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Orion Helical Fine Focus Adapter

Posted by mathieu chauveau   12/15/2005 08:05AM

Orion Helical Fine Focus Adapter
This is going to be quite a long review for such a small and cheap piece of equipment, because I think it can be very useful for some, and useless for others. So I’ll let aside the background explanations and introductions on how proper focus has been eagerly sought since the dawn of humanity, except to say I have no connection with Orion or the retailer I bought it from, Optique Unterlinden in Colmar, France.

------What it does------
As I said, when looking at pictures of Galileo’s first scopes, one’s breath is taken by the stark beauty of long-focus refractors, but one also cringes at the thought of focusing that thing, especially with that mount which would probably topple over if you hung your hat on it. These features can still be found on many commercially available telescopes today.

Until maybe ten years ago, your focuser was your focuser: you lived with it, and if ever you wanted to change it, you had few options. Now there are many after-market focusers available, available both for ATMers but also for people wishing an upgrade for their scope.

The typical upgrade focuser today is a Crayford-type focuser costing a couple hundred dollars, and which requires you to disassemble your scope’s stock focuser. The Orion fine focus adapter is not a focuser in itself. It acts as an add-on to a regular 2” focuser, and holds 1 ¼” eyepieces. And it’s only 45$.

-------Getting started-------
There is no installation per se for this thing: you just thread it in the place of your regular 2” – 1 ¼” adapter (if you were not using one of these, it’s unlikely you will use the fine focus adapter – go back to poring over ads for those megabuck Crayfords!). You can put it on and off during a session as you wish.

The first thing one notices is that the 2” skirt is pretty “shallow” and might not hold well in the 2” focuser. Experience shows that with typical 2” eyepiece holders, which either have two tightening screws or a compression ring, this is not a factor. I’ve never had the thing slip out. It’s also lightweight (too light to be weighted with my kitchen scale), probably no more than 2oz.
The second thing is that the 1 ¼” holder is a one-screw thing; like most people I have taken the habit of using compression ring systems. But this one holds tight, there is no wiggling of the eyepiece in the barrel, and I have no horror stories of eyepieces barrelling down to an untimely end.

Then – let’s get to it! You focus by rotating the thing. I usually have the fine-focus adapter screwed halfway up, make the coarse focus with the telescopes regular focuser, and then do the fine focusing with the FFA. There is 8mm of travel, meaning that all my eyepieces focus within the FFA’s range. The travel is very smooth, meaning you are not going to have the image shaking while focusing, and very precise too because you are not moving the eyepiece much with every slight turn. One might notice a little wiggling of the two parts: there is a little play between the female and male threads, which make the focusing so smooth. Some have criticized this wiggling, and it can be disturbing at first, but it does not affect the quality of the focusing (there is no up-and-down play), nor does it disturb the optical alignment (even when trying to twist the thing sideways, I could not cause star images to look miscollimated).

------in the field-------

I have used this adapter on four scopes:
- a Chinese 100mm achromatic refractor with a syntaglue-coated rack and pinion focuser
- a Takahashi FS102 which has perhaps the best rack-and-pinion focuser ever made
- an Intes MN61 maksutov-newtonian with a less-than-perfect Crayford focuser
- a Celestron 9.25, with your typical moving mirror focus system.

------100mm Achromat-------

On the achro, the improvement was that focusing was easier not only because the movement was smoother, but mostly because the mount was not taxed by my biceps giving 110% to rotate the focusing knob.

------Takahashi FS102------

On the FS102, believe it or not, the focusing is smoother with the FFA. Also, the FFA is not subject to gravity like a rack-and-pinion is – with the R&P, you have to fight gravity when moving the focuser upwards, which is towards inside focus; and the R&P focuser might even slip down a little with heavy stuff attached. Probably because it’s a helical system, the FFA does not slide down with gravity. So, this is a 50$ upgrade for a Takahashi, about the price of the FS102’s dust cap!

-------Orion / Intes MN61----

On the Intes, those excellent scopes are plagued by a sub-par Crayford which, at times, won’t focus at all (because the friction is either too tight or too loose). When using 1 ¼” eyepieces on it, I just achieve rough focus with the Crayford, lock it, and observe for the whole session with the FFA.

-------Celestron C9.25-------

On the C9.25, I use it mainly for high-power viewing, and I appreciate this gadget because I don’t suffer from the image shift of the C9.25, nor from the fact that the moving mirror makes focusing difficult (because the slightest rotation of the knob moves the focus a lot, due to the “magnifying” effect of the secondary).

It is the only case where I notice the limits of the FFA, because I usually observe planets seated, with the eyepiece horizontal, and a heavier one like a 6mm Radian will make screwing and unscrewing slightly harder. The movement is still smooth, but a little harder. The induced image wiggling is still less than what you would get with the focus knob.

Our alert readers considering that a $150 aftermarket focuser is a good deal will take notice that upgrading all four scopes would have cost $600 – here you can shuttle the adapter from a scope to another, if you own more than one.


- adds 9mm (1/3”) of back focus, so make sure you have at least 9mm inwards margin if you have a Newtonian or refractor (usually not an issue with a cassegrain)
- fits on a 2” focuser (meaning that if you use it on a telescope with a diagonal, it has to be a 2” diagonal).
- only holds 1 ¼” eyepieces

----What it does well:-----

- focus on scopes with stiff focusers
- focus on scopes with shaky mounts
- focus on scopes with shaky mounts and stiff focusers!
- Focus on any scope with lightweight eyepieces
- Focus on SCTs and MCTs where focus is achieved by moving the primary mirror, resulting in mirror shift and coarse focusing.

---What it does not do well:--

- hold heavy (over 200grams / 6oz) eyepieces under certain angles: when the eyepiece is horizontal and its weight puts the max torque on the “screw”, the screwing /unscrewing motion is not that smooth; the single-screw tightening might also be an issue. Same with Barlows.

--------Forget about it:-------

- placing a diagonal behind it
- 22mm panoptic or other very heavy stuff
- using it on a 1 ¼” focuser, or with 2” eyepieces (in case you had not understood yet)
- Photography, as the eyepiece holder rotates to achieve focus, and the weight of the camera might be a problem.


for visual observing with 1 ¼" eyepieces, one of the best way to spend 50 bucks in this hobby. For other uses, make sure you will not regret a more expensive model.