Oct 05, 2004: ASAS 002511

Michael Richmond
Oct 17, 2004

On the night of Oct 5/6, 2004 EDT, Tracy Davis and I used the RIT Observatory's 12-inch Meade telescope and SBIG ST8 CCD camera to monitor the cataclysmic variable star ASAS 002511; it's under study by the Center for Backyard Astrophysics.

The plan:

• the Meade 12-inch telescope with SBIG ST-8E CCD (no focal reducer); the plate scale is about 1.85 arcsec per pixel
• no filter
• 60 and 120 second exposure times (60 seconds for the first 17, then 120 seconds for the rest)

Notes from the night

• the sky was clear at sunset, but the data shows some evidence for clouds moving in and out for an hour during the night twice
• the target star, ASAS 002511, had faded so much that we increased exposure times to 120 seconds after looking at the early 60-second exposures

Here's a chart of the field of ASAS 002511 taken on Sep 18, 2004. The field is about 20 arcminutes wide. Click on the picture for a larger version.

Arne Henden's calibration of stars in the field indicates

• star A = GSC 599-255 has V=11.16
• star B = GSC 599-1272 has V=11.49
• star C = GSC 599-184 has V=12.06

  but all these bright stars are saturated on our long exposures, so we used to set the magnitude scale

• star G = GSC 599-628 has V=14.03
• the variable, ASAS 002511, is roughly V = 16 on this night

I measured the instrumental magnitude of each star with aperture photometry, using a radius of 5 pixels = 9.3 arcseconds, and sky defined by an annulus around each star. 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.

Below is a graph of the scatter in differential magnitude versus magnitude. Note that the brightest stars included in the solution may be slightly saturated. I made a series of preliminary solutions and discarded stars which had large scatter due to close companions, or due to their position near the edge of the frame, etc.

ASAS 002511 is the star near differential mag 2.8, with elevated scatter. The brightest unsaturated star (of mag V=13.1) has a formal scatter of 0.004 mag, but the scatter rises to about 0.04 mag at the brightness of ASAS 002511.

Light curves for selected stars in the field are shown below. ASAS 002511 is in green; star D in red, star G in blue, star F in brown.

Here's a closeup of the variation in ASAS 002511 itself:

I've made a table of the measurements themselves, with three different flavors of time. The differential unfiltered magnitudes from the ensemble solution has been shifted so that star "G" in my chart, GSC 559-628, has value 14.034, matching its V-band magnitude as determined by Arne Henden.

Here's the start of the table:

# Measurements of ASAS002511 made at RIT Obs, Oct 6, 2004 UT, 
#   made by Michael Richmond and Tracy Davis. 
# All data taken with 12-inch LX-200 + no filter + SBIG ST-8 CCD 
#    no focal reducer, so at native f/10 
# Each exposure 60 or 120 seconds long; tabulated times are midexposure 
#    and accurate only to +/- 1 second. 
# 'mag' is a differential magnitude based on ensemble photometry 
#    which has been shifted so star GSC 599-628 has mag=14.034  
#    (matching Henden's calibration of the star to V=14.034)  
# 
# UT day      JD-2,450,000  HJD-2,450,000   mag 
Oct06.03057      3284.53057   3284.53627  15.801 
Oct06.03149      3284.53149   3284.53719  15.857 
Oct06.03243      3284.53243   3284.53813  16.049 

• download the entire table (plain ASCII)

Last modified 10/17/2004 by MWR.