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New Helical Crayford Focuser that's cooking with GAS*

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#1 jtsenghas

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Posted 23 March 2015 - 11:12 AM

* -- Gravity Assisted Stabilization

 

Here's a thought and prototype I'd like to share on a modification to focuser designs that I think may have merit.  The concept could be used on both helical Crayford designs (which have the axes of the rollers only slightly tipped to the focuser axis) or even as a variant of the more traditional Crayford (which have all roller axes perpendicular to the focuser axis).  It can be slid to close focus, then rotated for fine focus. The thought for this struck me immediately after reading this thread  in which Pierre Lemay shared a link to Kineoptic Helical focusers and showed his similar handmade ones on his large ball scope.

 

The main design concept is to have a pair of widely spaced rollers (and a provision to apply gentle pressure against them) in two different sections as with traditional focusers, but to have the rollers in one plane reversed 180o to allow gravity to always work (in varying degrees to each roller) to increase the pressure of the drawtube against ALL the focuser rollers for ALL telescope altitude angles. The design intent is to use this on any alt-azimuth arrangement where heavy loads of big eyepieces, Paracorr, cameras, etcetera create large moments that make collimation maintenance difficult.

 

The prototype I'm about to show was built to fit on my folding hexagonal 10" dob which was originally designed with a 3" tall fully racked-in 1 1/4" focuser.  As a result, it is taller than most 2" focusers and has a full 2" range of adjustment.  Despite its heavy-looking design, it weighs in at just under 10 ounces, with the 1/8" wall thickness aluminum drawtube being more than half of the total.  Shorter versions would easily be in the 6-8 ounce range with these materials and considerably less if one wanted to start optimizing the design with better materials.  It needs just a few finishing touches including paint, a drawtube thumbscrew (possibly spring-loaded), and an eyepiece thumbscrew, but preliminary shop tests with a laser collimator without those are VERY encouraging.  I'd prefer it if the tube were anodized for surface hardness, but for my use and these low forces I think it should suffice as is.

 

First a few photos of the prototype, then a description of its construction and how it works. It is shown in the orientation it would be for a scope altitude of 45 degrees:

 

GAS HC1.jpg

GAS HC2.jpg

GAS HC3.jpg

 

That tan material the body is made of is a urethane board left over from a water jet fixture project at work.  It is one of the lightest density versions of the product. At 38 pounds per cubic foot it is about the same density as dried birch or ash wood.  I used this material because I don't have a machine shop, just woodworking tools, and wanted to make this in my own garage. I used my Shopsmith as a table saw, wood lathe and drill press to build this. This material turns, drills and sands almost like wood, and taps well for 1/4-20 threads or coarser. The focuser body is 1/2" thick, so it is rigid enough despite being made from such a plastic.

 

Directly opposite each of the four rollers is a Vlier screw with a Delrin tip. 

 

These are stainless steel body spring-loaded plungers of the lightest spring force available in the 1/4"-20 size.

 

Vlier2.PNG

 

The smooth spherical tip requires a force of only one pound to start depressing it, and 3.5 pounds to fully bottom them out.  I've adjusted them near the middle of the range.  I'll explain more on the construction below, I just mentioned these details early to explain how the focuser functions.

 

The design is optimized for a dob aimed at an altitude of 45 degrees.  I'd probably make future ones with slightly smaller angles about the vertical plane (perhaps 50o rather than 60o) to further reduce stresses on the roller screws now that I see how well it works, but all rollers and their opposing pushers are 60 degrees from the vertical plane that passes through the focuser axis at a scope 45 degree altitude for this prototype.

