Added Autofocus to my TV NP127is

[ChrisLX200]

With endless cloud/rain the only good side is I have plenty of time to make mods to my gear. I had bought a SharpSky kit consisting of one controler and two motors, one motor I attached to my 10" LX200 and last night completed the installation of the other on my TV NP127is. These stepper motors are quite large and rather than the standard method of using a direct-drive (with the motor attached onto the end of the focuser spindle, which would be rather ungainly IMHO) I slung the motor underneath the focuser body using a small bracket, the drive being transmitted by a toothed belt. I found I didn't need to replace the 10:1 focuser knob on the refractor because the belt was quite happy driving the rubber grip on the knob without any slippage. For the SCT this was a fairly straightforward job, for the NP127is it took some head-scratching to figure a strategy how to attach it to the scope. Fortunately there were already two threaded holes in the spindle casing which I could use (albeit American #7-32 thread!) and I made a bracket which was offset using a couple of bushes (adjustable for tensioning the belt) to get the distance right between motor and focuser spindle. The photos do a better job of explaining On the refractor I also added a small clutch arrangement, unscrewing the knurled brass thumbwheel on the end by half a turn allows it to spin freely thus I can manually focus, then quicky tighten it again to use the motor. This works so well I think I'll retro-fit the same system on the SCT focuser.

Anyway, the hardware works fine, but there is an issue with Sequence Generator Pro. The NP127is has a long-travel focuser and the number of steps to move it from one end to the other is something like 300,000 steps. The SharpSky ASCOM driver is happy with that and I can set an appropriate range for max travel, but SGPro ignores any request to move the focuser beyond 100,000 steps. It seems to be a fixed limit within the program, so from fully-in I can only move the focuser about 1/3 the way out under power.

This is the other motor attached to the 10" LX200:

The control box is attached to the mount base using velcro:

ChrisH

[ollypenrice]

Looks good. If you set the rough focus on your clutch mechanism does the limited number of steps matter at all?

Olly

[ChrisLX200]

Hi Olly, it's not a show-stopper - and that's how I'll be using it probably (rough focus by hand then call that point mid-range for the autofocus travel). However, that doesn't give you the absolute focuser position, only a relative position to where you start from. With an absolute position I could wind it very near to perfect focus each time the gear is set up with no need for manual focusing at all.

ChrisH

[ollypenrice]

But equally you could have a pencil line on the drawtube or a little plastic guage cut to fill the right amount of drawtube extension. (My solutions tend towards the rustic! What am I like?)

[Sp@ce_d]

Oh that looks quite neat. I'm expecting mine to arrive this week & was thinking it might be a squeeze to fit. So might have to do the same. Any recommended sources for the extra bits, toothed belt etc?

[cfpendock]

Toothed belts, pulleys etc from:

http://www.motionco.co.uk/pulleys-belts-c-25.html

Chris

[dmahon]

With an autofocus routine it doesn't matter much, as long as you start in roughly the right place. Ollie's pencil line (or masking tape) would be more than accurate enough - still no need for manual focusing.

[ChrisLX200]

Oh that looks quite neat. I'm expecting mine to arrive this week & was thinking it might be a squeeze to fit. So might have to do the same. Any recommended sources for the extra bits, toothed belt etc?

As stated - Motionco. I used the MXL belts and aluminium pulleys. They are inexpensive so I bought a variety of pulleys and belt lengths because I could not estimate exactly the sizes/lengths required.

ChrisH