Helical Focuser - 31mm fitting

[badhex]

Hi all

I've been looking at ways to improve the focuser on my Skymax 102 for imaging. I appreciate it's not the most ideal scope for the job but I have had some good results using a modded Lifecam, and well, one must work with what one has!

I came across a post a few months ago on Jim Easterbrooks site http://www.jim-easterbrook.me.uk/astro/scope/m4/borg/ about converting to a Borg helical focuser for fine focusing and decided I'd like to have a go at something similar - albeit on a somewhat lower budget

I got a bit trigger happy on that well known auction site and ended up buying this: http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/PqEAAOSwk5FU0uf-/$_57.JPG

It has a 31mm with 0.45mm pitch outer threaded fitting on the telescope side but I'm struggling to find anything that fits that thread. Ideally looking for something that would adapt it to T thread, or 36.4mm which is the size of the current diagonal holder on the Skymax - basically something more standard! Does anyone know what this 31mm thread actually is? Is it standard on some scopes?

I found (alas after I'd already purchased) this on 365 Astronomy which has an adaptor included to take it up to 36.4mm, I've asked them if such a thing is available separately. 

Anyone got any ideas?

[badhex]

Apologies, missed off the 365Astronomy link: http://www.365astronomy.com/1.25-Non-rotating-Helical-Micro-Focuser-with-T-thread-and-0.05mm-Scale.html 

[jpgr]

the link to the image of what you bought doesn't work.

assuming its similar to the one on 365astronomy can't you use the same rings that jim easterbrook used?

[keithatrochdale]

I have been impressed by the focuser's produced by Peter Drew from old camera lens, you can often find suitable lenses going for a song on e-bay.

Maybe he will share how it is done with us.

.

[badhex]

Apologies, here's the link to the original listing: http://www.ebay.co.uk/itm/Helical-Micro-focuser-for-telescope-1-25-034-eyepiece-holder-for-precise-focusing-/191538169628?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2047675.l2557&nma=true&si=wZSnVNwIJ4IWO9qqxkoJ0XpIji4%253D&orig_cvip=true&rt=nc

Jpgr, I was hoping that I could in fact do the same thing as Jim Easterbrook but the Borg helical focuser has a 36.4mm thread and so fits, whereas the focuser I have is only 31mm

Keithatrochdale, sounds interesting. I have seen some of these before. 

[jpgr]

according to the description, the focuser has a t2 female connector on the telescope side.

if your tube eyepiece holder is the same as my mak 90 it should have a t thread on it - in which case the focuser should screw straight on to it.

[badhex]

Hmm, not sure where you saw that - but it's definitely not T-thread on the telescope side, only the eyepiece side unfortunately. I'm not sure if this 31mm thread is unusual or I'm missing something.

[badhex]

Oh, and I think that the Mak 90 is the same connector as the Mak 102, from what I can see.

[jpgr]

sorry - i was looking at the tsoptics item and not the original.

funnily enough, i've just bought one of these helical focusers from skys the limit (before i saw the 365astronomy item).

the one i bought came with adapters for 1.25" and 2" fittings.

if you email them they might be able to get an adapter for you.

[badhex]

Thanks jpgr - will get in touch with them.

[happy-kat]

I can't find a helical focuser on Skys the Limit.

[Peter Drew]

As Keithatrochdale mentioned, SLR lenses can be used as telescope eyepiece focusers provided that once the glassware is removed there is enough aperture for fitting an eyepiece. To be honest, access to a lathe is pretty much essential to effect the modificatiion. Even the cheapest camera lenses seem to have excellent smooth action with non rotation of the eyepiece.   

[badhex]

Thanks Peter and everyone else for the replies. I very much like the idea of of the camera lens conversion I love a good project (my girlfriend and mates would say this would be project no. 5,643)! Sadly, I have no access to a lathe and moreover I have now go bought focuser, so will have to try and persevere

Will still need to find some sort of adaptor though. I'll see how the sky's the limit thing pans out. Happy-kat I haven't looked yet, but you say you were unable to find it? Jpgr are you able to provide a link?

