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Basic gearing requirements


Can I use an 'off the shelf' RA drive motor ?

You may be tempted to purchase an off-the-shelf 'RA motor', however few (if any) have an actual feed-back speed control. All those listed below are of the 'open loop' type = they all have a (manually operated) 'speed adjust' knob plus fitting instructions along the lines of "if the Stars drift, adjust the speed control". This works fine so long as the motor is immune to battery voltage drop or changes in loading due to gear resistance 'high spots' etc. etc.

Will any motor work with my mount ?

No = you have to get one with the correct gear ratios for your mount. Most mounts fall into one of the "EQ" types. You have to get a motor with the right gear ratio for yours.

EQ1 mount = "Motor drive M2 for telescope Seben 1000-114 + 900-76"

EQ3-2 mount = "Motor drive M1 for telescope Seben Big Boss 1400-150"

CG-2 or CG-3 (Celestron AstroMaster / Powerseeker ) = Celestron motor drive #93514

If you have the money, by all means start with a commercial kit and look to improve on it's control circuits (the kit should mean you can avoid having to make your own fitting brackets)

What do you mean by a "Hurry up and wait" control system ?

This is a basic type of control where the motor is set to run slightly 'faster' than needed ('hurry up') and then 'stops' until a precision timing clock allows it to continue ('wait'). In an electro-mechanical system, the 'stop' is after one revolution and the 'wait' is until the second hand of a clock reaches the 'one minute' point. In other words, the gearing is set up for '1 rev. per minute' = 1rpm.

Change-over relays exist that can switch in mS time periods and support cycle times up to 30 Hz, however, given the difficulty in generating head move pulses, I would suppest that 6 Hz with a 10% (1s) 'wait' is the practical limit of a mexchanical control system

How can I arrange a 1 rpm drive ?

If 'motor on' is generated once per minute (from the clock second hand) then 'motor off' must be generated when the telescope mount has moved through an angle of 360 / (60 x 24) = 1/4 degree.

Direct measurement (by mechanical means) of a quarter degree movement is all but impossible ... so instead we will take our 'off' signal from the final worm gear that drives the main head gear wheel.

Each turn of the worm moves the main gear wheel by 1 tooth.

If (by some miracle) your mount's main gear wheel had (4 x 360 =) 1440 teeth, then each turn of the worm = 1/4 degree !

However it is more likely that your mount will have a lot less than 1440 teeth .. and this means you have to 'sub divide' the worm turn ( i.e. generate multiple signals per worm revolution to get the required 1 off per minute).

If it has 770 teeth, you need 2 "off's" per worm turn. If 480, then 3, if 360, 4 and so on.

Teeth1440770480360288240206*180160144131 (130.9)120100
'off' signals12345678910111214
*actually this multiple requires 205.7 teeth, but 206 is 'near enough' (the 'drift' will be .3 in 206 slow i.e. -0.146%, 1 in 686 or -1 second of arc in approx 11.5 mins).

Will my mount's gear wheel / worm drive count be an integer division of 1440 ?

Unfortunately, most likely, no. You might think that the manufacturers would be sensible enough to choose an 'exact multiple', however making things easy for the 'amateur' also means 'generic' kits can be used and that would mean fewer sales of their own 'unique', proprietary, high margin (i.e. over-priced), motor drive.

What are the 'popular' mount gear/worm wheel counts ?

Quite a few 'popular' mounts are 'less than convenient' when it comes to RA gear/worm tooth counts ..

The EQ1 has 96 teeth.
The EQ2 has 100 teeth.
The EQ3-2 & NEQ3 have 130 teeth.
The HEQ5 & HEQ5 Pro have 135.
The CG-2 or CG-3 mount (used by Celestron AstroMaster / Powerseeker range) have 136 teeth..
The old EQ3 & EQ4, EQ5 & EQ5 PRO & Orion Skyview have 144 teeth.
The Meade LX and the Celestron CGE range have 180 teeth.

The above information has been 'gleaned' from the Internet over some time and is not guaranteed to be correct (many manufacturers seem very reluctant to publish their gear counts = I wonder why ?). Be sure to COUNT yours before investing in expensive gears / sensors / motors !

How can I drive a non-integer gear/worm count ?

One solution is to change both the clock ('on') count per minute and the worm shaft 'off' counts until some integer 'balance' is obtained - unfortunately, this typically requires much higher worm gear counts which are difficult to achieve mechanically.

The only real solution is to fit some additional gearing between the worm shaft and the motor, and take the 'off' signal from the motor shaft, although, again, you end up with signals at intervals of a few seconds at most.

eBay is a good source of geared motors - you will have to build your own speed control, however various motors with built in gearing rated at 2rpm in various steps up to about 200 rpm are available.

What's the 'ideal' RA gear / worm count (and how to drive it) ?

144 teeth = 1/10th rpm drive. Whilst specialist 1/10th rpm motors do exist they will be expensive - and building a sensor grating with 600 'off' marks (to balance a 1 second 'on' clock) is a task for the brave. So, in practice, I would fit an intermediate 60 tooth gear/worm (allowing the use of a 6 rpm motor and a 10 slot sensor).

How accurate does the gearing have to be ?

Low end mounts typically have very poor grearing accuracy - with a 'wobbly' tripod, precision sub-arc-second accurate gearing would just be a waste of money. Whilst you can replace the the commercial grears with your own (see, for example how on how to machine your own gears), a better solution is to use a feedback system to monitor the actual movement of the telescope and adjust as needed

Of course a feedback system only 'adjusts' the motor speed when the telescope tube is 'pointing' in the 'wrong' direction. Such a system will never be 'as good as' a decent set of accuratly cut gears, but should still be a lot better than the 'open loop' control provided by the mount vendor

How do I design my own drive control circuits ?

Those wishing to proceed with their own design are directed toward the very excellent SPICE simulations tools e.g. LTSpice, which is free for non-commercial use (and has an active Yahoo Users Group).

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