Photometric Standards and Atmospheric Extinction
Topics
- Broad band filters
- Narrow band filters
- Atmospheric Extinction
- IR Colours as a function of spectral type
Broad band filters
For ISAAC 1-2.5 micron imaging, standards are taken from:
- LCO/Palomar NICMOS. These stars have JHK magnitudes.
- UKIRT fundamental and extended lists.
It is important to appreciate that the ISAAC broad band filters differ from the filters used in these systems. The colour transformation between the ISAAC filter set and these systems will be reported soon.
In general, standards should be fainter than 11th magnitude.
For ISAAC 3-5 micron imaging, standards are taken from:
- Van der Bliek IR Standards. These stars have JHK magnitudes and spectral types. Some have L and M band magnitudes.
- UKIRT bright standards. These stars have JHKLL'M magnitudes.
Imaging with the M band filter is not possible. Instead, imaging can be done with a narrower filter at 4.66 microns. For imaging in this filter, standards should be fainter than 5th magnitude.
The L band filter is in reality L'. The original L band of Johnson has a central wavelength of 3.6 microns; L' has a central wavelength of 3.8 microns.For L' band imaging, standards should be fainter than 6th magnitude.
Narrow band filters
For 1-2.5 micron imaging, standards are taken from:
- Van der Bliek IR Standards. These stars have JHK magnitudes and spectral types.
Some care must be paid to the spectral type of the standard. Choosing an A0 star to calibrate Bracket Gamma observations may lead to a systematic error in the photometry. Stars should be fainter than 8th magnitude.
For 3-5 micron imaging, standards are taken from three lists.
- Van der Bliek IR Standards. These stars have JHK magnitudes and spectral types. Some have L and M band magnitudes.
- MSSSO Standards. These stars have JHKL magnitudes and spectral types. .
- A composite list of standards. These stars have spectral types, some have JHK magnitudes and some of these also have L band magnitudes.
- UKIRT bright standards. These stars have JHKLL'M magnitudes.
Stars should be fainter than 5th magnitude.
Atmospheric Extinction
| Filter | Average Extinction |
| Z | 0.08 |
| SZ | 0.09 |
| J | 0.11 |
| Js | 0.06 |
| H | 0.06 |
| Ks | 0.07 |
| L | 0.08 |
| nb_M | 0.1-0.2 |