VSP, not many comparison stars have Sloan magnitudes

Technology VSX
21

My opinion is the basic match-up of whatever comp stars and standard fields with ‘refcat2’ is fine. It is the most convenient way forward for the moment.

For my understanding, is this 1% of the magnitude value… uncertainties (1-sigma errors) should be equal or less then 0.1 magnitude?

 A 1% uncertainty is 0.01 mag (or very close).  As Brian K described, the trend is to tie everything to the GAIA absolute fluxes, which are thought to have systematics well down in the millimag range.  Then getting one's own data on that system mainly depends on various 'funnies' in one's instrumentation, including knowing filter+detector passbands accurately, gradients across a CCD field etc.  At the few-percent level you can get away with ignoring a lot of stuff, but below 1%, then 'everything' matters.

\Brian

22

I have cross-matched the ATLAS-REFCAT2 with the VSP and VSP standard field stars. See the two spreadsheets in this link:

Link is only three days valid: Unique Download Link | WeTransfer

  1. For all cases where the distance was smaller then 2 arcseconds and only one star was found, the ATLAS-REFCAT2 position and all magnitudes were added.

  2. In some cases I found two ATLAS-REFCAT2 stars. I decided to skip them. Probably it would require to combine the two star fluxes or maybe it is better to stop using these close doubles as comparison star. These entries are marked with “Warning 2 stars found at position…” You can easily check it at Vizier, .

  3. Some stars where not matched. I assume if I increase the maximum distance I will find more matches. Something to investigate later (next days).

  4. The yellow marked columns are from ATLAS-REFCAT2 catalogue.

  5. The blue area are the ATLAS-REFCAT2 epoch 2025 positions, calculated using the proper motion values. I’m not sure at what epoch the VSP, if any. It assumed it was 2025. there is no proper star motion available in the VSP.

  6. In red red marked area:

  • The position difference between ATLAS-REFCAT2 catalogue epoch 2025 and VSP in arcseconds is indicated in column R.
  • There is a check on magnitudes. g’-V-0.38*(B-V) ~0 at column S.
  1. The ATLAS-REFCAT2 C language program to access it did not work for me except for the positions close to ra=0 and DEC=0. Bug? I decided to write my own program for cross-matching. Cross matching takes only 2 or 3 hours on my computer. I could redo it for stars which have not been matched yet.

I hope this is useful for the AAVSO.
Han

23

Given some input list, one could also use TopCat (an amazing piece of software) or the CDS’s own ‘XMatch’ service to do this task.

\Brian

24

Thanks. I had a look to one of the Topcat introduction videos and it looks powerfull. I will try it for the next project.

In a few days I will try to fill the current “blanks” and increase the search cone for those without a match to 4 arcsecs.

The main thing to highlight is that I found several double stars at the AUID locations. I assume that the current B,V… magnitudes are a flux summation of those double stars. For the next version I will apply the following logic:

  1. First try to find a star within 2 arcseconds of the VSP position. If two or more stars are found within 2 arc seconds, flag the warning and do not report any magnitudes.

  2. Second try, (If there is no match) find a star within 4 arcsecs, if two or more stars are found within 4 arc seconds, flag the warning and do not report any magnitudes.

In the spreadsheet a check on similar magnitudes is applied.

Han