Last updated 2008 December 20
Astrometry - Obtaining an Observatory Code
1.0 Why ?
The Minor Planet Centre needs to be satisfied as to the accuracy of one’s observations before they will accept data relating to the positions of new or newly discovered asteroids.
2.0
How ?
2.1 Current observation Format
Their website, under ‘Guide to Minor-Body Astrometry’ carries details of how to submit observations of known asteroids for verification. To help would be astrometrists my own (successful) attempt is described in section 4 below.
All too frequently the MPC receive error-filled submissions of astrometry. Here is a little "how-to", created by Tim Spahr, describing what common pitfalls can be avoided and how to improve submissions to the MPC.
2.2 New Observation Format
‘A New Observation Format For A New Millennium’ issued by
the
According to Section 8 of the document the new format has been finalized but has yet to be introduced. Observations in the current format will continue to be acceptable for some, as yet unspecified, time after the new format is introduced.
3.0 Theory
Astrometry is the measurement of the position of a celestial object. Such measurements obtained over a period of time allow the motion of, and in the case of an asteroid its orbit, to be calculated. In the days of astrophotography this was a complicated and time consuming task involving the use of a mechanical plate measuring engine. Nowadays it requires a; calibrated CCD image, an accurate star catalogue (eg; USNO or UCAC), a PC and an astrometry software package (eg; COAA FitsX or AIP4WIN). In outline the astrometry software finds the position of the asteroid by;
(a) Generating a list of reference stars covering the area of the image
(b) Overlaying and aligning a map of those reference stars on top of the stars in the image
(c) Generating RA and Declination coordinates for the asteroid
The accuracy of the position so determined will, of course,
only be as good as the accuracy of the reference stars used. The Hubble GSC,
for example, does allow positions to be determined to the accuracy required by
the
4.0 An example
4.1 Target selection
Towards the end of 2000 I carried out the necessary observations in order to obtain an observatory code for my home observatory. As suggested on the MPC website I chose a number (six) of asteroids with numbers in the range 400 – 3000. Using Megastar I chose a selection that were on or to the east of the meridian. This ensured that they would be visible for a number of nights and thus avoid having to restart the exercise should I experience a period of cloudy skies. Asteroids chosen were;
3259 Brownlee Discovered by J Platt at Palomar in 1984
403 Cyane Discovered by A Charlois at Nice in 1895
1040 Klumpkea Discovered by B Jekhovsky at Heidelberg in 1925
1719 Jens Discovered by K Reinmuth at Heidelberg in 1950
1468 Zomba Discovered by C Jackson at
591 Irmgard Discovered by A Kopff at
(Discovery details from ‘Dictionary of Minor Planet Names’)
Megastar chart showing positions of asteroids chosen – the meridian is at approximately RA 21 h 30 m.
I chose the brighter asteroids thus allowing fairly short exposure times. Any tracking errors are therefore negligible.
4.2
Equipment
The method described here involved the use of;
- Orion 10” Newtonian Reflector on a Vixen GP German Equatorial Mount with a hand controller
- Starlight Xpress MX516 CCD camera
- Light tight cover for telescope tube (for dark frames)
- PIX_MX5 imaging software
- Guide star charting software plus the USNO-SA1.0 CD
- COAA FitsX software (for astrometry)
Your set up will most likely be very different but will need to include the above elements or the equivalent thereof.
4.3
Observations
The six asteroids were each observed over a period of one hour on two separate nights. It took approximately three weeks to complete all the observations. Having aligned the telescope on the target asteroid a typical observation run proceeded as follows;
(a) Set the PC to the correct time from the Speaking Clock (Please note that, now I know better, I use a GPS receiver connected to the laptop via a USB port. If you have an internet connection handy then a clock sync program can be downloaded).
(b) Enter the asteroid name and number into the FITS header.
(c) Set the Starlight Xpress MX5 software to; Continuous, 5 min delay between exposures, Autosave image as a FITS file. This allows all the image data to be saved including the date and time of the exposure. The MPC requires that the mid-points of the exposure times are submitted so check that your imaging software is assigning this time rather than the start time of the exposure for example. If this is the case then the correct time will need to be inserted in the submission e-mail.
(d) Obtain a trial image and verify the asteroid is clearly visible (don’t bother with image calibration for this).
(e) Image for a period of one hour (after the first few images were obtained I checked to ensure that they had
been saved correctly – it is also worth doing this every fifteen minutes or so to ensure all is well).
(f) Obtain dark frames (same exposure as images).
If you are using a software package such as Astrometrica which automatically detects and aligns reference stars it will be necessary to obtain dark frames and flat fields and calibrate each image to avoid multiple spurious detections. At the time of this exercise I was using COAA FitsX where the alignment of reference stars is manually performed and therefore the problem is avoided but I did obtain dark frames to clean up the images.
