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DSLR Photometry on Carbon Stars?
Hi,
I have another question regarding DSLR photometry. I was looking over a list of variable stars that should be bright enough for me to use my camera and zoom lens this coming weekend. I saw X Cancri, which looks bright enough and seems well positioned for me this time of year, but then I noticed it's B-V color index of about 3.3. My question is with a star this red, can we use the same technique of measuring the "tricolor G" magnitude and applying the Transformation Cooeficient to obtain a measurement that is comparable to a Johnson V measurement?
-Mike Durkin
Hi Brian, The D300 link refers to a modified camera with the IRblocking filter removed. The standard D300 will have a clipped IR response and is unlikelyto showany leakage of the filters in IR.Any camera modified by completely removing the IR block is likely to show this effect so an additional IRblock should be used when using these cameras to estimate Vmag. Fortunately the most common camera modification is to replace the stock IRblock with one with a slightly longer wavelength and sharper cutoff to include Halpha. These still cut off below the leakagewavelengths so should be ok for V mag etimates, though a different transformation coefficient might be needed Robin
Robin, My DSLR is a Hutech modified camera and has the common modification you mentioned. It is more sensitive than a stock camera to Hydrogen-alpha, but still blocks IR. Ican use the camera for daytime shots, but Ihave to apply a custom white balance or adjust the red color shift later in Photoshop. My calcualtion of the camera's Transformation Coefficient is 0.09 and the highest color index I used in the calculation was about 1.4-1.5. Ithought I read online somewhere that there were problems trying to get a DSLR Vmag measurement on stars with a high color index, but hopefully it is just a case of my faulty memory. Ithink Ineed to follow Brian's suggestion and find a bright (less than mag 8), non-variable, very red star that Ican test my camera out with. -Mike
Mike, I think it should be fine although I'm not for certain. The idea of the transformation coefficent is to correct for the differences between your camera's Green filter response vs. the standard photometric V filter response curves. Normally they are fairly close. I would be concerned if your camera has a big long-wavelength response like the D300 (see the second figure here http://www.maxmax.com/nikon_d300_study.htm) where you would expect there to be a modest contribution from the longer wavelengths. I read an article on astronomy.com (or someplace similar) that a company was going to produce photometric filters for Cannon cameras... don't know what happened to those plans. The good thing is that we can check to see how badly your camera performs on similarly reddened stars. To do so, simply find a non-variable star that has similar reddening and compare your DSLR-estimated mangitude with that listed in a catalog. If you can't find one easily, let me know and I'll dig one up for you to test with. Brian