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Infrared Photometry

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Posted by Aaron Price on April 15, 2011 - 3:16pm

Infrared photometry is yet another area where amateurs are encroaching on what used to be the sole domain of professionals. Two amateurs, in particular, have been collecting an amazing near-IR light curve of epsilon Aurigae.

Below is the J and H-band light curve of epsilon Aurigae over the last 1000 days, with the V band light curve also plotted for comparison. Most of the J and H-band data comes from Brian McCandless (MBE) or Tom Rutherford (RTH). They are using SSP-4 Infrared photometers.  Recognizing the scientific need, the AAVSO entered into an agreement with Optec in 2001 to develop the detector and distributed many to experienced PEP observers around the world. Since then, many have gone on to purchase their own from Optec.

So why would amateurs want to measure variable stars in the J and H band? The infrared bands give information about variable stars that cannot be determined from the visual bands. For example, a Mira type variable star measured in the infrared has a much smaller amplitude of variation. The infrared light curve relates more closely to the actual temperature and radius change of the star as it pulsates, unlike the visual light, which traces the opacity of the atmospheres. This gives theoretical astronomers a better understanding of the physical processes that occur in pulsating stars. (Mira variables also radiate the vast majority of their light at infrared wavelengths, and in infrared light, many of these stars are among the brightest in the sky, making their smaller amplitudes easy to detect.)

Observation in the infrared region of the spectrum has a few advantages over photoelectric photometry in the visual bands. The atmospheric extinction is much less in the infrared when compared to the visual. As a result, differences in airmass between the comparison and target star don't introduce as much error as in the visual bands. In a light polluted city sky, the infrared sky is much darker because most of the man-made light pollution is in the visible band. Also, lunar glare interference is less in the infrared. And some stars are far, far brighter in the infrared, making it easier to perform high signal-to-noise photometry. Some stars can even be seen in the IR during the daytime. Brian and Dr. Bob have been doing daytime IR observations of epsilon Aurigae during the summer months, when the star is lost to the glare of the Sun for most of us using eyeballs and regular PEP equipment.

In 2010, IR-PEP observers submitted over 300 J and H observations of the following stars: epsilon Aurigae, beta Lyrae, U Aurigae, NO Aurigae, NSV 2537, rho Persei, zeta Aurigae, PU Aurigae, R Lyrae, UU Aurigae, lambda Andromedae, R Leporis, and miu Cephei.

For more information, check out the following:

AAVSO Science Director Matt Templeton contributed to this post.

IR Photometry, Filters, Potential Project

Speaking of IR photometry, I was looking for flux calibrations today (so I could determine the number of photons coming into a detector for a simulator I'm writing) and found a nice paper discussing the different filters in use in IR photometryalong with anticipated extinction amounts (J and H ~ 0.015 mags / airmass with the MKO filters). I've done a lot of work with the SSP-4 and have even wrote some new control software for the instrument that really opens up it's capabilities. Dr. Bob and I even did some daytime observing with the SSP-4 this last summer! It's a good machine, but has a lot of oddities. Please be aware that there are lots of different filters in use in the IR and Arne's JHK standard's are based on a different set of filters than are in the SSP-4 (Johnson vs. MKO-like, I think). The calibrator magnitude differences are small, but worth paying close attention to if you are going for well calibrated photometry. I have a list of bright JHK standards for the SSP-4 converted into the MKO filter set, but have been looking for someone to help me improve the error bars (i.e. by taking much more data). Please let me know if you are interested in this project. Brian

Infrared Photometry

Aaron: Thank you for the mention above. I really like working in the IR for all of the reasons that you mentioned above plus one other-- almost no one else is doing it! Unlike working in the visible wavelengths where your data point might be just one of many (depends on the target, of course) and may not contribute much by itself, working in the IR means that your data may be the only data points at those wavelengths for that object. A point that really needs mentioning is the lack of light pollution in the IR-- data could be collected from a mall parking lot and it would look as good as if it were collected from a dark-sky site. If an observer's location is heavily light-polluted, this is something that should be considered. There are some negatives, though. Nights that look good in the visible range may be really bad in the IR-- water vapor is something that has to be paid attention to-- sometimes the data is all over the place even though it looks like a great night. Its worth it though-- I don't know if I would want to go back to CCD observing or not (I probably would, so I don't forget how ). Brian: Once the eclipse is over, I might be able to help with calibration, at least for the IR-bright targets (most of my observations are made with an 8-inch SCT, although I do have access to a larger telescope at times). Tom

IR Photometry Calibration

Hi Tom, Sounds good. In a few minutes I'll be sending you a private message so we can continue to talk about this. If anyone else is interested, do send me a message as we can use as much help as we can get. Brian

IR Photometry Calibration

Brian: If you've sent me a PM, I haven't gotten it. Tom


Hi Tom, I just sent you a message directly instead of through the CS website. Please let me know if it doesn't make it. Brian

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