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A method for determining the V magnitude of asteroids from CCD images


Hints and tips for using Astrometrica and Guide


1.0       Introduction


A paper of the above title has now been published in the 2009 June issue of the BAA Journal. Richard Miles approached Herbert Raab and Bill Gray to see if the V magnitudes so calculated could be incorporated in to their software, Astrometrica and Guide respectively. The outcome was positive in both cases and the results are described here. The paper described a very manual method for obtaining V magnitudes from CMC14 data. The latest version of Astrometrica has transformed what was a laborious

undertaking in to a very simple task.


2.0       Astrometrica


Astrometrica is widely used by both professional and amateur astronomers to determine

the positions of asteroids. A new version of Astrometrica, or later,

incorporating CMC14 data is now available from: It is

inexpensive and costs just 25 Euros for a single licence. This version is a major advance

in that, whether the asteroid is a Main belt asteroid or a fast-moving Near Earth Object

crossing many fields of view, the software is able to yield accurate magnitudes as well as

positions with the minimum of effort. It should be noted however that the Minor Planet

Center recommend, in their ‘Guide to Minor Planet Astrometry’, the use of the

USNO-B1.0 catalogue to obtain comparison star coordinates. The CMC14 catalogue does

not include proper motions and therefore its accuracy will degrade as we move away

from the epoch of the catalogue positions.  Users wishing to obtain the highest

photometric accuracy with Astrometrica should choose the following options accessed

via File/Settings:


CCD tab (Figure 1)


- choose either the Visual (V) or the Red (R) passband.  The software is set up to return

Johnson V magnitude or Cousins R magnitude for these two options. Saturation is set to 50000 to ensure any stars which saturate pixels are not included in the solution.



Figure 1, Settings/CCD tab


Program tab (Figure 2)


- set the magnitude Lower Limit to 15, 14.5 or even 14 (rather than a fainter limit) this

  can further improve the accuracy of the photometry


- under 'MPC Report', tick 'Magnitude to 0.01 mag'.


 - under 'Object Detection' the size of the 'Aperture Radius' selected also equals the size

   of the aperture used to perform photometry on objects on the frame.  Normally, users

   should adjust the value of the 'Aperture Radius' so that it is large enough to contain the

   image of the star, asteroid, etc. 


- under ‘Object Detection, Background from’ select ‘Aperture’ where comparison stars

  and asteroid (or other targets) are well separated or select ‘PSF’ for crowded fields and

  faint objects   


- under 'Residuals' you can set the 'Photometric Limit' as low as 0.20 mag without  

  rejecting a large fraction of potential reference stars .



Figure 2, Settings/Program tab


Finally, when working in unfiltered mode, users can report either 'V' or 'R' magnitudes

provided you select the option which is closest in colour response to that of your

unfiltered CCD camera.  For most astronomical CCD cameras, the 'R' magnitude option

is best, although Sony interline transfer chips are closest to 'V' in their response.  You

may experiment with both of these options by checking the results in terms of the

residuals reported in the Log file before deciding which is better for your camera.


So long as you have at least say 6-8 stars on each image, the photometry should be

accurate to better than 0.05 mag or better (provided the signal to noise of the asteroid is

adequate).  To improve signal to noise ratio use the Track and Stack facility to co-add a

number of image frames by keying in the speed and direction of motion (P.A.) of the

asteroid, or selecting the asteroid from the drop down list, and choosing 'Average' for the

final stacked image.  The ultimate accuracy of the frame-to-frame calibration (zeropoint)


depends on the availability of reference stars but can easily attain 0.02 or even 0.01 mag. 

Fortunately, the CMC14 catalogue numbers over 95 million stars of which about 60

million are suitable for use as reference stars and so it is usual to have plenty of useable

CMC14 stars in any one frame.


The disadvantage of using CMC14 is that it is not an all-sky catalogue, being limited to

Declinations between +50 and -30 degrees (apart from a gap between RA 5h30m and

10h30m for declinations south of -15 degrees).  This puts southern hemisphere observers

at a slight disadvantage.


2.1       Example


Five images of asteroid (115) Thyra obtained 2007 October 17th were calibrated and

stacked – Figure 3



Figure 3, Stacked image of asteroid (115) Thyra


The position and magnitude of the asteroid were measured and the results shown in

Figure 4. It can be seen that the calculated magnitude is 10.69.



Figure 4, Position and magnitude determination


This exercise was repeated for 3 further stacks of 5 images and the values of magnitude

derived were; 10.64, 10.64 and 10.61 giving a mean value of 10.65 +/- 0.03


2.2       Guide


Guide is a planetarium program and the CMC 14 catalogue data for a particular star can

be displayed by right clicking the mouse on that star and then selecting ‘More info’. The

CMC data is then displayed as shown in Figure 5.



Figure 5, CMC14 data available in Guide


The authors are extremely grateful to both Herbert Raab and Bill Gray for incorporating

CMC14 catalogue data in to their software.


Richard Miles and Roger Dymock


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