UT Mar 22, 2022: Photometry of RR Lyr star MU UMa (for Obs Astro class)

Michael Richmond
Mar 22, 2022

On the night of Mar 21/22, 2022, under good conditions, RIT undergrads Abagail von Plinsky, Kamryn Griffith, and I acquired images of the RR Lyr star MU UMa as part of their class project for Observational Astronomy.

The first hour or two suffered from light to moderate clouds, but the latter portion of the night was clear. We acquired nearly one full cycle of this star's variation.

MU 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 = 0.807, V = 0.747 yielding FWHM 2.8 at T ~= 38?
  • foc in B = 0.794, V = 0.745 yielding FWHM 2.5 at T = 34 / 1
  • later changed focus down by 0.024 as tempe dropped to T = 28 / -2
• camera ran at T = -20 C

Notes from the night:

• moderate clouds for first 90 minutes, clear after that
• bright waning gibbous Moon, but relatively far from target
• the camera rotated a bit after midnight; in order to run ensemble photometry properly, I had to de-rotate the measurements.
• the measurements of this object from ASASSN
show a peculiar result: when phased with the ostensible period, the measurements are scattered all over the place. Cutting the data into short chunks and phasing each set individually yields decent light curves, so apparently the period is changing.
• one can find a nice dataset from the MAST portal to TESS as well.

The object is located at

  RA = 11:35:36.73  Dec = +38:45:57.8   (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	11.105	10.553
B	12.321	11.787

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 clouds early, but clear after that.

The FWHM graph below shows a gradual increase as the field moved lower in the sky.

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 0.006 mag in V.

The change in zeropoint shows a steady climb due to airmass as the field set.

Last modified 3/22/2022 by MWR.