Imaging Vector Magnetograph Upgrade
The upgrade will achieve the full design performance of the IVM by permitting
high repetition rate magnetogram observations at the 0.5 arc second pixel
scale over the full field of view. The principal limitation is now the
speed of the data system.
Observing Data System
Existing configuration
Data and
Control Block Diagrams 9/22/97
Hardware upgrades
Existing software
Software change plans
Optics
Liquid crystal Fabry-Perot etalons
Liquid crystal Lyot filters
Hardware problems and enhancements
Issues
-
Pattern noise in dark frames
The cameras are specified as having read noise of around 40 electrons,
or about half a DN. We observe this if we just hook up the cameras in the
lab. However when the cameras are installed in the IVM, the rms variation
in a dark frame is about 2 DN for the data camera and about 3.5 for the
geometry camera. The noise appears to be highly structured. We have looked
at power
spectra of the data stream from each camera, in a few dark frames.
It's clear that there are power peaks which occur in both cameras, more
strongly in the geometry camera.
-
L2 focus drive
The servo motor for the L2 focus translation stage isn't driven directly
from the Bitbus card as are the other servo motors, but through a Newport
electronics package which provides limit stops. Through some software-hardware
mismatch, we are able to move the motor normally only in the positive direction.
When we try to move negative, most of the move will be executed but an
error is reported and the software requires re-initializing the motor.
This is time-consuming and annoying, though it can be worked around.
-
Fabry-Perot finesse
Initial laboratory testing of the Fabry-Perot showed very high finesse,
about 100, using an expanded laser beam. In use in the IVM, however, we
appear to realize more like 50. This isn't horrible, but the difference
is not explained, and it would certainly be nice if we could improve it
a little. Collimation of the incoming beam appears not to be a problem,
nor plate figure: it has been tested with the aperture stopped down and
the finesse is about the same. A possible suspect is electronic noise in
the plate spacing control loops.
Progress
A Hypermail
archive of communications between people working on the issues listed
above is maintained here, to record various hypotheses, tests and modifications.
Operations modes
Reduction Data System
Last modified: Tue May 5 11:38:13 HST 1998
