15
February 2006
William
Schuster
(a) First, see page 3, section 4 of the
Danish photometer manual by Dr. Poul E. Nissen 1984, “Description and Data for
the Danish 6-channel uvby-beta Photometer”, especially Fig. 3. There is a “Danish” or “Danés” notebook
with this manual in the control room of the 1.5m telescope. One section of this notebook is
dedicated to calibrating the grating angle and there are copies of many
previous calibrations. The Danish
photometer manual is online (without figures).
(b)
Use the smallest diaphragm, No. 5.
Important!
(c)
Use gloves or optical tissue when handling the cadmium lamp, to keep the lamp
clean and free of finger prints.
The lamps also last longer with this care.
(d)
Turn on the lamp ~15 minutes before beginning the calibration. Nissen’s manual recommends doing this
only 5 minutes prior to the calibration, but with 15 minutes the lamp is more
stable, facilitating the calibration.
Important!
(e)
Point the 1.5m telescope at an hour angle of 0h 0m (meridian) and a declination
of -22d 0', so as to use the reflector mounted on the inside of the dome. Illuminate this reflector with the
cadmium lamp.
(f)
Begin the calibration at 0.30 and proceed to 0.70 in steps of 0.01. Then, return to 0.20 before adjusting
the angle. This avoids mechanical
play. In other words, make the
final adjustment of the grating angle in the same direction as was used for
calibrating this angle. Important!
(g)
The calibration should be done as quickly as possible to avoid variations of
the lamp. The result is then
easiest to interpret. To help
avoid lamp variations, plug the cadmium lamp, its power supply, and transformer
into a regulated power outlet.
(h)
Do not forget to add 0.011 to the result from the graph to correct for the
difference in wavelength between the cadmium line and the “slot edge” of the
“b” channel that’s used to construct the graph, 4799.9A and 4801.0A,
respectively.
(i)
Before beginning the calibration, note the initial reading of the grating angle
(see the dial; to check the stability) and the dome temperature (for comparison
with future calibrations). Include
both data on the plot used for the grating angle calibration.