 

On this prototype at this scope angle the rollers furthest from the eyepiece are about 1 1/2" below the top surface at 10 o'clock and 2 o'clock. The closer rollers are 1 1/4" further from the base of the focuser, just below the top, and are at 8 o'clock and 4 o'clock.  This CAD wireframe view may make the roller arrangement clearer, although it shows the cylinders that correspond to the pushers 120 degrees from each roller (another option that should work):

 

GAS helical crayford5A.PNG

 

Here's the same view shown opaque:

 

GAS helical crayford5.PNG

 

All rollers are angled 3 degrees to the focuser axis, so the focuser advances at about half the speed (0.370" per turn) of a Kineoptics focuser. I made mine left-handed to turn in the opposite direction of my Paracorr--not necessary, but I wanted to slightly INCREASE focuser turning friction when pulling and pushing while turning the Paracorr.  The bottom rollers are elevated more than an inch from the base of the focuser to increase my focuser range.  For low-profile focusers these lower rollers could be mounted just above the plane of the base, which would be stiffer on lightweight versions.

 

Note that regardless of what angle the focuser is mounted on the side of the tube, as long as the focuser is mounted 45 degrees to the primary axis of the scope, gravity always works in the focuser's favor. When the scope is pointed at zenith the rollers nearest the eyepiece move from 8 o'clock and 4 o'clock to 6:30 and 2:30 for right-side focusers, or 9:30 and 5:30 for left side focusers. The rollers never cross a vertical plane through the focuser axis.  Similarly, the lower rollers move from 10 o'clock and 2 o'clock to either 8:30 and 12:30, or 11:30 and 3:30, again depending on whether the focuser is on the left or right side.

 

As the scope is pointed to the horizon things rotate the other direction, but again, the rollers never cross a vertical plane through the focuser axis. On my hexagonal scope the focuser would be angled 30 degrees from vertical when the scope is aimed at the horizon. On a big dob with the focuser parallel or nearly parallel to the scope altitude axis the focuser would be nearly horizontal when the scope is aimed at the horizon. In all cases none of the rollers cross a vertical plane through the focuser axis for all ranges of altitude angles.  The scopes with the focuser mounted more on the side simply see a greater amount of gravity loading at lower altitude angles.

 

Now, as long as the center of gravity of the combination of drawtube, adapter, eyepiece, Paracorr, camera, etc. are further from the base of the focuser than the point on the focuser axis midway between the planes of the rollers, gravity will always increase the pressure on each roller and never oppose any of the gentle pushers opposite the rollers for all altitude angles. A thumbscrew could be added in the plane of the top rollers and pushers to lock things down, but that thumbscrew would not be required except to keep the focuser from sliding or turning inadvertently.  In the photos you may see the threaded hole I have already made that is at the top when the focuser is aimed at 45 degrees. When I add a thumbscrew there I may make it a spring loaded Vlier screw of a stiffer design than those used so far. I've already found a moderate pressure setting on my four screws that keeps my drawtube from sliding with two pounds on it when pointed vertically.

 

I think I'd like to loosen those four existing light-duty spring-loaded Vlier screws slightly, and use one stiffer spring-loaded one on a thumbscrew as a more traditional focus lock. That would also make it easier for me to slide the focuser to close focus, and then rotate it for fine focus. I can do that already, but I could do so more easily if I loosen those screws some.  The coefficient of friction of the Delrin to the aluminum drawtube is only about 0.2, so as soon as I rock the tube slightly off the rollers I can slide it easily.  Because those stainless steel rollers (1/8" ID, 3/8" OD, 5/32" long bearings) are almost square to the axis of the focuser the slightest pressure of the screws and gravity keep the tube from slipping.

 

Anyway, I want to encourage others to experiment along these lines. I've recently learned from the greats on these forums how critical collimation is as we progress to faster mirrors. With the heaviest accessories being added to the focusers of the biggest of these dobs, collimation becomes ever more critical. My laser tests in my shop have me increasingly confident that a design that inherently WANTS to stay against the steel rollers when all other pressures are lessened is preferable.  This doesn't have to be much more complicated a change than a rearrangement of rollers.  It is critical, however, that with a focuser with a drawtube that can be tipped a little manually, all pressure points must be in the same planes as the rollers.

