UT Nov 15, 2020: Photometry of RT And in V and I

Michael Richmond
Nov 15, 2020

On the night of Nov 14/15, 2020, under fair conditions, I acquired V-band and I-band images of the eclipsing variable star RT And, which one of my capstone students will analyze. I caught one eclipse near the start of the run, which was terminated a bit early due to clouds.

The other star in the field which I suspect to be an eclipsing binary showed no variations tonight.

RT And

You can find some basic information on this star at the SIMBAD's page for it.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera
• V filter and I-filter alternating, at 10 sec and 20 sec, respectively, 292 images each
• images binned 2x2
• no guiding
• focus in V at 1.546. Decided to put focus at 1.596, to defocus the V-band a lot and the I-band a little. Over the next few hours, as temperature dropped, the FWHM decreased, then increased again.
• camera ran at T = -20 C

Notes from the night:

• no Moon
• no clouds in the first half of the run, but a few cirrus passed over later, followed by heavier clouds that cut the run short
• temperature was cold: around 47 F / 9 C at start (6:02 PM), and 31 F / -1 C at end (11:25 PM).
• this combination of exposure times and focus position worked well: in neither passband was RT And saturated

The object is at

  RA = 23 11 17.9   Dec = +52 59 59.2    (J2000)

A chart of the field based on pictures tonight is shown below. The size of the chart is about 38 x 26 arcminutes.

The stars "A", "B", and "C" appear in the charts and tables for this field made by the AAVSO. see

• RT And chart from AAVSO
• RT And photometry from AAVSO

I used the star labelled A = "102" to shift my measurements to the "B" magnitude scale. It has V = 10.159, according to the AAVSO chart and table. It also has magnitude i = 10.127 according to the APASS catalog. I used this i-band magnitude as equivalent to I-band magnitude for the preliminary calibration.

Here's a picture of the TV with the finder's field of view when pointing at RT And:

The dark current was normal this evening:

The sky value shows evidence for clouds in the second half, growing with time.

The number of objects detected.

I defocused using a larger value of the focus position at the start of the run. As the temperature dropped, note how the FWHM decreased as that become the best focus, then increased as the telescope shrank even further.

Using aperture photometry with a radius of 9 pixels (binned 2x2, each pixel is 1.25 arcsec, so a radius of 11.25 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 0.009 mag in V with 10-second exposures, and about 0.008 mag in I with 25-second exposures as well. I marked all the bright stars as possible variable.

Last modified 11/15/2020 by MWR.