I am having difficulty in deciding what values to use for the J and H magnitudes of Alf Tau (Aldeberan, HD 29139) which I use as a comparison star for near IR measurements of Alf Ori (Betelgeuse). Below are listed the values from the Joint Astronomy Centre (UKIRT) and 2MASS. The quoted error for UKIRT is 0.05 mag, and for 2MASS it is listed as 0.194 Mag in J and 0.170 Mag in H.
UKIRT Alf Tau J= -1.86 H= -2.64
2MASS Alf Tau J= -2.10 H= -2.78
I have only entered 3 observations for Alf ORI to date, and I have used the UKIRT standards. I note that the other IR observations in the databased have used the 2MASS standards. I am a beginning PEP observer and would appreciate any guidance in this area.
I did one run of all sky photometry using 4 of the brightest standards listed in Henden, 'JHK Standards for Small Telescopes', JAAVSO Vo. 31, 2002. (The stars used were HD6860, HD 29139, HD 34029, HD 62509). I originally used the UKIRT values and got a slope very near to 1 when plotting the insrumental versus the catalog magnitude (for the J band I got a slope of 0.998 (ideally it would be one) when I used J=-1.86). When using J=-2.10 the slope was 0.824 which would indicate poor agreement between the -2.10 value and the other 3 standards. Not a lot of points but it would seem that the -1.86 value for J is in better agreement with the other standards. For H I used 5 stars and got a slope of 0.981 for the -2.64 value and 0.930 for the -2.78 value, which at least indicates consistency between the UKIRT standards.
Does anyone have additional comparisons between Alf Tau and other standards that could be used to help reslove this issue? Is there any protocol for establishing comparison stars and values in the amateur IR PEP community?
Jim
Hi Jim,
2MASS is saturated on bright stars, which is why its magnitudes are so discrepant (and why it has such high reported errrors). In addition, there are a dozen "standard" J & H filters, so the results will vary depending on which one you choose. Nonetheless, here are the values that I know about.
UKIRT: J= -1.86 H= -2.64
2MASS: J= -2.10 H= -2.78
TMSS: J=-1.84 (from Johnson, 1966)
Shenavrin 2011: J~-1.85 H~-2.63
So I'd use the UKIRT numbers.
Arne
Arne,
Thanks much for the help. Makes a lot of sense. I use an SSP4 photometer with the J and H filters supplied by OPTEC. I get good fits with the J and H standards in your paper with my current setup at least for the bright standards. I am waiting for a really good night to further confirm the fit between my equipment and these standards, as I do have some issue with the dimmer standards (although it could just be a S/N issue). Data I have taken to date would indicate that transformation coefficients are not necessary to achieve the 0.05 magnitude accuracies typical of my skies in Vermont.
Jim
KJMB