I used 10x50 binoculars for a long time to measure variables brighter than 7.0, picking from the Binocular Program and some alert notices. Disappointed with the huge dispersion in visual observations, I'm a now beginner in DSLR photometry (and still a binocular observer). I would like to make sure I'm doing things right, so please help me improve this workflow.
Target: The same stars I observe visually. This past months some examples are epsilon Aur, TU CVn, Y CVn, psi1 Aur, UU Aur ....
Equipment: Canon 600D + 50mm fixed lens. I measured linearity, gain and saturation values for ISO 100, 200 and 400. I took 24 bias at each of these ISOs.
Process:
- Take 25 lights of 12 seconds at ISO 400.
- Take 16 darks and 16 flats at ISO 400, and the same focus.
- Calibrate and extract B, G1 and G2 channels.
- Stack (average) these channels in 5 groups of 5 lights (1 minute total exposure each). I also stack (average) G1 and G2 in one G.
- Chose 1 check star + 6 comparison stars for each target. These comparison stars are spread in a wide (within reason) range of V mags and also B-V indexes.
- Use Maxim DL to perform aperture photometry on the 5 lights (G, then B) and export all the data to CSV.
- Use my own spreadsheet to make sure all the stars are in the same line in a scatter plot of V magnitudes, and also in a scatter plot of B-V indexes.
- Take the data to Mark Blackford's spreadsheet in order to transform my TG and TB instrumental magnitudes into V and B.
Things that still annoy me: The stars with higher B-V indexes usually show higher errors in the expected value from regression analysis of the B-V index. Error is minimized when the comparison stars have different B-V indexes to perform a good regression analysis, but I'm still not completely happy with the results.
I'm also in doubt whether I should report the observation as "transformed" or not, as I don't use a standard field to transform the magnitudes.
Any ideas for improvement, or tricks you find useful in a similar workflow, are very welcome. Thanks!
Arturo
That looks like a very good workflow indeed.
As for reporting "transformed" magnitudes: Yes, if you work with a DSLR (TG and TB filters) and want to report V and B magnitudes, your magnitudes must be reported as transformed.
IMHO, it cannot be helped that this method will not work so well for very red stars. And some of your target stars are indeed extremely red like Y CVn. To start with, they will be comparatively faint in (T)B so that hurts the SNR of that measurement and hence the color term in the transformation to start with. But I guess far more important is that the whole concept of transforming magnitudes between filters with a simple linear ansatz works best when the star in question has a spectrum that is similar to a smooth "black body"-spectrum for some given temperature. But very red stars like Y CVn have spectra that in part look more like a sawtooth pattern, because the lower temperatures allow molecules to persist in the star's vicinity that then cause pretty broad absorption features in the spectrum, including already the green part of the spectrum.
This is not limited to DSLRs of course, it's true for any filters that are sufficiently different. As an example, the Hipparcos astrometry satellite mission was doing photometry as a kind of by-product, published in the Tycho catalog. The actual blue and green filters used on board the satellite were quite different from the standard B and V filters (tho their green filter is very similar to DSLR TG filters), so a transformation to B and V was needed. They provide a standard transformation but even they caution that this simple transformation is *not* valid for very red stars.
You can read about the Tycho catalog's transformation approach in this document, Appendix 4 to section 1.3 https://www.cosmos.esa.int/documents/532822/552851/vol1_all.pdf . You could try to work out correction terms for very red stars like they do, but you will find it hard to find comparison stras that are as red as Y CVn and are not variable stars themselves....it is complecated.
So for very red stars, maybe it's better to skip the transformation and report TG magnitudes (tricolor Green with V zeropint). They will be off compared to V measurements by others, but should be comparable to other TG measurements. Or just look for target stars that are not so extremely red, of course, because your DSLR measurements will be more in line with others for those stars.
CS
HB
Wow, that was very helpful for understanding the physical problem!
And it gives me some peace of mind :) . I observed that stars with B-V indexes above 2.5 (Y CVn, UU Aur and X Cnc) showed a significant deviation from the linear regression. While stars up to 2.0 (psi1 Aur is 1.970) are a closer match.
These are some plots of my B-V indexes (instrumental vs. catalogue), the yellow triangle is the target star: link to image.
So I will follow your advice and avoid very red stars. Thank you!
Arturo