Table of contents




Please note that this document is the result of combining two earlier documents on this web site and therefore the examples used do refer to different asteroids. The reason for doing this is to pass on experience gained in the past year or so and to simplify some of the original procedures.


1.0              Introduction


Imaging faint objects is, in very simple terms, a matter of a long enough exposure be it with a CCD or film camera. Faint Near Earth Asteroids present the observer with two additional problems;

-         they are fast moving

-         you can’t see them either visually or on a single CCD image

(In comparison supernova hunters have a much easier time – those things shine so brightly that it must be a bit like a film premiere with the camera flashes firing off as the stars arrive – only joking guys !)


This document describes how to find a suitable target for follow-up observations, ascertain the position(s) of the asteroid (astrometry) using Astrometrica and then submit the results to the Minor Planet Center. One can use the MPC NEO Confirmation Page as a source of potential targets but my set-up is not capable of reaching down to the faint magnitudes (usually) of these objects. The orbit of objects on the NEO Confirmation Page is, by definition, not yet well defined. On the couple of occasions I have tried to image these asteroids I failed due to the actual track falling well outside my 12 x 8 arc sec field of view.  


‘A practical guide on how to observe NEOCP objects’ by Sergio Foglia, Peter Birtwhistle, Luca Buzzi, Gianni Galli and Gianluca Masi can be viewed here.


The Minor Planet Center has a facility which can be used to determine the probabilities that your observations belong to NEOs. At least two observations of each object must be entered ensuring that each observation has the same designation and that all observations are formatted correctly.


2.0              Theory


The matrix of pixels accumulating photons in a CCD camera can be compared to a similar matrix of buckets catching rainwater in a field. The more rain that falls, the more water there is in the buckets and, likewise, the longer your exposure the brighter will be the object on the image.


Now imagine a very small rain cloud moving so quickly across the field that it deposits only a few drops of water in each bucket. You could run after it with your bucket and collect more water but, if you can’t see it and therefore don’t know where it started from or its speed and direction you are somewhat scuppered.  However, if by some magic, you could ascertain its initial whereabouts and subsequent motion you would be (continuing with the agricultural theme) in clover.


That ‘magic’ is the internet and the sites it enables you to access. Inserting the relevant parameters in to Astrometrica (Herbert Raab’s excellent astrometry software) enables you to automatically process a set of calibrated images in to a single image showing the NEO and enabling its position to be determined.


3.0              Equipment


My current set-up includes;


Telescope and accessories

-           Orion 10” Newtonian reflector on a Vixen GP German Equatorial Mount

-           Skysensor hand controller

-           Starlight Xpress MX516 CCD camera

-           Light Bin (for flat fields)

-           Light tight cover for telescope tube (for dark frames)


Observatory laptop PC and accessories

-           Astroart imaging software

-           Megastar software for star charting and telescope control

-           TAC 32 software (for handling GPS input)

-           Garmin eTrex Venture GPS receiver (for accurate timekeeping)

-           D-Link Xtreme G 2.4GHz Wireless Cardbus Adapter

-           Radmin software (for handling wireless link to PC)


Study desktop PC

-           Astrometrica  (using the on-line USNO-B 1.0 star catalogue)

-           AIP4WIN (for generating master dark frames and flatfields)

-           Guide for plotting astrometry results

-           D-Link Xtreme G 2.4GHz Wireless Router

-           Radmin software (for handling wireless link to PC)

-           Megastar software for star charting and telescope control


4.0       Target selection


4.1       Overview


Try the brighter asteroids first and then go fainter to see just what you can achieve. Then, having proved your equipment and procedures, you can move on to follow-up observations and discovery ! 


