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Building a New Type Low Cost Refractor Focuser

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#1 BillB9430

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Posted 02 April 2020 - 12:39 AM

Building a New Type Low Cost Refractor Focuser

 

     Problem-

When planning a focuser for a fast refractor, a long focuser tube allows different eyepieces (perhaps with diagonal and/or Barlow) to come to focus and extends to focus nearby objects for bird or wildlife viewing, while a relatively short focuser tube is needed to avoid cutting into the wide light cone and causing vignetting. The common solution to these obviously opposite requirements is to use a very large diameter focuser tube.  Unfortunately, unless you can find something like the discontinued Jaegers 2.7" "Superfocuser" on the used market, the cost of such a focuser plus a similarly large external diagonal is high. These problems are good motivation to try a different ATM approach to the traditional refractor focuser setup.

 

     Requirements-

An optimal focuser might feature (1) long focus travel to accommodate differing eyepieces and image distances, (2) a large mirror or prism with arrangement to minimize of "choke point" vignetting/partial edge-of-field illumination, (3) the ability to fine focus, (4) comfortable right angle viewing, (5) correct (or at least erect) images, (6) ease of construction without expensive machine shop equipment, and (7) low cost parts and materials. My attempt to meet those criteria has resulted in a scope that looks a bit strange compared to a standard refractor, but may have elements useful to other ATMs - hence this post. Like most "new" developments in telescope building, this type of refractor focuser is based on previous ones, but put together in a different way.

 

     History-

The first focus idea is from a 1950's vintage refractor spotting scope that focused by just sliding the chromed focuser tube in a ring of firm felt inside the scope tailpiece. (Idea: Sliding metal on felt) See photo A.  I used the sliding idea to coarse focus my first ATM Newtonian back in 1963 by mounting the diagonal and eyepiece on a section cut from extra scope tubing that functioned as a "sled" focuser, sliding on felt pads and constrained by rails cut from narrow tube sections that are screwed to the tube. (Idea: Sliding focuser on main tube.) See photo B. The third idea is from a thread on CN in December of 2010 with posts by "Arnold3" and "Mirzam".  They both use a large Newtonian-like diagonal in the bottom of the refractor tube and focus the eyepiece using a reflector type focuser mounted on the side of the tube, near the bottom.  (Idea: Diagonal mirror within refractor main tube.) The thread containing their posts of this great idea is here:     https://www.cloudyni...to#entry3882048
An article in the March 2020 S&T Magazine by Jerry Oltion features Art Gamble's nice ATM refractor using that same configuration.

 

 

Photo A.jpg Photo B..JPG

 

     New Focuser-

Combining and modifying the above ideas results in a new refractor focuser design that uses a big diagonal mirror or prism within the tube, like the Arnold3/Mirzam arrangement, but has that diagonal, along with its attached eyepiece/focuser, able to move longitudinally in the tube. This is similar to my old external-sliding Newtonian sled focuser, but in this case, the sliding occurs internally, with a rigid diagonal-holding "cage" sliding inside the tube and carrying along the eyepiece and fine focuser that protrude out through a wide lengthwise tube slot. The diagonal assembly slides smoothly on attached firm felt strips that bear against the inside of the aluminum scope tube to permit coarse focusing. This design could be called a Longitudinal Slide Diagonal Focuser, or LSDF for short. See Photo C and D for LSDF refractor.

 

Photo C.jpg  Photo D.jpg

 

 

The "different" part of the LSDF is a hollow wooden cylinder that slides within the main scope tube and carries the large 45˚ diagonal mirror or prism and the fine focuser with eyepiece on top. The sliding assembly is built-up from wooden rings, screwed together to form the hollow cylinder of requisite length. The back ring and end disk are spaced apart by 3 large dowels, forming a diagonal holding "cage". The back ring and end disk are cut flat on top to support a focuser board atop the "cage". This board protrudes outside the tube through a wide slot cut all the way to the bottom end of the scope tube. The focuser board holds a short 2" helical focuser that carries the eyepiece.  The focuser board also serves, along with the forward rings, to seal the slot from dust and light entry. Adhesive backed firm felt strips applied circumferentially to the cylinder allow the whole assembly with included diagonal mirror (or prism) and eyepiece to slide somewhat snugly fore and aft within the scope tube.

 

The eyepiece holder can be of any type, but needs only minimal travel, so a short-throw, low profile helical focuser is ideal. Besides having a large mirror, keeping the eyepiece field lens close to the diagonal mirror or prism also makes it easier to attain good edge-of-field illumination with no vignetting, since there's not much draw tube length to cut into the light cone. The bottom of the fine focus eyepiece holder baseboard is screwed to the rings inside the tube, barely overhangs the slot width, and can be gripped to move the whole assembly along the tube for coarse focus.

 

A flanged wooden end cap disk, fit BEFORE the tube is cut and screwed to the bottom end of the tube, holds the bottom tube walls concentric after the slot cut is made. This is important since the tube walls at the end tend to "splay out" slightly when the slot segment is removed and will be pulled back into alignment by the screws reattaching the end cap. The fact that most of the focuser is made of wood enables "low-tech" amateur construction as well as reducing weight. Since the sliding diagonal assembly moves within the rigid main scope tube, the problem of a "drooping" focuser tube when the eyepiece is at maximum distance from the objective is eliminated.

