HI All. Im just starting out. My plan is to work with standard fields a few nights before I get into variables. I want to know is the process the same as for variables? I plan to use NGC 7790 and take sets of 5 frames with B and V at interval exposure times (like 60s, 120s, 180s). Here are some questions, in new particular order.
Do I need one comp and one check?
Why is it suggested that beginners do not use ensembles?
When calculating variance, do I use comp, check, or target.
For standard field, is it better to calculate error using my stars and known mags or using my 5 frames to calc the deviation?
4.1) Do I calc sd on raw images or calibrated images?
4.2) Do I calc sd on each filter?
Any tips for evaluating accuracy will be appreciated.
I find the hardest part of analyzing science frames, is finding my stars on the star chart. Cant this be done with software?
What is the standard software most members use for photometry? Would VPHOT be best choice for a beginner?
Lots of questions there.
Unless you plan to transform your data right away, you won’t need a standard field like NGC 7790. Try T CrB first. There are plenty of mostly untransformed data there that you can compare your results to.
7. VPHOT works well and finds the stars in question 6.
6. VSP plotter plots a variable in the middle of the chart. But you point your telescope using the given RA and DEC.
2. Beats me!
One target, 1 comp, 1 check will yield good accuracy, but many comps averages out the lack of transforms and the possible variability of comparison stars.
3,4,5 just work up your data in VPHOT and submit it with WebOBS.
4.1 always calibrate your images and use those calibrated images.
Compare your data to other observers. If you want less scatter, average your images. An average of 5 to 10 works pretty well for medium stars ( 5 to 10 magnitude ) with medium cadence ( 20 to 120 seconds).
Many comps will average out a lack of transforms only if the average of the comp B-V values appproximates the B-V of the target.
If the comps in an ensemble were all redder than the target (or bluer than the target, for example if the target were a very red long period variable) the ensemble may not compensate for a lack of transforms. It might be better instead to choose one comp closest in B-V to the target.
Of course, if the transformation coefficient Tv_bv were very close to zero, any ensemble of well chosen comps would do, even for non-transformed photometry.
The above statements could all be tested by experiment.
I presume it is to set conditions for beginners to more easily understand the principles underlying differential aperture photometry.
In addition, if errors occur finding the cause is easier.
Understanding photometry starts with knowing about instrumental magnitudes, how they are calculated from raw counts or ADUs, and the formula for the calculation of relative magnitude from instrumental magnitudes and the known magnitude of the comp star. To achieve that, a beginner should start with a target, one comparison star and one check star.
I just can’t agree that the most basic facts needed to gain an initial understanding of photometry can be regarded as complicated.
There are resources everywhere to help beginners and more advanced observers, not least the AAVSO web site and the guides and manuals located there. If anyone needs more, books are available.