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Is My Focuser Fickle?

It’s a clear, dark, beautiful night.  The seeing is off the charts and the transparency is even better than normal. And so far the scope you’re using has failed to split about half the double stars on your list.

Is it the optics?  Maybe.  But it could also be the focuser.

One of the most over-looked items of essential astronomy equipment — and to me one of the most critical — is a good focuser.  Too much tension on the gears is an obvious problem — you can feel it every time you turn the focus knob.   Every bit as frustrating — and worse because it tends to go unnoticed — is poor gearing.

If your focuser is geared improperly — as in too fast — you won’t be able to make the very minor adjustments in focus which are required to pull faint secondaries out of the glare of bright primaries, or split very close doubles.  What happens is that when you turn the focus knob a slight amount, the gearing is so inadequate that the focuser moves right past the zone of critical focus.  And more than likely you won’t even realize it’s happening.

I’ll give you an example.  I have a C102 — a 102mm Celestron achromatic refractor — that was performing abysmally on double stars.  It seemed like about seventy-five percent of the time it couldn’t split two stars if they were at opposite edges of a wide-field eyepiece.  I had just about decided the optics were to blame — except that every now and then it would surprise me and split a difficult double.  After a couple of nights of that, I realized the problem was in the focuser gearing.  Just by pure luck, I would occasionally land on the critical focus, and it would seem like the scope was performing beyond it’s 102mm limits.  But if I de-focused and then attempted to focus once more — or swapped eyepieces — it was usually impossible to find that perfect focus again.  The focuser would move right past the critical point so quickly my eyes couldn’t detect it.

So how did I cure it?  First, I tried changing the focuser tension by adjusting the four screws on the metal tension plate that pins the focuser to the gears.  If you try that, loosen all four screws and then re-tighten them in an X pattern — top left, bottom right, top right, bottom left — just to the point where they become snug.  Then rack the focuser in and out a few times and check the screws once more.  One or two will probably be loose, and the others will likely be very tight.  Take the pressure off the tight ones by loosening them again and then adjust them so they’re snug once more — then tighten the loose ones until they’re also snug once again.  Repeat that process several times until the screws remain snug — if nothing else, you’ll at least have a focuser that turns more smoothly.

For a variety of reasons that doesn’t always work.  It worked well on an Orion 90mm refractor — but I had no luck at all with it on the C102.  So I took a deep breath, ordered a two-speed focuser that cost more than the scope, installed it — and found it transformed a poor scope into an excellent one.  Since then, I’ve used that scope to pry more than a few faint secondaries out of the glare of much brighter primaries.  Take a look at the posts on OΣ 111 in Orion and Propus in Gemini as examples.

Over the years, I’ve used several achromatic refractors, and almost every single one of them had poorly geared focusers.  That list includes Meade’s AR-5 and AR-6, and Celestron’s C102 and C6R.  I ended up installing two-speed focusers on all four of those scopes.  Without exception, everyone of them became an absolute joy to use when the new focuser was installed.

The beauty of a two-speed focuser — or even a very well geared single speed focuser (Astro-Physics has an outstanding one) — lies in its ability to achieve a critically perfect focus.  Astro-photographers are very familiar with critical focus, but I suspect a lot of visual observers don’t realize how critical “critical” is.  But if you want to coax that last possible photon out of your scope, you’ll be a whole lot more successful a whole lot more often with a top quality focuser.

To literally “see” what I mean, try this on a dark, moonless night, assuming you have a two-speed focuser, or a very good single speed model.  Point your telescope at a star field that is reasonably dense — not the heart of the Milky Way, and not one with a first magnitude star in the field.  A good one to use is the open cluster Steph 1, which surrounds Delta (δ) Lyrae, but almost any field will show something similar to what follows.

Bring the scope to a focus on the brighter stars in the field.  Now, look away from the bright stars toward the fainter parts of the field, and turn the fine focus knob very, very slowly.  At some point — and it won’t take more than a few seconds, you’ll see a few very faint stars begin to appear.  As you very gingerly advance the fine focus knob a bit further, you may catch some others popping into view, especially with averted vision.  If you continue to nudge the focuser along, the faintest of those stars will begin to fade from view.  And all that time, if you check the brighter stars in the field, you’ll find no change — or very little — in their appearance.  And you’ll also find you’ve only turned the focus knob a millimeter or two, if that.

The point at which the faintest stars in the field pop into view and stay there is what I’m referring to here as critical focus.  Try that same exercise using only the high speed focus knob, and unless you have an outstanding focuser, coaxing those very faint stars out of the background will be very difficult, if not impossible.

Is a two-speed focuser a necessity on a long focal length refractor?  I’ve found they can be a huge help, especially when working at the optical limits of your scope.  While the depth of focus is shorter — and consequently more critical — in a short focal length scope (let’s say f/7 or faster), even with the greater depth of focus provide by longer focal lengths, there is a small area of critical focus waiting to be mined.

