After I stack and calibrate by images (usually four - BVIR and occasionally a U) I upload them into VPhot. I then select and image from the uploaded VPhot list, Identify the target star (usually using GCVS) and then load the comp stars. All this seems to be “standard” procedure. Then I select one comp star and one check star - by displaying the photometry report and then selecting the two stars with the “least red” and remove the others. I then choose one as check and the other as comp. I repeat this process with each of the other three images and continue the process through transformation and submission, as outlined in the VPhot manual. When I upload a new image set for a new observation, I repeat this same process for choosing the comp and check stars. This means that the check/comp stars may be different for each observation because of target mag changes (I know they will be different for each filter and I assume that is ok). The VPhot manual urges consistency so I wonder if my process is ok or do I need to use the same comp/check stars across observations per filter?
Lane:
- I recommend using an ensemble. Comps should cover a range of about ± 2 mags of the target. IF you are monitoring Miras, this can get problematic since the target mag range can cover 7 mags! You can still delete comps that show large target error (red highlight).
- One comp may be different than that single comp used by others and a potential systematic bias will make your target mag different than others. Thus, we see the typical scatter in the light curve generator. An ensemble will generally average out such variation and make everyone’s data more consistent. Less scatter!
- The key is that your camera response is quite linear over quite a large mag / flux range. Ideally your mag range would have SNR of many hundreds (>100 to <saturation) but it is not unreasonable to use comp SNR > ~20 with a random error of 5% (0.05). Again, look at / check your target results for each comp!
- There is generally no reason to create a different sequence for each filter. Ideally you would select exposures to yield similar SNR for the target in each filter. For very red targets this again gets problematic since most AAVSO comps are yellow/green stars (0.3<B-V<0.9).
- In fact, using the same comps over time makes your own result more consistent. Do you want your target mag to reflect real variation of the target OR do you want any target variation to reflect possible differences in the comps?
- Yes, you always use one check. That is just what the AAVSO Report format requires.
- Comments/questions?
Ken
Ken, I’m not ignoring you, your input is very thoughtful as always - I need some thinking time. I will start using ensembles but I’m trying to figure out how to arrive at a common comp list across filters. Or, am I overthinking this (as usual)? …Lane
Interesting. I have tried to stick with the same Co, Ck sequences since I started just over a year ago. Initially I took out outliers which gives the illusion that you are reducing errors. I take Ken’s point about a fixed ensemble makes your own results more internally consistent. It does however cause some problems with more than one filter. I only observe in B and V at present and of course the Co’s Ck’s and targets can differ by several mags. It must be even more difficult if you add R and I filters.
I guess you are at the mercy of the accuracy of the Co’s, which I use as a contributing factor to error calculation since the Time Series method as learnt in the VPhot course (for multiple filters) does not yield useful error estimates. I did find the error calculations the most difficult aspect of the course and struggle to follow posts. A similar “How to” for errors would be useful. What do others think?