Subject: GRIM II thermal backgrounds

From: Alan Watson

Submitted: Sat, 8 Feb 1997 17:04:26 -0700

Message number: 35 (previous: 34, next: 36 up: Index)

There has been some concern that the 3.5m and GRIM II have a higher
thermal background than expected. On the engineering run last week,
Mike Ledlow and I measured the background in the K filter as a
function of airmass. We find a good linear fit from 1 to 2 airmasses of

    K background = 4180 + 2330 X        DN/s/arcsec^2

This measurement was taken at an air temperature of about 275K. Using
the zeropoint of 22.24 derived on the same night, this translates to
background of 12.8 mag/arcsec^2 at 1 airmass. We can compare the
instrumental and atmospheric contributions to the background at 1
airmass using the measurements quoted by Wainscoat & Cowie (1992, AJ,
103, 332) for UKIRT and the UH 2.2m, also at 275 K:

                    APO 3.5m    UKIRT       UH 2.2m

    instrument      13.2        12.9        13.8        mag/arcsec^2
    atmosphere      13.8        14.1        13.9        mag/arcsec^2

On the whole things don't look too bad. The 2.2m measurements indicate
that there may be some room for some improvement, especially since the
coatings on the 2.2m were apparently not especially clean at the time
the background was measured. The additional 0.6 mag of emission reduces
our 3.4m aperture to 2.6m. The camera on the 2.2m had an optimized
cold stop that blocks out the high emissivity central hole in the
primary; I don't know if GRIM II does.

We also roughly determined the background at Ks from single
measurements at 1 and 2 airmasses. We find

    Ks background = 2300 + 2240 X       DN/s/arcsec^2

Using the zeropoint of 22.37 derived on the same night, this
translates to 13.2 mag/arcsec^2 at 1 airmass, about 0.45 mag fainter
than at K. This is similar to the difference Wainscoat and Cowie find
between the K and K' filters.

Alan Watson

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