UT Aug 05, 2021: Photometry of V627 Peg (and a faint eclipsing binary)

Michael Richmond
Aug 05, 2021

On the night of Aug 04/05, 2021, under very good conditions, I acquired images of the cataclysmic variable star V627 Peg. One can find information about it at

• AAVSO Alert Notice 747.

Near the end of the night, moisture started to condense on the dewar window. The relative humidity was very high (around 96%), but this is still disappointing. I had to discard data after that point. Perhaps I can find a way to prevent this in the future ...

V627 Peg

Christian Knigge is soliciting observations of this cataclysmic variable star at all wavelengths. The AAVSO has advertised for optical measurements, and I'm joining the effort.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera
• V filter, exptime 30 seconds
• images binned 2x2
• guiding with 3-sec exptime, worked well
• focus
  • 0.232 yields FWHM = 1.6 at T = ? / ?
  • 0.232 yields FWHM = 1.8 at T = 62/16
• camera ran at T = -17 C for 2-sec darks and flats, T = -17 C for 30-sec darks and target images

Notes from the night:

• sky was clear all night
• waning crescent Moon had no effect on sky brightness
• guiding with 3-second exposures worked well, put target at (1112, 458) to get the bright guide star
• moisture started to condense on the dewar window around 3:10 AM, near image 430. The giveaway is a dark circular patch near the center of the screen, surrounded by a lighter annulus. The relative humidity was very high, around 95%, so this is perhaps not surprising. The ATIK camera manual mentions an internal heater for the window; I will try to enable it the next time I use the camera, if I can find the original software that came with the camera.
• at 3:54 AM, MaximDL just stopped taking images all by itself. I didn't notice for some time. There were no error messages, but the program wasn't responsive when I tried to use it. I had to kill it with Task Manager.
• the skunk moved around in the back yard and inside the fenced area

The object is located at

  RA =  21:38:06.63    Dec = +26:19:56.0     (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. See

• this chart from the AAVSO
• this list of stars and photometry from AAVSO

I'll use star "D" to shift my instrumental magnitudes to the V-band scale. It has a V-band magnitude (according to AAVSO chart X26768AQ) of 12.002, and (B-V) = 0.692.

The star marked "X" is an eclipsing binary of type EW, with a period of 0.36 days and amplitude of about +/- 0.3 mag in V-band. I just noticed it myself for the first time in tonight's data (perhaps because I used 30-sec exposures for the first time on a good night), but it appears in the ASASSN database.

• ASASSN-V J213808.68+261704.4 / VSX J213808.7+261704

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 at all.

The FWHM graph below shows small variations; I re-focused at image 377, around JD 431.76.

Using aperture photometry with a radius of 7 pixels in a V filter (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 0.007 mag after I removed a few images with large outliers. Conditions were very good.

The change in zeropoint shows only a few outliers; guiding helped.

Here is the light curve of the object and several field stars in the V filter; I've shifted the instrumental magnitudes so that star "D" = 000-BJV-433 on AAVSO chart X26767AK has the value given by AAVSO as its V-band magnitude.

Here's a closeup, also showing the light curve of star "X", the EW variable just south of the target.

I have submitted these measurements to the AAVSO, CBA, and VSNet.

You can download my measurements below. A copy of the header of the file is shown to explain the format.

# Measurements of V627Peg made at RIT Obs, UT 2021 Aug 5, 
#    in good conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 30 seconds long, V filter. 
# Tabulated times are midexposure (FITS header time - half exposure length) 
#    and accurate only to +/- 1 second (??). 
# 'mag' is a differential magnitude based on ensemble photometry 
#    using a circular aperture of radius 7 pix = 8.8 arcseconds.  
#    which has been shifted so AAVSO 000-BJV-433 has mag=12.002 
#    which is its V-band magnitude according to AAVSO chart X26767AK.  
# 
# UT_day             JD            HJD        mag    uncert
Aug05.06826     2459431.56826  2459431.57245  13.467  0.037 
Aug05.06877     2459431.56877  2459431.57296  13.558  0.040 

• download ASCII text file, UT 2021 Aug 05, V627 Peg from RIT Obs

Last modified 8/05/2021 by MWR.