From data obtained on 2/7/2004, of the optical flux standard BD+26 2606 (G166-45; SP1446+259; V=9.7, J=8.7, H= 8.4, K=8.4 Alonso et al. 1994 A&ASS 107, 365), with the widest 'slit' (1mm pinhole), grism #1 and WFC, we obtained the following preliminary results.
At 1250 nm (approximate center of J band), the system efficiency (atmosphere, telescope, instrument, detector) is about 4%. At 1030 nm (approximate center of the Yn filter NOT #222), the system efficiency is about 3%, but these 3% are divided over the 6th and 7th order. Extrapolating this to H and K, using estimates for instrument and detector efficiencies, we get a total system efficiency for H and K of 5% and 6% respectively.
In the J band (6th order), the above star gives about 110 electrons/Angstrom/second, in the reduced spectrum. In the Z band (Yn filter, 1030nm, 7th order) we get about 75 e-/A/s. In the H band we observed approx 90 e-/A/s. In K we observed about 70 e-/A/s.
Science-grade array (SWIR3)
I have extrapolated the above results to derive estimates for the spectroscopic JHK efficiencies and countrates when using the new science array (installed Dec. 2007).
| Expected count rates and efficiencies for science-grade array | |||
|---|---|---|---|
| J band | H band | K band | |
| system efficiency | 7% | 8% | 9% |
| photons/sec/Å for SP1446+259 | 190 | 140 | 105 |
In June 2009 the star SP2209+178 was observed to calibrate the Yn-band efficiency.
| Measured efficiency for science-grade array | ||||
|---|---|---|---|---|
| date | Yn band 7th order | Yn band 6th order | J band 6th order | J band 5th order |
| June 2009 | 3.5% | 4.7% | 6.3% | 8.2% |
In June 2011 the star SP2209+178 was observed to calibrate the Y- and Z-band efficiency.
| Measured efficiency for science-grade array | ||||||||
|---|---|---|---|---|---|---|---|---|
| date | Z band 8th order | Z band 7th order | Y band 7th order | Y band 6th order | J band 6th order | J band 5th order | H band | K band |
| 12 June 2011 | 3.7% | 3.5% | 4.7% | 4.6% | 4.5% | 6.3% | 7.0% | 5.6% |
| photons/sec/Å | 175 | 150 | 190 | 180 | 150 | 200 | 175 | 105 |
Exposure-time calculator
Currently, the above estimated efficiencies and countrates for the science-grade array are used in the exposure-time calculator.
John Telting (jht)