UT Nov 09, 2020: Photometry of RT And

Michael Richmond
Nov 9, 2020

On the night of Nov 08/09, 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 the start of a dip.

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 1.454, focus in I at 1.474. Decided to put focus at 1.504, to defocus the V-band and concentrate a bit in I-band.
• camera ran at T = -20 C

Notes from the night:

• 3rd quarter Moon only rose near the end of the run, was pretty far from the field
• no clouds at all
• temperature was very warm (nice!) -- around 68 F / 20 C at start (6:10 PM), and 47 F / 8 F at end (12:46 AM).
• the FWHM varied from 4.0 to 3.1 to 4.0 pixels in V-band over the course of the run, so I guess as the temperature dropped, we moved into and then past focus. That suggests that as temperature drops, the focus position moves to smaller numbers.

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.

I used an aperture of 9 pixels for photometry in both V and I.

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 0.005 mag in V with 10-second exposures, and about 0.005 mag in I with 10-second exposures as well; however, the signal-to-noise for most stars was visibly lower in the I-band, so increasing the I-band exposure time would be a good idea. I marked all the bright stars as possible variable.

The outliers in the photometric solution are due to my adjustments of the dome slit.

Last modified 11/9/2020 by MWR.