UT Mar 21, 2022: Photometry of RR Lyr star BK UMa (for Obs Astro class)

Michael Richmond
Mar 21, 2022

On the night of Mar 20/21, 2022, under good conditions, RIT undergrads Jonathan Lutzer, Ashley Martsen, and I acquired images of the RR Lyr star BK UMa as part of their class project for Observational Astronomy.

There was a small mistake early on, when we didn't set up the camera software properly, so the first 90 minutes of data were taken without the proper filters. After that, however, things went smoothly, and we saw a large portion of one cycle of this pulsing star.

BK UMa

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
• I focused mid-way between the best positions for B and V filters
  • foc in B = 0.812, V = 0.791 yielding FWHM 2.0 at T ~= 44 / 5
  • foc in B = 0.791, V = 0.749 yielding FWHM 2.4 at T = 37/ 2
• camera ran at T = -20 C

Notes from the night:

• a few light clouds early, clear after that
• bright nearly full Moon, but relatively far from target
• first 90 minutes of data taken without proper filters, due to an oversight when setting up software. Argh.
• the camera rotated a bit after midnight; in order to run ensemble photometry properly, I had to de-rotate the measurements.
• no skunks. Rats. Where are they?
• new computer in dome has name "cos-dome01".
• this object has a nice set of measurements from ASASSN

The object is located at

  RA = 10:50:18.94  Dec = +42:34:08.5   (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.

Label	B	V
A	10.799	10.168
B	11.825	11.088

I'll use star "A" to shift my instrumental magnitudes to the B-band scale.

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 no clouds during the filtered portion of the night (the one dip occurs when I moved the panel in the dome slit).

The FWHM graph below shows a decrease early, as the field rose, followed by an increase later, as the field set. I made one adjustment at about 659.73.

Using aperture photometry with a radius of 7 pixels in B and V filters (binned 2x2, each pixel is 1.24 arcsec, so a radius 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 B.

The change in zeropoint shows a steady climb due to airmass.

Last modified 3/21/2022 by MWR.