Welcome to the TransformApplier (TA), a tool prepared for the AAVSO by "super-volunteer" George Silvis (SGEO), to make it easy to submit transformed CCD observations to WebObs. We, members of the AAVSO, want to submit transformed observations because these can then be used more easily by professional astronomers. We're amateurs, but sophistocated amateurs, and this tool will help us go the next step.

TransformApplier is a feature included within the VPhot application.

How does it work?

With TransformApplier, all you need is a file of data in AAVSO extended file format and the transformation coefficients for your system. You can use a tool such as Transform Generator (TG) to help you compute your coefficients if you have not yet done so.

Once you have set your coefficients into your VPhot telescope profile, simply load your observations into the program, hit the Process button and you have a transformed, AAVSO extended format, file ready for submission. Please note: Ensemble observations are not transformable with this tool.


  • The chart ID must be to VSP photometry, not the picture chart
  • Comp and check stars should be identified by their AUID's. If you have used the chart labels, that will probably work too. The problem is that there are often duplicate labels on a chart. TA will try to identify the labels and warn you if there is a duplicates problem. Worse comes to worse,you can insert the comp stars magnitude into the observation file by adding a comment line before the first line that that refers to that comp that looks like: "#CREFMAG= <mag> <err>"

For more information

TA's "Help" tab contains further details about input file requirements and information about how the program works and exactly what calculations are being performed to transform your data.

And there are Notes for new users put together by Ed Wiley (WEY) to walk you through the TA process. See the attached files below: The notes in pdf and word format and a zip of the example data. Thank you Ed!

What does the transformation process do?

In a nutshell: It adjusts your instrumental magnitudes to the standard system.

The transforms you've developed by plotting instrumental magnitudes of known stars vs their standard magnitudes, charactierize your instrument, they describe how it is different from the "standard" photometry. Your instrumental data falls on a line which is described by its slope, or "transform coefficient".

If your instrument was perfectly standard, your primary coefficients would be 0 and your secondary coefficients would be 1.

When you measure a variable star, you want to determine the standard magnitude. You do this by first re-creating your standard line with the point determined by the comp star and the line defined by your transform coefficient running through it. The transform process adjusts your target star instrumental magnitude so that it falls on this line. It does this simultaneously for all the stars in the transform group.

The attached PDF document was presented at the 2015 AAVSO conference and offers a visulization of the transform process.

The AAVSO Guide to CCD Photometry (published in September 2014) also provides an explanation of transformation illustrated with a simple spreadsheet example (see Chapter 6).

Help is available!

If you have problems, issues or questions with or about the TA, you are welcome to contact the author, George Silvis, at SGEO @ The AAVSO Software Development Forum is also available for questions, comments, or suggestions.

For more general questions about the transformation process or other aspects of photometry, please feel free to post to the Photometry Forum.