Single Channel Infrared Photometry with a Small Telescope

Introduction

The recent development of the SSP-4 Infrared Photometer has opened a new window for the small telescope observer, that is, light with wavelengths beyond one micron. Up until now, the infrared wavelengths were strictly the domain of the professional astronomer and large telescopes. The SSP-4 photometer allows an amateur or professional with a small telescope to precisely measure starlight in the J (1.25 micron) and H (1.65 micron) bands. This paper presents a brief overview of the development, design, and error estimates for the SSP-4. Initial observations and status of the Infrared Photometry Group of the American Association of Variable Star Observers (AAVSO) are also covered.

SSP-4 Photometer Development and Design

The development of the SSP-4 was a joint effort between the AAVSO and Optec, Inc., the designer and manufacturer of the instrument. The AAVSO has taken deliver of the first five instruments and the observer team is lead by the author. The AAVSO members that were most actively involved with Optec in the development of the photometer were: Janet Mattei, Arne Henden, Bob Wing, and Doug West.

The SSP-4 is very similar in external design to the venerable SSP-3. However, some of the internal electronic components are very different. The most obvious difference is the detector, the SSP-4 uses an Hamamatsu G5851 InGaAs PIN photodiode, where as the SSP-3 uses a silicon photodiode. The InGaAs detector is thermoelectrically cooled to -40C within the SSP-4. Figure 1 is the photosensitivity curve as function of wavelength for the detector. The peak response of the detector is approximately 1.75 micron. More technical information about the SSP-4 can be found on the Optec, Incorporated web site www.optecinc.com.

Figure 1 Spectral Response of the InGaAs Detector

The outline of the SSP-4 photometer is shown in figure 2. The operation of the photometer is straightforward and the user interface is intuitive. A flip mirror and alignment eyepiece are used to center the starlight onto the detector.

Figure 2 Outline of SSP-4

Figure 3 Picture of SSP-4

The control panel for the SSP-4 is shown in figure 3. The gain, detector temperature, and integration time are set through a menu system that appears in the red LED window. Once the operating parameters have been input, that is, gain, integration time, and detector temperature, then the counts are read from the red LED window. A computer can be interfaced to the SSP-4 for data collection. A software package for computer control is supplied with the photometer. Not completely shown in the picture is the manual slider for the J and H band filters.

Filter system for the SSP-4

The J and H band filters for the SSP-4 are built to the Mauna Kea Observatories (MKO) system (Simons and Tokunaga). The bandpasses of the two filters are slightly narrower than previous filters to avoid contamination by water vapor in the atmosphere. This filter system represents a compromise between the competing factors of throughput and photometric performance and has been endorsed by a working group of the International Astronomical Union. In a paper by Arne Henden (Henden 2003), a good presentation is given on the history and development infrared filters. Figures 4 and 5 show the MKO bandpasses and the atmospheric transmission for the two filters.

Figure 4 MKO J Band Filter Curve

Figure 5 MKO H Band Filter Curve

Estimation of the Error

This section develops the relationship between the signal-to-noise ratio (SNR) and the error in magnitude for a single five second integration with the SSP-4. As with any scientific instrument it is necessary to understand the limitations and errors of measurement. The SNR is calculated from the ratio (average counts for n measurements)/(standard deviation for n measurements). Note that the standard deviation is for a single measurement and not for the mean. Seven or eight five second integrations per band were taken for each star. The sky background count was an average of three five second integrations. The telescope used was a Meade LX200 0.2m SCT and the observing setting was suburban. At the observing location in Mulvane, Kansas, USA, the typical naked eye visual limiting magnitude is 4.5.

In table 1, the columns labeled "J Band SNR" and "H Band SNR" represents the average SNR that would be expected for stars with magnitudes between -2 and 4. These numbers are derived from an exponential curve fit. The columns with labels "J Band Error" and "H Band Error" represent the expected error (one sigma) associated with each value of SNR. The formula for calculation of the estimated error is 1.0857/SNR.

