Posted: Thu Mar 02, 2006 7:09 am Post subject: tracing flats with APFLATTEN
I found the following question and answer (see below) in your forum but the solution does not work for me. Do you have any other suggestions how to solve the tracing problem in apflatten? Even using a standard star traced with aptrace as a reference doesn't work, though aptrace does a great job on the standard star and a crappy job on the flat field.
Additionally, I find that some parameters work better for some orders of an echelle fram but not for other. Is there a way to change the aptrace options for each order?
Any advice would be appreciated.
From: firstname.lastname@example.org (Trevor Price)
Date: 26 Aug 1998 14:18:36 -0700
Greetings Iraf Guru,
My quandry deals with echelle-apflatten . I have been following the
echelle Iraf manual. I have used the parameters for apflatten that appear
in the manual. Adjusting were obvious for Hires(Keck) specifics. I have
also modified numerous paramters in aptrace. No matter what combination of
parameters I choose apflatten cannot trace my flat fields at all. I have
tried aptrace on a standard star and it worked very well. I imagine since
the flat field echelle orders are plateau profiles instead of gaussian's
that the tracing algorithim has difficulty. In any case is it possible for
the traces to be used from a refernce image like apertures.
>From valdes Thu Aug 27 15:39:27 1998
Date: Thu, 27 Aug 98 15:39:26 MST
From: valdes (Frank Valdes)
Subject: Re: Apflatten
The tracing algorithm in the APEXTRACT tasks does not depend on the profile.
Probably the parameter you are not setting correctly is the profile width.
This width should be the full width, that is the width from minmum to minimum
across the flat field profile.
In any case, you can easily use traces from a reference image. A reference
aperture includes both the width information as well as the trace. What
you would do in APFLATTEN is specify the stellar profile image that you
successfully traced as the reference image. Then you can use the
automatic resize option or get into the edit step of APFLAT and adjust
the widths appropriate for the flat field. You can also use the recenter
option either non-interactively or from the edit step. You can operate
on all the orders in the edit step by using the 'a' key to select all
I would need to see some example data to understand why the recommendation does not work. To repeat, the key parameter is the width. This must be set so that the edges of the flat are included in the width. For you information the method used for the centering, where tracing is basically repeated centering at different points along the dispersion, is given by the help page center1d .
If you want some help by having me look at some sample data email me at email@example.com with a link to getting an image.
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