Hello friends,
I have a general question regarding choosing comp stars in VPHOT, and some concerns regarding some measurements of the nova V1405 Cas in infrared.
My main question is: What is the best way in choosing comp stars? How to figure out which ones are suitable for performing measurements, and which ones that are not? If there is a document describing this, I would be very thankful if you can point me to it.
This question comes from the fact that when looking carefully at the comp stars, they can differ quite a bit. In addition, when performing measurements over time I observe that some comp stars give consistently lower or higher values than other comp stars.
More importantly, I ask this question because of a discrepancy in infrared measurements of the nova V1405 Cas. Check the light curve figure below, in particular the measurements after maximum. My measurements are highlighted by the plus sign (by Pablo Vallejos). In general, if you only consider my measurements they appear to be consistent, i.e. no weird jump between adjacent measurements or so. This is because I use the same comp stars. However, there are other users that report values that are consistently lower than my values by about 0.2 magnitudes. I am not sure why this is the case, which brings me to the next question:
Could it simply be that they use other comp stars?
At the current magnitude of the nova, it is difficult to find comp stars with good SNR in comparison with the nova. Check the image below of my 10 companion stars for V1405 Cas. Note that there is one comp star (102) that give a target estimate that deviated by about 0.2 mag.
In picking comp stars, I aim for the ones with highest SNR, if possible above 50. I also try to pick stars with B-V between 0.3 and 1, and if possible prioritize the ones with B-V ~= 0.5. I also deselect stars that give a target estimate that deviates too much.
I should perhaps point out that I use a focal length of 650 mm, giving a big FOV. I know that other users, in particular, SCT users, have a narrow FOV and very few comp stars.
I would appreciate a lot if some experienced user can comment on my observations and question above, it would really help me out here.
Thank you in advance!