UT May 14, 2021: Photometry of AM CVn

Michael Richmond
May 16, 2021

On the night of May 13/14, 2021, under very good conditions, I acquired images of the cataclysmic variable star AM CVn. One can find information about it at

• SIMBAD

AM CVn

Joe Patterson of the CBA requested data on this star, so I gave it a try. The star is at the faint limit for time series photometry with our equipment, but I wanted to test it.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera
• no ("clear") filter, 60 second exposures.
• images binned 2x2
• guiding with 7-sec exposures
• initial focus at T = 15 C (?) in clear at 1.048, yielded FWHM 2.4 pixels; then added 0.005 to focus for each decrease in temperature of 1 degree C; ended at focus = 1.087 with temperature T = 7 Celsius.
• camera ran at T = -20 C

Notes from the night:

• clear all night long
• small waxing crescent Moon set before I started collecting data
• temperature reported as 81 F / 27 C at start (7:44 PM) (due to sun shining on dome during the day), falling to 43 F / 6 C at closing (3:53 AM).
• MaximDL froze once very early, before starting observations of the target. It seemed to freeze just as I clicked a command button at the same time that a frame was reading out ... not sure that was the cause, or a coincidence.
• used guider at times, but it tended to fail after 10-20 minutes when telescope was near zenith. Typically, stars would appear to move to the south in a sudden motion, and they would drift to the east slowly. I turned off guider when it seemed to be causing more problems than it helped.
• frequent small adjustments in focus to follow the temperature worked pretty well -- FWHM remained between 3.5 and 4.5 pixels

The object is located at

  RA =  12 34 54.6   Dec = +37 37 44.1   (J2000)

A chart of the field is shown below. The size of the chart is about 19 x 26 arcminutes.

I've marked the location of several comparison stars as well.

  star       AAVSO               B          V          
-----------------------------------------------------------

   A        APASS 8699612      12.226     11.639        

   B        APASS 8699621      12.952     12.290        

-----------------------------------------------------------

 

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 that the sky was clear.

The FWHM.

Using aperture photometry with a radius of 7 pixels in a clear filter (binned 2x2, each pixel is 1.24 arcsec, so a radius of 8.7 arcsec) for all exposures, 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 for 60-second exposures, close to the best I've seen for this field.

The image-to-image zeropoint shows frequent outliers, due to trailing.

Here is the light curve of the object and several field stars in the clear filter; I've shifted the instrumental magnitudes so that star "B" = APASS 8699621 has the value given by APASS as its V-band magnitude.

A periodogram -- thanks to the NASA Exoplanet Archive tool! -- shows a strong peak close to the known 525-second (= 0.006076 day) period.

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

# Measurements of AM_CVn made at RIT Obs, UT 2021 May 14, 
#    in good conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ATIK 11000. 
# Exposures 60 seconds long, clear 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 = 8.7 arcseconds.  
#    which has been shifted so APASS_8699621 has mag=12.29 
#    which is its V-band magnitude according to APASS.  
# 
# UT_day             JD            HJD        mag    uncert
May14.06286     2459348.56286  2459348.56498  13.942  0.019 
May14.06366     2459348.56366  2459348.56578  13.912  0.019 

• download ASCII text file, UT 2021 May 14, AM CVn from RIT Obs

Last modified 5/15/2021 by MWR.