 

Captioned photos in the following posts show how the parts are made and go together, hopefully clarifying just how the refractor LSDF works. Although I've now made several refractors of the LSDF design, the one illustrated strives especially for low cost and building in a home-workshop setting. See following photo posts. 

- Bill Blunk  AKA BillB9430


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#2 BillB9430

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Posted 02 April 2020 - 12:42 AM

LSDF Photos 1-4

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#3 BillB9430

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Posted 02 April 2020 - 12:44 AM

LSDF Photos 5-8

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#4 BillB9430

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Posted 02 April 2020 - 12:46 AM

LSDF Photos 9-12

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#5 BillB9430

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Posted 02 April 2020 - 12:48 AM

LSDF Photos 13-16

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#6 BillB9430

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Posted 02 April 2020 - 12:50 AM

LSDF Photos 17-20

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#7 BillB9430

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Posted 02 April 2020 - 12:53 AM

LSDF Photos 21-26

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#8 BillB9430

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Posted 02 April 2020 - 12:55 AM

LSDF Photos 27-29

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#9 BillB9430

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Posted 02 April 2020 - 12:57 AM

LSDF Photos 30-32

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#10 BillB9430

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Posted 02 April 2020 - 01:05 AM

Additional LSDF Building Information and Tips

 

The LSDF can be made with a minimal set of power tools. The ones used in the above illustrated "economy" build for the Jaegers 5" f/5 RFT refractor with a Hastings 5" tube were a router with circle cutting jig, drill press, and miter saw. Router circle-cutting skills learned from making Dob mounts are useful here. The correct tension for coarse focusing the slider disk assembly can be adjusted by thinning the 1/16" thick adhesive back firm felt strips (from McMaster-Carr), so you have some leeway there. The wooden end cap, however, needs to be fitted precisely. Most router circle cutting jigs adjust only in 1/16" increments, so if your router-cut flanged end disk is cut a bit large, use a short length of 1/4-20 all-thread, washers, and nuts to secure it in the drill press and use a file or fine rasp held against the spinning disk rim to carefully reduce its smaller, non-flange diameter until that section will just smoothly insert in the unslotted tube. If you have a lathe, use that.

 

Note on Tooling for Making Wooden Disks:

A key part of this method for LSDF construction is making precision diameter wooden disks. I first did this with a low cost ($20) adjustable two blade fly cutter on my drill press. While this did work, the process was difficult to adjust precisely and, with cutters spinning at large radius, uncomfortably dangerous. I also tried using a metal lathe to finish rough sawn disks. That process, though accurate, was slow and necessitated massive cleanup of wood dust from the lathe when completed. The best and easiest method for precision disk cutting seems to be a plunge router with a 1/4" "upcut" plunge bit and commercial circle-cutting baseplate. (I have a Jasper). A standard router with the same bit and a homemade circle jig would also work, but perhaps less conveniently.
The other important disk assembly idea once the disks are made is to center drill them to snugly accommodate a piece of 1/4" all-thread and use that with nuts and washers to clamp the disks in precise alignment when drilling all the various pilot screw holes. Drywall screws come in different lengths, are easy to pilot drill and countersink in wood, and are low cost -so work nicely for assembling the wood pieces.

 

Adhesive Backed Felt Strips

Use 1" wide 1/16" thick adhesive backed firm felt applied around the wooden cylinder. This is available in rolls from McMaster-Carr. For the back slider disk, you'll need to cut a strip 3/4" wide with scissors to fit the disk edge before removing the paper backing. Thin these strips after application, lightly using a sanding drum on your drill press to remove some wool and achieve a smooth but snug sliding fit in the scope tube. Save the leftover 1/4" strip cut so you can apply it, inset underneath, parallel to the side edges of the focuser board to lightly touch the cut metal slot edge and block dust entry there. Do this after the final outside painting of the focus board, since you will want to use masking tape there when painting. Since the focuser boards shown in the photos are not yet painted, the little strips are not applied.

 

Diagonal Holder Block Details:

 

The prototype and the second LSDF scope built for friends use big WWII surplus Amici prisms instead of flat diagonal mirrors. See Photo 27 C. This works well for low power correct image terrestrial viewing and also allows seated correct image viewing with the scope horizontal and tube rotated so the eyepiece is to the side. (It also lets the observer see the North America nebula looking like the map rather than its mirror image.) Sadly, such Amici prisms in their ready-to-use military mounts are nearly impossible to find now. The mirror shown in the above "budget" build is a $2.50 Kodak F.S. mirror from Surplus Shed and gives an erect, though R-L reversed image when viewed from above. It is secured to the block with bent, padded metal clips. Surprisingly, this very low cost mirror is adequate for powers under 20x or so for RFT or terrestrial use, and can be easily replaced by a better quality Newtonian elliptical diagonal mirror of ample size if higher power viewing is planned. The one shown in Photo 27 B is a half-inch thick 2.7" minor axis 1/10 wave Newtonian secondary attached to the wooden block with RTV silicone cement.

