Hello all,
for quite some time, I was primarily focused about time of minimum (or maximum), so I didn’t really care about the actual photometry values of the stars I was observing. Recently, I was involved in projects where the actual values matter. Having uploaded my photometry files in AAVSO database, I am surprised to see that all my values are offset. For example, for SN 2026aaiv, all my untransformed V data are offset by 0.4 Mag, which is quite a lot. To my surprise, after applying transform coefficients, it is even worse.
Can someone shed some light on this?
Thanks
– Normand (RNDA)
You haven’t said anything about what equipment you are using. Mono camera with photometric filters? RGB camera (DSLR or OSC astro camera)?
Your problem is that your measured variable star magnitudes are offset from those measured by others, if I have interpreted your post correctly.
You may already have done it, but you need to know if you can measure magnitudes accurately for check stars, and even better, photometric standard stars. For the latter it is very instructive to select several as targets and others as comp stars. Assign several target - comp pairs and see how close to the catalogue mag your measured mag is. Doing this pretty soon reveals if there is some systematic problem.
thanks for your reply. Yes, you’re right, I should be more specific about my photometry setup and process.
I use a mono CCD camera (with a KAF-1603ME sensor) with Johnson/Cousins photometric filters (BVRI), cooled at -15°C. To do the photometry, I use Phoranso (latest release), with all standard calibration frames (bias/dark/flat/dark-flat). In Phoranso, I first created an ID Card for the variable star I am looking at, with one check star, one “Reference star” and a bunch of comp stars.
You read my post correctly : when I compare my measures with others in AAVSO light curve generator, my measures are almost always offset compared to the others. Can’t figure out why.
Phoranso allows you to select one or more reference stars (from which the target, check and coparison star mags are calculated), a check star and one or more comparison stars (these are like additional check stars in Phoranso). Are your check star and comparison star measured magnitudes close to catalogue values, or are they offset from catalogue values?
Another thing you might try is redoing the photometry on your images using the same comp star as those other observers with whom your measurements disagree.