 

As far as things I'd do differently in the future (besides building in a REAL machine shop with all metal), I'd like to move the rollers a little closer to that vertical plane--maybe five or ten degrees.  This would reduce slightly the forces on the small screws that support the bearings and make the design slightly less sensitive to dimensional variation in the drawtube.  As long as no roller crosses that vertical plane in use, loading should never be against those gentle pushers.  I'd also likely increase the bearing size slightly to 1/2" with 3/16" holes (and 3/16" axles) and have them mounted on the outside of an aluminum body and extend through notches in it. That would allow the focuser body to be another aluminum tube with an ID only slightly larger than the OD of the drawtube.  The top rollers and pushers could be mounted beneath a stiffening ring at the top of that body tube, making the structure resemble the "top hat" UTA shape known for a good stiffness to weight ratio.

 

This design could also be used even better on an alt-az scope with a rear-mounted focuser. Since there would be no rotation about the focuser axis those bearings could be separated by a more conventional +/- 45 degrees rather than 60 degrees.  AT a stretch, if a locking swivel were built into the base that allowed rotation of the body about the focuser axis, this could be used on an equatorial mount. Opportunities to add error to collimation would occur with such an arrangement, but it could be done carefully.  These concepts could also be used on a more conventional construction with longitudinal rollers and a typical Crayford drive shaft on the upper plane.  I want others to think about setting things up not to require fighting gravity and subsequently applying point forces to drawtubes that exceed the weights of the accessories by several times just to lock them down.

 

Here are a few more pictures of construction of my prototype. The aluminum tubing arrived with some serious scoring on the outside and had to be sanded and polished on the lathe. I also gave it enough of a chamfer on the bottom edge to facilitate assemble. Parallelism of the outside of the drawtube is critical with this design; a taper would generate varying amounts of tipping of the focuser axis throughout its travel. Machinists would call this a tight cylindricity tolerance--not just roundness, but constant diameter along its length to a close tolerance. I used round headed brass wood screws to hold the bearings. I spun them against a fine flat file to taper the shaft from about 0.130" just below the head to about 0.123" just above the threads. I then tapped the screws through the 0.125" ID bearings until they jammed on the taper, raising a slight burr as they did so 0.040"-0.060" below the head.  That eliminated longitudinal play in the bearings in the assembly.  1/8" shoulder bolts would be preferable if metal were used, but this body was made of plastic. All screw holes were drilled with the drill press table tipped 3 degrees.

 

GAS HC4.jpg

GAS HC7.jpg

GAS HC6.jpg

 

Now we're cooking with GAS!


Edited by jtsenghas, 24 March 2015 - 12:42 PM.

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#2 Oberon

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Posted 23 March 2015 - 07:31 PM

Very interesting. Good thinking 99.



#3 Pierre Lemay

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Posted 23 March 2015 - 08:49 PM

The idea of forcing the weight down on the bearings further from the tube makes a lot of sense for an alt-azimuth mounted scope like a dob. Good thinking. It should make the drawtube more stable, even with 3 pounds of eyepieces hanging on the end. Unfortunately I can't use this on my ball scope since the eyepiece holder is spinning all over the place, in all orientations. I really depend on the central teflon tipped bolt squeezing the draw tube against the four bearings to limit any slop.  Thanks for sharing.



#4 jtsenghas

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Posted 22 May 2015 - 03:12 PM

Here's brief update on the prototype of that focuser regarding a few changes and improvements after two months of tinkering with it intermittently. I thought it would be worth going into more detail after just posting on the "What did you work on today?" thread.

 

I had neglected to consider that the Paracorr 2, which I bought just as that focuser was reaching completion would require about another 1/2" of in-travel, and I had to remove that 1/2" long ring from the top of the drawtube as a temporary measure for using it with the Paracorr.  I also was disappointed with the roundness of the 2" I.D. extruded pipe I had purchased for the drawtube in hopes of avoiding machine shop work. The result was that my laser collimator was describing circles about my center spot as I turned the focuser drawtube itself (not the laser within the drawtube). Since a Paracorr requires a much tighter focuser axial error to avoid tipping that Paracorr lens too much and degrading its performance I knew I owed the brilliant guys at Televue better than that.

