Observing Fast-Moving Asteroids
Bruce L. Gary, Last Updated 2015.01.30

This web page is meant to provide observing and processing tips for observing fast-moving asteroids, e.g., 2004 BL86.
Suggested processing procedure is at http://brucegary.net/BL86/calibration.html 

Current Observing Status


2015.02.03, 03:50 UT:  B.Gary observed 5.5 hrs on Feb 02, UT (2 rotations), and obtained a phase-folded LC that matches the other ones. This means that BL86 is not a "tumbler."

2015.01.31, 02:00 UT:  P. Benni's Jan 29 data has residuals off the 2.608-hr rotation model that exhibit a 3.08-hr variation, with semisamplitude 5.5 mmag, as shown by the graph below.  THis might be due to the rotation of the smaller of the binary components. So far we haven't seen any "mutual events" (transits, eclipses or shadow fadings), so apparently the binary orbit spin axis wasn't oriented in a favorable way for such events to be seen from Earth at our observing times.


2015.01.30, 22:20 UT:  P. Benni's Jan 29 R-band data, a continuous run lasting 10.7 hours, provided an excellent solution for rotation period. I used it in combination with my Jan 28 data (3.6 hrs) to refine the rotation period even further. Below are both data sets plotted as a phase-folded LC. Rotation P = 2.608 0.005 hours.  A slight difference in the phase-folded LC shape can be expected due to the different observing geometry, being one day apart.




2015.01.30, 04:50 UT:  B. Gary reports a rotation period of 3.8 hours, based on one night's 3.5 hr observing session. The light curve is complicated, as the graph below shows. This result has to be tentative since there is no phase overlap; i.e., the observing session is shorter than the solution rotation period. We need a longer data set to be more certain about the period. We have plenty of long data sets, but it will just take time to process them. (Incidentally, I can produce a visible albedo spectrum, from ~ 450 to 850 nm, for any of the times in the graph, below, because this LC was produced from the zero-order images taken with a SA-100 transmission grating.)

2015.01.29, 18:00 UT:  P. Benni obtained 10+ hours last night (Jan 29, UT). I've revised my description of how to process FOV image segments to produce a long, combined LC (see link at top of this web page, in the abstract section). Below is a pic of the Benni observatory for these observations; I admire anyone with this level of dedication!

The Paul Benni Observatory, as configured for time-critical observations in the Massachusetts winter.

2015.01.29, 00:20 UT: P. Benni is observing. hoping to see a mutual event and at least confirm rotation period.

2015.01.28, 23:40 UT: The data coffers are FULL, so no more observations are needed. Last night J. Garlitz got 6 hours, T. Kaye got ~ 5 hrs &, I got ~ 4 hrs (of SA-100 observations). Here's an updated plot of albedo & geometric albedo, and another plot comparing geometric albedos of BL86 and Vesta.


 

2015.01.27, 21:20 UT: Radar images have been reported to show a diameter of 325 meters for the primary and 70 meters for the secondary. The solid angles are in a ratio of 22:1, which means any mutual events are going to be no greater than ~ 4.6% (50 mmag).

2015.01.27, 21:20 UT: B. Gary has processed some of last night's SA-100 transmission grating observations. Only 1 of 2 BL86 sequences, and 1 of 3 solar analog star sequences, have been processed, so this is a preliminary result. Nevertheless, the overall conclusions are solid: 1) the albedo spectrum exhibits a 0.9 micron (Band I) absorption feature, caused by olivine and pyroxene, and 2) geometric albedo is high, at ~ 22% at the highest albedo wavelength (750 nm). These two aspects of BL86, which are rare among asteroids, resemble Vesta! 


2015.01.27, 21:15 UT: J. Garlitz reports observations (12") from 04.2 to 10.3 UT with a G filter (as in RGB) and clear filter.

