A Method to Improve the Tracking of Celestron Nexstar

8i, 8iSE and 8SE Series SCTs and Possibly All Other Telescopes in This Series

(Issued as an Article to “Cloudy Nights” Sept. 2007. Updated Jan. 2010)

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 within 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 onspur gears for slewing and tracking, this can be compromised by less than accurate “Anti-Backlash” settings. This 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 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 (OTA), to ascend when tracking towards the meridian and thereafter, descend.

According to that still excellent, (but now dated), book “The Nexstar User’s Guide”written by Michael W Swanson, the Nexstar 8i ; contrary to its (negative) default setting, should adopt a positive “GoTo Approach” on both axes for use in the Northern Hemisphere in order, in his words, “to take advantage of the natural backend heaviness of these ‘scopes” and consequentlyemploy a“Right and Down” movement of alignment stars, (NOT the tube), as viewed through the eyepiece, in making a final approach to its centre to initially align the telescope.

Having made this alignment, it infers that the “Right and Down” approach should 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 thereby 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.

Due obviously to limited resources, it can be claimed only to relate to Nexstar 8i, 8iSE and8SE series telescopes although almost certainly itwill apply to the 6SE which shares the same mount. Validation of this method’s use for the whole of the Nexstar series will require further investigation by individual owners but there is nothing to suspect that its application will not work for any of these spur gear driven ‘scopes.

The method itself relies on three factors andhas 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

The choice of Altitude GoTo Approach according to owner preference. Azimuth GoTo Approach to be set to positive for Northern Hemisphere users.

Factor 2

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. 5, 4, 3, and 2]). Short delays in drive take-up are acceptable.

Factor 3.

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

Procedure.

a) After alignment and with a Positive Altitude GoTo Approach, initiate a “GoTo” and then centralise the chosen object in any eyepiece by moving it in a “Right and Down” direction if the object’s position is Pre-Meridian. If however its position is Post-Meridian, move the object“Right and Up” to centralise it in the eyepiece. If an overshootof the centre of the eyepiece is inadvertently made, just retrace the movement.

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.

Note again however, that the default Altitude GoTo Approach setting on the tested ‘scopes is Negative.

Should the operator prefer to retain this setting, then the centralisation of objects will need to be reversed. That is to say that when an alignment is being carried out in the Northern Hemisphere, the alignment stars should be brought to the centre of the eyepiece using a final “Right and Up” movement of the star itself.

Thereafter, having gained a successful alignment, all objects viewed pre-meridian should be centralised in the same manner, (“Right and Up”) but those which lie post-meridian should be centralised in the eyepiece with a ”Right and Down” movement.

Final Notes.

It is recommended that before applying this method, Nexstar owners first determine which Altitude GoTo Approach setting (i.e. Negative or Positive), works best for them in terms of alignment success rate and subsequent “GoTo” accuracy. Once selected, this method may be applied accordingly to enhance tracking.

However, even in applying this method, there will inevitably be some drift of the object from the centre of the eyepiece but this should now be minimal and not be such as to require frequent adjustment thereby impairing the enjoyment of viewing over a protracted period. Significantly improved tracking stability is the result according to all testing to date by using this method.

Acknowledgments.

I am indebted and extend my sincere thanks to both Tony Dralle and Bob Griffiths for their invaluable help in compiling this simple but hopefully more effective method of improving tracking in the Nexstar 8i, 8iSE and 8SE series telescopes which possibly, and hopefully may apply to those of the entire Nexstar series.

T. Bridgwater.