UT Jul 24, 2021: Photometry of V627 Peg

Michael Richmond
Jul 24, 2021

On the night of Jul 23/24, 2021, under good conditions, I acquired images of the cataclysmic variable star V627 Peg. One can find information about it at

• AAVSO Alert Notice 747.

The star is fading slowly, by about 0.15 mag per day, but is still very bright: V = 10.4 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
  • 1.215 yields FWHM = 2.0 at T = 72/22
  • 1.224 yields FWHM = 2.8 at T = 63/17
• camera ran at T = -17 C

Notes from the night:

• no clouds all night; well, a few light moisture clouds to the North near dawn, but they weren't near the field.
• full Moon made sky bright, but target is bright, too
• didn't try using guide camera; the short (15-sec) exposures mean it isn't needed. The telescope drifts very slowly, so I only needed to make two small corrections in RA during the entire run.
• no problems with MaximDL -- phew.
• I did perform a "fake polar alignment" at the start of the night. I removed all cables, so that the telescope could rotate freely, and told it to do a polar alignment. But I didn't try to center Polaris -- I just told the telescope that Polaris was centered when it finished slewing. It then moved to where Arcturus was supposed to be, and it was indeed within a degree or two. All done!
• the skunk was active all night in the back yard, so I had to check carefully as I walked out and back.

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 no clouds.

The FWHM graph below shows smooth changes. I made one focus adjustment.

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 24, 
#    in good 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
Jul24.08196     2459419.58196  2459419.58568  10.477  0.009 
Jul24.08226     2459419.58226  2459419.58598  10.499  0.009 
Jul24.08285     2459419.58285  2459419.58657  10.509  0.009 

• download ASCII text file, UT 2021 Jul 24, V627 Peg from RIT Obs

Last modified 7/24/2021 by MWR.