In summary;

-         decide which area of sky is suitable for observing

-         check the approximate RA and Declination of that area for the period of observation

-         choose a target using the web sites described below

-         obtain orbital elements and an ephemeris from the Minor Planet Centre

-         plot the track of the target using a star charting software package such as Megastar and verify against the MPC ephemeris


4.2       Spaceguard


4.2.1    Spaceguard Priority List


The Spaceguard’s Priority List. can be accessed directly or by following the links from the Spaceguard Foundations homepage via the Spaceguard Central Node, the Spaceguard System and the NEO Coordination System web pages.


I start with the Priority List and then link to others (MPC, JPL NEO Program, Near Earth Object Dynamics) in order to assemble the necessary information for observing, and to find out more about, the target asteroid.


I found it useful to sort the list by RA having first established, using a planisphere, which area of the heavens was best placed at the time I intended to observe – in this case 17 - 19h RA. Arranging the list by RA to start at 17h gave the following results (portion of list only);




Inserted in this categ.





Sky Uncert. in arcsec

End of Visibility


Data for 2002 Jul 18, 0:00 UT



2002 LX

2002 Jul 9

17h 07m





2002 Jul 29


2002 MT3

2002 Jul 17

17h 07m





2002 Aug 25


2002 MR3

2002 Jul 14

17h 52m





2002 Aug 12


2002 NV16

2002 Jul 15

18h 30m





2002 Aug 30


2002 MT1

2002 Jul 10

18h 49m





2002 Oct 11


2002 NN4

2002 Jul 15

19h 27m





2002 Oct 7


2002 MN

2002 Jun 20

19h 29m





2002 Jul 30


2002 LH3

2002 Jul 15

19h 44m





2002 Nov 8


From the above list I selected 2002 NV16 as a suitable target, it being in the right area of sky and of a magnitude, I knew from previous experience, I could successfully image.


4.2.2       Orbits


Following the links to the JPL NEO Program site produces an orbit diagram, shown below, which can be manipulated using the slider bars to the right of and below the diagram. The data listed shows this asteroid to be an Apollo type, perihelion

<= 1.0167 au and semi major axis >= 2 au. As the diagram shows, this type of asteroid spends most of its time outside the Earth’s orbit but just dipping inside around perihelion.



4.2.3       Orbital Elements


To be able to point the telescope in the right direction in order to image the asteroid one needs to know its position and motion extremely accurately. Such data is available from the Minor Planet Center’s website.


The MPC’s Ephemeris Service yielded the following data for 2002 NV16 (portion of table shown). When completing the form select the ‘MPC 8 Line’ format as this will yield both orbital elements and an ephemeris.

Minor Planet Ephemeris Service: Query Results

Below are the results of your request from the Minor Planet Center's Minor Planet Ephemeris Service. Ephemeredes are for observatory code 940.

2002 NV16

Epoch 2002 Nov. 22.0 TT = JDT 2452600.5                 MPC
M  44.04520              (2000.0)            P               Q
n   0.71562296     Peri.  179.32679     +0.99867012     -0.05141017
a   1.2378838      Node   183.62690     +0.04701083     +0.93888971
e   0.2202507      Incl.    3.50877     +0.02116544     +0.34035732
P   1.38           H   21.4           G   0.15           U   6
From 203 observations 2002 July 13-Aug. 14, mean residual 0".53.

Last observed on 2002 Aug. 14. Elements from MPO 35583.

Further observations? PHA : None needed at this time. Desirable between 2002 Sept. 13-Nov. 2. At the first date, object will be within 60 deg of the sun.

Date       UT    R.A. (J2000) Decl.   Delta    r     El.    Ph.   V  
Sky Motion     Object    Sun
                                                                        "/min    P.A.  Azi. Alt.  Alt.
2002 07 17 21   18 30.61   +15 45.6   0.099  1.092  138.5  38.0  18.1   2.24   329.3  317  +48    -7
2002 07 17 22   18 30.52   +15 47.5   0.099  1.092  138.5  38.1  18.1    2.24   328.5  338  +54   -13
2002 07 17 23   18 30.44   +15 49.4   0.099  1.092  138.4  38.1  18.1    2.23   328.2  003  +55   -17

4.2.4       Plotting the track


To confirm that the asteroid is actually where you think it is it’s a good idea to cross check the track as determined by, say, Megastar, with the MPC ephemeris as listed 3.1 above.


