A New Method to Improve the Tracking of Celestron
A New Method to Improve the
Tracking of Celestron Nexstar
8i, 8iSE and 8SE Series SCTs.
There is no doubt that the Celestron
Nexstar 8i, 8iSE and 8SE series of SC telescopes are excellent value for money
in both optical and mechanical performance. However, there is equally no doubt
that being mass produced to a specific price range, compromises must be made
and none more so than in the use of spur gearing rather than worm drives for
slewing and tracking as is the case with their more expensive brethren.
Inherent in such spur gearing is an
unwanted feature known as backlash which can be defined as free play between
the teeth of the gears and which consequently causes varying delays in
Fortunately, some degree of compensation
for this can be made by adjusting the "Anti-Backlash" settings provided by the
hand control. .Unfortunately however, the elimination of backlash or at least
attempts to minimise it, can prove to be a tedious and frustrating task, so
much so that most owners appear to opt for and are satisfied with, just
achieving a smooth movement of an object within an eyepiece at the expense of
some response delay.
In view of the fact therefore that these
telescopes, like many others, rely on spur gears for slewing and tracking, they
can be compromised by less than accurate "Anti-Backlash" settings. This new
method attempts to offer a solution to such potential inaccuracies which may
cause the drifting of objects in the eyepiece within only short periods of
Background to the New Method.
I would like to claim that the concept of
this method was entirely based on scientific reasoning but like so many others,
it was discovered by accident combined thereafter with perhaps a little logic
and relates to the fact that the altitude axis allows the optical tube assembly
to ascend when tracking towards the meridian and thereafter, descend.
According to that excellent book "The
Nexstar UserÕs Guide "written by Michael W Swanson, the Nexstar 8i should have
a positive "GoTo Approach" on both axes for use in the Northern Hemisphere and
equally employ a "Right and Down" movement to making a final approach when
placing a chosen alignment star in the centre of the eyepiece to initially
align the telescope. Having made this alignment, it infers that the "Right and
Down" approach may be used continually thereafter. The method fully supports
this apart from the continual application of "Right and Down" movement
This method changes the latter approach
according to the position of the chosen object in the sky at any time and
arguably offers greater stability against object drift. It is based on studies
made into the behaviour of various objects to drift viewed in both Pre- and
At present, due to limited resources, it
can be claimed only to relate to Nexstar 8i, 8iSE and8SE series telescopes
although it almost certainly will be applicable to the 6SE. Application to the 5i, 5iSE and 5SE
series will require further work to establish validity.
The method itself relies on two factors and
at the present time, has only been validated for Northern Hemisphere users. I
am confident however, that it will work for users in the Southern Hemisphere
with the necessary reversal of settings and application.
Satisfactory "Anti-Backlash" settings. (i.e.
smoothness of object movement in both altitude and azimuth axes within the
field of view particularly at low viewing speeds [e.g. 4, 3, and 2]). Delays in
drive take-up are acceptable.
A normal initial telescope alignment has
been made according to the userÕs preferred method (e.g. Skyalign, Auto Two
Star Align etc.).
After alignment, initiate a "GoTo" and then
centralise the object in any chosen eyepiece by approaching its central
position using the "Right and Down" hand control buttons if the objectÕs
position is Pre-Meridian. If however its position is Post-Meridian, use the
"Right and Up" controls.
Additionally, if an object is set to track
when Pre-Meridian but transits the Meridian, employ the necessary "Up" change
to the object once the drift (downwards) is identified.
Even in applying this method there will
inevitably be some drift of the object from the centre of the eyepiece but this
should not be such as to require frequent adjustment and therefore impair the
enjoyment of viewing over a protracted period. Good stability is the result
according to all test indications to date by using this method.
I am indebted and extend my sincere thanks
to CN members, Tony Dralle and Bob
Griffiths for their invaluable help in compiling this simple but hopefully more
effective method of improving tracking in Nexstar 8i, 8iSE and 8SE series telescopes.