On the night of Apr 04/05, 2021, under good conditions, I acquired images of the cataclysmic variable star AM CVn. One can find information about it at
The results were decent, but I did introduce some error by refocusing in the middle of the observations. I also used a mixture of exposure times -- better to stick with just one.
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. The discontinuity is due to the change in exposure time.
The FWHM: note improvement when I re-focused.
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 the 60-second images, and then a 9-pixel aperture (binned 2x2, each pixel is 1.24 arcsec, so a radius of 11.2 arcsec) for the 30-second images, 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 with a 60-second exposures, 0.007 mag for 30-second exposures.
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 Apr 5, # in good conditions, # by Michael Richmond, # using Meade 12-inch LX200 and ATIK 11000. # Exposures 60, then 30, 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 9 pix = 11.2 arcseconds. # which has been shifted so APASS_86996216 has mag=12.29 # which is its V-band magnitude according to APASS. # # UT_day JD HJD mag uncert Apr05.02976 2459309.52976 2459309.53392 13.879 0.020 Apr05.03061 2459309.53061 2459309.53477 13.902 0.020
Last modified 4/06/2021 by MWR.