To save time, if you have a GO-TO feature on your telescope, you can image one asteroid, switch to the next, image it and so on. In this way you may be able to complete the whole exercise on fewer evenings (remember that each asteroid must be observed on two evenings/nights)
4.4 Image processing
(a) From the dozen or so images obtained for an asteroid during each of the two evenings it was observed I selected three images taken roughly fifteen minutes apart.
(b) Each image was processed by subtracting the dark frame and then stretching as required to make the asteroid and surrounding stars clearly visible.
(c) The image (see below)was then saved with ‘IP’ (Image Processed) added to its original file name assigned by Pix_M5.
The position of the asteroid was measured using the astrometry feature in COAA FitsX. This uses an overlay of reference stars created by Guide star charting software from the Hubble Guide Star Catalogue (see below and the comment about accuracy in 3.0 above).
(d)
The results were recorded for later inclusion in the
email to be sent to the
|
Image number |
Date |
Time |
Formatted date/time |
RA |
Dec |
|
IMG104IP |
16 Oct 2000 |
19:22:40 |
2000 10 16.807407 |
22 47 04.12 |
+05 00 12.2 |
|
IMG110IP |
16 Oct 2000 |
19:52:40 |
2000 10 16.828356 |
22 47 03.71 |
+05 00 03.9 |
|
IMG115IP |
16 Oct 2000 |
20:15:26 |
2000 10 16.844051 |
22 47 03.35 |
+04 59 57.9 |
< ASTEROID
403
Image IMG110IP and Guide star
chart
(Partial) List of reference stars
GSC # RA (J2000) dec mag
571 1103 22h46m47.457s +04 56.3723' 9.12
571 837 22h46m35.049s +04 57.7980' 13.50
571 837 22h46m35.028s +04 57.7902' 13.06
571 885 22h47m07.718s +04 58.4802' 14.09
571 885 22h47m07.752s +04 58.4874' 14.56
571 889 22h46m41.560s +04 58.6278' 14.22
571 889 22h46m41.596s +04 58.6338' 14.54
571 952 22h46m44.544s +04 58.7970' 11.61
571 952 22h46m44.522s +04 58.7928' 11.25
571 977 22h46m33.484s +04 57.7086' 13.75
571 977 22h46m33.456s +04 57.7050' 13.32
571 1015 22h47m34.965s +04 56.6862' 13.76
4.5 Verification of results
It is wise to check ones results prior to submission.
Megastar or Guide or an equivalent can give you a very good idea of where the
asteroid should be provided that recently downloaded orbital elements are used.
Alternatively one can go to the ‘Orbits
Diagrams’ page on the ‘JPL NEO
Program’ web site and obtain an ephemeris (the site covers all asteroids,
not just NEO’s).
4.6 Submission of results to the
4.6.1
Submission
data
My results were submitted in the following format. The MPC requirements are very specific and must be adhered to. They are explained in detail in the ‘MPC Submission Information’ web pages. The observations must be submitted as ASCII files – these can be generated using Microsoft Notepad for example.
---------------------------------------------------------------------------------------------------------
COD XXX
CON R. Dymock, 67 Haslar Crescent, Waterlooville, Hampshire, England, PO7 6DD
CON [roger@dymock36.freeserve.co.uk]
OBS R. Dymock
MEA R. Dymock
TEL 0.25m f/6.4 reflector plus CCD
NET GSC
ACK Minor Planet Astrometry
00403 gC2000 10 16.80741 22 47 04.12 +05 00 12.2 XXX
00403 gC2000 10 16.82836 22 47 03.71 +05 00 03.9 XXX
00403 gC2000 10 16 84405 22 47 03.35 +04 59 57.9 XXX
and etc
--------------------------------------------------------------------------------------
The first time observations are reported additional
information is required by the
(a) Postal (Snail-mail) address.
(b) Observatory name and site.
(c) Observatory position – longitude, latitude and height above sea level
(d) Details of telescope set-up
This
information may be submitted using the COM prefix on each line. Further details
may be found on the
MPC website under ‘Guide to Minor Body
Astrometry, Section 10, The observation
format requires an observatory code. I don't have a code. How do I get one?’
4.6.2
Explanation
of codes and format
The codes and format are explained in the following MPC web pages;
Format For Optical Astrometric Observations Of Comets and Minor Planets
Packed Provisional Designations
Publishable Notes For Observations Of Minor Planets
New- And Old-Style Minor Planet Designations
In brief;
COD Observatory code (not yet assigned at this time)
COM Observatory details
CON Contact details
OBS Observer
MEA Measurer
TEL Telescope details
NET Catalogue used
ACK Enables the MPC to automatically acknowledge receipt of observations
Each subsequent line describes one observation and includes; object designation, notes, date, time, RA, declination and XXX for an unassigned observatory code
4.7
And finally…
A few days later I received a short email to the effect ‘Your site is now observatory code 940’ (plus a slap on the wrist for submitting the observations in the wrong format !).
5.0
Conclusion
It really isn’t too difficult – all you need are a few clear nights to obtain the images and the time and patience to process them.