For mid-infrared ground-based astronomy, ambient temperature optics and the associated support structure of the telescope are the most significant contributors to background radiation. Minimizing the radiation by reducing the emission from these sources can help achieve optimal performance for scientific observations by reducing the background noise. Thus, characterizing the emissivity from the optics can be useful for tracking changes and making improvements of the infrared performance. LWS detects thermal radiation from three ambient temperature surfaces, the dewar window, the secondary mirror and the primary mirror.  The Keck I telescope primary mirror is coated with bare aluminum and the secondary mirror is gold coated.  The spaces between the 36 segments, which total approximately 0.5% of  the primary mirror area, contribute background radiation as well. It is of particular interest to measure the emissivity of a segmented mirror when making comparisons to other infrared telescopes.
 
 

A good technique for measuring the Keck I emissivity is to use LWS in low -resolution spectroscopy mode to acquire spectra of the background in the 10 µm region, which is dominated by the telescope sources of radiation. The spectra provide a direct measure of the total emissivity from the sky and telescope as a function of wavelength. In order to characterize the emission of Keck I, measurements were made in July of 2002 following completion of periodic primary mirror segment aluminum recoating. The July measurements also followed replacement of the gold-coated f/25 secondary mirror and the replacement of the LWS potassium bromide dewar window.  Thus, background radiation from these three optics should have been at or nearly at their minimum. The mirror temperature was recorded at 277°K at the time of the measurements.

The spectra were reduced by subtracting dark frames from the summation of the raw spectra and were fit to a wavelength scale using telluric emission features. Calibration measurements of a 99.9% emissive black body source at ambient temperature (277°K) that were acquired simultaneously were reduced in a similar fashion.  The dotted line in Figure 1 is a plot of background radiation divided by the black body calibration spectrum as measured by LWS. The ratio of the actual background spectrum and the maximum background (black body) is a measure of the emissivity from all background radiation sources, including the sky. In order to determine the contribution from telescope optics alone we must try to separate out the sky portion. To do this we use a model of background radiation that includes a Mauna Kea sky emission and fit it to the LWS data. The solid line of in Figure 1 plots a model of background emissivity that consists of  the sky and a 7.5% emissive black body divided by a 100% emissive black body Planck function.  At its minimum, the total detected emission is approximately 8.0% and at this wavelength the sky emission is nearly negligible at less than 0.5%. The model fit shows that there is a 7.5% emission after the sky portion is removed. Estimating the dewar window to be 1.0% emissive results in the determination that the Keck telescope emission is 6.5 +/- 1.0 %.
 

R. D Campbell
Posted on 7 January, 2003
randyc@keck.hawaii.edu