2015.01.27, 17:40 UT: No more accurately calibrated images are needed since we have sufficient "phase curve" coverage. The only remaining task is monitoring for "mutual events." Just produce a LC for each FOV & send to me. We're looking for brief fades of very small depth, so observing unfiltered, or with any preferred filter, is OK. T. Kaye and I will observe some more with our SA-100 transmission gratings to get visible spectrum confirmation of the interesting result that V. Reddy got from IRTF observations (more on that later).

2015.01.27, 17:40 UT: J. Gregorio got 11 FOVs from 23.0 UT to 01.7 UT using V-band & 4-sec exposures.

2015.01.27, 09:50 UT: B. Gary & T. Kaye are still observing. I have 3.0 hours worth of g'r'i' images, and a visible spectrum image set using the SA-100 transmission grating. The SA-100 observations consisted of a solar analog (SA) star near zenith, BL86 for 20 minutes, another SA star, another BL86 20-minutes and a third SA star. These spectra should be capable of producing a solution for albedo vs wavelength, from 400 nm to 1000 nm, and will show a 920 nm absorption feature (Band I) if it is present. T. Kaye also obtained some SA-100 observations with a 32" telescope. We can be assured of having a wide range of phase angle data for V-band phase function and a visible spectrum.

2015.01.27, 09:30 UT: Y. Ogmen obtained 4.5 hours of V-band observations with his 14" telescope in Cyprus.


2015.01.27, 03:00 UT: V. Reddy reported results of an IR spectrum that he took last night with the NASA IRTF. The spectrum covering 0.6 to 2.6 microns and shows something very interesting, and unusual.

2015.01.27, 02:00 UT: B. Gary & T. Kaye are running. BG getting g'r'i' now (EL = 12 deg). SA-100 visible spectra when EL is higher.

2015.01.26, 22:50 UT: B. Gary has processed 1/2 hr of r' images and created the following rotation light curve (from 3 FOVs). The fitted period of 1.8 hrs can be misleading, since LC variations are rarely a 1-component sine wave. The consensus rotation period is now ~ 2.6 hrs (based on S. America observations).  The g'-band LC looks the same. The i'-band data is only partially processed, but so far it's consistent with the other two bands. J. Garlitz processed his 1-hr of V-band images to produce a LC, and a small variation may be present (but data is noisy due to low elevation).

2015.01.26, 19:15 UT: V. Reddy passed along the following radar image, showing that BL86 is a binary. It's either from JPL's Goldstone Tracking Station (thanks to Lance Benner).

2015.01.26, 18:30 UT: Y. Ogmen started observing (~18:20 UT). Clear but dew prone.

2015.01.26, 17:00 UT: J. Gregorio is starting to observe w V filter, 4-sec exposures. Clear now, but clouds coming. V. Reddy got IRTF images at ~12 UT.

2015.01.26, 09:20 UT: J. Garlitz (Oregon, 12") obtained some images, but EL was low & seeing was poor. K. Schindler (San Francisco, 135mm EFL & FLI CCD) obtained some images during a clearng and created an animation.

2015.01.26, 08:37 UT: B.Gary & T.Kaye (Southern Arizona) obs'd for a few hours, but there was only one clearing, lasting ~ 1/2 hr. During the clearing image quality was good enough for deriving g'r'i' mag's (& estimating V-mag) for phase angle of ~ 55 deg. BL86 was very close to the JPL Horizons & MPC listings. Persistent clouds, plus satellite IR images of clouds, led to decision to shutdown of both observatories at 08:30 UT. Below is a 20x23 'arc crop of 4 images averaged, taken at 07 UT, with the asteroid showing as a 4-dot sequence moving northward.


4 images averaged, then cropped to 20x23 'arc, northeast upper-left.

