Near Earth Object Rotation Light Curves and Magnitudes
Observations by Bruce L. Gary, Hereford, AZ

Summary of RLC Results

    NEO # & name     Dates w/ Observations

    2013 XY8                         3C10
    Apophis (2004 MN4)   Many dates
    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


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. GaryNothing on this web page is copyrighted. This site opened:  2008.11.16 Last Update:  2014.01.10