Spectroscopic Standards


Flux calibration in the IR

Calibration of spectroscopic data in the IR is a complicated procedure thatrequires care. In the optical, one uses spectrophotometric standards that have tabulated fluxes and wavelengths. In the IR, there are no such standards.There are some pseudo-standards, in the sense that tables of flux versus wavelength do exist for some stars. However the fluxes are either derived from models (for DA white dwarfs) or from a scaled version of the solar spectrum (for solar analogs). These standards are for space based missions and are not particularly useful in calibrating ground based data.

The most prominent feature in IR spectra are the telluric features of the Earth's Atmosphere. Unfortunately, some of the telluric lines do not scale linearly with airmass, so it is necessary to observe a star, which we willcall a telluric standard, at the same airmass and with the same instrument setup as those of the science target. This standard is divided directly into that of the science target. Ideally, the spectrum of the telluric standard should be known, so that features belonging to it can be removed. However, this is never the case, so one has to use standards in which the spectrum is approximately known.

As part of the ISAAC calibration plan, we use two types of telluricstandards, hot stars and solar analogs.

The spectra of hot stars, those hotter than B4, are relatively featureless and are well fitted by blackbody curves. So, by knowing the spectral type ofthe star, one uses a blackbody curve with the appropriate temperature to fit the continuum of the standard. The spectra of stars that are cooler than A have many more features and the continua are not well fitted by a blackbody curve for wavelengths below 1.6 microns. Fitting the spectra of these stars is more complicated; however, some astronomers use them to remove telluric features.

Unfortunately, hot stars, do contain some features, usually lines of hydrogen and helium, that that can be difficult to remove. If the region around the hydrogen and helium lines are of interest, then one can observe a late type star, which should have weak hydrogen and helium lines. This star is then used to correct for the helium and hydrogen absorption in the spectrum of the hot star. Some hot stars also have emission lines or are in dusty regions. These stars should be avoided. The V-I colour of the star can be usedas an indicator of dust. For stars hotter than A0, it should be negative. Andlastly, hot stars tend to lie near the galactic plane, so there may situations where there are no nearby hot stars.

Solar analogs are stars with spectral type G0V to G4V. These standards have many absorption lines in the IR, particularly in the J band. The features can be removed by dividing by the solar spectrum that has been degraded to the resolution of the observations. This can be very tricky with ISAAC as the spectral resolution is variable.

The final step of the calibration process is to set an absolute flux scale.The most common procedure is to use the broad band magnitudes of the target.The filter band passes are then convolved with the spectrum to determine the appropriate scaling factor.

Alternatively, one can use the standard, which is measured through both the 2" slit and the slit that was used for the science target, to set the absolute scale.

Lists of Telluric Standards

For SW spectroscopy the preferred source of telluric standards is the Hipparcos catalogue. These stars have V band magnitudes, V-I colours and spectral types. However only a minority have JHK magnitudes. This means that we have to infer the IR magnitudefrom the spectral type, which leads to uncertainties of 5-20% in the absolute flux calibration. If a more accurate flux calibration is required users should provide their own standards.

In the near future, we will provide a WEB based interface which browses the Hipparcos catalog for suitable stars. In the meantime, you can use SIMBAD to search for suitable stars or you can search the lists provided below.

Stars of the required magnitude can also be found in the list of Van der Bliek IR Standards. These stars have JHK magnitudes and spectral types. Some have L and M bandmagnitudes.

For LW spectroscopy, where the Hipparcos stars are in general too faint, we also take telluric standards from:

Appropriate magnitude ranges forstandards

ISAAC has four spectroscopic modes; medium and low resolution spectroscopy in the short wavelength arm and medium and low resolution spectroscopy in the long wavelength arm. The range of magnitudes that is appropriate for each mode differs.

