Pentax 75 SDHF refractor - How do I remove focuser knob?

[opticalpath]

I'd like to motorise the focuser on my Pentax 75 SDHF but there's no obvious way I can see to remove a focuser knob. There's no visible grub screw - it may be hidden somewhere under the dimpled rubber part of the knob but I'm reluctant to start cutting.

Has anyone else done this?

Thanks

Adrian

[swag72]

Adrian, there's a few small allan key's underneath the scope on the focuser barrel by the focuser knobs - Do these do anything? We took the focuser apart the other week and there was no obvious way from the inside to remove the knobs that we could see.

[opticalpath]

Adrian, there's a few small allan key's underneath the scope on the focuser barrel by the focuser knobs - Do these do anything? We took the focuser apart the other week and there was no obvious way from the inside to remove the knobs that we could see.

Hi Sara; thanks for the suggestion. I'm looking at these four small screws, but I think they are perhaps just retaining (or adjustment) screws for the fat aluminium-coloured cylinder (the bearing?) that the thinner spindle itself runs in. Or maybe they go into grooves in the spindle to adjust out any sideways movement. They don't seem to be involved in how the knob is attached to the spindle, though.

I'm sure I remember reading somewhere about a hidden set-screw under the dimpled rubber grip but I can't find that reference now. Francis (Fwm81) reckons it should be possible to lift and roll back the rubber grip, maybe with a little heat to help.

BTW, did you have a problem with your focuser - what did you find when you opened it up?

I found it a little tricky to get exact focus with this scope; it could really do with a 1:10 slow-motion drive. Hence my wish to add a stepper motor and belt drive. I've got this on my MN190 and it works a treat.

Adrian

[ollypenrice]

If you don't manage it, Adrian, adding a lever or wand to a single speed knob can be helpful. I did this with an early Genesis.

Olly

[swag72]

Following Olly's advise I fitted a rather crude wand devise, but have since had a friend make me something. It makes a massive difference in the focusing as you can move it in such small increments now.

I didnt have a problem with the focuser, we opened it up to see if it could be adjusted somehow and give me slightly more drawtube when I was initially looking at the Optec. The drawtube fitted nice and snugly on the teeth of the focuser and in the middle of the focuser knobs was a small 'wheel' that the focuser runs on.

Meanwhile I watch with interest!! BTW, have now got all adapters and hope to have a go at focusing tonight.

[opticalpath]

If you don't manage it, Adrian, adding a lever or wand to a single speed knob can be helpful. I did this with an early Genesis.

Olly

Thanks Olly. I have done just that in the past! My favourite device for this is one of these lightweight clamps that can be quickly clamped on to the focuser knob in at a convenient position to provide a lever like you described:

But I want to motorise it for another reason too - so I can focus remotely from the warm indoors! I've got an Arduino controlled stepper on the focuser of my MN190 and it has really spoiled me.

The Pentax does need a little tweak to focus when I change filters. The PC-stepper control should allow me to check and adjust focus as needed without to-ing and fro-ing out to the telescope so much, perhaps even just program in the number of clicks needed to change focus between filters. Luxury .... and a time saver, not just laziness, honest!

Adrian

[ollypenrice]

Thanks Olly. I have done just that in the past! My favourite device for this is one of these lightweight clamps that can be quickly clamped on to the focuser knob in at a convenient position to provide a lever like you described:

But I want to motorise it for another reason too - so I can focus remotely from the warm indoors! I've got an Arduino controlled stepper on the focuser of my MN190 and it has really spoiled me.

The Pentax does need a little tweak to focus when I change filters. The PC-stepper control should allow me to check and adjust focus as needed without to-ing and fro-ing out to the telescope so much, perhaps even just program in the number of clicks needed to change focus between filters. Luxury .... and a time saver, not just laziness, honest!

Adrian

Wott?? Get outside, man, and turn blue. It's good for you!!!

Olly

[opticalpath]

Wott?? Get outside, man, and turn blue. It's good for you!!!

Olly

You'll be telling me to put an eyepiece in the drawtube next . (I'm sure I have one somewhere .... )

Adrian

[opticalpath]

Solved ..... thanks to Francis (fwm891)

The dimpled rubber grip is actually a rather thin band that sits snugly in a recessed channel cut in the circumference of the focuser knob. It can be prised up at the edge and rolled back and off the knob without damage. Only a smear of adhesive and the tight fit hold it in place. With the rubber grip removed, two deeply recessed grub screws (1.5mm hex) are revealed that hold the knob on the shaft.

Adrian

[Coco]

This looks like a solution, a nice gear wheel?

http://www.astro-foren.de/showthread.php?9378-Halb-APO-Pentax-75-SDHF-75-500

[Coco]

This looks like a solution, a nice gear wheel?

http://www.deepfield.at/equipment/

[opticalpath]

Thanks Guy. That's certainly a neat solution and would give very fine control.

I'll probably go with this arrangement used on my MN190, though, using a stepper motor and toothed belt drive. It's working well and the belt and pulleys are easily obtainable.

Adrian