UT May 19, 2021: Photometry of AM CVn

Michael Richmond
May 19, 2021

On the night of May 18/19, 2021, under 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 = 22 C in clear at 0.988, yielded FWHM 2.3 pixels; then moved focus to account for temperature changes, though not very well :-( ended at focus = 1.015 with temperature T = 14 Celsius.
• camera ran at T = -17 C, as it was too warm at the start of the night to reach -20 C

Notes from the night:

• clear all night long
• first-quarter Moon pretty bright in southwest almost the entire run
• temperature reported as 80 F / 26 C at start (8:38 PM) (due to sun shining on dome during the day), falling to 56 F / 13 C at closing (3:34 AM).
• no MaximDL freezing. Hooray!
• started with guider, but it didn't do very well, allowing stars to drift to south. Turned it off and didn't use it after the first 30 minutes; instead, shifted telescope to account for long-term drift
• made an error while shifting field at start of run to find guide star: mis-pointed so target was near northern edge of field of view. Fixed after 10:45 PM = image 34
• probably overcorrected for focus shift due to temperature; moved from 0.988 to 1.047 around 1:59 AM, but an actual focus check at 2:40 AM showed best position was around 1.015

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 drifted up over time; I should have made real focus checks instead of simply moving focus based on temperature.

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, which surprises me; I thought the sky was relatively bright, due to Moon.

The image-to-image zeropoint shows some 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 19, 
#    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
May19.08602     2459353.58602  2459353.58778  13.941  0.023 
May19.08683     2459353.58683  2459353.58859  13.932  0.023 
May19.08764     2459353.58764  2459353.58940  13.933  0.023 

• download ASCII text file, UT 2021 May 19, AM CVn from RIT Obs

Last modified 5/19/2021 by MWR.