By George Silvis SGEO
Transformer has moved ! Go to http://code.google.com/p/transformapplier/
for the latest updates.
As you work your way up the learning curve of CCD photometry one of the final refinements is to develop transform coefficients for your telescope/filter/CCD camera setup. There are several references from the AAVSO web on how to do this and I’ve included some of them below. Once you have your coefficients you need to apply them to your observations. This can be a confusing process and usually involves transcribing your data to various spreadsheets. I offer here an application that is meant to make that process of applying your coefficients simple, consistent and easily traceable.
There is still open discussion on how exactly you should apply your coefficients. Starting from the AAVSO website you have the article by Benson, Priscilla. "CCD Transformation Coefficients", AAVSO Information Sheet, www.aavso.org, 1993. and then Lou Cohen’s article that tries to explain it further and correct minor errors : Cohen, Lou. "Computing and Using CCD Transformation Coefficients", AAVSO Information Sheet, www.aavso.org, 2002. I’ve found Bruce Gary’s webpage which lays it out even clearer, at least for me, as it also extrapolates the technique to various filter combinations: CCD TRANSFORMATION EQUATIONS FOR USE WITH SINGLE IMAGE (DIFFERENTIAL) PHOTOMETRY . Arne Henden, the head of the AAVSO, is working on refining this process and presumably standardizing it for AAVSO observers.
The process I have is to take the file with your untransformed observations and have the Transformer application scan it and rewrite it with the transformed observations. Care is taken to include in the output file comments fully describing the process applied to the data. Here is an example of an input file and the resulting output file . I am using AIPWin for developing my observations for the AAVSO and include the whole night’s worth into one file, building that file from the AIPWin output. The only application specific dependency is that the Transformer requires the standard magnitude of your reference star so it looks for as AIPWin posts it, as “#CREFMAG= x”. Your coefficients are collected by the application and saved in an INI file.
The application is a Windows app written with Borland BCB5. Please download and try it. If there are problems or errors, do email me and I will get them fixed straight away. If you would, please send me your input file so I can see what went wrong.
Transformer , version 1.2
Source code available too under GNU GPL
Plans for enhancement:
- More filter combinations.
- Code up alternate transform equations and allow the user to select.
- Have the application work in drag-drop mode, where you can drop your text file on the application and it will automatically save the transformed file.
Thanks
George Silvis
Update log:
- 1.1
add BVI combination
- 1.0
bug fix, recognize MTYPE==STD
- 0.9
Add VI combination
- 0.7
Skip readings with VMAG=<xx, Allow filter grouping with GROUP field,
Allow save of Un-transformed window
- 0.6
Handle ENSEMBLE observation records
- 0.5
improve error and warning msgs , test for missing coefficients and warn,
- 0.4
tidy up doc, explain variables
- 0.3
fix bug with crefmag. Don’t need to repeat CREFMAG line for a
given CNAME/FILTER combination
- 0.2
add VR, VRI, BVR and RI filter
combination formulas
- 0.1
original release
If your CCD capture software does not include the FILTER= reference in the FITS header you can use this utility to do so. Create a bat file, eg. B.BAT with the line: modhead %1 FILTER B. Now when you when you drag and drop your image file on to B.BAT the FILTER= B line will be added. ModHead can be used to add or change any keyword in a FITS file. Execute it with no parameters for help on it use.
ModHead is a utility that is part of CFITSIO