For example — I was using a 60mm f/16.7 (focal length of 1000mm) on Polaris a few nights ago, and found I was having trouble seeing the 9.1 magnitude secondary.  Normally I can pry it out of the glare without too much problem using a 60mm scope, but sometimes poor transparency or too much moisture in the air can make it difficult.  I wrestled with the old single speed focuser for a good five minutes, nudging the focus knob along until finally the secondary popped into view.  Just a slight nudge past that point — and it was gone again.  A two-speed focuser would have saved me a lot of time.  Unfortunately, I don’t know of any that are made for 60mm scopes.

So the moral of this whole story is this:

Having a focuser that will allow you to reach the critical focus I’ve described above will very often make all the difference in the world in how well you succeed in splitting difficult double stars — and as a bonus, it can turn a poor performing scope into a real treasure.


16 Responses

  1. Terrific – and very helpful post John.

    I wonder how you feel about electronic focusers? Have you had much experience with them? Are they a good – and perhaps cheaper – substitute than going to a two-speed? Can they overcome the kind of gearing problems you mention?

  2. My only experience with an electronic focuser was on a Meade ETX-90 — it had a focuser with virtually no ability to nudge it into the critical zone. The electronic focuser was a slight improvement, but the gearing on that scope was really so poor it was almost beyond any hope of improvement. And I found the whirring of the focus motor tended to shatter the magical silence of a peaceful dark night.

    There are a variety of electronic focusers available, and I suspect that the best do a very good job. Orion offers a less expensive model that fits many of the focusers on their scope, which generally gets favorable comments.

    The one thing I noticed immediately when I had the electronic focuser on the ETX-90 was a feeling of being separated from the scope by a technological barrier. When you operate a focuser manually all the time, you take the tactile sensations that come from adjusting it for granted.

    What I mean is this: On a very smooth operating focuser, I find all the things that come with turning the focus knob blend into one sensation — the extension of your arm as you reach for the focuser, the feel of the focus knob on your fingers as you turn it, the immediate reaction of the image as your fingers slowly adjust the knob, and the visual recognition that takes place instantly as that happens. All that takes place in a fraction of a second, and I experience it all as one event operating on my tactile and visual senses at the same time.

    The electronic focuser leaped right into the middle of all that and disturbed it. No doubt it’s a matter of making the adjustment to a different way of focusing. But I like things simple, so I’ll stay with the old tried and true fingers-on-the-focus-knob-approach.

  3. Hi John

    Poor focuser gearing can be frustrating, for those out there who cannot afford a new focuser,I have found not using the focuser to really help in obtaining critical focus. I move the eyepiece to focus ,then tighten the thumb screw. I have a 60 mm meade refractor
    with very poor focuser gearing yet I can obtain sharp focus of epsion lyrae using this method. Compared to using a poor focuser moving the eyepiece is more controllable and enjoyable!!!


    • Hi Michael,

      That is a GREAT idea! I love it! I’ve adjusted an eyepiece that same way in a diagonal because it wouldn’t reach focus any other way, but I never thought of using that approach to get around a poor focuser. Absolutely one of the best ideas I’ve read in a long time. It really is ideal for a 60mm scope, since most of the focusers on them leave a lot to be desired.

      Now that I think about it, another approach might be to use a helical focuser — such as the one Orion used to sell, and may still have — which drops into the diagonal ahead of the eyepiece. There may be a problem with vibration when adjusting it, but it would get around the poor focuser issue and still be reasonably affordable.

      At any rate, thanks for the idea. That one is very much appreciated.


  4. Hi John and Greg,

    Interesting discussion on focusers. I have been engaged in chasing down the DSC list for awhile now, and have a few scopes to work with. One of those is a 60mm F15 achromat, and I have yet to succeed is seeing Polaris’ companion in that scope. Part of that may be my less than stellar sky (30 miles due south of Boston), but I also believe that part of the problem lies in the optical train of the scope. I think the quality of the objective lens is very good, if not excellent, but the focuser leaves something to be desired. Specifically I’m speaking about the focuser tube being able to look down the middle and retain collimation. Every low-cost scope I’ve encountered thus far seems to have the same lack of attention to this particular aspect of focuser construction. I wonder if addressing this issue would reduce the effect of poor gearing on achieving critical focus?

    I’m currently in the process of overhauling a recent acquisition, that being a Meade 390 refractor. It’s not done yet, but I have completed the focuser portion, and you can see my approach to resolving what I see as a major focuser isssue here: http://assne.org/board/viewtopic.php?f=5&t=2993 I can’t wait to complete this project and see how it works out. This scope exceeds the critical formula for spurious color elimination in an achromat refractor (2.88xD*), and I’m looking forward to exploring some doubles with it in the near future.

