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DSLR photometry of close binary, Delta Cep
Hi everyone,
Earlier this month I took some wide field images of the area around Cassiopeia and Cepheus for photometry measurements. I was able to make a measurement of Delta Cep, however my measurement includes both components of Delta Cep, the variable plus it's fainter companion at magnitude 6.3. My question is, since I can't split the components of Delta Cep in a DSLR camera, should I submit the combined magnitude or should I calculate the magnitude of the variable primary by mathematically removing the fainter companion (a difference of 0.12)?
Thanks,
-Mike Durkin
del Cep is a tough target for wide-field systems, with its 40 arcsec neighbor. Usually, working with bright stars avoids most of the blending complications since it is statistically less likely to have another bright star nearby, but del Cep is an obvious exception.
When you have these blended cases, there are three main ways to submit your data:
1) use a measurement aperture large enough to include both stars, and place a note on the observation stating "this measurement is a blend of the variable and the 6.3 companion."
2) use a mathematical method to remove the contribution from the companion. Since it is a couple of magnitudes fainter than the variable, it only contributes ~0.1mag anyway, so removing it with 10% precision results in 0.01mag photometry. So the mathematical method works pretty well for such cases. Lew Cook wrote a nice Excel spreadsheet to perform the operation; it can be found at:
http://www.aavso.org/sites/default/files/software/nemesis.xls
The trick is to use the best magnitude for the companion. I would probably use V=6.224, as that is what the converted-Tycho magnitude becomes (Kharchenko catalog). Whatever magnitude you use, you need to report it as a note as well, along with indicating that you used such a technique - this enables a researcher to reprocess your data if they have a better estimate.
3) you can actually do a pretty good job of deblending objects with very specialized software, such as the ISIS image subtraction routine. I do not recommend it for the faint of heart, but it is fun to play with digital images and see what can be extracted.
Arne
Thanks Arne,
Unless there's a specific reason not to, I think I'll stick with the combined measurement, even though doing the math to remove the companion isn't hard. I did find a slightly different V magnitude for the companion (6.307) from a different catalog which makes a difference of about 0.01 when calulating the magnitude of delta Cep A.
I took a quick look at the web page for the ISIS software you mentioned. I didn't see a lot of documentation, but the impression I got was that I would need to have multiple images covering different magitudes of delta Cep in order for the software to recognize the star as a variable.
Thanks,
-Mike Durkin
Hi Mike,
If the companion is well characterized, then removing it mathematically seems ok, but you need to indicate that somehow in the data that you submit as the 0.12 mag is probably catalog-dependent. We should probably get an official comment from the AAVSO as this situation is likely to come up frequently in the future.
Brian