Efficiency
By measuring the signal from standard stars we can measure
the efficiency of the entire system.
Measurements
Measurements of standard stars were taken on Nov 3, 2000 UT
using the Thirty Inch under fair conditions.
The total integrated counts over sky in a 10 pixel square box
was measured.
The magnitudes were taken from
Hunt et. al. JHK Standards
for Array Detectors.
The Zero Point Magnitude Mzp is defined such that:
Mstar = Mzp - 2.5 Log10(DN/sec)
| Star AS02 - SAO 054271 |
|---|
| Filter | Signal (DN/sec) | Mag | Mzp |
|---|
| J | 5000 | 8.78 | 18.0 |
| H | 5760 | 8.77 | 18.2 |
| Kshort | 3060 | 8.77 | 17.5 |
| Star AS040-5 |
|---|
| Filter | Signal (DN/sec) | Mag | Mzp |
|---|
| J | 2560 | 9,49 | 18.0 |
| H | 2965 | 9.42 | 18.1 |
| Kshort | 1560 | 9.40 | 17.4 |
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 = 35 e- /DN
| Theoretical Zero Point
|
|---|
| Filter | Lambda | DLambda | 0MagFlux | Mzp |
|---|
| micron | micron | W/m2/micron | |
|---|
| J | 1.25 | 0.16 | 3.07E-09 | 19.0 |
| H | 1.63 | 0.31 | 1.12E-09 | 18.9 |
| Kshort | 2.14 | 0.30 | 4.07E-10 | 18.1 |
The central wavelengths and bandwidths are taken from
the IR consortium curves for our filters.
Note: there was an error in width of the J filter the analysis
of the Mark I camera.
The 0 Mag flux is taken from
Vega
Efficiency
The efficiency of the system is the difference between the observed
Mzp and the perfect Mzp.
| Observed Efficiency |
|---|
| Filter | Mag Diff | Efficiency |
|---|
| J | 1.0 | 0.40 |
| H | 0.7 | 0.52 |
| Kshort | 0.6 | 0.58 |
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
- Three mirror reflections: 0.92 each
- Quantum efficiency: 0.65
- Atmosphere: (guess) 0.90
The product of this is 0.32.
We are doing better than theoretical.
Last Revised November 3, 2000