I would then display the Megastar chart on my observatory laptop and compare it with CCD images to verify that the required field was being imaged.



The other piece of information I needed for the Track and Stack feature in Astrometrica was the motion of the asteroid in terms of direction or position angle (degrees) and rate (arc seconds per minute). This is listed in the MPC data shown above.


4.2.5       Checking for other asteroids


It is also worthwhile checking to see if there are any other asteroids in the vicinity by using the MPC’s MPChecker facility. An example of the output obtained is shown below. This data can also be used as a rough check on the accuracy of ones own astrometry as mentioned in para 7.0 below.


Here are the results of your search(es) in the requested field(s):

The following minor planets, brighter than V = 19.0, were found in the 15.0-arcminute region around R.A. = 09 35 45, Decl. = +35 14 13 (J2000.0) on 2002 04 16.89 UT:

 Object designation         R.A.      Decl.     V       Offsets     Motion/min                           
(38071) 1999 GU3         09 35 43.1 +35 13 34  16.7   0.4W   0.6S   5.7+   7.4+    

The following minor planets, brighter than V = 19.0, were found in the 15.0-arcminute region around R.A. = 09 35 45, Decl. = +35 14 13 (J2000.0) on 2002 04 16.90 UT:

 Object designation         R.A.      Decl.     V       Offsets     Motion/min  
(38071) 1999 GU3         09 35 49.8 +35 15 20  16.7   1.0E   1.1N   5.7+   7.4+    


4.3          Lowell Observatory


Potential targets can also be identified using Lowell Observatory’s Asteroid Services ie; Hierarchical Observation Protocol and Critical List of Asteroids


4..4.        Minor Planet Center


4.4.1       Services for Observers


The MPC’s ‘Services for Observers’ includes Date of  Last Observation of NEO’s not seen in a while with easy access to ephemeredes  and NEA Observation Planning Aid. These pages can be customized to suit the observer’s circumstances.


As an example the following settings were input on the ‘Date of Last Observation of NEO’s not seen etc’. page on 27 June 2005;



RA limits  to    Decl. limits  to 
Elongation range  to    V magnitude range  to 


What kinds of objects are to be returned?

    Numbered objects

    Multi-opposition unnumbered objects

    Single-opposition unnumbered objects

Which page is to be customized?

    Dates Of Last Observation Of NEOs

    Dates Of Last Observation Of NEOs (R.A. order)

    Bright NEO Recovery Opportunities

    Faint NEO Recovery Opportunities

    Dates Of Last Observation Of Unusual Minor Planets

    Dates Of Last Observation Of Distant Objects

    Dates Of Last Observation Of Distant Objects (R.A. order)


These produced the following list of targets;


1988 NE             

2005 JF21           

2000 NF11           

2000 KO44            

2002 EX12    


4.4.2       Orbital Elements        


Of these 2002 EX12 appeared to be well placed and relatively bright so elements and an ephemeris were obtained using the MPC’s Minor Planet and Comet Ephemeris Service

Minor Planet Ephemeris Service: Query Results

Below are the results of your request from the Minor Planet Center's Minor Planet Ephemeris Service. Ephemerides are for observatory code 940.

2002 EX12

Epoch 2005 Aug. 18.0 TT = JDT 2453600.5                 MPC
M 352.76090              (2000.0)            P               Q
n   0.23458425     Peri.  217.92735     +0.82655784     -0.56270508
a   2.6037752      Node   176.24614     +0.55239277     +0.80660649
e   0.7675194      Incl.   11.31892     +0.10800168     +0.18096674
P   4.20           H   15.9           G   0.15           U   2


4.4.3 Plotting the track


This information could then be used to plot the projected track in Megastar.