 

I disassembled and reworked the tube of the focuser on my wood lathe, shortening it by 1/2" and making a new shoulder to fit into the base. I also ordered a short length of 2" Aluminum Schedule 80 pipe for a new drawtube.  This was actually 1.939" I.D. and 2.380" O.D., so it had plenty of meat to allow a replacement to be turned.

 

A friend with a big JET lathe turned a replacement complete for it this morning.  I redesigned it to be 1/2" shorter  to make sure the drawtube would not enter the light path (reducing travel from 2" to 1.5"), had a large chamfer put on the bottom end to allow it to be assembled easily without catching on the bearings or Vlier screw tips, and left a slight shoulder on the top to make sure it never cascades through the focuser onto my mirrors. I won't be reinstalling the original top ring now that I have enough metal for the threads of the eyepiece thumbscrews in the new shoulder.

I also sanded slight flat tips onto all of the Delrin-tipped spring loaded Vlier screws to reduce their contact pressure.  Despite their soft springs they had been making slight grooves in the aluminum of the original drawtube similar to what a ball point pen would leave. This is how the focuser looks now:

 

focuser w new tube.jpg

 

That thumbscrew lock which needs the nuts changed from an acorn nut plus jam nut is actually another 1/4" Delrin-tipped Vlier screw, but with a much higher spring force than the other four light duty ones that serve only to keep the tube against the rollers. This works sweet for setting the turning force required to a comfortable level and allows me to set the friction to just over what is required to turn the Paracorr tunable top. Sweet! 

 

Besides paint, all it needs is a couple of nylon screws at 90 degrees to each other for holding the eyepieces. I'm not going to deal with installing a brass compression ring, and I like the suggestion Don Pensack (Starman1) made on another thread to use nylon screws for this purpose.  I have plenty of nylon set screws for the purpose since I ordered a bag of 100 of them for $5 for my next scope to use on its unusual OTA.  Actually four of them were used and one can be seen in the above photo holding the base ring on. When lots of screws are needed on a scope, grams rapidly become ounces and ounces become pounds.  For many things these nylon screws are strong enough and they never corrode!

 

First light should be this weekend!


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#5 Oberon

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Posted 02 September 2015 - 11:00 PM

So how is this project developing?



#6 don clement

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Posted 03 September 2015 - 12:46 AM

A suggestion: use six roller bearings. On one pair of bearings mount those in cutout section of the solid housing so each of the pair of bearings can be pivoted by the compliant cutout and use the spring loaded Vlier  screws to put pressure on the compliant support of the two bearings. No sliding parts and no slop.



#7 jtsenghas

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Posted 03 September 2015 - 07:36 AM

A suggestion: use six roller bearings. On one pair of bearings mount those in cutout section of the solid housing so each of the pair of bearings can be pivoted by the compliant cutout and use the spring loaded Vlier  screws to put pressure on the compliant support of the two bearings. No sliding parts and no slop.

Don, I'm not sure I quite understand your proposed configuration. I had considered something that must be similar with six rollers and the additional ones on each plane spring loaded to serve as a pusher, replacing all Vlier screws, but that added mechanical  complexity and another pair of rollers that needed to be carefully angled. If I understand your suggestion correctly, you may be proposing a simple way to mount them that is sufficiently compliant on the "pusher roller".  

 

I actually like the option to slide the drawtube for rough focus for the big adjustments required when Barlow lenses or a Paracorr are either added or removed. This means that on a fast scope the angle of the rollers can be set for a fairly fine speed, but without the nuisance of spinning this numerous times for large adjustments.  The Vlier screws also allow me to add a controlled amount of friction to keep things from sliding or turning inadvertently. The four are as light as possible and the additional one at the top plane lets me adjust the pressure enough that several pounds of components won't slide due to gravity.

 

The reason I settled on Vlier screws 180 degrees from each roller is that I was thinking ahead to a much more skeletal design for the focuser itself. Except for the "thumbscrew" pressure point all rollers and pushers could be mounted on four slender parallel members. Something like this will probably be on my eventual lightweight build with a 12.5" mirror. I'll have to be careful that the top of the focuser is stiff enough against radial forces and those slender parallel members don't flex significantly also, but I believe I'm adequately addressing these issues.


