UT Sep 30, 2022: Photometry of XX And

Michael Richmond
Oct 01, 2022

On the night of Sep 29/30, 2022, I observed the RR Lyrae star XX And for a capstone project. Physics majors Joey Root and Mimi Harrison joined me at the start of the night to see how things work at the Observatory. It was fun to chat with them.

XX And

The star XX And is a variable star of the RR Lyr "AB" type. Its period may be changing slightly on long timescales -- that will be one focus of our study of it.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera
• B, V filters, exptime 30 seconds
• images binned 2x2
• no guiding
• focus values:
  • V: 1.668 yielding FWHM 2.0 at T = 52/11
  • B: 1.709 yielding FWHM 2.0 at T = 52/11
  I set the focus to be about 1.688, about 0.02 units smaller than the B-band value, in order to lie between the best focus for B and V.
  • V: 1.677 T = 42/6
  • B: 1.716 T = 52/11
  This time, I set the focus to 1.687.
• camera ran at T = -20 C

Notes from the night:

• mostly clear, but perhaps light cirrus growing throughout the night
• thin crescent low in the East after sunset
• no skunks -- sigh
• temperature dropped to T = 35/2 at the end of the run. Brrr.

The object is located at

  RA = 01:17:27.41  Dec = +38:57:02.0    (J2000)

A chart of the field is shown below. The size of the chart is about 31 x 26 arcminutes.

I've marked the location of several comparison stars as well. You can find reference magnitudes for these stars at the AAVSO:

• AAVSO chart for the field
• table of magnitudes from the AAVSO: sequence X27032O

I'm not sure yet which reference star(s) might be best to shift the instrumental magnitudes to the standard scale. These might be good choices:

Label	B	V
C = 111	11.575	11.101
E = 117	12.503	11.745

I took a photo of the finder TV's screen when pointing to this target; this could be a useful reference for the future:

The sky value shows a nice, smooth curve, indicating very few clouds.

The FWHM graph below shows only a small rise despite the drop in temperature.

Using aperture photometry with a radius of 7 pixels (binned 2x2, each pixel is 1.24 arcsec, so a radii of 8.7 arcsec), I measured the instrumental magnitudes of a number of reference stars and the target. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame.

Sigma-vs-mag plots show that the floor was about 0.008 mag in V, and 0.007 in B.

The change in zeropoint shows a few brief episodes of light clouds near the end of the night.

Photometry shows the star fading over the entire night. Note the "shoulder" in the decline.