Recommendation needed: non-rotating helical focuser

[Richard136]

Hi all

I'm looking for a helical focuser to assist in fine focusing. Ideally non-rotating, but will accept a rotating one.

There's seemingly a huge array of these things and a bit confusing how to attach to the scope.

I'm looking for either a 1.25, or preferably a 2" version, that simply fits into the standard 1.25" or 2" eyepiece slots. Alternatively something that screws directly onto the focuser of my scopes (but how to know what thread those use; have several scopes all with different threads?)

Also: since these devices obviously lengthen the path, do they actually work for visual on refractors and Newt's? (assume CATs are fine since the mirror moves)

Am lost in the woods atm in terms of threads and diameters, etc. Help?!

 

Edited December 10, 2021 by Richard136

[johninderby]

The Svbony helical, focuser is pretty good and not too expensive. You can get it to fit a T2 or SCT fitting or with a 2” nosepiece like this version. It is non rotating. I have the T2 version on my mini mak.

https://www.amazon.co.uk/Svbony-Finderscope-Astronomical-Photography-Observation/dp/B0836Z8YZV/ref=sr_1_9?keywords=svbony.+helical%2Cfocuser&qid=1639145078&sr=8-9

Edited December 10, 2021 by johninderby

[Cosmic Geoff]

I have a Svbony helical focuser (non-rotating) and a Chinese one that looks the same. You can get these things with various different threads on the inner end, to suit your kit.  Mine screws onto the focuser of a 102mm f5 Startravel, adapting it for a planetary 1.25" camera.  The extra length is no problem if you can omit a diagonal used for visual.

Edited December 10, 2021 by Cosmic Geoff

[StevieDvd]

I have the one John has linked to. Note there are 2 versions that SVBony do, the more expensive one is aluminium adding roughly £10 to the price of the one I have.

So many adapters make it essential you select the right one, as the adapters on their own are quite expensive compared to the focuer & adapter pricing.

The 2" one I bought was based on the fact that a standard 2" to 1.25" adapter had the same filter thread to fit on the SVBony spotter scope, and it did indeed fit perfectly.

[Richard136]

Thanks, all.

I had identified the SVBony versions on Amazon, and also noticed the adapters are costly compared to the rig itself.

Think I may go for a version with 2" SCT thread and then buy a 2"-to-T2 adapter to enable it to used on both the SCT and the refractor.

[wookie1965]

5 hours ago, johninderby said:

The Svbony helical, focuser is pretty good and not too expensive. You can get it to fit a T2 or SCT fitting or with a 2” nosepiece like this version. It is non rotating. I have the T2 version on my mini mak.

https://www.amazon.co.uk/Svbony-Finderscope-Astronomical-Photography-Observation/dp/B0836Z8YZV/ref=sr_1_9?keywords=svbony.+helical%2Cfocuser&qid=1639145078&sr=8-9

Will any like this fit a Tal 100rs and then I could put a 1.25" eyepiece in please. 

[johninderby]

7 minutes ago, wookie1965 said:

Will any like this fit a Tal 100rs and then I could put a 1.25" eyepiece in please. 

I’m not sure what thread is on the TAL draw tube? 🤔

[vlaiv]

6 hours ago, Richard136 said:

I'm looking for either a 1.25, or preferably a 2" version, that simply fits into the standard 1.25" or 2" eyepiece slots.

Here is one - but you'll have to add M48 to 1.25" (or 2") adapter for eyepiece side:

https://www.teleskop-express.de/shop/product_info.php/info/p9360_TS-Optics-2--Non-Rotating-Helical-Focuser-with-M48-connection.html

It comes with 2" nose piece or M48 on telescope side if your focuser has that thread (that is standard 2" filter thread).

[Richard136]

Thats the snag isn't it: always need an adapter!

[vlaiv]

4 minutes ago, Richard136 said:

Thats the snag isn't it: always need an adapter!