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Introduction

There are 5 section on this web page: 2004 BL86 Overview, Observations Needed, Processing, Analysis and Interpretation. For now the only ones to consider are the first two. The others will be important after the several-day observations have been completed. Here's a hint of what's on this web page: 1) BL86 isn't observable for mid-latitude northern hemisphere observers until Jan 26.2 UT (i.e., Sunday night at 10 PM, Arizona local time), 2) among your various telescope configurations choose the one with the largest FOV, 3) use a V-band filter (R-band, or even Clear will be acceptable), 3) adopt an exposure time that keeps the asteroid at the safest maximum counts (i.e., ~ 40,000 for sharp images, probably < 5 seconds), 4) be prepared to change FOV location often because BL86 is moving fast for the first few days.

2004 BL86 Overview

BL86, as I will refer to it, is fast-moving and bright. It can be observed by amateurs for over a week, but there are ~ 4 days where phase angle changes make observing most important. These are UT dates Jan 26 to Jan 29. I will refer to observing dates as Day#1 = Jan 25/26, etc. Here's an overview graph of magnitude and declination (DE):


Figure 1. Magnitude and declination for Jan 24 to 29 (UT dates).

For an observer at latitude of +30 deg, for example, where the southern-most observable object is at DE = -40 deg (EL = 20 deg), BL86 can't be observed until Jan 26.2.

The next graph is another overview, showing phase angle and rate of motion versus date.


Figure 2. Phase angle and rate of motion for Jan 25 to 29. 

Since northern hemisphere observers won't be able to begin observations until ~ Jan 26.2, we will be limited to a phase angle coverage of ~ 48 degrees to 2 degrees. Those all-important small phase angle observations, e.g., under 10 degrees, will be confined to Jan 27.00 to 27.45 UT.  European and USA observers are well-placed for these observations. (USA observers: that's Monday night, from 5 PM MST to 4 AM MST).

Each observer might want to check with the JPL Horizons web site before each observing session to get the latest ephemeris: http://ssd.jpl.nasa.gov/horizons.cgi#top. Here's an example of information that will be available (depending on your settings):


Figure 3. Example of asteroid info that can be obtained from the JPL Horizons web site.

You can download a spreadsheet containing this info for the interval Jan 24 to Jan 29 at: link.  Since the uncertainty on RA/DE is small the above spreadsheet should be adequate for this project.

I suggest using TheSkyX for planning a night's observations, as well as real-time guidance. For example, I used it to create the following observing schedule for my site:


Figure 4. Basic info for the first 4 days of BL86 observations, for my Arizona observing site.

Observers in Europe and the USA will be measuring magnitude for observing sessions that are as long as possible. It will be exhausting staying awake all night in order to change FOV to keep the asteroid track within the FOV, so surrendering to sleep before dawn is expected. Two of us will be measuring the spectrum at low resolution in the visible (Tom Kaye and B. Gary). Dr. Vishnu Reddy will be obtaining spectra in the infrared (1 to 3 micron) using the NASA 2-meter IRTF on Mauna Kea on one night. We will compare all data with rotation phase since it is possible that the spectrum at visible and IR wavelengths will vary with rotation. 

I will of course be available for questions and suggestions. My e-mail is B L G A R Y at umich dot edu.

Observations Needed

Exposure Time

The goal is to obtain a set of images for an observing session in which the asteroid is not "smeared" into a long trail and for which a circular photometry aperture yields SNR > ~5.

Suppose the rate of motion is 2.0 "arc/second (which equates to 2.0 'arc/minute and 2.0 degree/hour). If your PSF is 4 "arc, then a PSF crossing time is 2 seconds. A good "rule of thumb" is to keep exposure time to no more than the PSF crossing time, which for this example would be 2 seconds.

FOV Changes

It's good to have a big field-of-view (FOV). For example, if your FOV is 15 'arc square, and rate of motion is 2.0 'arc/minute, then a 12 'arc track occurs in 6 minutes. There's a big advantage in having a large FOV; my prime focus (HyperStar on 14-inch Meade) has FOV = 48 x 71 'arc, so a north/south movement of 40 'arc, for example, occurs in 20 minutes for this rate of motion example.

