American Association of Variable Star Observers (AAVSO)
Thu, 01/18/2024 - 13:34

Hi, what do I have to do if there are no reference stars shown on the chart? The variable star is HQ CAM and the chart reference is X35672UK. Field width is 60 arc minutes.




American Association of Variable Star Observers (AAVSO)
finding comp stars

Hi Steve,

Two routes:
- You can ask the Sequence Team to identify comp stars for you in the 60' fov
- If you are using VPhot you can bring up your image and have APASS stars displayed. From these you can build up an ensemble sequence.


American Association of Variable Star Observers (AAVSO)
HQ Cam comp stars


By all means give George's APASS suggestion a try, but I'm not sure how faint the APASS data go.   

Here's another plan.  If you reset the chart plotter to a minimum magnitude of 14.6 you'll find two Pan-STARRS comps: 

000-BPH-018  Labeled 143 

000-BPH-019  Labeled 146

These would be appropriate for this target with a minimum magnitude of 14.5 using one for the comp and the other for the check star.  At least this would get you started.  This may be all you need depending on your project.  Perhaps the sequence team has access to more Pan-STARRS comps if you need them.


American Association of Variable Star Observers (AAVSO)
Yes, reference stars

     In cases like this it is helpful to learn some stuff about photometry and available star catalogues.  Taking this star as an example, I think of what would I do?  The telescope I use has a 0.4x0.4-deg field.  So I first have a look at sky survey images available, such as from the Goddard SkyView utility.  Looking at these, I see bunch of medium-bright stars off somewhat to the northwest, so I wouldn't center the telescope on the variable itself, but somewhat in the direction of those stars. 

     Next I go to the CDS-Strasbourg's VizieR catalogue-query utility, and look at APASS DR9 and also the ATLAS 'refcat2' photometry catalogues, which are items ii/336 and j/apj/867/105/refcat2 in that set-up.  HQ Cam seems to be an early-type star, so the comps are probably best chosen from among relatively blue, probably F-type stars that are common in any field.  One can use the coordinates feature in the SkyView image-display to get good-enough (few arcsec) coords for the stars.  Use that to search candidates star-by-star in the two star catalogues until you find a few good ones --- you can hope for 3 or 4 such stars without too much work.  Cut-n-paste the coords into VizieR to see whatever data are available in the two catalogues mentioned.  You might add item b/vsx in the search so as to include VSX as well, in case one of your candidates is a known variable (which you might want to observe anyway).  If the photometry errors in APASS are large or data simply missing (often the case), adopt refcat2 values instead.  Look at the reported errors in both cases.  Convert from refcat2 Sloan griz values to BVRI as necessary using the formulae in this paper:

Be sure to record the names/coords of the comp stars so things are traceable.  The AAVSO chart team must be going through a similar process to identify comp stars, but this way you get to learn something about it.


American Association of Variable Star Observers (AAVSO)
Finding Comps the Aladin Way

The strategy I use when I don't request comps from the Sequence team.

1. Plate solve a good image from my run. (I use ASTAP.)

2. Open Aladin (free, downloaded from CDS) and import the image, best to immediately reverse B&W so you have black stars.

3. Bring the Aladin FOV to approximately the size of the AAVSO star chart and manipulate the chart to match the image or plot a new chart that can be oriented correctly.

4 Call up the APASS catalog directly in Aladin. (Typing APASS in the Bottom left "Select.") Make sure to only select "In View" or you might get the entire catalog. All APASS stars are marked by symbols overlain on the image.

5. Select likely comps by selecting on various marked APASS star in the field. You get access to all the data on that star below the image.

6. You can also call up other catalogues (e.g., GSC) for names to identify the comps in the AAVSO report that are stable.