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Transformation

CBLA's picture
CBLA
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Joined: 2011-07-16

I posted this a few days ago, but I think it got overlooked because it was at the bottom of an old thread so I'm reposting here.  Just want to make sure I have calculated my transfomation coefficients correctly:

I have been working over the past few days to determine the transfomation coefficients for my system (I have a B and V filter).  I realize that this is a topic that pops up periodically, so I tried to learn as much as I could from other people's experiences in the forum.  I wound up imaging M67 using the process described in Priscilla Benson's paper.  Attached is my excel worksheet with my results. I feel pretty confident that I did the process correctly which means I probably made a mistake.  I would appreciate feedback on any glaring errors.

I am unsure of the best practice for using the coefficient information.  I have already entered my values into VPhot (Tv, Tb, and Tbv).  For a single target should I take 3 images in each filter, stack each group of 3, and then perform the transformation?  Typically, I have used time series to get a better idea of the uncertainty, but that doesn't seem to be an option here.  I just want to make sure that after having gone through the process of figuring out the coefficients I am using them correctly.

Thanks

Blake

AttachmentSize
B-V Transformation coefficient M67 2-17-14.xlsx20.62 KB
transformation coefficients
HQA's picture
HQA
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Joined: 2010-05-10

Hi Blake,

What is your equipment setup - telescope, filter vendor, camera, location?

In general, this looks good.  You have quite a bit of blue response; note that the zeropoint for the B equation is almost the same as for the V equation (down 0.22mag), which is better than most setups.

VPHOT currently does a good job of transforming pairs of images, say one B and one V.  It does not do transformed time series.  It does do ensemble transformation.  So it is not a perfect solution yet; we're working on a couple of alternatives for observers.  For your case, as long as you have reasonable signal/noise in each pair of images, I'd transform the 3 sets individually and then average the results to get a mean magnitude and standard deviation for reporting.

Arne

Thanks, Arne.  I have an 8
CBLA's picture
CBLA
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Joined: 2011-07-16

Thanks, Arne.  I have an 8 inch Celestron SCT mounted on a CG-5.  I am using an Orion Starshoot G3 CCD with filters purchased through Astrodon.  I live in Charleston, SC (32.78 N, 79.93W).

Transformation Tool
WGR's picture
WGR
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Joined: 2010-08-17

Hello Blake and all;

Just to augment what Arne said, we have a team of HQ staff and volunteers working on a transformation tool for observers.  It will read an Extended AAVSO format, apply previously determined coeficcients, average n images, and write an AAVSO Extended format file, ready for submission to WebObsn along with an option to submit.  It will do time series, but not ensemble.  

We have had discussions, visits, generated and revised a specification, and met face to face at HQ.  The team is working on the implementation/coding.  The Team consists of Arne, Matt, Mike Simonsen, Sara Beck, Rebecca, George Silvis, Dick Sabo, Will, Doc and myself.  The tool will access the VSD to get the standard values, a saved file of up to 3 sets of transformation coefficients, and provide that data to the LCG, which will change to be able to plot transformed, untransformed, and both data.  This will require some minor mods to the AUID and LCG.  The project has lots of impacts.  Not quite as simple as one might think upon first reflection.

This tool is an applier of coefficients, which can be used with your determined coeficients, AAVSOnet coeficients or other.  The tool to determine coeficients is separate, and being discussed, but is not part of this effort.

Gary, WGR

 

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