Filter Choice

A requirement for doing anything with a fast-moving asteroid image set is to achieve calibration of all asteroid brightness measurements. This requires use of background stars for calibration, and in any image there will be more V-mag's to work with than for any other band. Therefore, if SNR for short exposures is sufficient when using a V filter, then use it. Second choice is R-band (i.e., Rc-band, or pretty picture R band). Third choice is unfiltered. It's possible to convert any of these magnitude measurements to just one band, and it required knowledge of the asteroid's color (or spectrum), which I'll measure. Therefore, choose any of those filters (V, Rc, R or clear).

You'll know how to estimate SNR for any filter with your telescope system, but here's a crude guide that I use: SNR = 3 (2.512^(LM - mag)), where LM is limiting magnitude (see graph, below) and mag is the asteroid's magnitude. Consider the example of a 14" telescope, 2-second exposures, FWHM = 4 "arc and asteroid mag = 10.0: LM ~ 19.2 for 60-sec exposures, so for 2-sec exposures LM ~ 19.2 + 1.25 Log (2/60) = 17.3; therefore, SNR = 3 (2.512^(17.3 - 10.0)) = 2500. This example illustrates that there will be plenty of SNR for BL86 on Jan 26/27.  A V-band filter has a "throughput" of ~ 23%, which requires SNR to be reduced by the ratio 0.23 using the above equations (i.e., SNR = 570).


Figure 5. Limiting magnitude for 60-second exposures, unfiltered vs. telescope aperture and seeing.

Guiding

When exposure times as as short as a few seconds, which will be the case for fast-moving asteroids, auto-guiding won't be necessary. Hence, in all images there won't be any "smearing;" all stars and the asteroid should be "sharp." This is helpful for the processing phase.

Processing

There are two choices for processing: 1) send images to me for processing (using a dropbox), or 2) process the images to the stage of having calibrated magnitudes for the asteroid for each image of an observing session, and send that to me.

In order to compare V-mag observations from different observers it will be important to apply "CCD transformations" to photometry readings. I know that sounds like a lot of work, but forget about the classical way of doing this, described everywhere (and at the AAVSO web site). There's a  much simpler method for doing this, too often neglected by old-fashioned purists. It's a simple procedure: 1) apply a "first magnitude offset parameter" to all asteroid measured mag's so that they are approximately zero, 2) for each reference star in the FOV being used for calibration, plot "measured (instrument) magnitude minus true magnitude" versus the star's color (your choice, e.g., B-V, or g'-r', etc), 3) create a curve (or straight line, or whatever shape mimics the reference star plot), and adjust it vertically, using a "second magnitude offset parameter" until the curve crosses zero at the asteroid's color, 4) adjust the "first magnitude offset parameter" until the reference star data fit the curve that you decided represents their variation with star color. True mag's refer to the AAVSO APASS BVg'r'i' set, and can be obtained from the UCAC4 catalog (or the AAVSO web site, or the Vizier web site).

If you really want to get good quality calibrated mag's for the asteroid, maybe you have the patience for the procedure I use. Otherwise, use whatever CCD transformation procedure you like to convert your instrumental mag's to calibrated V-band mag's.


Analysis

I'll perform most of the "analysis." This will consist of combining data sets from different observers, and putting them on the same magnitude scale. I'll construct a plot of "V-mag vs. phase angle" and begin analyzing it for determining albedo and size (the slope of the phase function can be converted to albedo, and this can be used to estimate size). More description later.


Interpretation

Dr. Vishnu Reddy will perform the interpretation, possibly with the collaboration of Dr. Jian-Yang Li. Our job is to GET THE DATA!

BL86 Observer Team

Yenal Ogmen, Cyprus, 14"
Joao Gregorio, Portugal, 12"
Paul Benni, Massachussets, 11" & telephoto
Tom Kaye, Arizona, 32" & 5"
Bruce Gary, Arizona, 14"
Vishnu Reddy, Arizona, 2-meter IRTF (Mauna Kea) & 14"
Joe Garlitz, Oregon, 12"
Karsten Schindler, California, telephoto lens with FLI CCD


Location of observers


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