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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 Minor Planet Centre and dated 2001 June 19 describes a new format for submitting astrometric observations of minor planets, comets and natural satellites.

 

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 Minor Planet Centre.

 

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 Johannesburg and L Boyer at Algiers in 1938

591      Irmgard            Discovered by A Kopff at Heidelberg in 1906

 

(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 Minor Planet Centre eg;

 

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 Minor Planet Centre

 

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 [[email protected]]

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 Minor Planet Centre ie;

 

(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;

 

Observational Details

Format For Optical Astrometric Observations Of Comets and Minor Planets

Examples Of Observations

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.

 

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