Near Earth Object Rotation Light Curves and
Magnitudes
Observations
by Bruce L. Gary, Hereford, AZ
Summary of RLC Results
NEO # & name
Dates w/ Observations
5693 93EA
9525, 9528, 9529, 9601, 9603, 9604
138883 00YL29
9501,9505,9506,9508
5011 Ptah
9423, 9422, 9421,
9420, 9419
010416 Kottler
8b20, 8b21, 8c28, 8c29, 8c30, 8c31, 9101, 9102, 9108, 9110,
9111, 9112, 9113
2006SZ217
8c11
1620 Geographos 8b04
2335 James
8b15
162900
8b18,
8b17
Introduction
This web page is meant to record a series of observations of NEOs with
the goal of establishing their rotation light curves (RLC) and
r'-magnitudes. The list of NEO candidates is provided by Brian
Skiff, of the Lowell Observatory.
Links on & from this Web Page
Telescope calibration (HAO example)
Observing & image analysis
procedures
Sample RLC Result
Summary of Results
Hardware, Observing & Image
Analysis Procedures
The entire procedure for obtaining RLCs can be thought
of as three parts: observing, image analysis and data analysis.
Hardware. The telescope is fork-mounted on an equatorial
wedge (no meridian flips for me!). In order to reduce image rotation
during an observing session the telescope's polar axis has been adjusted
with an accuracy of ~2 'arc. MaxIm DL (v 4.62, later 5.03) is used to control
the telescope, wireless Craycroft style focuser, image stabilizer (SBIG
AO-7) and CCD camera (SBIG ST-8XE). A tip-tilt image stabilization mirror
(A SBIG AO-7) is used to keep the star field fixed to the CCD's pixel field.
Master Flat. At least 20 flat field images are made starting
shortly after sunset. Exposure times are adjusted manually to keep the
maximum counts within the range 40,000 to 50,000. Each flat field exposure
is calibrated using a dark frame exposure with the same exposure time.
I stop taking flats when exposure times exceed ~20 seconds. Only those
flat field frames with exposure times greater than 1 second are used in
producing a master flat for that night. I use median combine with level adjustment,
and sometimes and also use the average flat (provided I don't see artifacts
in any of them). Before starting the flat field exposures I set the CCD
cooler to about half way between ambient and what I expect to achieve for
asteroid observations. I also adjust the focus to what I expect would be
the correct setting for the telescope's temperature (based on previous
nights of focus versus temperature calibrations). I adhere to the rule
"Every night must have it's own set of flat fields!"
Master Dark. I'm more relaxed about using a previous night's
master dark frame than a master flat frame. However, I try to use a master
dark that was made using the same CCD temperature setting as will be
used for the asteroid observations, and I also require that the exposure
times be approximately the same. Calibration of asteroid images employ
aut-scaling to adjust for both.
Master Bias. I use a master bias frame that is within
a few weeks old. It is made from ~20 bias images.
Observing Procedure. Asteroid observations are
started ~55 minutes after sunset. This corresponds approximately to
"nautical twilight." The CCD's FOV is chosen so that a bright star is
within the autoguider's FOV ("bright" means 12th mag). When mirror movements
exceed 10% of its range of motion the observing program nudges the telescope
drive motors. Exposure times are typically 100 seconds, which is short
enough to assure that asteroid motion during the exposure is much smaller
than typical PSF FWHM (3.0 to 5.0 "arc for 100-sec exposures). Typically
only a few stars are saturated for this exposure time (my CCD is linear
up to ~52,000 ADU).
Image Analysis. More later...
Calibration. Most of these observations are made with a Celestron
11-inch telescope located inside a "sliding roof observatory" at my
Hereford, AZ site (MPC observatory code G95). Some were made with a Meade
14-inch, but it's controller card failed 2008 December 2. Each telescope
is used unfiltered, but the effective bandpass wavelength is similar
to the r'-band's effective wavelength. But since unfiltered is much broader
than r' it is important to not use stars for reference that differ greatly
in color from that for asteroids. For example, it has been suggested
that since asteroid color is typically J-K = 0.42 (V-R = 0.40, g-r
= 0.57) only stars with a similar color should be used for calibration.
I accept stars with J-K between 0.19 and 0.65 (corresponding to B-V between
~0.38 and 1.05). The Carlsberg Meridian Catalog is used for assigning r'
magnitudes to ~ a dozen reference stars. Here's a plot of the correction
needed to convert apparent r'-mag to true r'mag versus star color for my
Celestron 11-inch telescope.
Example of true r'-mag minus apparent r'-mag versus star color.
The range of colors between the blue ticks are used as a criterion for
accepting a star's correction.
Light Curve Creation. Excel...
Folded Rotation Light Curve. B...
Sample Result - 1620
Geographos
The following RLC is used to
illustrate the results for one NEO. The purpose of this project
is to compile a list of r' magnitudes, periods and variation amplitudes
for NEOs that have not already been observed for this purpose. 1620
Geographos is a well-known, high amplitude RLC NEO, so it serves
here to illustrate what this project endeavors to accomplish in
the context of previous observations. My intent is to conform to the
format given here for all future NEO rotation light curves.
Rotation Light Curve of 1620 Geographos.
A detailed explanation is given in the text. 8b04GBL1 (data file
for download)
The lower panel plots air mass
and "extra losses." The measured flux from a dozen or more nearby
stars is fit by an extinction model and the residual flux is interpreted
as a loss. Contributors to loss could be cirrus clouds, dew or frost
accumulation on the corrector plate, or PSF broadening due to seeing
changes, focus degredation or wind shaking the telescope. Since
a small and fixed photometry aperture is used to process all images
for an observing session changes in FWHM will produce changes in "loss."
The extinction model is fit to the sum of fluxes of all nearby stars.
In addition to the normal extinction term, K [magnitudes/airmass],
a temporal term is available for use, as necessary. In this example there
were negligible losses because the sky was clear, humidity was low,
winds were calm and seeing didn't vary much during the observing session.
The upper panel there are 4 plots. First, a magnitude
from each image employs small orange corsses. Groups of 3 of these
are median combned to produce the large red circle symbols. A running
median combine is shown by a blue trace. Finally, a thin black trace
is a "model fit" that employs the following parameters: average r' magnitude,
rotation period, amplitude of periodicity having period of 1/2 rotation
period (Amp1), amplitude of periodicity with period 1/4 rotation period
(Amp2). In addition, there are 3 parameters related to calibration and
observing conditions: offset, slope [magnitudes/hour] and air mass curvature
[magnitudes/airmass]. Using the model fit it is convenient to calculate
a "period average magnitude" that is unaffected by the observing session
not exactly equaling a rotation period (or unequal spacing of observations).
The information box in the lower part of this panel gives the photometry
aperture radius (in pixels, usually ~1.5 to 2.0 x FWHM), the 2-minute
equivalent RMS of the measurements (in mmag), the percentage of measurements
that were used after rejecting data that exceeded a loss criterion
and neighbor RMS (outlier) criterion, the exposure time for each image,
the group size used in producing the large red circle symbols (median
combine), the slope parameter and the air mass curvature parameter.
All graphical presentations of NEO RLCs will have this format.
Finally, below each graphical RLC there will be a
link for downloading a data file of the measurements. The format
will consist of header lines (object, filter, observer name, etc)
and data columns for JD, magnitude and loss.
WebMaster: B.
Gary. Nothing
on this web page is copyrighted. This site opened: 2008.11.16, Last Update: 2009.06.05