Probably because these are most often used for imaging, but addition of M48/2" adapter should be easy thing to do:

https://www.teleskop-express.de/shop/product_info.php/info/p4201_TS-Optics-Adapter-from-M48x0-75-to-2--Receptacle.html

That is simple compression ring, but if you want something more fancy - Baader does one with twist lock action.

https://www.teleskop-express.de/shop/product_info.php/info/p9516_Baader-2--ClickLock-Adapter-Clamp-with-M48-connection-on-telescope-side.html

[wookie1965]

1 hour ago, johninderby said:

I’m not sure what thread is on the TAL draw tube? 🤔

No I am not or even if there is a thread I will check tomorrow thanks John. 

[IckleImp]

Not trying to derail the thread, what type would I need for my 102 Skymax. Would it change the focus distance and could I still use my diagonal?.

My Travelscope 70 has an M42 thread. Would I be able to use one on that also?

Thank you

[StevieDvd]

2 hours ago, IckleImp said:

Not trying to derail the thread, what type would I need for my 102 Skymax. Would it change the focus distance and could I still use my diagonal?.

My Travelscope 70 has an M42 thread. Would I be able to use one on that also?

Thank you

There is an M42 adapter option, that would probably be before the diagonal on the Travelscope and change the focus a little. 

A little trickier on the Skymax as the diagonal has a dedicated connection to the rear of the scope and I doubt it would be a good idea to have a 1.25" helical focuser inserted to the existing 1.25" side of the diagonal.  Unless you can get the 1.25" part of the diagonal off and match the adapter for the helical focuser in it's place.

 

[vlaiv]

3 hours ago, IckleImp said:

Not trying to derail the thread, what type would I need for my 102 Skymax. Would it change the focus distance and could I still use my diagonal?.

My Travelscope 70 has an M42 thread. Would I be able to use one on that also?

Thank you

https://www.teleskop-express.de/shop/product_info.php/info/p7051_TS-Optics-1-25--and-T2-Micro-Helical-Focuser---T2-connection.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p9107_TS-Optics-T2-Reversion-Adapter-with-full-length-T-thread--female-.html

You'll be able to reach focus on Skymax without issues, even with diagonal as scopes with moving mirrors have quite extensive focus range.

It will however slightly alter focal length of instrument - as it depends on separation between mirrors, but this effect is very slight ~50mm on 1300mm that you already have, and there will be slight change in spherical aberration - as it also depends on mirrors separation, however this is even smaller change and you will not notice anything (change is probably order of 1/10th of the wave - and it really depends on what is perfect position anyway, so you might even improve spherical correction).

With Travelscope 70 - it depends on how much back focus your scope has. Above combination is likely to eat up around 60mm of optical path and if you need to extend current focuser ~60mm or more when viewing with diagonal in place - then it will work.

By the way - that helical focuser that I linked can be found on AliExpress for about $25. I haven't checked on Ebay - but it is likely you'll find one there as well for less money - just make sure it is T2 version (there are several different versions with different threads).

For example:

https://www.aliexpress.com/item/4000922964606.html?spm=a2g0o.search0304.0.0.59342d532mDgqJ&algo_pvid=011411a1-af3e-4bab-b563-a5f45344b2bd&algo_exp_id=011411a1-af3e-4bab-b563-a5f45344b2bd-0

[IckleImp]

Is the Skymax also a T2? 

[IckleImp]

This is an M42 × 0.75 I assume this is the same as a T2.

 

https://www.ebay.co.uk/itm/SV108-Full-Metal-durable-1-25-High-Precision-Double-Helical-Focuser-Brass-Ring-/284553551239?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=710-127635-2958-0

Edited December 11, 2021 by IckleImp

[vlaiv]

1 hour ago, IckleImp said:

This is an M42 × 0.75 I assume this is the same as a T2.

Yes - there are two types of metric pitches for threads - fine and coarse.

M42 is usually refers to M42x1mm - coarse metric thread, while T/T2 refers to M42/0.75mm - fine metric thread.

Astronomy equipment uses T2 - or M42x0.75 as standard thread, while old lenses use M42x1mm as their thread - like this one for example: