Wed, 03/31/2021 - 06:33
The example of transformation in the CCD Photometry guide only uses a single comparison star but I think it's normal to use multiple comparison stars. Is there any information about how the transform works with multiple comparison stars?
Steve
Steve,
Yes, the CCD Photometry Guide only describes applying transforms for single comp star photometry. It doesn't even mention transforms in ensmble photometry. The reason: it's complicated. I recommend the following discussion:
https://www.aavso.org/transforming-ensembles
The practical solution for those of us doing ensemble photometry is to use photometry software that does this for us. VPhot, and perhaps also AIP4Win, can apply transforms for ensembles.
Phil
Steve
Thanks Phil
Steve
I use personally-created XL workbooks containing several spreadsheet templates for dealing with transforming the data and calculating the results from ensemble photometry. There are two template workbooks, one for entering raw data from AIP4Win and the other for data from AstroimageJ.
They are designed for reducing data on stars with 'longer' periods, meaning one V magnitude measurement per night per star. They assume the target is at a high altitude, and thus do not deal with atmospheric exctinction, but columns of extinction-corrected data could be pasted into the data sheets.
Essentially, the entire table of raw data (output from AIP4Win or AIJ) is copied and pasted into the first two spreadsheets (one for the green channel, the other blue) of the workbook. The relevant parts of the raw data are automatically copied into the appropriate sheets, for calculation of the V magnitude and B-V colour index of the variable and check stars. The sheets are designed to deal with data from 10 consecutive images, with the final results being the means of 10, together with the standard deviations. Currently, they require 4 comp stars and a check star.
One template workbook is created for each variable star, and thus contains the relevant V and B-V data of the appropriate comp stars.
The sheet that does the photometry has already been loaded with current transformation coefficients.
OK, it's a lot of work, and you need to know the formulae for calculating transformed magnitudes.
But once it's done and the spreadsheets are created, the templates can be used for any variable, provided the relevant comp star data is entered. Then all that is required are two lots of copy and paste (one each for green and blue channels), and instantly all results are available.
The above was possible for me only because of the book on Astronomical Photometry by Arne Henden and Ronald Kaitchuk, and AAVSO papers on transformation coefficients by Priscilla Benson and Lou Cohen.
Roy
I've made up an…
Thanks Roy,
I've made up an excel spreadsheet for calculating the Transform coefficients and I want to extend it to do the actual transforms. I can make the spreadsheets myself but I'm not sure how to handle multiple comparison stars.
Steve
I calculate a…
Steve,
I calculate a transformed V magnitude for the variable star using each comparison star in turn (there are 4 comp stars). Thus, at this step, there are 4 V magnitude values. Each of these four V magnitudes is the average of 10 measurements (1 measurement on each of 10 images). The final V magnitude is the average of these four. The final standard deviation is calculated from the 40 individual V magnitude measurements. I do the same for the check star.
B-V values for the variable and check star are also calculated.
Hope this makes sense.
Roy
Thanks Roy, That makes perfect sense and it's how I would have done it. Someone further up the thread gave me link to another thread about ensemble photometry. I read it but didn't quite understand all the issues. There seemed to be several methods of handling it.
Steve
Steve, I've not used any of the other methods in that link.
Roy