Table 1 - Estimated SNR and Error as a Function of Magnitude

Initial Observations with the SSP-4

Initial observations with the SSP-4 and a 0.25 SCT through 17 June 2003 are presented. The observation technique was as follows:

1. Set gain =10, Detector temperature = -40 C, and integration time = 5 seconds. Set dark current greater than 100.
2. Eight observations with J filter of star.
3. Three observations of sky with J filter.
4. Eight observations with H filter of star.
5. Three observations of the sky with H filter.
6. Next star and repeat steps 2-5.

In a typical observation session, each star is measured at least twice in each filter band. Each observation has been reduced with the Henden and Kaitchuck Astronomical Photometry Software For IBM-PC (http://www.willbell.com/). This software takes into account color differences between the comparison stars and airmass differences. The average airmass extinction coefficients and color transformation coefficients were calculated over four nights using Henden standard stars and stars from the UKIRT list. Additional information about how the color transformation and airmass extinction coefficients were determined can be found at http://www.aavso.org/observing/programs/pep/report3.shtml.

Table 2 contains initial observations with the photometer. Table 3 gives the magnitudes of the comparison stars used in the reduction of the observations. Each of the column headings is self explanatory except for the "Est Error" column. The error was estimated by taking the standard deviation of the mean. Taking at least two observations per target and using two comparison stars results in at least four estimates of the target magnitude. The standard deviation of the mean of these multiple observations becomes the estimated error. All of the observations in table 2 have been submitted to the AAVSO or Association of Lunar & Planetary Observers (ALPO).

Table 2 - J and H Band Observations with the SSP-4

Table 3 - Catalog Magnitudes for Comparison Stars in Table 2

Infrared Photometry of Algol

The triple system eclipsing binary Algol has been observed by the AAVSO Infrared Photometry Group. Figure 6 is a partial phase plot of the primary and secondary eclipses. More observations are planned on this system before publication in an astronomical journal. Typical errors are 0.05 magnitude. This plot was generated using the ephemeris 2441773.4894 + 2.8673285E. Zeilik, Bayliss, and Heckert have previously reported infrared photometry of Algol.

Figure 6 Infrared Light Curve for Algol (beta Per). The boxes represent J magnitudes and the stars (*) represent H magnitudes.

Current Status of AAVSO IR Photometry Group

The AAVSO Infrared Photometry has five active observers, they are Jim Wood, Dirk Terrell, Doug Hodgson, Ken Luedeke, and Doug West. Michael Koppelman is a former observer with the SSP-4. Arne Henden and Jerry Persha are acting as technical advisors to the group. Hundreds of J and H band observations have already been submitted to the AAVSO database for archive. The most actively observed stars are delta Sco, R Leo, Mira, Algol, S Vir, and Rho Cas. Since many of these stars have very long periods (excess of one year), publication of the groups results in astronomical journals won't be until early 2005.

Acknowledgements

The author would like to thank the following for their help in preparation of this paper: Arne Henden, Jerry Persha, and Michael Koppelman.

References

• UKIRT IR Photometry of Selected Bright Stars http://www.jach.hawaii.edu/JACpublic/UKIRT/astronomy/calib/bright_stds.html
• Kaitchuck, R.H. and Henden, A.A., Astronomical Photometry Software for the IBM-PC, Version 2.0, 1992, published by Willmann-Bell, Inc.
• Zeilik, M., Bayliss, L., Heckert, P., "Infrared Photometry of Algol", IBVS 1787, May 1980.
• Henden, A.A., "JHK Standards for Small Telescopes", JAAVSO, Vol. 31, No. 1, July 2003, pages 11-20.
• Simons, D.A. and Tokunaga, A., "The Mauna Kea Observatories Near-Infrared Filter Set. I. Defining Optimal 1-5 Micron Bandpasses", PASP, Vol. 114, Issue 792, February 2002, pages 169-179.
• Gerzair, D.Y., et. al., Catalog of Infrared Observations, Edition 5, 1999. Available through Vizier at http://vizier.u-strasbg.fr/viz-bin/ftp-index?II/225