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Observing >> Variable Star Observing and Radio Astronomy

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freestar8n
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Reged: 10/12/07

Re: UCAC4/APASS SDSS u'g'r'i'z' differences new [Re: StupendousMan]
      #6266765 - 12/22/13 11:54 AM

Hi-

Well there are many little details involved, but yes it is unfortunate to have a "primed" label on a filter. I dropped the prime in places, so G refers to the true g' magnitude of the star, and the residual compares the table value to the calculated one based on the calibration.

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.

All these calculations are happening in code I wrote, so there could be issues - but the fundamental thing is that I'm using a catalog that isn't intended for photometry. It aims to be "10% absolute and 5% differential" - and I'm just trying to find out how good it is in practice.

For the nova, there are AAVSO standard stars nearby in BVR and if everyone uses them the results will be matchable - except for filter differences and emission lines. But there is no similar standard for ugriz. I want to avoid empirical conversions from BVRI to ugriz, so I am trying to use a system strictly based on ugriz.

Frank


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: UCAC4/APASS SDSS u'g'r'i'z' differences new [Re: freestar8n]
      #6266950 - 12/22/13 01:59 PM

Interesting thread - coming from the other world of double star observing I am often puzzled by inaccurate magnitudes for secondaries and a bit less often for primaries. I then look up APASS, UCAC4, Tycho II etc. to get a clue what is going on - for wide pairs most often but not always successful but for close pairs ...
The idea of doing ground based measurements of magnitudes within an error range of 0.05 or even less makes me a bit uneasy as conditions may change within minutes (not to speak of hours, days or months) to a degree far larger.
I would be happy to get magnitudes such precise from any calaogue as I am confronted rather often with estimated discoverer magnitudes off to one magnitude or even more.
Wilfried


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freestar8n
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Re: UCAC4/APASS SDSS u'g'r'i'z' differences new [Re: WRAK]
      #6267930 - 12/23/13 04:27 AM

Anyone doing photometry over a period of time in the atmosphere will be limited by how stable the sky is, but you can look at the behavior of the residuals in all the fields to get an idea for that stability. I chose nights that looked very good and took data over about an hour. My key point is that the overall systematic error in my measurements appears to be less than the error introduced by using the APASS values - which again isn't too surprising. I'm just trying to determine what the errors are.

For double stars you should be able to get very accurate relative values without much effort.

Do you have a need for more accurate magnitudes of doubles that you look at?

Frank


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: UCAC4/APASS SDSS u'g'r'i'z' differences new [Re: freestar8n]
      #6268025 - 12/23/13 07:02 AM

Frank, when preparing observation sessions with ~20 objects per session from the WDS catalog I have in average at least one double with questionable magnitudes. Most of the time I get useful data via Tycho II/UCAC4/APASS and often there are also photos giving a clue when comparing with other stars nearby with known magnitudes - but there always remain some pairs especially baffling as all these sources do not work.
Some examples are STF450 (companion assumed to be fainter than advertised +9.1mag but no data to be found in other sources), STF1881 (companion assumed to be fainter than advertised +8.81mag but no data to be found in other sources) and LEO55 (advertised magnitudes are Tycho II origin, but both values are for the position of the primary which seems to be wrong by ~2 arcseconds to make things even more unclear).
Wilfried

Edited by WRAK (12/23/13 07:04 AM)


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freestar8n
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Re: UCAC4/APASS SDSS u'g'r'i'z' differences new [Re: WRAK]
      #6268058 - 12/23/13 07:41 AM

Hi-

Well doubles pose other challenges, but it's interesting if no one has tried to re-measure these guys. I might give it a try if conditions work out for me, but my sky time is extremely limited. Also - I am only working with Sloan filters - and clear filter - but they may be adequate for the needs here.

It may need a combination of video and ccd methods but it could be interesting. Basically I would calibrate the field using APASS values and ccd if there are references nearby - then use those as a reference for video measurements.

So I'll keep it in mind as a project to try.

Frank


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StupendousMan
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Reged: 08/21/05

Re: UCAC4/APASS SDSS u'g'r'i'z' differences new [Re: freestar8n]
      #6268535 - 12/23/13 01:02 PM

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

Quote:


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?


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freestar8n
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Re: UCAC4/APASS SDSS u'g'r'i'z' differences new [Re: StupendousMan]
      #6268573 - 12/23/13 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.

Frank


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StupendousMan
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Reged: 08/21/05

Re: UCAC4/APASS SDSS u'g'r'i'z' differences new [Re: freestar8n]
      #6269117 - 12/23/13 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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