Efficiency
By measuring the signal from standard stars we can measure
the efficiency of the entire system.
H Measurements
Measurements of two standard stars in H band were taken on June 9 UT
on the Thirty Inch under good conditions.
The total integrated flux over sky in a very large aperture
was measured.
Several different exposures at different times were used and
the values converted to DN/sec.
Star DN/sec Mag Mzp
HD10560 48911 +/- 500 6.7 18.4
SAO106-1024 1246 +/- 300 10.7 18.4
The magnitudes were taken from
KPNO and UKIRT lists
The Zero Point Magnitude Mzp is defined such that:
Mstar = Mzp - 2.5 Log10(DN/sec)
J Measurements
Measurements of two stars were taken in J band on July 29 UT.
Star DN/sec Mag Mzp
A28-0 (s1) 15079 8.46 18.6
A29-0 (s13) 1458 10.23 18.1
The agreement between these two observations is very poor and
not understood.
Theoretical
The Mzp of a perfect system is computed from the following factors:
- Telescope area (a 30 inch diameter circle in these calculations)
- Photon NumberFlux from Vega (0 Mag)
- Bandpass of filter
- Conversion Gain = 31 e- /DN
Filter Lambda DLambda Flux 0Mag Mzp
micron micron W/m^2/micron
J 1.25 0.28 3.07E-09 19.8
H 1.65 0.30 1.12E-09 19.0
Ks 2.2 0.35 4.07E-10 18.4
The central wavelengths and bandwidths are taken from
the IR consortium curves for our filter.
The 0 Mag flux is taken from.
Vega
Efficiency
The efficiency of the system is the difference between the observed
Mzp and the perfect zp.
From the above two numbers at H, we get a difference
of 0.6 mag which implies that the efficiency is 58%!
The difference at J is around 1.3 mag which is about 30%.
Compare to the theoretical value.
- Cassegrain form factor efficiency due to 11" secondary blockage: 0.89
- Filter transmission: 0.95
- Window transmission: 0.92
- Telescope mirror (guess): 0.90
- Quantum efficiency: 0.65
- Atmosphere: (guess) 0.90
The product of this is 0.40.
Last Revised September 15, 2000