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 fromthe filters used in these systems. The colour transformation between the ISAACfilter 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 bedone 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 acentral wavelength of 3.6 microns; L' has a central wavelength of 3.8microns.For L' band imaging, standards should be fainter than 6thmagnitude.

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 A0star to calibrate Bracket Gamma observations may lead to a systematic error inthe 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

Table 1. IR Extinction on Paranal