Astrometry 101 - Timing is the most critical parameter

The time associated with a precise astrometric position of a moving object in an image such as an asteroid or comet is the most critical observer controlled parameter. The position of the object can be derived very accurately from the relative positions of several dozen reference stars. But the time associated with that position is absolutely necessary to achieve small residuals less then 1 arc second. Timing errors of are probably the most serious type of astrometric error.

The faster a object is moving in the sky, the more critical this becomes.

Timing is of little importance for non-moving objects or objects whose image properties are not changing quickly with time, such as a supernova or a variable star or almost any other stellar object. Quasars would be an exception to that.

The accuracy required to obtain low residuals of comet or asteroid observations requires that the image capture program perform this time measurement automatically. The time measurement should be stored in the image header, so that it will not be altered by the image calibration process. The date-time-stamp given to an image file when it is saved by the computer operating system is not good enough for precise astrometry of moving objects.

SBIG cameras store the time of the start of image exposure and exposure duration in the header of either SBIG formatted images or fits images. It is intentionally set up so as to make modification of that difficult or tedious. Generally it should never require manual edit.

These are the lines containing keyword-value pairs in a SBIG image header which record date, time, and duration of exposure.


Exposure = 36000
Date = 12/28/09
Time = 11:23:44

Duration of exposure is in hundredths of a second. 36000 hundredths of a second.= 360.00 seconds
Note the precision.
The time of mid-exposure is then calculated by an astrometric software program.  In this case 180.00 seconds is added to the start time of 11:23:44 or 11:26:44 on 12/28/09 UT. The time of mid-exposure is the official time of an observation. It is this time along with the position that is submitted to the Minor Planet Center. The date and time is formatted as YYYY MM.DD.FFFFF where YYYY is 4 digit calendar year, MM is month o year, January is 01 etc., DD is day of month, and FFFFF is fraction of the day. eg: 2009 12 28.47690

There are many ways to synchronize a computer operating system with a time standard such as a NIST time server. For most astrometry of comets and asteroids, a program such as Dimension 4 set to update periodically will do the job. For very fast moving objects, the timing of the shutter on the camera can affect the astrometry. That would be a  more advanced topic.

For example, consider an object moving at 0.01 arc sec/second, a typical value for many bright comets and asteroids. A 5 second time error will then cause a  0.05 arc sec residual. For an object moving at 0.1 arc/second, a 5 second time error will then cause a  0.5 arc sec residual (too much). On the other extreme, for example Pluto, a slow mover, moving at 0.0005 arc/sec, a 5 second time error will then cause a  0.0025 arc sec residual which is probably negligible.

An observer should be very familiar with their image scale (or pixel resolution) and the time it takes for an object to travel one pixel in distance. One should understand quantitatively what the limits of your timing system and image acquisition are. Astrometry has no room for sloppiness. Precision time is essential.

Dimension4 software is freely available from the Thinkman software web site at:
 http://www.thinkman.com/

The synchronization history can tell you how far off your operating system gets off during times that the computer is off as well as how it varies with your computer usage. Performing CPU intensive tasks may be affecting time synchronization.

Astrometrica is a research grade shareware program astrometry:
http://www.astrometrica.at/

Guide to Minor Body Astrometry: http://www.cfa.harvard.edu/iau/info/Astrometry.html


Pixel scale in arcseconds = (206.265) * (pixel size in microns) / (focal length in mm)

Pixel time = pixel scale / target rate

If pixel scale is 2.5 arc sec per pixel, and an object is moving at 0.01 arc sec/sec, then it takes

2.5 /0.01 seconds or 250 seconds to move one pixel.

If pixel scale is 0.5 arc sec per pixel (very small), and an object is moving at 0.01 arc sec/sec, then it takes

0.5 /0.01 seconds or 50 seconds to move one pixel.

If pixel scale is 2.5 arc sec per pixel, and an object is moving at 0.10 arc sec/sec, then it takes

2.5 /0.1 seconds or 25 seconds to move one pixel.

If pixel scale is 0.5 arc sec per pixel (very small), and an object is moving at 0.1 arc sec/sec, then it takes

0.5 /0.01 seconds or 5 seconds to move one pixel.

From these examples, one can deduce that the smaller the pixel scale, the less time it takes to travel one pixel. This also means that the smaller the pixel scale, the less time you have for your CCD chip to integrate the light from the target to reach any given magnitude. The shorter your exposure limits, the more critical time becomes.

RESIDUALS STATISTICS FOR OBSERVATORY CODES: 

http://www.cfa.harvard.edu/iau/special/residuals.txt

The IAU MPC provides guidance on how to get an accurate time:
http://www.cfa.harvard.edu//iau/info/Astrometry.html#time

Guide to Minor Body Astrometry has been updated

The Guide to Minor Body Astrometry was updated December 11, 2009 for further clarification:

• on the use of "K" for astrometry obtained from stacked images
• on the number of observations to make per night
• on the time span for a night's observations of an object new or followup
• submission of single positions

http://www.cfa.harvard.edu/iau/info/Astrometry.html



Special thanks to Peter Birtwhistle, J95 for alerting us all to this change.



