A method for determining the V magnitude of asteroids from CCD images
Hints and tips for using Astrometrica and Guide
A paper of the above title has now
been published in the 2009 June issue of the BAA Journal.
undertaking in to a very simple task.
Astrometrica is widely used by both professional and amateur astronomers to determine
the positions of asteroids. A new version of Astrometrica, 18.104.22.1686 or later,
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
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
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
- 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.
million are suitable for use as reference stars and so it is usual to have plenty of useable
The disadvantage of using
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.
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
Guide is a planetarium program and the
be displayed by right clicking the mouse on that star and then selecting ‘More info’. The
The authors are extremely grateful to both