UT Nov 10, 2020: Photometry of RT And

Michael Richmond
Nov 11, 2020

On the night of Nov 09/10, 2020, under good 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 in the middle of the run.

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, 436 images each
• images binned 2x2
• no guiding
• focus in V at 1.400, focus in I at 1.430. Decided to put focus at 1.450, to defocus the V-band and concentrate a bit in I-band; within an hour, increased the focus position to 1.500 to avoid saturation in V-band.
• camera ran at T = -20 C

Notes from the night:

• waning Moon rose after I closed
• no clouds at all
• temperature was very warm (nice!) -- around 75 F / 24 C at start (5:00 PM), and 52 F / 11 C at end (12:07 AM).
• I focused again after ending the measurements of RT And, finding best positions of 1.478 in V and 1.498 in I. Those are about 0.070 larger than the values at the start of the run. So, as the temperature dropped by 15 C, the focus INCREASED by 0.070. In other words, the focus change is very roughly -0.005 units per degree C.

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 no evidence for clouds; good.

The number of objects detected.

Note the big changes in FWHM as a result of my changes in the focus position, required to avoid saturation. I discovered that using apertures of radius 7 or 9 led to a discontinuity in the photometry before and after JD 143.47. Increasing the aperture radius to 11 pixels (in both V and I) seemed to yield acceptably uniform measurements.

Using aperture photometry with a radius of 11 pixels (binned 2x2, each pixel is 1.25 arcsec, so a radius of 13.75 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 with 10-second exposures, and about 0.009 mag in I with 25-second exposures as well. I marked all the bright stars as possible variable.

Last modified 11/11/2020 by MWR.