Skip to main content

New user - V vs Sloan r'

5 posts / 0 new
Last post
New user - V vs Sloan r'

I am just getting started in doing photometry, so I am looking at this in a more long term view. I have been practicing for the last year using my current gear, that is only a L filter (LRGB) on my system. My question is pretty simple, should I go ahead an get a V filter to build a UVBRI chain or would it be more future use to go with the Sloan filter group r' to build toward ugriz group?

I plan to start simple, and move toward 3 filter group after at least a years worth of the single filter.

The Sloan and V filter for my 2" STL camera fw are pretty close in cost (close enough after all other costs).

Any recommendations?


HQA's picture
V vs. r'

Hi Keith,

APASS is currently calibrating the sky at B,V,g',r',i'.  The reason we chose those filters is to give continuity with the majority of the previous photometry (done in BV), plus connection with most of the professional observers and surveys (which are concentrating on the Sloan filters).  It turns out that r' and i' transform pretty nicely into Rc and Ic, so a combination of B,V,r',i' is nearly as good as B,V,Rc,Ic.

So my suggestion for filters is to start with V and then add B for a color index.  Filters beyond B and V get a little more uncertain, as we are definitely in a transition right now.  My preference at this moment would probably be to stick with the APASS filter set since you will have the best choice of comparison stars using that, and I'd probably choose i' as my third filter.  Three years ago, my answer would have been different; three years hence, and it will probably be different again!


Filter choices

Thanks Arne, that makes a lot of sense!


lmk's picture
Lack of universal standards

I'm not a CCD operator, but from what I have experienced over the years, and before, there just an endless variation in filters, passbands, detectors - electronic, chemical, biological, etc. Even sky surveys, both space and ground based have sometimes opted to use custom filters for some odd reasons! All this has made life very complex and messy for people who want to make use of the historical data...Difficult and problematic transformations, comparing "apples and oranges", much headaches.

I guess visual and V may have the most observations historically, but doubtfully a clear majority. I do not understand the reasons why professionals recently went the ugriz route vs. UBVRI? But as Arne mentions, these standards seem to be changing every couple of years. Good grief.

This is just one of the many reasons I have avoided the whole electronic photometry world and stayed purely a visual observer.

Mike LMK

Look at the band pass

I have no specific knowledge of why the Sloan filters were choosen for SDSS but if you look at the band pass of the Sloan filters you can start to get a clue.  Nice sharp roll-offs, almost vertical shoulders and almost no overlap.

Generally when measuring anything, there can many reasons for choosing the equipment.  It's what you have and there isn't any money in the budget for new stuff.  It's the best you can do at the time.  It's optomized for making a specific measurement.  and so on. 

I would guess that in the case of UBVRI it falls at least partially inder the category of it's the best they could do at the time.

Science is complex.

Jim jones

Log in to post comments
AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484