Edited by jtsenghas, 03 September 2015 - 07:40 AM.


#8 jtsenghas

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Posted 03 September 2015 - 07:56 AM

So how is this project developing?

I've probably gotten that prototype version adequately tuned on my Tardiscope that I'll leave it alone. It does give me a low enough focuser profile that it folds up in my packaging inside out without interference. If you look at the second photo on my entry (#12) from the competition page from Stellafane this year you can see the hole in the packaging it folds into.

 

I was able to show at Stellafane that even if I load up that focuser with a Paracorr 2, barlow and heavy eyepiece it maintains collimation when turned even with the thumbscrew loose.  A certain Stellafane prize-winner from 1958 named Al Nagler reacted with horror that I had a Meade eyepiece in that Paracorr, however.

 

The key to a successful lightweight version will be making the top of the focuser very rigid against the radial loads of the drawtube against the rollers, especially at those oblique angles that have a large mechanical advantage. To that end I plan to water jet a couple of rings from some carbon fiber plaques left over from some R & D work I participated in and make the focuser structure look a bit like a miniature UTA.

 

All in due time. Now it appears I'll have to find a good way to mount such a focuser much closer to the secondary mirror than I was originally planning...


Edited by jtsenghas, 03 September 2015 - 08:17 AM.

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#9 jtsenghas

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Posted 03 September 2015 - 08:49 AM

All in due time. Now it appears I'll have to find a good way to mount such a focuser much closer to the secondary mirror than I was originally planning...

 

DUH! I could flip the focuser so that it extends into the tube and the base is still mounted to the same surface! This also solves the problem of how to mount the one "thumbscrew", which now can be on a small block on that base (but still in the same plane as the nearest rollers and pushers).  An eyepiece fully racked could be against what used to be the back of the base and only a small depression would be required for the "focuser board"!

:hamsterdance:

 

This also means that the focuser could be mounted onto a larger UTA ring for a more standard build than the one I'm currently designing, and still have an adequately low focal plane for a typical large fast dob!


Edited by jtsenghas, 03 September 2015 - 09:41 AM.


#10 don clement

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Posted 03 September 2015 - 01:16 PM

You really only need 5 roller  bearings with just one of the roller bearings supported by a  spring loaded compliant hinge.



#11 jtsenghas

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Posted 03 September 2015 - 01:32 PM

From a kinematic standpoint I agree. However from a practical standpoint I'm not sure I do because the extra pressure against all rollers provides convenient friction and helps to prevent any rocking when the eyepiece is handled, especially near the extremes of the range where two rollers approach that central vertical plane.



#12 jtsenghas

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Posted 03 September 2015 - 01:36 PM

Don, are you considering that compliant bearing to be above the plane of the nearest rollers so that it tips the tube for pressure against all four rollers (although not in equal amounts)?  That may have merit but doesn't lend itself well to my planned skeletal design.


Edited by jtsenghas, 03 September 2015 - 01:38 PM.


#13 don clement

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Posted 03 September 2015 - 02:23 PM

Only need one roller bearing to supply pressure if  5  roller bearings are used. The tube rotates so not good for focusing a camera with.



#14 Fuzzystar

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Posted 04 September 2015 - 09:34 AM

I have not posted much at all.

 

A picture that caught my eye is that you are using a mandril (correct term?,  is that the wood part that the metal is around in the lathe?) to hold the metal tube.

 

Also is that a Shopsmith?

 

I am presently making an adapter out of PVC.  I am using a wooden mandril to hold it to the wood lathe. The PVC slides onto the wood mandril with a tight fit that is strong enough to hold the PVC. I am also doing this on an old Shopsmith (greenie).

 

I am making the adapter to improve the alignment of my collimator (red laser and  barlowed).

 

Paul



#15 jtsenghas

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Posted 04 September 2015 - 09:53 AM

Yes, a mandrel (with an "e") is the correct name for any spindle around which work is fastened for turning applications. I'm a mechanical engineer with a tooling background, however at home I have only woodworking tools. So yes, technically speaking, that spindle I turned to hold the original tube could be called a mandrel.  It isn't wood, by the way, but more of that urethane tooling board the prototype focuser is made from. My material specification for that tool was "what have you".

 

Yes,  that is a Shopsmith Mark V. It was bought in 1985 during the brief interval in my life in which I was both an employed engineer and a bachelor.  It was bought new at a serious discount, because Shopsmith was moving out old inventory to make room for the newer Mark VI machines that have the same headstock and base, but larger tables and improved accessories. If you look in the background of those photos you can see the cart with chair casters that I park the bandsaw and belt sander on that also go on that same machine.

 

I used that "mandrel" to polish the OD and part the ends square on an extruded tube I hoped would be sufficiently round.  I often turn quick cones and faceplates and mandrels for reworking things on that woodworking lathe with files and such.  The greatest limitation for using it for such non-woodworking applications is that it doesn't really have a sufficiently low speed. As it turned out (see later posts) the roundness wasn't sufficient to maintain collimation well enough with rotation that I had to break down and buy a thicker-walled pipe and have another drawtube turned by a friend on a good metalworking lathe. 


Edited by jtsenghas, 04 September 2015 - 10:00 AM.


#16 don clement

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Posted 04 September 2015 - 01:26 PM

Are you going to harden the metal tube? IMO that is a problem with point or line contact rollers on soft metal.



#17 jtsenghas

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Posted 04 September 2015 - 02:53 PM

That's a good point, and one I thought about before construction. I now have two good drawtubes (one for the lightweight next version) turned from 6061-T6 aluminum. The first isn't showing any digs yet, but I'd prefer an anodized surface. I considered stainless steel as well, despite the cost and more trouble to machine. One advantage of aluminum over steel (besides weight) is that the friction is slightly greater, so less contact pressure is needed to avoid slippage. Anodized aluminum may be the optimum way to go.



#18 don clement

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Posted 05 September 2015 - 12:34 AM

Hard anodizing perhaps. Even so hard anodized aluminum may not stand up to the enormous pressure of point or line  contact with roller bearings.  With a  Crayford and this type of helical  focuser accuracy and repeatability depend on accuracy of the reference surface (i.e. tube). With a soft tube that reference surface will change every time focus changes with the enormous pressures involved.  I can see with a Crayford adding hardened and precision ground inserts that act similar the hardened races of a roller bearing. With this type of helical the entire tube must be hardened and hopefully precision ground as a reference surface. Perhaps with this type of helical there is rotation when focusing  and will never be noticed as it will be used for visual  and not for imaging. Imaging will definitely demonstrate how good the accuracy and repeatability is of a focuser particularly when autofocus software is used and a record of how focusing is recorded in the V curve.  

That said I have been experimenting with a threadless  leadscrew using three or six roller bearings on a small diameter hardened and precision ground rod.  In addition to the hardened rod for the roller bearings to bear against there is a flexible rubber shroud around the threadless leadscrew and roller bearing assembly to  seal and prevent grit and dust contamination which will surely destroy the accuracy and repeatability of any reference surface.


Edited by don clement, 05 September 2015 - 12:35 AM.


#19 jtsenghas

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Posted 05 September 2015 - 07:30 AM

One point you may not be recognizing, Don, is that with this arrangement, particularly if the two planes of rollers can be spread sufficiently apart, is that those roller loads have been considerably reduced compared to typical helical Crayfords.

I don't see it having an application in motorized focusers, or anything like you are doing with a focusing primary.

It's clear that the later refinements to the Kineoptics focuser to spread its rollers were to reduce such loads somewhat for heavy eyepieces. That design still applies much greater loads to the rollers than mine because it still has a thumbscrew providing a force several times greater than the weight of the accessories to keep the tube against all four rollers.

You can simulate the roller contact points of my design by holding an eyepiece with four fingertips as you look through it horizontally. Two thumbs touching in the near plane and two middle fingers in the far plane. For mounting on a dob these contact points would necessarily be more than 45 degrees from a central plane (see my long initial description) and this represents the situation of a scope aimed at 45º altitude.

