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#1 WRAK

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Posted 13 September 2013 - 10:53 AM

To hopefully solve at least partly the never ending puzzle of correct magnitudes for faint companions (WDS is annoying often not this reliable, Tycho is less then precise, same goes for Hipparco, AAVSO has lots of empty spots, ...) or faint stars in open clusters for trying to determine TML I ordered now the UCAC4 catalog for use with AstroPlanner.
Any experience with UCAC4 in terms of reliability of given magnitudes?
Wilfried

#2 Cotts

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Posted 13 September 2013 - 02:34 PM

If you say a catalog such as the WDS (or any other catalog such as those you listed) has incorrect magnitudes by what authority do you declare them to be incorrect?

It is true that these catalogs disagree with each other but there may not be one catalog that is error free, UCAC4 included.

Dave

#3 WRAK

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Posted 13 September 2013 - 03:48 PM

... by what authority do you declare them to be incorrect?...


Simple facts of observation - if I cannot resolve for example a 3" +9.5/9.8mag double (did I say Jonckheere?) with my 140mm refractor then the data is wrong. Or if you have three +11.3mag stars in an open cluster plain to see obviously different bright then again. And the higher the magnitudes the higher seems the error rate in the usual used catalogs. UCAC4 is certainly not free of errors (how could that be with this number of entries) but I hope it is for higher magnitudes the best available catalog. If you know something better please let me know.
Wilfried

#4 Ed Wiley

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Posted 14 September 2013 - 12:39 AM

Magnitude errors of UCAC4 run about +/- 0.3. But you also have to remember that they are not Johnson-V magnitudes, one would have to transform them. Also note that UCAC4 is meant to be a positional catalog and not a primary photometric catalog. Also, Tycho magnitudes are different from Johnson V. Finally you have the problem of bo-boos from saturation, magnitudes derived from older blue plates and etc.

There might be a research project lurking. Someone interested who encounters such a problem could do a visual estimate of magnitude using companion stars. Then one could see how the estimates compare with the suitably transformed (to Johnson V) magnitudes in various catalogs.

Just a thought...

Ed

#5 3c_273

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Posted 20 September 2013 - 12:53 PM

FWIW, If a UCAC4 star shows its visual magnitude is from the APASS survey, I've found no errors in my small visual sample I've done. I also think that the USNO regards them as definitive, as far as magnitude estimates go. The link to that survey:

http://www.aavso.org/apass






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