    Someday I’ll get around to addressing the issues in the 60mm focuser, and I’m hoping it brings out that last little bit that I just know is hiding in there.

    Mike McCabe

    PS – Thanks for the effort you guys put into this site. I’ve enjoyed it much.

    • Hi Mike,

      Thanks very much for your comments on the site and the radio show — they’re very much appreciated.

      I read your piece on the Meade 90mm focuser and you really had my attention for the whole thing. Wish I had that kind of machining facility — it would certainly open up a whole new avenue of approach to many problems I run into with telescopes and accessories.

      I have what looks like that very same focuser on an Orion 90mm f10 and was able to improve it by working the tension back and forth on the four screws in the focuser tension plate as I described in the post above. It still suffers from gearing that is too fast, but at least now I’m able to coax the focuser into the critical zone with very careful movements of the focus knob.

      Also, I’m very familiar with the drawtube problem you refer to on the older 60mm scopes. Many of the cheaper models are severely afflicted with the drawtube droop. The better models have a delrin-like material that sits inside the focus knob end of the drawtube and takes up the space between the inside of the focuser body and the drawtube. If done well, it eliminates about 98% of the droop and provides a very smooth surface for the drawtube, allowing it to glide smoothly through the focuser.

      That approach should solve the problem you referred to with your 60mm scope. If the spacing between the inside of the focus tube and the drawtube is small enough, you might try using a very smooth grade of scotch tape. Clean the inside of the focuser so the tape will stick, and then apply it length-wise at three evenly spaced locations around the inside of the focuser. You may have to build up three or four layers of it to reach the point where most of the droop is eliminated, and yet still allow the drawtube to glide through smoothly.

      At any rate, I’ve had success with that a couple of times. The non-sticking side of a good grade of clear scotch tape provides a very smooth surface for the drawtube to slide on.

      Hope your skies are clear!


  5. Thanks for the kind words Mike – and for an important addition to this discussion.

    I checked out your link on the ASSNE Bulletin Board – very impressive – and your “tinkering” skills are so far beyond my own I can only stare in envy. But I have several classic scopes and I have often wondered about all that play in the focusers. John will probably have something useful to add.

    I also have trouble splitting Polaris with anything less than 80mm – though I have done it, most of the time I can’t. John has darker skies by about a magnitude and doesn’t seem to have any problem with it using a 60mm.

  6. Great information John as always. Your experience with the 4 inch Celestron really drives home the importance of smooth focusing to achieve maximum performance. One other point that might be discussed is vibration induced in the image due to stiff focuser or a less than adequate mount or both. I have an old Bausch & Lomb 8001 SCT that has excellent optics, but the stiff focusing mechanism made it a real chore to achieve critical focus at high magnifications. I am into webcam planetary imaging using this scope with 3.2X barlow amplification(F32). The image scale on the CCD chip is equivalent to using a 1.6mm eyepiece visually. Merely touching the focus knob sends the image on the laptop dancing making focus a trial and error procedure. The problem was solved by using a JMI motofocus which smoothly and precisely achieves focus. Needless to say it is also great visually on close doubles making precise focus a breeze. Visually at the eyepiece it is beautiful to watch the expanded diffraction disk of a star in excellent seeing slowly shrink down to a precise round airy disk at 450x,… and do so without any vibration, jerking or movement. I also have a low cost 90mm Meade DS Series go-to refractor that was very difficult to focus at high power due mainly to mount vibration. I solved that problem by modifying a Tasco 1603 electric focus to fit the meade . No problem achieving precise focus since I don’t have to touch the scope. Even though I would prefer a rock solid mount and a smooth precise focuser, electric focus motors have a place. The only scope I have that does not benefit from an electric focus is my 5 inch F9.3 achromatic on an Antares pedestal. The mount and pedestal are solid enough to make manual high power focusing relatively easy. For those who do not prefer electric focusers an alternative is a helical focus adapter that Orion still may be selling. It is a 2 inch to 1-1/4″ eyepiece adapter with a fine thread helical focus for the eyepiece.

    Clear Skies,

    • That JMI electric focuser sounds rather appealing, Karl. I could be tempted to give that a try. Your description of it in operation is terrific.

      I wholeheartedly agree that vibration, whether from a stiff focus mechanism or an inadequate tripod, makes things unnecessarily difficult. I tend to use more mount than I need, but I still run into it when using one of the old 60mm scopes on their original mount/tripods. Those beautiful, thin, highly polished wooden legs are very appealing to the eye, but practically speaking, they’re just not strong enough to eliminate vibration — and some of the older “classic” mounts leave a bit to be desired as well.