[jpgr]

skys the limit focuser: http://www.ebay.co.uk/itm/Skys-the-Limit-1-25-2-Telescope-Micro-focuser-set-/161051352119?pt=LH_DefaultDomain_3&hash=item257f689c37

but the 365astronomy unit (http://www.365astronomy.com/1.25-Non-rotating-Helical-Micro-Focuser-with-T-thread-and-0.05mm-Scale.html) looks just as good for half the price.

i emailed skys the limit about attaching the focuser directly to the scope, as shown on the jim easterbrook page, and they think it should be possible.

what i find confusing is - the 365astronomy focuser page states:

Telescope side thread is 31mm with 0.45mm pitch.
M36x1 (36mm x 1mm pitch) threaded adapter to attach to a telescope is included.

If you wanted to attach it via T-thread or M48, we would recommend the following adapter: http://www.365astronomy.com/M48-M48-Positive-Converter-with-T2-and-M36.4-Negative-Threads.html.

however the internal thread of this adapter measures M36.4 which i assume screws onto the telescope side of the focuser.

they seem to be measuring the same thread in two different ways - maybe someone can clarify this.

[badhex]

Hmm intriguing. so, it looks like the Sky's The Limit one comes with 1.25" or 2" adaptors that you put into your normal eyepiece holder, then the focuser screws into that.

It does look identical to the 365 Astro one but with eyepiece holder adaptors. 

I assumed that the 365 astro adaptor is basically a step up ring - internal thread on the ring should match the 31mm thread on the focuser, with the external thread of the ring being 36.4 which seems to be a more standard thread size (and compatible at least with Borg stuff and the Skywatcher.)

http://www.jim-easterbrook.me.uk/astro/scope/m4/borg/parts.jpg

The above photo shows the disassembled skywatcher eyepiece holder - the knurled ring and retaining ring would then fit onto the 36.4mm thread of the step up ring.

Phew. Complicated.

[happy-kat]

Thank you for the links, I have had the 365 bookmarked for a while.

[jpgr]

badhex:

did you ever get the eyepiece tube apart?

ive had a go at the retaining ring but its starting to show a little wear and still hasnt budged.

i dont want to wreck it if i can help it.

[badhex]

Not tried mine yet. I understand they can be quite stiffly attached. Careful application of WD40 maybe?

Do you have a lens spanner? I don't have a proper one - but did make one for cracking open the lens cell of a 'frac from a piece of 1x1 with a couple of thin nails hammered through. I then blunted the nails so as not to damage the retaining ring, perhaps you could do something similar: 

http://stargazerslounge.com/uploads/monthly_08_2014/post-8274-0-80553700-1407417598_thumb.jpg

[jpgr]

i dont have a lens spanner.

i tried using two allens keys sandwiched between the leaves of a closed hinge but stopped when i heard grinding noises - plus it wasnt moving.

i dont know how much force and leverage is needed and dont want to totally ruin the retaining ring.

i shall leave it to soak in some wd40 and try your wood and nails approach.

[jpgr]

i left it for a couple of days with 3 in 1 oil applied.
the wood and blunted nails worked perfectly.
the 36.4mm ring is reversible so any scratches disappear when you turn it over.

the 45mm knurled ring binds against the body of the focuser, so it needs a 45mm od 36mm id washer or something similar. i've found a rubber grommet that i might be able to do something with.

did you have any luck finding your M36x1 ring?

[badhex]

Awesome, glad that worked. Nice to know that two nails and a bit of wood does the job of a £40+ tool!

No joy on the M36 ring. I still don't know what this 31mm x 0.45mm pitch is actually supposed to be for! 

[jpgr]

referring to the image at http://www.ebay.co.uk/itm/Skys-the-Limit-1-25-2-Telescope-Micro-focuser-set-/161051352119?pt=LH_DefaultDomain_3&hash=item257f689c37,

the ring with two notches on the focuser unscrews. the inner thread is 31mm the outer thread is 36.4mm.

the retaining ring, from the telescope eyepiece holder, screws onto this notched ring and holds the 45mm knurled ring in place.

the 45mm ring binds on the focuser so needs a washer in between it and the focuser body. ive found a site that cuts individual washers to a specified size. once ive double checked on the dimensions needed, i'll order one and post the results.

try http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=1535 or http://www.alpineastro.com/eyepiece_adapters/eyepiece_adapters.htm for your missing 31mm to 36.4mm adapter. if they dont provide the exact item they may be able to suggest something or someone else (31mm circlip?)

[badhex]

the ring with two notches on the focuser unscrews. the inner thread is 31mm the outer thread is 36.4mm.

Yeah this is exactly the bit mine doesn't have. I'd guessed that this is how it would all fit together earlier in the this thread - thanks for confirming. Will try the suppliers you mentioned - thanks!