Pointing accuracy is the ﬁrst and foremost requirement for any astronomical observation.
With this report an eﬀort has been made to study and document various ways of modeling
an alt-az telescope’s pointing accuracy, obatining statistical inferences from the ﬁt models,
peering into the physical causes of the modeled errors and providing a comprehensive list
of shortcomings and limitations of the approach used. The study has been made using
the data obtained from 3.6m Devasthal Telescope. A kinematic model has been used to
model the pointing errors, vector form of least square method used to ﬁt the model and
various statistical parameters obtained that allow one to asses the goodness of ﬁt of the
model and provide solution to some common pointing diﬃculties that may arise during
an observation e.g. the imager being used is not aligned with the telescope’s rotator’s
axis. At the end several other sources/causes of error that may aﬀect the accuracy, have
With amateur telescopes when one wants to look at a bright object, say a planet, all that
needs to be done is, swing the telescope to the location where the planet is approximately
located and manually move it until it is focused in the eyepiece. But what if the target
object is faint? i.e. a deep sky object. Then the usual method is to use a ﬁnding-chart.
This approach (although still being used at some places) is highly ineﬃcient simply
because of the sheer amount of wasted time.
Instead, large and professional telescopes have stepper motors & encoders (either
absolute or incremental) and have physical readouts. Here all an observer needs to do
is enter the desired location, usually as (RA, Dec), and the telescope (hopefully) points
to that direction. Although this approach points the telescope with minimum eﬀort, it
is riddled with errors - the celestial target may or may not appear in the FOV (Field
Of View) and even if it does, it is certain that it will not appear in the center of the
FOV/crosshair. Taking the simplest of examples, it is possible that the encoders have
some zero-errors or the bearings have runout. The direction where one wants to look
is diﬀerent from the direction the telescope’s looking. Therefore there arises a need
to introduce some correcting oﬀset in the values that are to be input. Or still better,
generate a model that takes into account various errors that can be present (both static
and dynamic), takes the desired coordinates as input and gives the corrected coordinates
as output - the coordinates which should actually be entered in order for the image to be
centered in the FOV.
1.1 Ways to point a telescope
The term ’pointing’ itself refers to the initial acquisition of the target. Depending on the
type of target (bright/faint) and the accuracy demands (large observatories often need
sub-arcsecond pointing accuracies) there are diﬀerent ways to point the telescopes.
1. Blind pointing : Also known as dead reckoning, this is the most basic way wherein
one enters the target coordinates and the telescope moves (usually to some diﬀerent
coordinates since certain errors always present). This is generally used when the
accuracy required is not too demanding (several arc seconds is acceptable) e.g. in
case of wide-ﬁeld imaging.