I wonder if anyone has any experience transforming DSLR or similar tri-color image magnitudes to Sloan filters rather than Johnson/Cousins? From a look at the passbands it seems logical that a two filter g’/r’ would be reasonable.
Just thinking….
Cliff
Eventually we will all move to Sloan filters. That is the way the professional world is going.
This is interesting; there’s another recent posting talking about the new QHY camera that comes with a built-in filter wheel with Sloan photometric filters and asking if that would be a sensible way to go for photometry (discussion is here). If Sloan filters really are the way things are going maybe this would make good sense as an amateur photometry setup…
Brian (SBQ)
However particularly for amateurs just starting out in photometry there is much more BVRI data than Sloan data (for example in the AAVSO database) for new observers to compare observations with.
I certainly agree with that. It is not only the case for the AID, but VSD as well for comps and standard fields. I suppose mine was a bit of an academic question of bandpasses matching.
Greetings,
From a transform methodology point-of-view and the observed filter response compared to the Sloan filter is reasonably similar, the color transform terms should be determinable just like normal.
Standard pretty picture filters like RGB or a DSLR/CMOS camera RGB transform fairly well into the Johnson-Cousins system using the normal standard fields and good sampling of the Bayer matrix as a practical example.
However, there are no SLOAN system standard magnitudes for any of the major standard fields as of todays check of the AAVSOs standard field page, M-11, M-67, NGC 7790 and SA-110 were checked. I know this has been one of the pending AAVSO projects to be done.
The other thing to consider with everyone going to SLOAN system (SDSS) filters is that they were designed for survey use. Specifically “The SDSS (SLOAN) was designed primarily as an extragalactic survey”. This paper SCAN-9601313.pdf discusses the original design, system, and transform of SDSS photometry to the Johnson-Morgan-Cousins system.
Of course, practically we may be forced to go to SLOAN filters because you won’t be able to buy Johnson-Cousins anymore.
Jim (DEY)
I would say the Sloan system, or actually its precedessor cooked up by Thuan and Gunn, was designed to let older red-biased CCDs go as faint as possible on galaxies while minimizing the effects of night-sky airglow. It is not a Vega-based system, but instead works from spectrophotometric fluxes of stars. It was not designed to get optimal kinds of astrophysical information about stars. The Fukugita et al transformations are not based on any data from an actual camera system on a telescope but instead from models and spectrophotometry of a like three stars (RTFP).
The practical problem right now is that there are no consistent Sloan standards. The original SDSS telescope data are different from the data for Landolt stars observed by Allyn Smith (2002), which was as intended. These are both inconsistent with the ATLAS ‘refcat2’ system, which nevertheless is the most useful all-sky catalogue at the moment. The southern SkyMapper catalogue seems to have offsets of at least ~0.05 mag relative to refcat2 star-by-star, but those folks admit having calibrations problems. We have no published description of APASS g,r,i, but it does have known field-to-field and zonal errors of 0.2-0.3 mag.
Note that the Sloan r,i, and Cousins R,I passbands are sufficiently similar that transformation between the two is not problematic. You will still have to determine the transformations for any specific telescope/camera system. The paper by Kostov & Bonev that I have cited here several times shows the best transformations that I know about. Meanwhile if you want to stay on the native Sloan system, I would adopt ‘refcat2’ data (be sure to specify the stars in one’s reports) and hope that things will improve in a year or two once the final GAIA data are issued.
\Brian
Blockquote The Fukugita et al transformations are not based on any data from an actual camera system on a telescope but instead from models and spectrophotometry of a like three stars
Okay, I’m going to try this from memory: HD 84937, BD +26 2606, and BD +17 4708.
Did I get them right? 
MWR
Michael is probably right! (You were there for some of this, yes?) My memory is of reading the original Thuan-Gunn paper and them saying they based the flux zero-point on observing BD+17 4708 at the Palomar 200-inch on one nice night, and deciding to scale everything to that.
Arlo Landolt suggested BD+17 4708 is a long-term variable. But his mean value is nearly identical with what Nancy Roman observed in the early 1950s(!), and his data overlap in time with Hipparcos, which shows no variation. I now have about 90 nights over the most recent five seasons (Lowell 1.1-m telescope + CCD), and my V-filter differential photometry shows it flat with 0.0035 mag rms.
\Brian