Reference star identifier recommendations from AAVSO?

Science Photometry
1

When trying to reduce HADS observation sequences, the AAVSO star plotter charts usually give me reference stars that are way too bright for good measurement (like V mag 8-9 when you try to measure a V 12-13 HADS star), so basically unusable and saturated.

In order to produce ENSEMBLE measurements with sufficient reference stars, several options and catalogs are possible: Gaia DR3 is obviously very accurate, but then you have to transform to V or R-band. Gaia star identifiers are too long for convenient use in AAVSO reports.
Another option is the APASS database, which has direct photometry of the reference stars, but the star plotter doesn’t seem to have an easy option to select APASS stars.

My questions:

  • what are the AAVSO recommendations for reference or check star identifiers to use in an AAVSO report, in case the plotter tool didn’t give useful stars? (APASS?)
  • do you recommend specific tools to select fainter reference stars, and possibly filter them on intended magnitude range and color index near the variable target?

Best Regards,
Tom

2

Tom,

VSP normally displays all viable calibration stars within the FOV to whatever limiting magnitude you specified. Have you tried increasing the magnitude limit to V~10? Otherwise, please let us know what object(s) you are experiencing issues with and we can inspect the area to see if there are any additional calibration stars we could add to the sequence.

Brian

3

Hi Brian,

thanks for the quick reply.
I attach an obvious example of V2008 Aql (HADS).
The field is 30’, the mag limit is 16.5, the plot around V2008 Aql gives only 1 calibration star (000-BCK-689 = mag 6.3), so completely unusable if the star to measure is about 12.0.
Instead of adding a few fainter stars in such a case, wouldn’t it be better to use APASS or Gaia DR3 stars? Typically an ensemble of half a dozen good reference stars gives a much nicer lightcurve with smaller error bars for the variable star in several photometry analysis programs (like for example Astro Image J), I measured it by handpicking a few suitable Gaia DR3 stars.

Very interested to hear your recommended best practice for such cases,

Tom

V2008Aql-example
V2008Aql-example1230Ă—1530 116 KB

1 Like
4

Hey, Tom.

I do not know if my answer is valid, but I also ran into a situation like yours in the past. I use VPhot however, and what I suggest is the following workflow.

On one of my uploaded images at VPhot, I choose it to see a preview.

Afterwards I select the pull down menu at the upper right of the screen named Catalogs and choose the option Load AAVSO Comp Stars, and I get this image:

image
image1603Ă—906 306 KB

Then I go to Simbad, look for my target star, find out the B-V value (and write it down), and then I select the image of my target star that appears at the right side of Simbad’s window. I zoom out until I get a similar FOV of the image above.

Next, I look for the comp stars that have similar color, check their B-V value. If this value is close to that of my target star, I choose it. I repeat this process as needed. Once you have your comp and check stars, you are good to go.

Now, when you have a very narrow FOV like yours, you can look for “alternative” stars in APASS or Gaia DR3. The idea is that you pick up stars of similar magnitude and color index of your target star, but also be careful to be sure that they are not variable, or at least that they almost don’t vary. If you are not certain about this, you can run a few observations and check their magnitude to see how they behave. If there is very little variability or none, then you are good to pick those stars as a comp or check stars.

Please be aware that this is what I have been doing, and this practice has been giving to me good photometric results (a good deal of this was taught to me by Ken Menzies some time ago), and of course this procedure is by no means something that the AAVSO officially recommends. But I think that this procedure is kind of safe enough for you to get good quality photometry.

Hope this helps, at least a little.

Cheers,

Enrique Boeneker (BETB)

5

My FOV is just 24’. But I begin by asking the comp team for comps within 10’. Most times I get new comps in a day or two. If I get just one comp, I’ll APASS a check star. There are rare occasions when they are unable to find some comps in the field. Then I pick through APASS for small B-V and magnitudes that work for me. I delete comps that are variables or that give me trouble when I look at the VPHOT photometry statistics.

1 Like
6

Sebastian and I recently brought this issue up to the sequence team. Although we haven’t settled on a specific solution, our current recommendation is that they identify calibrators stars within 30’, 1 degree, 3 degrees, and 15 degrees to accommodate CCD/CMOS, wide-field sensors, and visual observers. We are presently testing this with beta Aurigae (V~1.9) which is featured in observing campaign 859 and very challenging to observe for a myriad of reasons, including a lack of nearby reference stars.