On the night of Mar 20/21, 2021, under fair-to-good conditions, I acquired images of the cataclysmic variable star AM CVn. One can find information about it at
The results aren't great, due to bright moonlight and persistent trailing, but they do show clear evidence for some of the known periodic signals in the star's light.
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:
Notes from the night:
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. Note the large decrease in brightness as the Moon sets.
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), 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 0.004 mag with a 60-second exposures, but about 0.018 for AM CVn.
The image-to-image zeropoint shows jumps due to trailed images.
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 "A" = APASS 86996126 has the value given by APASS as its V-band magnitude.
A periodogram -- thanks to the NASA Exoplanet Archive tool! -- shows a strong peak at 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 Mar 21, # in fair-to-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_86996126 has mag=11.639 # which is its V-band magnitude according to APASS. # # UT_day JD HJD mag uncert Mar21.04352 2459294.54352 2459294.54802 13.891 0.018 Mar21.04438 2459294.54438 2459294.54888 13.910 0.018 Mar21.04523 2459294.54523 2459294.54973 13.862 0.018
Last modified 3/21/2021 by MWR.