Subject: 1.2 arc second DIS slit with 2 arc second occulting bar

From: Alan Uomoto

Submitted: Wed, 03 Oct 2001 11:01:25 -0400

Message number: 527 (previous: 526, next: 528 up: Index)

DIS SLIT WITH OCCULTING BAR

A 1.2 arc second slit with a 2 arc second occulting bar is available
for DIS.  This slit is narrower than the others so you might consider
it even if you don't need the occulting bar.

FLAWS

This slit was made to test a new cutting technique.  Because we used a
rejected slit blank it has the following flaws:

1. One corner of the plate is unpolished and non-reflective.  This is
   probably not of practical importance; the central 2-3 arc minute
   diameter region is good.  Stars will "disappear" when they move into
   the bad corner, however.

2. Cosmetic imperfections.  There are scratches on the surface.

I believe neither flaw will interfere with typical use.  Only the
slitviewer image is affected.  The spectrograph sees an excellent slit
with no compromises on science data.

WARNINGS

The occulting bar is at field center.  Remember this when using the
slitviewer software to center an object because the default will put
your object on the occulting bar.  Russet McMillan says "Anyone using
the centering offsets recommended by slitview will need to CHANGE the
recommended X offset by several arcseconds to keep their target out from
under the bar."  Unless, of course, you want that object blocked.

The current DIS configuration, with the original cameras and blue
Tektronix and red Texas Instruments CCDs, has pixel scales of 1.1 arc
seconds per pixel in the blue and 0.61 arc seconds per pixel in the red.
This new 1.2 arc second slit is undersampled in the blue.  On the red
side, DIS images of a star in the slit show adequate (just) sampling.

Scattered light performance is unknown.  In particular, we don't know
how much light from a bright star on the occulting bar will get into
the spectrograph at large field angles.  I worry about reflections and
scattering off the guider camera and its mirrors and filters.

Alan Uomoto

October 3, 2001

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