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
I was looking for a standard-field with Sloan magnitudes to test my own software. The only one I could find in the VSP is Landolt 94.
Position 02:55:19.99, 00:28:12.0
Seven standard stars of that field SA94 that have documented SG, SR, SI magnitudes
The data seems to come from ,APASS DR10 sloans’’
Han
Again, it is quite reasonable to simply adopt the Pan-STARRS ‘refcat2’ Sloan g,r,i photometry for any field. You can search for stars using the CDS VizieR catalogue query utility looking at item j/apj/867/105. Down below (fainter than) mag 10 or so, it is quite reliable. It will certainly be better than anything from APASS.
\Brian
Thanks. These values are accurate. Uncertainty around 0.02 magnitudes. That’s better then transformed Gaia DR3 values to Sloan which are around 0.1 uncertainty.
Any idea why these Pan-STARRS Sloan values are not added to the VSP?
Han
Hi All,
I’d like to take advantage of this discussion because the topic has intrigued me for a long time. Of course, Sloan filters are very precise; their frequency cutoffs are their strength. That much is clear.
However, working frequently with binaries, I also find many advantages with JC filters, advantages I haven’t yet found with Sloan filters.
For example, we found anomalous V-R values on an EA binary, even though the spectra were perfectly normal. We think this could be a trace of the cooler secondary star.
V-I and V-R values are indicators of nebulosity. B-V values provide a color index used to calculate the temperature of our target star. Furthermore, tables have been compiled, and these tools are invaluable in analyzing the stars we observe. (See: http://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt)
Do we find the same advantages with Sloan filters?
Actually the rectangular passbands of current commercial Sloan filters are fairly disastrous in terms of transformations to a standard system. See papers by Andy Young about this. But it is a differential world, so it may not matter much; hardly anyone actually cares about accurate transformations any longer.
In re Cousins R,I versus Sloan r,i: the passbands are similar enough that whatever you’re getting from Cousins filters should be obtainable from Sloan filters.
\Brian
@bskiff wrote:
Actually the rectangular passbands of current commercial
Sloan filters are fairly disastrous in terms of
transformations to a standard system.
See papers by Andy Young about this.
Thanks for posting this so that I didn’t have to I agree with you, of course: filters with sharp vertical edges make it difficult to transform ordinary stellar measurements onto a standard system.
One of the relevant papers by Young is
Transformability of Imaging Filters in Four Photometric Systems at KPNO, CTIO, ESO, and CAO
Reading Andy young’s work can be discouraging, since he makes it seem as though it’s not possible to do accurate photometry. Looking elsewhere, particularly work by Ben Taylor and Mike Joner, Peter Stetson, Petr Harmanec, and (yes) Arlo Landolt suggests there are practical things one can do. This mainly means observing more standard stars (or fields) on any night you think is actually cloud-free. Then in the reductions the transformations are likely to involve not just a simple linear color term, but quadratic or cubic coefficients, plus terms in two color-indices — at least to test for those to see if the coefficients are significant. Thus a transformation for V might involve terms in both B-V and V-R (say). It would not be surprising to see such effects in tri-color cameras since none of the passbands is quite right.
Instead of polynomials, Arlo got around this problem, especially for B-V and U-B colors, by doing linear fits first, then looking at the residuals to apply piecewise linear fixes for different color ranges for each color-index. The figures in the main 1992 standards paper show residuals before/after such corrections, which involve the two sets of filters he had to use.
Very instructive in this regard is the tutorial by Taylor & Joner:
…which outlines broadly applicable statistical tools.
\Brian