CFHT, Instruments, Detectors, IR, Redeye Manual: Chapter 5.


Infrared Photometry Guidelines

Accurate calibration of fluxes recorded with Redeye is of course needed by most programs. We have accordingly listed a number of standard stars in this chapter for Redeye users to have available during their runs. These standards are scattered across the entire sky and tend to lie fairly close to the celestial equator. Those interested in extremely accurate photometry (~1%) should consult the references listed at the end of this chapter. These references are also helpful for transforming Redeye measurements into the variety of photometry systems in use around the world today.

Unlike photometry at visible wavelengths, the atmospheric extinction at J, H, and K is generally quite small (< 0.1 mag. a.m.-1) and for most programs that remain under ~2 air masses, can be neglected in photometric reductions. As an example of the dependence of extinction on air mass, Figure 5.1 shows model differential extinction for observations from Mauna Kea through J, H, and K, filters (kindly provided by Gene Milone). This plot is for demonstration purposes only and serves to show the relatively weak dependence of extinction with air mass. Since the atmospheric conditions leading to this extinction are highly variable from night to night, programs requiring exceptional (~1%) photometry should use conventional means to monitor extinction as a function of air mass on a nightly basis. Programs that do not require photometry to better than *0.05 mag can safely neglect conventional extinction curves when reducing photometry data.

Figure 5.1 - Theoretical relative extinction in magnitudes is plotted for J, H, and K filters for typical Mauna Kea atmospheric conditions. Vertical offsets between J, H, and K values are arbitrary. For all passbands extinction varies by < 0.1 mag for air masses under 2.

It is important to note that in just the past decade infrared astronomy has gone from being dominated by single element photometers to array detectors with extreme sensitivity. When combined with the fact that Redeye is on a a 3.6 m telescope, most of the infrared standards compiled before 1980 are not useful today since they cannot be imaged by Redeye without saturating the detector. The practical limit to the brightness of standards is K ~ 7 mag, which with Redeye on CFHT corresponds to a ~0.5 sec exposure. Clearly this is not an ideal exposure time since for exposures *1 sec scintillation becomes a noise source and detector noise dominates the sky background. This problem can be alleviated somewhat by defocusing the telescope enough to spread the standard star flux across a large number of pixels. Unfortunately this degrades the pupil image at the cold stop, altering the camera's throughput slightly between the time standards and science targets are imaged. For this reason, a list of fainter standards derived from the UKIRT set is listed at the end of this chapter. While recording images of standard stars observers are urged to dither the telescope a few arc seconds between exposures since the probability of a star falling on a bad pixel is significant.

Absolute calibration of infrared measurements is possible through a number of sources. An absolute calibration of standard stars is particularly helpful for checking the throughput of Redeye and comparing it with nominal values (see Chapter 2) to confirm that the cameras are working properly. The formal definition of observed energy flux integrated over all wavelengths is:

where F0(*) is the incident flux at the top of earth's atmosphere, A(*) is the transmission of the earth's atmosphere, Q(*) is efficiency of the combined telescope, camera optics, and detector, and T(*) is the bandpass of the filter used. In practice, evaluating each of these terms is non-trivial and simple approximations should be used to calculate actual system throughputs, in combination with existing absolute photometry. Accordingly the flux, in units of photons per second from a 0 magnitude star, can be derived from the formulae:


The reason for specifying two formulae is because the throughput is a function of the collecting area of the telescope, which changes significantly depending on which upper end (f/8 or f/35) is in place. Taking the ratio of the 0 magnitude absolute flux values listed in Table 5.1 with those derived from standard stars yields the system throughput, from the top of the atmosphere to the detector. Also note that the gain (e-/ADU) of the Redeye read-out electronics can be found in the FITS header of each image, which is needed to convert from measured electrons to photons. The F* and F* values are from Wamsteker (1981).

Table 5.1 - The absolute flux calibration for the J, H, and K bands is listed, along with relevant data used in equations 5.2 and 5.3.


Elias, J. H., Frogel, J. A., Mathews, K., and Neugebauer, G. 1982, "Infrared Standard Stars" A. J., 87, 1029.

Casali, M., Hawarden, T. 1992, "A Set of Faint JHK Standards for UKIRT", JCMT - UKIRT Newsletter, Aug. 1992, 33.

Landolt A. U., 1983, "UBVRI Photometric Standard Stars Around the Celestial Equator", A. J., 88, 439.

Turnshek D. A., Bohlin, R. C., Williamson, R. L. II, Lupie, O. L., Koornneef, J., and Morgan, D. A. 1990, "An Atlas of Hubble Space Telescope Photometric, Spectrophotometric, and Polarimetric Calibration Objects", A. J., 99, 1243.

Wamsteker, W. 1981, "Standard Stars and Calibration for JHKLM Photometry", Astro. & Astrophy. 97, 329.

Elias Standards

The list of standard stars originally published in Elias et al. (1982) has been reproduced here to provide a set of moderately bright infrared standards for Redeye users. Note that the very brightest stars listed by Elias et al. were omitted in this reproduction since they cannot be imaged without saturating the detector at CFHT. The majority of the stars in this list are A type, with a handful of nearby M dwarfs and distant K and M giants. Note that some of the stars have a significant amount of proper motion and care should be used in identifying them in the sky. Most of the magnitudes are accurate to within ~1%.

UKIRT Faint Standards

A fainter set of standards than those listed by Elias et al. are available from the UKIRT set of "Faint Standards" (Casali and Hawarden 1992). These stars range in K magnitude from around 8 - 14, hence are well suited for a NICMOS camera on a 4 meter telescope. They were selected from the lists originally compiled by Landolt (1983) and HST (Turnshek et al. 1990). All measurements were made with UKIRT's UKT9 InSb single channel photometer and have rms errors of ~3%. CFHT observers should note that it is not clear what systematic differences exist between standards derived through this type of InSb photometry and CFHT's Hg:Cd:Te arrays, but the difference is probably comparable to the photometric errors.