Helical vs. fine-focuser

[The Warthog]

I have some difficulty with my standard Celestron Newt focuser, in that I find it difficult to find the exact spot at which the focus is best, what with the little shiver imparted to the tube by my fat fingers, and the fact that the focuser wheel is an imprecise instrument.

I am embarking on building an 11.5" dob (when my wife isn't looking, as I have been replacing all the door in the house lately...) and am wondering if a helical focuser would work better, in terms of reaching a precise focus, or if the difference in a 10:1 fine focus type focuser justifies the extra expense. Remember that I am not doing any significant amount of photography, I'm jus' lookin' at stuff.

Thanks for any suggestions.

[John]

Hi WH,

I used to use a helical focusser on my 1st 6 inch newt, many years ago. It worked very well for fine focussing but was a but of a pain if you needed to move the focus position in or out a fair distance as the movement was quite slow. In those days my eyepiece collection was far from par-focal

I spotted this accessory from Orion (USA) a while ago which might be of interest - it seems to be a helical adaptor that you put into a normal focusser so you then have the best of both worlds. It might be the sort of thing that you are looking for both for your current scopes and your new project:

http://tinyurl.com/655sku

Regards,

John

[Reggie]

If focus is your issue you might want to try looking at this thread it might be an alternative for you.

My 6SE mount/ota also suffers from terrible shakes when touching the focus knob which makes focusing a real pain, to get over it I made an electric focuser. The other thing you might like to try is to make the diameter of the focus knob larger as that will give you more precision, I went through many round plastic household objects before I found something suitable.

[The Warthog]

Thanks very much, guys! John, that helical adapter is just too cool! If my C6N had a 2" focuser, I'd get one now. Reggie, I've used a Hartmann mask with other scopes, and if I wasn't so lazy, I'd have made one for my 6" already.

[Reggie]

Hi Warthog, I realise it looks similar to a hartmann but its really a hybrid and gives much much better results, the spacing of the slots is worked out to a forumla according to focal length. It's more of a cross between a hartmann and a stiletto focuser. btw. the helical won't help with scope wobble but they do look very cool.

[The Warthog]

Hi Warthog, I realise it looks similar to a hartmann but its really a hybrid and gives much much better results, the spacing of the slots is worked out to a forumla according to focal length. It's more of a cross between a hartmann and a stiletto focuser. btw. the helical won't help with scope wobble but they do look very cool.

Thanks for that. The scope wobble is less of a problem than the fiact that I find myself going from just one side of perfect focus to just the other side a lot. I think either a Helical or a 10:1 would give me more wiggle room.

[A McEwan]

I'd suggest the Skywatcher auto electric focuser: http://www.firstlightoptics.com/proddetail.php?prod=swautofocuser

I've got two, one on my 8" Dob and one that was on my 6" Newt. They can also fit refractors. It makes focusing completely vibration-free, and has variable speed so it is dead easy to find perfect focus. And it's cheap!

Ant 8)

[Astronut]

There's always the dual speed Moonlite - you can fine focus with just your little finger with around 3lbs of glass grenades attached to the tube (eyepiece, powermate + paracorr) - no vibration or wobbly images at all. Silky smooth

[Reggie]

I found that it was the wobble that was causing me to go from one side of focus to the other, the mounts fault, not the scope but frustrating going over and over to get good focus.

[Astro_Baby]

I'd second that Astronut - I have mine fitted now. The fine focuser with 1:10 is AMAZING, smooth even with the 38mm Panaview on it which is no lightweight EP.

I havent gone to the trouble (yet) or making sure the focuser is aligned because to pull that off I need a laser collimator and if I add any more expense I'll find myself single again

[Astronut]

Mel: You don't need a lazer collimator to mechanically align a focuser body to OTA. The method I used was careful measurement of the hole for the focuser in relation to the tube end and a piece of paper wrapped tightly around the circumference of the tube. By carefully marking the hole position on the paper and the halfway point of the circumference on the paper (while flat on the table) you can spin the paper 180 degrees around the circumference and mark a point on the opposite side of the tube that is exactly opposite the centre of the focuser hole. A small sixteenth inch drill hole in the tube at this point marks the spot and the focuser body can be tilted/adjusted with a crosshair sight tube in the focuser aligning on the small hole on the opposite side (with the secondary removed of course).

That's not a very good explanation and I can't find any links that explain the measurement method. In any case, it's not absolutely critical to get the focuser body at perfect right-angles to the OTA. As long as it looks near enough, that's OK. The optical alignment is what matters which is taken care of when you collimate.

The only reason I went for perfect mechanical alignment of the focuser body is because I'd stripped the OTA bare. In any case, the final collimation adjustment (using the Catseye autocollimator to get the multiple triangle reflections stacked) is best done ie. easier and more precise, by slight adjustment of the focuser alignment which slightly throws off any perfect mechanical alignment you initially started with.

[Astro_Baby]

Thansk for that astronut. I messed about trying to come up with a perfect 'quaring' system and gave up because I couldnt work out how you'd know where the exact centre of the focuser hole was with any reliability other than a laser. In the end I decided best to leave it and just use a set square to test the focuser body was at 90' to the tube and then tested with a bubble level on the top of the focuser to see if it was square and true.

Collimatated aftewwards as normal and collimation all squared up with the Cheshire tho a cats eye system is definitely on the shopping list after the extra weights, RDF, flocking, better EPs, fan module etc etc etc

You know how it is - the list goes on and on and I thought this was a cheap hobby

[The Warthog]

That's two very good ways of doing the job! Actually, people lined up their focusers for centuries before the laser was invented, so there should be good ways of doing it. I like Astronut's for its simplicity.