4.4.4       Checking for other asteroids


It is also worthwhile checking to see if there are any other asteroids in the vicinity as in 4.2.5 above.


4.5          Italian Organisation of Minor  Planet Observers


Follow-up Astrometric Program. Add further information when this item is updated for the ARPS handbook.


5.0          Imaging


5.1          Setting up the telescope


Correct set-up is all important for accurate astrometry. Some tips;

-         ensure the PC clock keeps good time (to the nearest second)

-         ensure your telescope is correctly polar aligned (to avoid trailing on the images)

-         focus your telescope (to obtain sharp images)

-         set the exposure time to avoid trailing and saturating pixels


Because the asteroid will not be visible the telescope needs to be aligned on the star field containing the asteroid. Having input the orbital elements in to my laptop computer I plot the path of the asteroid for the time I would be at the telescope – see Megastar chart above. Once the telescope has been polar and three point aligned I use the telescope control feature in Megastar to position the telescope on the target.


5.2          Acquiring images


Having eyeballed the field to ensure I am actually where I want to be I use the frowned-on method of removing the eyepiece and replacing it with the CCD camera oriented so that the CCD image is the same as that in the eyepiece ie; north up. You do have to be careful not to move the telescope while doing so but this has only happened to me once so far. I then make any fine adjustments necessary to ensure that the images look reasonable and that the asteroid tracks across the centre of the field of view while the images are being acquired (having said that I usually finish up with the thing on the edge of the image !)


In his article, Astrometry of Near Earth Objects, Stephen Laurie quotes the following formula;


               Exposure time (mins) = FWHM of star image (arc secs)/rate of motion of asteroid (arc secs/min)


Astrometrica can be used to calculate the FWHM for a typical star on any given image.


In the case of 2002 NV16 the maximum exposure time = 4/2.24 = 1 min 47 secs. If I was using a guider such as the Starlight Xpress S*T*A*R 2000 I might be tempted to use this exposure time but experience suggests 1 min max to avoid trailing (assuming saturation is not a problem).


5.3          In from the cold !


Once I have acquired the first few images and checked that all is working as it should I retire to the warmth of my study and monitor proceedings using a desktop PC (shown below) with a wireless link to the laptop PC in the observatory. The observatory laptop screen, keyboard and mouse are replicated on the desktop PC enabling imaging and the telescope to be controlled in comfort.


NEO’s move quite quickly in comparison to main belt asteroids and thus it is necessary to occasionally reposition the telescope along the predicted track of the NEO. I try and position the telescope so that the NEO remains in the same field of view for as long as possible. This maximizes the number of images that can be obtained in that one position and ensures that I have enough for Astrometrica to track and stack and bring the NEO in to view.



6.0          Image processing


6.1              Calibration


I use AIP4WIN  to create master dark frames and flat fields ie;

a)      Median combine the dark frames = Dark Master

b)      Ditto the Flat Fields = Flat Master

c)      Ditto the Flat Darks = Flat Dark Master

d)      Subtract the Flat Dark Master from the Flat Master = Corrected Flat Master

e)      Save the dark and flat masters


6.2              Track and Stack


To quote from Astrometrica , Tutorial III, ‘Astrometrica can also be used to stack a number of images, adding up the signal of a faint object. When stacking images, the images can be shifted to compensate for the motion of an object (asteroid or comet), which will enable the user to get reliable positions even for very faint objects (so that the object cannot be captured in a single short exposure), or fast moving objects (that would trail in a single, longer exposure)’. I would also make the point that some objects cannot be imaged with a single long exposure as they are too faint/moving too fast to leave a trail.


If the stacked images are still slightly trailed then use the facility in Astrometrica to slightly vary the motion rate and PA to see if the image can be improved. I look for the highest value of Signal to Noise Ratio (SNR), low FWHM and Fit RMS.


A single (calibrated) image typical of those obtained is shown below.



