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UCAC4/APASS SDSS u'g'r'i'z' differences

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#26 StupendousMan


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Posted 23 December 2013 - 01:02 PM

Thanks for your reply. Could you clarify one item, please?

The sdss and apass values don't include a magnitude (M) estimate, so I use the aperture based magnitude from UCAC4 as the reference for my clear filter values.

If you are interested in doing careful photometry -- and clearly you are -- why are you including measurements made with an unfiltered or "clear" filter? Perhaps they are handy for some good purpose; I could see them being used for asteroid rotation measurements, for example. How are you using them?

#27 freestar8n



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Posted 23 December 2013 - 01:17 PM

I was interested in how a C filter, combined with g' and r', could allow an estimate of M. It turns out it works pretty well.

All the stars in ucac4 have a magnitude value, but only some of them have g' and r'. If I calibrate with g', r' and c using g', r' and m values - I have a lot of info - and error bars on the estimates.

I'm not driven by a use-case here - I'm mainly driven by an interest in getting a feel for the errors and uncertainties in doing photometry - particularly with Sloan filters in 2013, where UCAC4 happens to be convenient. I assume standards will change over the next years and I hope u'g'r'i'z' remains relevant - but who knows.

For me, the uncertainties with 8" sct and 5x10s exposures in the 10-13M range appear to be limited by the standards used for calibration rather than the errors in my measurements. That was one of the things I set out to understand. I therefore don't need to spend time improving my flats or something.


#28 StupendousMan


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Posted 23 December 2013 - 05:47 PM

I see. Fair enough.

If "All the stars in ucac4 have a magnitude value" refers to the "f.mag" field in the UCAC4, then for the benefit of the general reader, let me explain. The "f.mag" field in the UCAC4 refers to measurements of stars with a filter which transmits light in "orange" range, between 579 and 642 nm. This is roughly between the standard V and R filters, in the spectral region at which many CCDs are most sensitive. It ought to be a pretty decent match to the sensitivity of a typical CCD without any filter, though problems will arise if one observes very red stars in this manner.

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