 |
The emissivity in K band of telescope/NAOMI/INGRID is ~ 100%, i.e. the background is 2 - 3 mags brighter than when INGRID is used at the Cass focus. This severely restricts the K-band science which can be done with NAOMI. The H background is ~ same as that at Cass.
Tom's 6/01 experiments with a cold soup ladle showed that the % of the
K counts coming from each part of the light path is:
%
Inside INGRID 7
Foreoptics 20
NAOMI 5
DM 12
Derot mirrors 13
Derot windows 10
Telescope 23??
---
100
Summary of actions suggested at a meeting 4 Nov 2002 between TG, AZ and CRB:
Actions not requiring INGRID:
- Obtain transmission curves for the elements in the foreoptics - any
absorption dip in K (i.e. emissive)? [TG].
- Make a rough sketch of the inside of INGRID, showing the approx
temperatures of each surface [AZ].
- If the mirror surfaces in the derot were 96% reflective rather than
98% (as expected for Ag), this would explain much of their
measured emissivity. They were done at RGO, maybe they were coated
with something which ensures long life, but is emissive.
Silver-coat the tertiary and the 3 derot mirrors, at ING, by
spring 2003 (and consider doing the latter at Denton NJ,
proprietary coating) [TG].
Actions with INGRID at Cass:
- Measure background counts through broad-band and narrow-band K filters,
to measure the strength of the total foreoptics + filter absorption
dip (water) in K band [AZ]. Difficult to separate effects of an abs dip
in the foreoptics from one in the filter, but one will be seriously
emissive, the other won't.
Actions with INGRID cold in test focal station:
- INGRID's aperture is much larger than that of the foreoptics, so
emission from the INGRID-side back surface of the foreoptics might
be leaking into INGRID. Try painting it black to see if the
counts rise (they shouldn't) [TG].
- Repeat soup-ladle test with Cass foreoptics, to see if there is
significant emission from those [TG/AZ/CRB].
- Repeat the soup-ladle experiment with NAOMI foreoptics, to confirm
results obtained in GHRIL 6/01 [TG/CRB].
Actions with INGRID open in test focal station (when new
narrow-band and broad-band filters installed, chip tilted??):
- Check baffling, check colours and shininess of surfaces, check
thermal contacts, install additional baffling? [AZ/TG].
|