UT Jul 22, 2021: Photometry of V627 Peg

Michael Richmond
Jul 23, 2021

On the night of Jul 21/22, 2021, under fair conditions, I acquired images of the cataclysmic variable star V627 Peg. One can find information about it at

• AAVSO Alert Notice 747.

This was my first attempt to measure this star. The results are pretty good, in large part due to its brightness (V = 10.3 or so).

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 15 seconds
• images binned 2x2
• no guiding
• focus in V
  • 1.224 yields FWHM = 2.5 at T = 68/20
  • 1.234 yields FWHM = 2.8 at T = 66/19
• camera ran at T = -20 C

Notes from the night:

• clouds passed through late in the run, but were isolated clumps
• waxing gibbous Moon made sky bright, but target is bright, too
• troubles with guide camera: at first attempt to take an image, MaximDL never read the image -- it just kept waiting. After re-starting MaximDL, the guider did work properly for a short time only.
• MaximDL froze at image 25. I killed it, re-started, and did NOT use the guide camera any more.
• After restarting MaximDL, I noticed that the telescope control handpaddle was also frozen. This is a new problem I've not seen. The telescope wasn't tracking, either. In order to fix the problem, I had to cycle power to the telescope. For the rest of the night, it behaved properly ... but I will have to run through the alignment procedure at startup the next time I use it.
• the skunk rambled around the back yard early in the evening, then moved into the fenced area close to the dome around 3 AM. I had to move very carefully.

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.

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 small clouds coming late in the run.

The FWHM graph below shows smooth changes, due to a gradual drop in temperature and due to the change in airmass.

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 0.006 mag after I removed images with large outliers.

The change in zeropoint was driving by airmass.

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.

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 Jul 22, 
#    in fair conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 15 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
Jul22.09782     2459417.59782  2459417.60145  10.245  0.007 
Jul22.09881     2459417.59881  2459417.60244  10.255  0.007 
Jul22.09913     2459417.59913  2459417.60276  10.244  0.007 

• download ASCII text file, UT 2021 Jul 22, V627 Peg from RIT Obs

Last modified 7/23/2021 by MWR.