NGF-S Handset

Linnhe Web Site for Mark S Baines and of Linnhe Observatory, Highlands, Scotland

Copyright 2002-2010 Mark S Baines All Rights Reserved
Last Updated: 04 03 2010


Most Schmidt-Cassegrain telescopes, such as the Meade LX200, suffer from image shift when focussing. This is caused by the focussing mechanism having to move the main mirror to bring about focus. Crayford-type focusers mounted on the rear of the telescope alleviate this problem and also give a finer focussing control. The JMI NGF-S focuser is one of these and comes fitted with a motor to control the drawtube into which eyepieces and other equipment is mounted. The handset controller comes with two red push-to-make buttons to control the direction of movement of the drawtube - in and out of focus.

When observing, I normally hold this controller in one hand pushing the buttons with my thumb. I have always felt this was clumsy and slow especially when chasing the seeing, and so I went about fitting a single control so that my thumb wouldn't have to keep jerking side-to-side to push the buttons. The answer was a 'non-locking both ways centre off toggle switch' which acts rather like a joystick (though admittedly only up and down.)

I decided to keep the buttons and I also needed to keep the very important braking action of the motor that the push-to-make buttons provide (this clever design prevents the motor running on when a button is released, something that is all to obvious when the focuser motor is plugged into the LX200 control panel and controlled from the LX200 handset.) This modification only requires three additional components besides the toggle switch all of which fit neatly into the existing handset case.

The image below shows the NGF-S handset controller with the toggle switch fitted above the existing two push buttons.

Modified NGF-S handset controller image
Modified NGF-S handset controller

1    Sub-mini SPDT toggle switch, non-locking both ways, centre off, (ON) OFF (ON) - (Maplin FH03D)
2    Micro-miniature relay, 6V - (Maplin FM89W)
1    Metal film resistor, 39 ohm - (Maplin M39R)

All the components were supplied by Maplin Electronics in the UK and as these are very common components I'm sure you'll be able to find them wherever you live.

The relays are tiny - w9.7mm x d6.8mm x h8.7mm. Anything bigger would be a problem fitting in the case. There is room next to the battery but I placed everything at the top end, in and around the existing push buttons. You may have to shave a little off the corner posts where the lid screws onto. Everything is held into place with the single core wire I always use - it's easier to strip and handle and stays in place.

All the original components are kept and the new ones and wiring attached to them. Below is a diagram showing how everything is wired together. Be careful which way around you wire up the relays - one 'end' is not the same as the other. The 'switch' end is at the bottom nearest the push buttons.

The new additional resistor should be 40 ohm but I could only get a 39 ohm. Doesn't make much difference. The point of this is that the relays are 6V and the battery supply is 9V and so this resistor is needed to adjust the supply to the relays. It is attached to the battery negative wire - you'll find a suitable place near the potentiometer where the other resistor is fitted.

Place the toggle switch so that it throws forwards and backwards rather than side to side. I find this the most convenient and easiest to use one-handed with my thumb.

Before you begin, make a note of which terminals on the push buttons go where, especially to the power supply and to each other. Use the same terminals when wiring in the new components.

Circuit diagram image
Circuit diagram

When fitted, simply push the toggle switch forward to move the focuser motor one way and backwards to move the other. When you let go, the switch returns to the centre and in the off position like a joystick.

Oh, and stick a little hook on the back to hang the handset from the LX200 rear cell handle bar rather than letting it drop to the floor. I fashioned one from a brass picture hook.

 


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