      What I’ve done is either mount the 60mm scopes on a larger scope, or clamp them in a clamshell, add a Vixen-style dovetail, and then use them on a larger, more sturdy mount. I’ve managed to find a couple of Unitron 60mm clamshells that work great for that, although I’ve need to add a little felt to them because the Unitron tubes are slightly larger than sixty millimeters. Removing a clamshell clamp from an old 60mm mount works just as well.

      There’s always a way to deal with this kind of stuff — sometimes half the fun is finding a better alternative to the existing situation!

      • Hi John. I agree with you about some of the older classic refractors. I have a mid 1960’s 60mm F15 made by Towa and marketed by Sears. It has a smooth focuser but has the extendable drawtube and suffers from the droop you mentioned earlier. To solve that problem I merely push the drawtube all the way in and install an extension tube (an old cheap barlow housing with lens removed works well) in its place so the scope will reach focus. With the drawtube all the way in there is no light cut off and the full aperture of the lens is visible when peering into the diagonal. The equatorial head and tripod are adequate but just barely. The slow motion cables are spiral wound spring steel and induce a swaying vibration to the mount when released after turning. The solution is to use heat shrink tubing (available at Radio Shack ) over the spring steel flexible cable. At least the tripod is wood which dampens vibration much better than the newer aluminum tripods included with low cost scopes today.
        My Apogee 5 inch F9.3 refractor is using a CG5 clone mount attached to an Antares pedestal. The original rectangular leg aluminum tripod that came with the mount was terrible. Focusing at high power was a real chore. I converted to a wood tripod and vibration was much less but still too excessive. I finally went to an Antares pedestal mount. Although not as portable as a fold able tripod, it is a very stable platform and the scope is a joy to use. With this scope in excellent seeing I was able to split the triple star system Zeta Cancri at 295-328X whose AB components at the time (2004) were at .96″. Without the pedestal I doubt if I would have been able to focus accurately enough to reach Dawes limit for the aperture.

        Clear skies,

  7. Have read all of this post and the Replies three times- i have one scope ,an ED 80, with a typical Synta R&P focuser. Have to say it has never “drooped” down but it is easily rocked upwards. I have carried out the suggested procedures but to eliminate the upwards problem requires everything to be so tight the movement is too stiff. At all times it does show completely round discs out of focus which do focus down to a point. Now I`llget to the real point. More than a year ago i bought a Baader 1.25″ prism diagonal. It is part of their “T 2” system which is intended to be configured anyway you like to suit –binoviewers, cameras etc. i just wanted a visual diagonal whichich is what i got. Where you insert the eyepiece has 3 screws to tighten against the internal ring. Maybe 2 more than you need for a normal eyepiece. However if you take a grip of the screws or the knurled rim the are set into, you can turn the eye piece through one and a half complete rotations which moves it vertically upwards 6.5mm. It has its own built in helical focuser! OK- first target-Alnitak. Clear skies to all, rich.

  8. Interesting, Rich. I wasn’t aware of that diagonal, but having that amount of travel should be ideal to fine tune your focus. I’ll look into that little devil. It would be perfect on a 60mm scope that accepts a 1 1/4″ diagonal, and I’ve got a few of those.

    Good luck on Alnitak — get all three of them!

  9. Thanks John, i did just that last night- obviously “C” Alnitak is a welcome but easy taget . The real bonus of being able to get critical focus was that the thick obscuring diff. ring i used to see all too often now became much thinner and more complete leaving a clear view of a round , hard and very definite “B” just 2.5 sec. away. Also at a lower power had great view of the “mist” at the bottom of Kembles Cascade. Liked the ref. to the “Triskthing”from Startrek .-shows well even in the finder. Took me a year to find out this diagonal has this fine tune ability. I believe the actual prism glass is Zeiss so if you have an excellent 60mm front end, this could be a good match at the rear end. BTW am seeing Mars like never before! Clear skies and regards, rich.

  10. Great to hear that, Rich. There is no under-estimating what can be done if you can fine tune the focus to reach that very narrow critical point. Things happen there that just can’t be seen anywhere else. I’ve been trying to get that message across for a while. It has to be seen to be believed!

  11. I’ve put an electronic focuser on my AR6 and it became a true double star killer . Since then I have replaced many focusers but the one that I would really like to change out is my 80mm /1200mm Towa but here we run into a problem because of the long drawtube and a new focuser not reaching far enough .

  12. Hi Mike,

    Without a doubt, a two speed or electronic focuser gives you an extra edge on difficult double stars. I prefer the two speed over the electronic models, but a good quality electronic focuser can do an excellent job.

    I’ve got an 80mm Towa also, and unfortunately there isn’t much you can do with regard to replacing the focuser because of that long draw tube. But the long f/15 focal length helps with obtaining a sharp focus.


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