Table 1. Magnitude range for standard stars
Mode IR Magnitude Range
SWS1-LR 7-9
SWS1-MR 5-7
LWS3-LR, SW filters 7-9
LWS3-LR, LW filters 4-6.5
LWS3-MR 2-4.5

The magnitudes in this table are IR magnitudes, not visual. To convert between the two, use the values listed in the following table (See Bessell and Brett, PASP, 100, 1134 and Koorneef, A & A, 128, 84).

Table 1. Typical colours of dwarfs and giants.
B8V -0.15 -0.35 -0.05 -0.04 -0.09 -0.03 -0.04 -0.05
A0V 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
A5V 0.27 0.38 0.06 0.02 0.08 0.02 0.02 0.03
F0V 0.33 0.70 0.13 0.03 0.16 0.03 0.03 0.03
F5V 0.53 1.10 0.23 0.04 0.27 0.04 0.04 0.02
G2V 0.68 1.46 0.32 0.05 0.37 0.05 0.05 0.01
K0V 0.88 1.96 0.45 0.08 0.53 0.06 0.06 -0.01
K5V 1.22 2.85 0.61 0.11 0.72 0.10 0.11 -
M0V 1.80 3.65 0.70 0.17 0.86 0.14 0.17 -
G0III 0.81 1.75 0.37 0.07 0.45 0.04 0.05 0.0
K0III 1.00 2.31 0.54 0.10 0.63 0.07 0.08 -0.03
K3III 1.36 3.00 0.68 0.14 0.82 0.10 0.12 -0.06
M0III 1.78 3.85 0.83 0.19 1.01 0.12 0.17 -0.09
M4III 2.55 5.96 0.93 0.25 1.17 0.18 0.21 -0.15

The solar spectrum

A high resolution (R=40,000) spectum of the sun is provided as six indivdual spectra that cover the SZ (1.05 microns), J, H, K, L and M atmospheric windows.

These spectra were created from data that was kindly made available by the NSO/Kitt Peak Observatory. If you use this data, we would ask you to add the following acknowledgement.

NSO/Kitt Peak FTS data used here were produced by NSF/NOAO.

The original data comes in many individual segments that typically cover a few hundred Angstroms. To make the use of the data, we have combined them into 6 spectra that cover the SZ, J, H, K, L and M atmospheric windows. This required the removal of the continuum with a 2nd or 3rd order polynomial and this has proved to be problematic near the Hydrogen lines, which usually cover several segments.

Telluric Features

Below 2.3 microns, telluric features appear as absorption lines in the spectra of astronomical objects. Above 2.3 microns, telluric features appear as as both absorption lines and as sky emission lines. These features can, withcare, be used for wavelength calibration and they are probably the most accurate way of calibrating M band and medium resolution L band spectroscopic data.

A high resolution (R=40,000) spectrum of the Earth's telluric features is provided as six indivdual spectra that cover the SZ (1.05 microns), J, H, K, Land M atmospheric windows.

These spectra were created from data that was kindly made available by the NSO/Kitt Peak Observatory. If you use this data, we would ask you to add the following acknowledgement.

NSO/Kitt Peak FTS data used here were produced by NSF/NOAO.

These data are for a site that is considerably wetter than Paranal. Nevertheless the data should be useful for identifying telluric features andfor wavelength calibration. We have made prelimiary measurements of the atmospheric transmission above Paranal and the results are provided below.

The K band spectra were calibrated with arcs and have an accuracy of about 1 Angstrom. This spectrum does not represent the true transmission spectruma bove Paranal at these wavelengths as it as been processed in such a way that the transmision at longer wavelengths is over-estimated. However, it should beuseful for identifying telluric features.

The L and M band spectra were calibrated from the telluric features themselves and have an accuracy of a few 10ths of an Angstrom. The spectra arein ADUs/second and have been normalised to zeroth magnitude. The stellar features of the star, which was used to derive these spectra, have not been removed. The specrtal type of the star was A7V.

Sky Emission Spectra inthe Thermal IR.

These spectra were taken with the 0.3" slit and are in ADUs/second. Thewavelength scale of the L and M band data were calibrated with the features themselves and have an accuracy which is a few 10ths of an Angstrom.