Subject: ARCES flatfielding

From: Julie Thorburn

Submitted: Thu, 15 Jul 1999 14:15:48 -0500

Message number: 13 (previous: 12, next: 14 up: Index)

Don York has posted to these mailing lists about apparent flatfielding
problems with the echelle.  These problems manifest themselves as a medium
scale fringe-like pattern in extracted spectral orders, which is easily seen
in tungsten-quartz lamp exposures and is less apparent in stellar exposures.
The ripple is characterized as an undulation of amplitude 1-2% about the true
echelle blaze function on spatial stretches of 10 to 20 pixels, thus limiting
the maximum signal-to-noise ratio on this length scale.  Possible optical and
mechanical explanations for the ripple were proposed, tested, and eliminated
as contributing factors.  

The ripple has now been demonstrated conclusively to be an artifact of the
default order extraction method used in IRAF's echelle software package.  In
particular, the algorithm used in aptrace does not cope well with minimal
sampling of spectral orders in the direction perpendicular to the dispersion.
Ripple is introduced in regions of strong order slope or curvature due to
aliasing.  While ultimately a detector with smaller pixels would reduce the
observed effect, a software fix to the relevant IRAF packages is definitely
warranted but may require some time to implement.  

Don and I will keep everyone posted.

Julie Thorburn


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