Zero Points

Standard stars are observed every night in the J, Js, H and Ks filters. For the NB filters, standards are observed as required.

Standard stars are imaged over a grid of five positions, one just above the center of the array and one in each quadrant. The recipe finds the standard (it assumes that the star in the first image is near the center), computes the instrumental magnitude, and then uses the standard star database to determine the ZP, which is uncorrected for extinction.

The standard star database contains about 1000 stars with magnitudes in the J, H, K, Ks, L and M bands, although most stars only have magnitudes in a subset of these filters. The standard star database is currently accessible through an eclipse command called std. In the (near) future, we will provide a web interface for accessing this list.

If you want to check out which stars are present in the ISAAC database, use the std command. You can search the database by name, position, magnitude range, catalog name, etc. Stars are currently taken from the following catalogs:

• Arnica
• ESO Van der Bliek
• LCO Palomar
• LCO Palomar NICMOS red stars
• MSSSO Photometric
• MSSSO Spectroscopic
• SAAO Carter
• UKIRT extended
• UKIRT fundamental

The implemented recipe is the following:

For any couple of consecutive images (image1, image2):

1.

diff = image1 - image2

2.

Locate in diff the star around the expected pixel position (provided by the FITS header or by an external offset list).

3.

Compute the background around the star, and the star flux.

4.

Store the flux result in an output table.

Apply steps 2 to 4 to the inverted image image2-image1. This yields 2(N-1) measurements for N input frames. From this statistical set, the highest and lowest values are removed, then an average and standard deviation are computed. The conversion formula from ADUs to magnitudes is:

zmag = mag + 2.5 * log₁₀(flux) - 2.5 * log₁₀(DIT)

where:

• zmag is the computed zero-point.
• mag is the known magnitude of the standard star in the observed band.
• flux is the measured flux in ADUs in the image.
• DIT is the detector integration time.

Note that neither the extinction nor the colour correction are included in the ZP. The average airmass is given in the output result file, together with individual airmass values for each frame.

The average extinction on Paranal for the J, Js, H, Ks and NB_M filters, is available from the ISAAC web pages.

The correspondence between the filter in which the observations were taken and the filter in the standard star table is listed below. The two filters are reported in the QC parameters: QC.FILTER.OBS and QC.FILTER.REF. This correspondence completely ignores corrections due to filter mismatch, and, in some cases, these corrections are substantial.

ISAAC filter	Reference filter
Z	J
SZ	J
Js	J
J	J
SH	H
H	H
K	K or Ks
Ks	Ks or K
L	L
M	M

ISAAC narrow band filter	Reference filter
NB_1.06	J
NB_1.08	J
NB_1.19	J
NB_1.21	J
NB_1.26	J
NB_1.28	J
NB_1.64	H
NB_1.71	H
NB_2.07	Ks
NB_2.09	Ks
NB_2.13	Ks
NB_2.17	Ks
NB_2.19	Ks
NB_2.25	Ks
NB_2.29	Ks
NB_2.34	Ks