: Frank Valdes <valdes@noao.edu>
Date: July 13, 2005 3:40:46 PM MST
To: Brendan Sheehan <bsheehan_nuig@yahoo.co.uk>
Subject: Re: exposure time for addstar
Hello Brendan,The author of ADDSTAR has left our project and I am not very
knowledgable of the details. However, it is my reading of the help
what sets the counts in the added stars is the magnitude you
specify. The magnitude is relative to the zeropoint given in the psf
image under the keyword PSFMAG. So if you want to add a 15th
magnitude star (assuming you don't use the random stars option) you
set a photometry file with something like100 100 15Then you will get a 15th magnitude star relative to the magnitude it
set from the original photometry and PSF generation. If you want to
scale to something that has a longer exposure time you either change
the magnitude in the photometry file (decrease the magnitude for the
equivalent of a longer exposure) or the value of the zeropoint in the
header of the PSF image (increase for the equivalent of a longer
exposure). Note that the magnitude change relative to a 1 second
exposure would be Dmag = 2.5 log10 (300.).I hope this helps. Just it case it may be useful to you, you might
be interested in the ARTDATA package and the STARLIST and MKOBJECTS
tasks.Yours,
Frank Valdes