Arne,
A slightly higher (brighter) bias in my Sloan i' data has been nagging me. If you take a look at the .PDF I attached you will notice that in Sloan i' only, after transforms I'm running about 0.04 magnitudes brighter than the mean. My first guess would be that this is simply the slight difference in Johnson I and Sloan i' filter band passes. Of course, the comp stars I'm using are Cousins Ic.
I wouldn't think that this is a difference between other folk's non-transformed data and my transformed results as transforms have if anything, made the my submitted measurments < 0.02 magnitudes dimmer.
I suppose this might be attributed to the fact that I'm at 8,300ft and the less atmospheric extinction, less water vapor at this altidude might account for the slightly brighter values. However, I am using the recommended altitude extinction values in the Transform Applier.
As an aside; my transformed V-band measurements come out right on the V-band mean for the same star at the same time.
Any thoughts on this?
-- John
Hi John,
There are several possible reasons why your data should be slightly brighter at Ic than others. The one that looks most "promising" is that you were using comp 000-BLS-549, while most of those with the fainter measures were using 000-BLS-551. It could be that, at Ic, there is a slight error in one or the other of the comp star Ic values. If you are able, you might try using 000-BLS-551 and see if your photometry better matches the others.
The other potential solutions are along the lines you've discussed. Slight difference in the Ic vs. i' bandpasses leads to the offset for this particular star, especially since you are at altitude. Transformation errors. Lack of transformation for the main part of the submitted data. .Saturation in the i' filter for either target or comp.
Arne
Thanks so much for taking the time Arne,
I will followup on your suggestions and post any clear findings.
--John
All,
Indeed as Arne suggested using 000-BLS-551 for a comp like most everyone else brings my SI measurments down about 0.04 mag and a bit more in line with the mean. I will keep my SI exposures shorter for future observations as many of my past frames exceed the linearity of my camera for 000-BLS-551.
--John (OJJ)
Hello John
Thanks for following up on this and posting your result. So often, questions get asked, but the follow never happens, or never gets posted and shared. Great Job.
Gary (WGR)