Notes on using the Halle Lyot H-alpha filters
Bart de Pontieu, Sep 2002
Using the Halle Lyot H-alpha filters
During the September 2002 run, two Halle filters were used for H-alpha, one
of these is no. 139 which has a knob indicating Angstroms
on it. The other one has no. ??8 (the number is obscured) which has at each
end a turnable ring which changes the polarization.
Each had an H-alpha prefilter in front:
H-alpha, 10 Angstrom filter
marking on side : 656.5/1.0 NM 3002 Barr #1
CWL = 6565.2 A, BW = 9.7 A, Barr, July 2002
peak transmission = 82%
wavefront : rms = 0.023, Peak-Valley = 0.15 waves
H-alpha, 10 Angstrom filter
marking on side : 656.5/1.0 NM 3002 Barr #2
CWL = 6565.8 A, BW = 9.8 A, Barr, July 2002
peak transmission = 83%
wavefront : rms = 0.025, Peak-Valley = 0.15 waves
Details regarding set-up of filter 139:
For no. 139 we had to tilt the prefilter a bit (a few degrees) to get nice,
bright H-alpha linecenter images. The knob shows a gauge for
temperature, and (at the bottom) for setting the temperature. We set it at
41.34C, which is the temperature on the paper attached to the
filter.
There is another knob for changing the bandpass from 0.5 to 0.7 Angstrom.
The knob to "tune" the filter shows values from 1 (red) to 1
(black). This knob can be turned over 4 revolutions of that range. We "found"
linecenter by taking steps of 0.5 Angstrom and recording the
images. Analysis of the brightness and contrast shows a nice cosine curve
as a function of the steps taken. There are two positions of
highest contrast, which roughly correspond with highest brightness. The highest
contrast images looked like H-alpha linecenter
the best. The position we used can be described as follows: First turn the
knob all the way to the red, until you hit the end stop. It hits
the end stop at 1.0 (red). Then turn the knob towards the blue (which is
the only way you can turn it at this point) over two full
"rotations", i.e., go through +1.0 (red) two more times. Then continue until
you hit the first 0.55 (black) after that, and that's the
position we used as line center. Tuning to the blue and red from there is
easy. If you want 0.4 to the red, tune the knob to 0.15 (black), if
you want 0.4 to the blue, tune the knob to 0.95 (black).
Details regarding set-up of filter ??8:
The temperature was set to 39.13C, which again is the temperature written
on the paper attached to the filter.
This filter has a circular ring at each end of the filter with numbers on
it (degrees of the polarizers). We will call the ring closest to
the Sun ring 1, and the other one (at the camera end) ring 2. We first tuned
this filter to H-alpha linecenter by eye. The values we
ended up using were 0 degrees on ring 1, and 150 degrees on ring 2. We then
tuned this filter to the blue wing, by comparing it by eye
with the blue wing of filter no. 139. The values we found for this position
in the blue wing (roughly -0.4 Angstrom on filter 139) was
280 degrees on ring 1, and 85 degrees on ring2. Note that these values are
roughly 90 degrees off from the linecenter values. The red wing
can presumably be found by turning (one or two of) the rings to 90 degrees
off in the other directions. Turning the polarizers in general
changes the brightness of the images in a non-uniform way.
Time-stamp: <2002-12-06 15:27:00 dettori>
Peter Dettori < dettori@astro.su.se
>