Subject: DIS Grating Repeatability

From: Gordon Richards

Submitted: Sun, 25 Jan 1998 00:28:37 -0600 (CST)

Message number: 39 (previous: 38, next: 40 up: Index)

Some time has been spent recently looking into the repeatability of
the DIS grating tilts.  Obviously, if the gratings are not moved, then
the question of their repeatability is moot.  It is with this fact in
mind that we wish to make the community aware of a situation in which
the gratings will move even though the observer has not intended to
move them.

Let's say you move DIS to low res and set the blue and red grating
tilts to 4400 and 7700 Angstroms.  Now, 4400 and 7700 are not at
integer positions on the stepper motor, so instead it puts the
gratings at 4391 and 7705, which corresponds to steps 534 and 496.
Fine, no problem.  The problem is this: If you now move to imaging to
focus and then back to low res and again use 4400 and 7700 (which will
be the defaults on your screen) instead of 4391 and 7705 (or steps 534
and 496), the gratings WILL move because Remark thinks that you have
changed the tilt even though 4391 and 4400 correspond to the same
stepper motor position. 

The simplest way to solve this problem is to always use step positions
rather than central wavelengths to reference the grating tilts.  This
is because DIS determines the tilts based on steps rather than central
wavelength, since a step is the quantity that is physically meaningful
to the camera.  An alternative would be to hit "sync" after
configuring your setup; however, this assumes that only one grating
configuration is required.  Thereafter, when returning to
spectroscopic mode, Remark will have the correct information, both
steps and the corresponding wavelengths to ensure that the grating
position does not change.  Additionally, one could reference the
reported wavelength instead of the input wavelength.

It is our feeling that this problem probably affects a large fraction
of the observers using DIS to do spectroscopy and we hope this helps
fix any problems that people may be having.  Our apologies if everyone
but us already knows this, but we were unaware of the problem
ourselves and are concerned that others were unaware as well.

Gordon Richards
Charles Corson
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