The threat to the earth from collision with an asteroid or comet is well known. The detection of these objects has recently taken great strides with professional programs, such as LINEAR and NEAT, finding them in significant numbers. However the area of follow up astrometry is struggling to keep up with this abundance of new discoveries.

A particular problem relates to the fainter objects – those of about magnitude 19 or greater. The number of amateur observers able to reach these magnitudes declines rapidly and these objects are less well observed. Fainter observations are important for two reasons. First there are many new objects being found at these magnitudes. Second even for brighter objects it is vital to get the longest observation arc possible to give the most accurate orbit. This means observing the object when it has moved away from the earth and is faint.

I have recently started a programme of NEO observation from Church Stretton (observatory code 966) aimed particularly at these fainter objects. Initial results have been encouraging and observations as faint as magnitude 20.7 have been submitted to the minor planet centre.

For these observations I am using an f11 C14 SCT with an aperture of 36cm. The Celestron focal reducer gives a more useful f ratio of 7.8 while maintaining image quality. The C14 is mounted on the Astro-Physics AP1200 mount. This is a fully computer controlled GOTO mount with excellent tracking characteristics. No guiding is necessary for short exposures up to 60s and stars with FWHM of about 2.5 arc seconds can be obtained under average conditions with 2.0 arc second or better under good conditions. The camera used is an SBIG ST7E which gives 765x510 pixels at a pixel scale of 0.67 arc seconds per pixel. The field of view is quite small at 9 x 6 arc minutes.

With modern CCDs magnitude 19 or 20 may not seem particularly challenging. For example the above set up can reach magnitude 20 in less than 10 minutes of exposure. The difficulty with NEOs is the combination of faintness with rapid motion. Once the image of an asteroid or comet begins to trail then further exposure decreases rather than increases the limiting magnitude. The optimal exposure can be calculated by dividing the typical FWHM of a star image by the rate of motion of an object. For star images of 2.5 arc seconds the maximum exposure for a main belt asteroid moving at 0.5 arc seconds per minute would be 5 minutes. A typical NEO rate of motion might be 2.5 arc seconds per minute giving an optimal exposure of 1 minute. However many objects travel faster than this and in some cases the optimal exposure is just a few seconds. Needless to say these fast objects present an extreme challenge.

The way of increasing the integration time is to use a well known technique of stacking many short exposures in such a way that the moving object remains still and the stars trail. I have written a program to do this. First the software co-adds the images normally so that the stars remain fixed and the object trails. A second run then re-adds the images using the same transformations as the first run but with an adjustment to allow for the object motion. A star catalogue can be fitted to the first image whereas the object position is measured from the second image. I use the USNO’s A2.0 catalogue which is available on the internet. This has the advantages of much more accurate star positions than the guide star catalogue and also goes down to magnitude 19 or below so it is usually possible to find sufficient stars even in sparse fields away from the milky way.

Although I wrote my own software I have since learnt that I was partly ‘reinventing the wheel’. The Windows version of the Astrometrica package by Herbert Raab also uses this method of stacking images. Others seeking to measure faint NEOs are encouraged to use this program which is available on the Astrometrica site.

One concern was that even if faint targets could be imaged the quality of astrometry would be poor. In fact this has not been the case and accuracy has been of the order of 0.5 arc second even for 19^th and 20^th magnitude targets with only a very few residuals greater than 1 arc second. The accuracy is significantly better than my previous astrometry of brighter main belt objects done a few years ago with a different telescope and different methods – I attribute this mainly to the fact that the pixel scale is smaller.

The main source of targets is from the Minor Planet Center’s comprehensive web page. These are the golden years of NEO discovery and large numbers of objects are being discovered by LINEAR, NEAT and other surveys. Once a promising candidate is identified an ephemeris is put on the Minor Planet Centre’s Confirmation Page. Some of these Confirmation Page objects will be real NEOs, some will actually be main belt asteroids and some will not exist at all. The MPC also puts potential comets on the confirmation page – these are not flagged as such to avoid biasing observers. It is necessary to review the Confirmation Page frequently throughout the night as it changes as new objects are added, orbits are updated or objects are taken off.

My overall rate of success with the Confirmation Page is significantly lower than 50%. This is mainly due to my small field of view missing objects. Also many objects turn out not to exist (these are often confusing shown as lost due to lack of observations on the web site).

Doing confirmation page work is absorbing, exciting and frustrating! The hope is to have your observations published in the MPEC e-mail bulletins and I have had several such observations published. To achieve this you not only have to find the object but also get your observations reduced and submitted almost immediately and certainly on the same night. MPECs are issued when the MPC have sufficient material and in Europe we have some very productive NEO observatories, such as Klet, a few hours to the east of us. Many a time I have observed an object only to find that the MPEC has been issued before I have been able to submit the astrometry. On the other hand this is perhaps the only area of astronomy where you can make an observation and have it published before you go to bed!

As well as Confirmation Page objects I aim is to spend significant observing time doing more general follow-up. For efficient follow ups you should only observe an object when an observation at that time would significantly improve the orbit. Most bright objects are over observed and additional observations have only a very marginal improvement on the orbit. A good source for objects that do need follow up is the MPC’s page on NEOs that have not been seen for a while. Another very useful source of information is the Near Earth Objects Dynamics site ‘NEODyS’ from the University of Pisa. This has a special risk page which shows those objects with ‘virtual impactor’ solutions – that is where there are orbits consistent with the observations that could hit the Earth. See: NEODyS: risk page for further details.

These are key targets - normally when additional observations are available the impacting solutions disappear. There is also a wealth of information for each NEO on this site including a listing of all observations and residuals, and summary pages for each observatory which are useful for monitoring the quality of your astrometry.

Two months into my program some 27 NEOs have had positions measured along with several other confirmation page asteroids which turned out not to be NEOs. NEOs observed include some Potentially Hazardous Objects (PHAs) which can come within 0.05AU of the Earth and some objects which had impactor solutions at the time of observation. Plans for the future include extending the software to permit long integrations of the faintest and fastest moving objects. Consideration is also being given to replacing the ST7E with a larger format camera which would be more efficient for confirmation page and recovery work. And I hope to go fainter yet!

Although other British amateurs have done some NEO work many astronomers have been of the opinion that only brighter objects could be observed from Britain due to ‘murk’. I hope I have disproved this and would encourage other amateurs to join this challenging and rewarding line of work.

Internet: slaurie@compuserve.com

Stephen Laurie

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