To get Johnson's B-V, as I said, you need to apply a transformation, like the classical:
B-V = zp+k*(B'-V')
where B'-V' =-2.5*log(B"/G") B" and G" being the ADU's (or e- ) count of the star after the photometry process (the said "intensity" from the .dat of IRIS, mode 3 ; in fact it's not a light intensity but something proportional to a photon-count for a CMOS sensor ! )
This is needed as the centroids of the DSLR are at a smaller distance from each other than the Johnson's B and V pass-band response ones. I do not have the 450D coefficients at hand now, I will put it in a post this evening when I am back home. As I said that works an acceptable way but except for spectral types like M and also some K and O.
Last night was very clear, but with moon, the extinction was so low I got into saturation with a couple of images ! (plus some moon background) I had to make some additional observation to replace them...
Clear Skies !
Hi Roger, HB
I've just published on my Gdrive 2 spreadsheets, Eps Aur 2012-10-23-15-manual.ods and Eps Aur 2012-10-23-14 greens only.ods .
The first one has been filled with photometry data from a finalG.pic image obtained following the tutorial way, splitting the final image in R-G-B layers at the end of stacking of 15 images. The procedure was fully manual, since I don't trust my IRIS scripts anymore.
The second one has been filled with photometry data on finalG.pic image obtained by separating the G layers from original pics after the preprocessing stage and then registering and stacking 30 green images only. This time too, I worked completely in manual.
The resulting image has been scaled with ASCALE command before photometry because its resulting X-Y size is half the size of the finalG.pic image obtained following the tutorial way
The inner circle size in both cases has been chosen with the rule 3 x max(FWHM).
As you can see, the measures of the second spreadsheet has higher errors ( > 30 mmag ) so there should be something wrong, maybe I had to split the GRBG channels before preprocessing?
I'm planning to buy a SkyWatcher Startravel 120, a 120/600 acromatic refractor and I wonder if I could do something good connecting my Canon 1100-D to it for photometric purposes.
I already used it for visual observations and I like its wide field; but it's also true that increasing magnification, the chromatic aberration starts to become noticeable so I wonder how much this aberration could affect photometry, once I connect my DSLR to it.
I do not see any reasons to not use achromatic refractors. I also make photometry with an old one without problem. If you apply a "green only" technique (like the CS one) the chromatic aberrations will not affect your results, the blue and red roll-off of the green band-pass would just change a little the profile of the PSF but nothing significant compared to the large defocusing we are using !
There are two effects of the chromaticism, the different focus depending the color, but also the lateral chromaticism that deviates more or less the position of the spots in radial direction. Then if you use a three colors technique, like my VSF, you can see a litlle bit different position for the red and blue photometric apertures. But ok, with my old achromate it's perfectly negligible, I use the same position for the three colors.
Anyhow a 120 mm pupil will provide you much more photons , about 50 times what is available with a 50 mm lens at F/2.8 ! It also provides a much better ratio between the stars photon-count and the background. A larger aperture means also less scintillation. All good ! But be careful with the saturation, sometimes on bright stars it's useful to apply a neutral filter, it allows to remain at long enough exposure keeping the benefit of low background and reduced scintillation.
A side effect (that is not chromaticism related) is the fact such F/5 refractor would have a non-flat image field. By the way you could experience a different focus depending the distance of the stars from the image center. This could conduct to increase the defocus more than ideal in the center. I have to take care of it, including with my Nikkor 200 mm lens. If truly a problem a flat-field corrector could be used in the case of the refractor.
I am new to DSLR astrophotography in general and I am looking to add DSLR photometry to my observing program in the future. Last night, with my Canon EOS 1100D rebel, I was able to take 10 successive images of the star field in and around UMA which contains some of my visual program stars. Camera settings were 10 sec exposures at ISO 1600 of raw images. I was able to image familiar stars down to about 8th magnitude. I was really quite surprised how easy it was. I also have taken two dark frames. Sure enough, I could see a couple of hot pixels in the images. I have two questions for any DSLR gurus who may have advice:
1) AIP4win users: Do I have to buy the $99 manual from the publisher in order to get the access code to AIP4win v. 2 (which is installed already). I'd like to play around with stacking images etc.
2) Is it appropriate to upload a couple of my images for comment from forum members?
Yes. The software is an addon to the "Handbook of Astronomical Image Processing", and I think it's worth the bucks if you ever want to to do pretty-picture atsrophotography. The book goes far beyond explaining the software.
Definitely useful. I doubt that you will be able to upload raw images directly to this forum as message attachments (because of file-type and file size constraints), but there are of course alternatives like google docs or other file hosting services. It's definitely more useful to upload raw or FITS images and not compressed formats like JPEG, of course, for photometry purposes.