I like the elegant slide to close focus (sometimes tipping slightly off the rollers to make sliding easier) and then rotate to fine focus. I think this has potential for ultralight builds that require a Paracorr for visual use because when a Paracorr is used the focuser is only tweaked a little anyway because the Paracorr is designed for a fixed height and tunable top. If I have a big focus change with changes of eyepiece (like removing or adding Paracorr) I generally push it or pull it to a slight in-focus position and then turn it for fine focus. My untreated soft aluminum drawtube doesn't seem to be suffering from roller pressures thus far.

Edited by jtsenghas, 05 September 2015 - 07:35 AM.


#20 Oberon

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Posted 05 September 2015 - 07:38 AM

One thing that stopped me looking at helical focusers seriously is my Dioptrix. I hate wearing glasses at the eyepiece and only get really sharp views with it.   :(



#21 jtsenghas

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Posted 05 September 2015 - 08:18 AM

That, I can understand. You would have the image vary from correcting for astigmatism to doubling your amount of astigmatism for every 90º rotation.

I'm blessed with no noticeable astigmatism in my preferred observing eye, and only slight farsightedness that never required glasses before the age of 45. I simply adjust focus outward a pinch and don't observe with my glasses.

#22 Starman1

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Posted 05 September 2015 - 05:23 PM

The one "killer" of helical focusers is that I have yet to see even one, from anyone, that maintains perfectly focuser axial error collimation during rotation.

IF one is going to use a helical focuser, and IF one even cares about collimation (ESPECIALLY if one uses a coma corrector), THEN the collimation tool holder will HAVE to be perfectly centered in the focuser.  If pushed to one side by the setscrew, it will immediately be out of collimation when the focuser rotates 45 degrees.

 

Edit:

This is because the helical focusers I've used all rotate the eyepiece.  If it's a helical NON-rotating focuser, collimation should be maintainable.


Edited by Starman1, 05 September 2015 - 05:25 PM.

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#23 jtsenghas

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Posted 05 September 2015 - 06:11 PM

Your point is well taken, Don.

 

I don't have 2" collimation tools yet and see how errors can really stack up. The 2" adaptor that came with the Paracorr II is definitely a slightly tighter fit on both O.D. and I.D. than the one that came with my previous Antares helical focuser, and I like that it has two thumbscrews 90 degrees apart. I also have two nylon screws on my homemade drawtube 90 degrees apart based on suggestion that I believe YOU once posted, and both the Paracorr and that adaptor are a tight fit to that drawtube. I'm glad I didn't use a compression ring on the drawtube. To minimize any eccentricities I orient both my nylon screws opposite the adaptor thumbscrews when I install my collimation tools. 

 

 My original drawtube on this focuser was out of round enough that rotating the focuser made the laser describe circles about 10 mm in diameter on my primary. Not anywhere nearly good enough.  My newer drawtube reduces that to just over 1 mm diameter, suggesting a FAE of about half that. Turning the laser collimator 90 degrees and 180 degrees and retightening it yields no real noticeable changes, so it appears I tuned my collimator well while it was in a V-block and making a spot on a wall target 15 feet away.

 

Still, this prototype is a little springier than I would like despite its wall thickness, in part because the rollers aren't supported at both ends of their axes, and in part because of the material it is made of. There IS a reason I just call it a prototype. I can't wait to see how it works out when I make version 2.0 (it will be a while) for my next scope.  This one will have the axes of the rollers supported at both ends  in blocks mounted to  good aluminum ribs oriented radially and wide rings made of carbon fiber sheet molded composite plaques to hold their ends.


Edited by jtsenghas, 05 September 2015 - 06:12 PM.


#24 Starman1

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Posted 05 September 2015 - 06:38 PM

JT,

Sounds good!

Don



#25 don clement

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Posted 06 September 2015 - 10:41 AM

I don't see it having an application in motorized focusers, or anything like you are doing with a focusing primary.

 

 

 

Have you tried using a motorized rotating focuser to autofocus an imaging camera?


Edited by don clement, 06 September 2015 - 10:42 AM.



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