Affiliation
American Association of Variable Star Observers (AAVSO)
Sun, 03/19/2017 - 12:40
I'm trying to figure out how the little transform check box near the top of the Photometry Report works. I think I understand the theory, but how/where do I input the color index for the variable? (The B,V,I magnitude transforms are already in my telescope setup in Admin.
I couldn't find anything about this in the latest VPhot users guide. The Help page within VPhot has a very brief description, but doesn't explain how to input the color index.
Phil
I believe you first need to go to the images screen, select 2 observations with different filters (that gives you the variable color) and then select transform. Then the photometry report would indicate that they have been transformed.
Haven't ever done it myself; I use TA. :)
Someday we'll get VPhot to do 3 and 4 color transforms.
George
Hi Phil:
I generally consider this box as an indicator rather than an executor but let me explain. This box effectively lets one conduct single filter transformation of a target. Usually, one would conduct the normal two filter (e.g., B&V) transformation provided by the Transform tool link on the image list.
However, IF you only have a V filter image AND you know the color (e.g., B-V) of a target one can still transform the single V magnitude. You would enter the know/assumed color of the target in the target/comp star sequence that you have created previously. Look there for this color box. If you enter a color value for the target and save the sequence, you are able to get a transformed magnitude and the "transform" box you are referring to will be checked and additional transformation info will be shown.
Try this out and reply if I have not been clear. Beware that you must know the color from other sources, or determined it yourself previously, and confirmed that the target color does not change over the time duration and cycle that you are measuring! Yes, this does reduce its usefulness but it is possible.
Make sense?
Ken
Ken,
Thanks, that works.
I used a recent set of V and I measurement of a very red star. Using the V-I color index from these measurements, I edited the sequence with this color index for the target. Then, using just the V image with a check in the "transform" box I compared this reported transformed V magnitude with the transformed V magnitude determined using the standard VPhot method using two image. The results were identical.
For me, this looks like a good tool for demonstrating (for pedagogical purposes) the effect of transforms on magnitude estimates. By clicking and unclicking the box it's very quick and easy to see the difference.
Phil
But I am still using TG and TA. Where did I go wrong? Does VPHOT do it all, or just the TA part, TG still being used to get the coefficients?
Ray
Ray:
VPhot only does transforms on pairs of images with the normal 2 filter technique. The single filter transform technique Phil was discussing has several drawbacks that I mentioned.
TA is useful for time series transformation. TG helps generate the coefficients.
Ken
Ray,
Using the VPhot transform box, at least the way I did it, was actually more work than using the standard method in VPhot for applying transforms. I was just interested in seeing if (and how) it worked for possible use in a CCD class to demonstrate the effect of apply transforms.
If you're doing CCD photometry and you're not using VPhot, you should try it. I think you'll love it. I'd recommend you sign up for Ken's class (in April) immediately, before it fills up.
Phil
I took the CHOICE VPHOT class awhile ago. Excelent class. The application is very usable too.
ray