Alphabetic notes for observations can be found here:


http://www.cfa.harvard.edu/iau/info/ObsNote.html


Format For Optical Astrometric Observations Of Comets, Minor Planets and Natural Satellites
http://www.cfa.harvard.edu//iau/info/OpticalObs.html



Examples of valid comet and minor-planet observations:
http://www.cfa.harvard.edu//iau/info/ObsExamples.html

General Observing Practices: Minor Planets and Comets

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EDITORIAL NOTICE.

General Observing Practices

The number of observers submitting astrometric observations to the MPC has risen rather dramatically in the past year. This has been accompanied by a rather worrying and troublesome increase in poor observing practice, with many sub-standard quality observations reported to the MPC.

Observers should strive to provide the best quality observations to the MPC. Poor quality observations cause the MPC significant extra work and reflect badly on the observer.

Some good practice advice follows:

• Observe each object at least three times over the course of an hour or so on each night. If the object is a known object, this can be relaxed to 30 minutes or more, as long as the motion of the object in that period is significant.
• Provide two nights of observation for "new" objects, obtaining three to six observations on each night, with at least one hour of coverage on each night.
• If you have a suspected new NEO, more than six observations may be useful if they are obtained over the course of several hours.
• In following-up interesting objects, provide good coverage of at least one hour.
• Never, under any circumstance, provide a single, isolated observation on a single night. A single observation shows no evidence of motion and there is no guarantee that the observer has not measured an image defect, a star or a variable object (star, nova or supernova).
• Stacked observations should always be marked as such and the individual images should be stacked so as to provide two observations, noting that an individual image can appear in only one stack. In very rare cases, a single stack may be all that is available: such situations will be handled on a case-by-case basis.
• Observations of "new" objects in support of discovery claims should be spaced by at least one and no more than five nights.

It is hoped that self-regulation by observers will be sufficient. If this does not prove to be the case by the end of this month, we will implement additional filters to reject automatically entire batches that contain single observations or new objects with insufficient nightly coverage.

"Corrected" Observations

Observers are informed that batches submitted with "corrected", "correction", "remeasured" or "remeasurement" in the subject line or ACK line are treated as being corrections to observations published previously and are filed by the automated routines for manual examination by MPC staff. The processing of such batches may be delayed.

It is also worth remarking that resubmission
of observations or batches that were rejected by the automated AUTOACK routines do not need to be indicated as resubmissions, as the MPC has no internal record of the original, rejected batch.

Observations of Dual-Status Objects

A number of objects are designated as both minor planets and comets. Examples include (2060) Chiron = 95P/Chiron and (4015) Wilson-Harrington = 107P/Wilson-Harrington. Astrometry of dual-status objects must be reported under the minor-planet designation, with the magnitudes reported in the asteroidal form. If observations are reported under the comet designation the AUTOACK routines will change the designation into the minor-planet designation. If there are "nuclear" or "total" magnitudes reported on the observations this causes problems further down the processing pipeline because minor planets cannot be marked with "N" or "T" magnitudes.

Observing at Remote Sites:

Observers who use multiple remote observing sites are requested to be extra vigilant in indicating where the observations were made. A number of observations have been received recently when, at the time of observation, the object was below or the sun was above the local horizon at the observing
site.

Indication of Observers, Measurers and Telescope Details

In anticipation of the short-term plans for automatic MPEC preparation by the MPC, we remind observers that information given with the OBS, MEA and TEL keywords in the observational header must conform to the formats described at http://www.cfa.harvard.edu/iau/info/ObsDetails.html. Observers who do not adhere exactly to these instructions will find that their observations on automatically-prepared MPECs will not be credited in the way they intended.
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Reference:

EDITORIAL NOTICE from 2009 DEC. 2 MPCs batch

Distant Minor Planets brighter than Magnitude 18 for December 2009

2009 WP104             RA 02 45 05.6 DEC +18 40 53 Mag 17.6 V
(136472) Makemake RA 12 37 06.8 DEC +27 31 12 Mag 17.0 V
(136108) Haumea     RA 13 44 56.1 DEC +18 12 55 Mag 17.4 V

From Twitter: @plutokiller: Looks like 2010 will be the year of Makemake



Photograph of Makemake taken by the Hubble Space Telescope on November 20, 2006.

From Twitter: @kpheider: Imaged Makemake on Thanksgiving November 26 2009 at 11:45 UT



Image of Dwarf planet Makemake by Kevin Heider taken at 2009-11-26 11:45 UT.
10 minute exposure of using 24" telescope LB-001 at LightBuckets in Rodeo, NM. Makemake is about apparent magnitude 16.9 in this image. Glaxy IC 3587 is visible above Makemake.

References:

Mike Brown on Twitter: http://twitter.com/plutokiller

Kevin Heider on Twitter: http://twitter.com/kpheider

Minor Planet and Comet Ephemeris Service: http://www.cfa.harvard.edu/iau/MPEph/MPEph.html

MPEC 2009-X06 : DISTANT MINOR PLANETS (2009 DEC. 15.0 TT): http://www.cfa.harvard.edu/mpec/K09/K09X06.html

File:Makemake hubble.png
http://commons.wikimedia.org/wiki/File:Makemake_hubble.png

Wikimedia Commons File:Makemake-LB1-2009Nov26-11UT.jpg: http://commons.wikimedia.org/wiki/File:Makemake-LB1-2009Nov26-11UT.jpg