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General Background.

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 operational response.

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 time.

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 Post-Meridian positions.

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.

Factor1

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.

Factor 2.

A normal initial telescope alignment has been made according to the userÕs preferred method (e.g. Skyalign, Auto Two Star Align etc.).

Procedure.

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.

Final Note.

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.

Acknowledgments.

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.