Stacking 12 images produced the following image. The stars are shown as a line of points each offset from the other as determined by the rate of motion and position angle as input to Astrometrica. The asteroid, 1999 GU3, is the single point in the bottom right hand corner of the image.



7.0          Checking astrometry


7.1          Using the MPC MPChecker


It is well worthwhile checking ones results to ensure they are reasonably close to the predicted position.


To get values for RA and Dec I use the MPC MPChecker facility as described in para 4.4 above. In this instance the RA and Dec listed in the MPC report in para 8 below lies between the RA and Dec values obtained from the MPChecker for times of 2002 04 16.89 and 2002 04 16.90 and therefore I could be reasonably confident as to the accuracy of my measurements.


7.2          Using Guide


I load my astrometric measurements in to Guide. Assuming that the images have been obtained at regular intervals the plot (below) will show a straight line of equally spaced positions (the gaps are due to some images being rejected prior to processing). The designation K03R10L is the packed format for 2003 RL10. Any obviously rogue results can be then be discarded before I submit a report to the MPC.




7.3          Using FindOrb


There were a number of postings on the Minor Planet Mailing List in autumn 2007 concerning the use of the measurement of residuals to verify the accuracy of ones astrometry prior to submission to the MPC. I asked Peter Birtwhistle for his thoughts which are:


‘I like to ensure I don't have any gross errors in the astrometry, e.g. by mistake a close star has been measured instead of the intended object etc., so I aim to try and make sure my positions are internally consistent, so RA and Dec flow evenly against the times of observation.


I used to do this by plotting RA against time and Dec against time in Excel but it is not always easy to spot a discordant position like that, especially when there is a reasonable amount of time between observations or the object is moving relatively fast.


The easiest, quickest way I find now is to bring the positions up in FindOrb (which can be found at at Using its Vaisala orbit option it will fit three or more positions making assumptions about the orbit that may well make the resulting orbit complete fiction, but that doesn't matter, it will have done a least squares fitting of the positions and displays the residuals directly. You can then easily see whether there are any big problems within the set of positions, though it is not always obvious which positions are good and which are bad.


If it does look like there could be a problem then I might re-examine the images to make sure I had identified the object correctly, or re-measure the object to make sure that the centre of light was properly registered during measurement (it's surprisingly easy using Astrometrica to click on an object and get the annulus *not* centred on the object - the resulting PSF curve is always distinctively 'wrong' in those cases but not always noticed...).


Alternatively, comparing against ephemerides should give you consistent *trends* in the residuals but only to as good a precision as the ephemeris, made worse by the inaccuracies in interpolation. So the residuals might be physically large (if the ephemeris was calculated from an orbit from relatively old observations) but the run in those residuals ought to be smooth. As it is so much easier to run up a Vaisala orbit in FindOrb I would only resort to the extra effort in comparing against an ephemeris if for instance I had only been able to get two positions of an object’.


8.0          MPC Reporting


Astrometrica then allows the position of the asteroid to be determined and produces a report, shown below, in the correct format for sending to the Minor Planet Centre (assuming the correct data has been input using File/Settings). Additional data is recorded in the Log file. Please note that no more than 3-5 observations per object per night should be reported to the MPC.



COD 940

CON Roger Dymock

CON []

OBS Roger Dymock

MEA Roger Dymock


TEL 10" Newtonian reflector + CCD

ACK MPCReport file updated 2002.06.27 16:36:53


    J99G03U  C2002 04 16.89207 09 35 45.25 +35 14 12.9          16.9 V      940



J99G03U is the packed designation for 1999 GU3


The day after submitting my results I check the MPC Electronic Circulars and data for my observatory in the NeoDys observatory list to see if my observations have been accepted.


9.0          Conclusion


There is probably as much if not more computer work than actual observing in all of this. However since we get far more cloudy nights than clear ones at least you will have something to occupy your time with when you can’t get outside !


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