ACS Monochromator Calibration

AUTHOR : G. Hartig

LOCATION AND DATE : BATC : Dec 20, 2000

Introduction

We illuminated the .39 m Acton VM 504 (serial 50434) monochromator with a Hg pen lamp, focussing the lamp onto the side entrance slit with a very fast lens to assure overfill. This side entrance slit will be used for RAS/HOMS calibration but the "end" entrance slit is used in the RAS/Cal delivery system, and a separate calibration should be performed in the future for this configuration. Both use the side exit slit.

Slit Calibration

Note that the entrance slit micrometer is difficult to read properly since the barrel rotation is not well adjusted wrt the stem markings. 0.000 is reached well below the 0 mark on the stem. The slit closed positions were checked by adjusting down until the throughput was effectively 0. This was found to be at 0.000 for the entrance slit and -0.001 for the exit slit. The monochromator should be operated with the two slits nearly matched. We suggest just setting them both to the same nominal widths on the micrometers since their 0-points are very close to nominal.

Wavelength calibration

We checked the wavelength calibration of gratings #2 (blaze 300nm) and #3 (blaze 750nm) with slits set the way they were for the calibration activities in late June 2000 i.e. entrance slit 0.095, exit slit 0.070. This calibration is somewhat misleading since the slits were not matched and the profile of each line is asymmetrical and dependent on the uniformity of the entrance slit illumination. Nevertheless we can say with certainty that the wavelength calibration is very close to nominal, that is, correct within about 10% of the narrowest passband (10 Ang) that we expect to use. The resolution calibration below indicates that for grating #3 at 872 nm the wavelength calibration is correct to well within the 1 Ang readout precision of the monochromator controller. This calibration should be repeated with matched, narrow slits.

Spectral Resolution vs Slit Width

Using the Hg 435.84 line in 2nd order (871.68), grating #3 :

Relative Grating Efficiency

The cross-over point, i.e. the wavelength at which the efficiency of grating #2 drops below that of grating #3 was determined to be at ~510 nm, estimated by scanning a tungsten lamp spectrum with each grating.