I have tried to find the answers to the following on the AAVSO web site and in the literature without success. I'd be grateful if someone could help.
The output of an APASS search includes the error (e.g., Verr) and number of observations (e.g., Vnobs).
Under the output table, the following statement is displayed:
"All negative errors means that the star was observed on one night only for that particular magnitude. Therefore, the error is purely Poisson rather than nightly scatter."
Therefore, what does a positive value error represent when the number of observations is 1? What does it mean to state that the number of observations is 1? Magnitude determination from a single CCD image? Magnitude determination from more than one image on a single night?
Roy
Hi Roy,
Do you have an example of n=1 and error>0? I'll check it out.
Arne
Hi Arne,
RA and DEC (2000) included:
TYC 8013 869, 23 24 40.68 -38 32 50.35
HD 207695, 21 51 23.91 -28 15 21.00
HD 207507, 21 50 03.92 -28 32 25.61
These 3 were in a sample of 11 stars I looked at in APASS (they were checks and comps for some photometry of mine, which is not in the AID).
The three Verr values were 0.004, 0.003 and 0.003 respectively, the lowest three in the group of 11. The Verr values in the other 8 stars ranged from 0.020 to 0.964. Vnobs in those 8 ranged from 2 to 14.
Roy
Hi Roy,
That is a strange one. APASS DR10 shows 5 V measures for the first star, and 3 V measures for each of the other two stars. So it is unclear to me why the merged catalog only picks the last V measure for reporting, or why it doesn't show as a negative error. That must be a bug that I will look for in the next release.
That said, all three stars are brighter than V=10. The nominal saturation limit for APASS is V=10.5, so these stars are likely saturated at V. They may be ok, but I would be suspicious of them. Have you looked at the Tycho magnitudes? Those magnitudes should be ok at V=9.4-9.8 like these are.
Arne
Hi Arne,
Thanks. Yes, I've looked at the Tycho magnitudes for all three stars, and the derived V and B-V are all very close to the APASS values. However, that is not the case for 3 other stars in the sample of 11 I looked at in APASS. The (absolute) differences in V mags for those are 0.222, 0.246 and 0.114. All others in the sample of 11 are OK, and are all 9th magnitude.
The star with APASS-Tych of 0.246 V is an 11th mag star with 14 obs in APASS, and the star with the APASS-Tycho of -0.114 V has a mag of 10.6 and 11 obs in APASS.
You wrote that APASS saturates at about 10.5. However, the description of APASS on the AAVSO web site states the upper limit is about 7th magnitude.
I should explain how this all came about. My targets are southern EW eclipsing binaries down to 10th mag, and most are 8th or 9th mag. I use field stars as checks and comps, selected to be as close as possible in V and B-V to the catalogue magnitude of the variables. I use Tycho magnitudes and colours. I use a 12 bit camera, thus very little well depth, hence the requirement of comps close in V to the targets. I calculate non-transformed V magnitudes because the aim is to determine mid eclipse timings from time series for Variable Stars South. My Tv_bv is about 0.033 so I can get quite close to the true V mags of the check stars (mostly less than 0.05 V, and often less than 0.03 V)
Most of my targets do not have any suitable AAVSO comps. Either there are none at all within my field of view (most cases), or they are not close enough in V mag and/or colour for my requirements.
I have not been submitting the data to the AID because I almost never use AAVSO comps for these EBs. Hence I thought I could use APASS stars as comps instead, but after this exercise I don't believe I should.
I have time series on southern EBs going back a few years. Perhaps I should submit the data to the AID, even though AAVSO comps are not used.
Roy
You might also have a look at the ATLAS 'refcat2' photometry catalogue as well as SkyMapper. The first of these is available through VizieR as item j/apj/867/105. Skymapper DR1.1 is also at VizieR (ii/358), but I would go their Web site to get more recent data/reductions. Do read the ATLAS source paper, which describes what was done especially to the southern APASS data to smooth out the rather bad field-by-field and zonal errors (Tonry et al did a complete re-reduction of southern APASS g,r,i images). SkyMapper has a brighter bright magnitude limit, but they, too, have some calibration issues. I would compare the various sources, including Tycho-2, to see how they stack up in terms of consistency. Depending on how finicky you are, you may be dismayed at how poor all-sky photometric zero-points are. At least on good clear nights, I would recommend including shots of Landolt or SAAO E-regions in the mix in order to ultimately derive improved values for field stars.
\Brian
Thanks Brian,
Comparisons made by me have been limited to Tycho-2, SAAO E-regions and APASS. Tycho-2 and the E-regions compare favourably for most stars sampled.
My lens and camera combination fortunately limits me to brighter stars, which include Tycho-2 stars with the lowest uncertainties, and of course the E-region standards.
Roy