Anybody here using a helical focuser?

[The Warthog]

I'm wondering how these are for fine-tuning your focus without the FOV quivering like jelly. My eps are roughly parfocal, so I only need a little bit of focus when I change, and I'm thinking if this does good fine focusing, it's a hell of a lot cheaper than a Crayford.

Any takers?

[Astroman]

I've used them, but it was a combination type. That is, the main focus was rack and pinion while ultra-fine focus was helical. It was really great, allowing extremely fine focussing. I used another, purely helical focuser once that was a major pain because my EP's were not parfocal, so it took quite a bit to focus from one to another and back. Once rough focus was reached, the helix made fine focus smooth and precise. (But the journey still s*cked.)

HTH

[chuenmanc]

I've got one on a Borg 77ED. It uses a combination of a drawtube to reach rough focus, then a helical for fine focus. It is great, but I find if the scope isn't well mounted, then the torque needed to turn the helical makes the FOV wobble a fair bit.

[paulestar]

I use one on my 10" Newtonian in combination with the standard rack and pinion focuser. I find it much easier to constantly twiddle the helical to tease out the right focus than the R&P which I only use for the coarse focus.

One issue I do face though is that the collimation seems to go off depending on whereabouts the helical is. Using the Chesher EP straight in the R&P I've got perfect collimation, but sticking it in the end of the helical (makes more sense as that's where the EPs go) the collimation can vary quite a bit as I turn it (but never as bad as enough to make me want to remove the thing altogether!)

Paul

[chuenmanc]

Paul, does your helical rotate the eyepiece by any chance? The Borg doesn't, which may explain why I don't have any collimation problems when focussing.

[The Warthog]

Thanks to all.

Chu, rotating the ep should make no difference, unless the centre of the rotation is not the same as the centre of the ep. (Which could happen easily, come to think of it...)

[dph1nm]

I love them - makes focussing really easy. I have always had one on my 8.5" Newtonian - used to be an old Fullerscopes 1.25" one with a brass drawtube, but the threads became rather loose (after 25 years!) which made collimation totally impossible, so this year I have replaced it with a cheap 2" Antares one from Rother Valley Optics. This seems to work well - the focussing is almost too fine (!) - although the mounting arrangement is a bit low tech (four screws with virtually no clearance from the focusser threads). I find them rather handy for focussing a camera, as you can take test shots at 1/4, half, 3/4 turn etc just by looking at the angle the camera is rotated at.

NigelM

[paulestar]

Hi Chuenmanc,

Yes, the EP screws into the helical, so everything rotates. I got it maybe 18 months ago...from memory I think it was from Orion for around £30. I first suspected that the whole focuser assembly wasn't square with the OTA, but I assume that since collimation is fine without the helical, then the helical is the problem. Still, last night I used it with the f5 10" and was able to tease out some Martian detail which perplexingly I couldn't get from my f8 150mm evostar (maybe too much glare / CA?), so I'll be sticking with the helical

Paul

[chuenmanc]

Aha

I guess in that situation it is down to how tight the tolerance is in the machining. Applying the set screw could then affect the collimation.

Borg's helicals are really well machined, but of course cost a lot more than £30