After cutting the five 2.5 x 3.0 inch blocks of pine (alternate grain direction) for the mirror holder block, center mark the back end block with straight edge lines from the corners. Using a compass, draw a 3/4" radius circle around that center on the block's back face. Locate and mark a drill point on the circle at the top, on the center line of the board. This will be for meridional diagonal adjustment. Locate the other two drill points at that same radius, but equally spaced on lower sides of the circle for lateral mirror block angle adjustment. The distance between each of the three marks should be the same. Drill the block center mark to fit a 1/4-20 bolt or all-thread rod and bolt squarely to the central hole of the back slider disk, with its top parallel to the top cut on that slider disk. Drill 1/4" holes through the block and slider disk at the three locations marked on the circle.  Removing the central bolt, the back block 1/4" holes just made can then be enlarged to accommodate 1/4-20 Tee nut barrels and drilled about halfway through from the side that will be towards the objective with a 3/4" Forstner bit to recess the Tee nut flanges. Install three 1/4-20 Tee nuts with the threaded barrels towards the back of the block (ie. towards the slider disk). Recess deep enough that the metal threaded barrel end is near the back wood surface.

Suggestion:  It is not much harder to make TWO such blocks at once, perhaps one for the cheap diagonal mirror described and another to use later when you obtain a precision Newtonian elliptical diagonal. Just repeat the drilling and Tee nut treatment of the block on the OTHER end of the five (using a spare rear slider disk that you made in steps 7 and 8), with the "triangle" of holes in the block inverted, so that the 45˚ cut will yield TWO identical mirror holding blocks that can each be attached to separate rear slider disks and used to conveniently change diagonal mirrors in the scope.

Glue the 5 blocks together with wood cement and clamp until set. The block is now ready for the critical 45˚ cut with the miter saw. Be sure to follow the safety precautions and clamp the piece securely for cutting, as mentioned in the captioned photos. Also cut a 6" long board 3.5" wide (or use a piece of pine 1 x 4 that comes in that width) for the focuser board to attach centrally atop the flats on the slider disks at the front and back of the diagonal "cage". It will be bored with a 2-3/8" hole centered above the middle of the mirror to hold the low profile fine focuser.

 

Low Profile Fine Focuser

The "flush valve" helical focuser was developed by the late Art Bianconi and described in his #6 and #7 CN posts from 2008, archived here:
https://www.cloudyni...-out-of-pvcabs/
It is an easy and economical way to build a short 2" focuser. The smooth inside of the threaded plastic piece is just slightly too large for 2" eyepiece barrels. Wrapping masking tape around the lower eyepiece 2" barrel for a friction fit is an "adequate" solution, but most will want to use a thin plastic or metal inserted sleeve to fit better, with a nylon screw to hold the eyepiece securely. A low cost insert can be made from a 2" to 1.5" PVC adapter, but producing the .030" wall thickness needed requires a lathe. Perhaps you can find other focuser tube lining materials. Another small nylon screw tapped through the threaded base with a small plug of felt on its tip will allow you to "adjust out" any thread slop. The plumbing thread, 11.5 TPI, is pretty fine and would be tedious if longer and not coupled with the coarse slide-to-focus of the LSDF. This helical focuser provides 3/4" travel.

For those who have a home workshop metal lathe, it is not too difficult to turn and thread a better helical fine focuser using multiple start threading with an aluminum barrel and a meshing UHMW polyethylene base. The lubricity of the UHMW base to the aluminum focuser tube eliminates the need to apply grease, so exposed threading is not a messy problem in use. The white tube and prototype scopes in the last photos use those, with 4.5 TPI threading and 1-1/2" travel.

Low profile 2" commercial helical, R&P, or Crayford style Newtonian focusers should work well with an LSDF and are a simple "buy and bolt on" alternative, but increase cost. See Photo 27 B.

 

Diagonal Collimation Adjustments

 

Once the 45˚ block is completed and FS mirror secured to it, small springs, O-rings, or Belleville spring washers inserted around the three adjustment screws between 45˚ block and back slider disk will provide slight clearance for diagonal angle adjustment, as needed.  The three attaching screw holes in the back sliding disk can also be converted to slots to allow some diagonal rotation adjustment, if needed.

 

Sizes and Sources

 

The three LSDF refractors I've made so far have all used Jaegers 5" f/5 objectives and the configuration works well for that size RFT lenses. The concept should also be easy to apply to larger and to somewhat smaller apertures, as well as different focal ratios. Three inch or smaller scope tubes may require an LSDF allowing only 1.25" eyepieces. Except for an occasional lens that turns up on the used market, Jaegers objectives are no longer available for these scopes. Fortunately, Surplus Shed in Pennsylvania usually has a supply of 4, 5, and 6 inch achromats in various focal lengths with relatively low prices that should be a good substitute for the old Jaegers objectives and work well in an LSDF ATM scope.

A fast refractor using the LSDF resembles a Newtonian reflector, but views in the opposite direction. If you make one of these unconventional-appearing scopes, friends who see it from a distance may ask why your scope is "looking at the ground". Ha!

I hope these ideas will provide a little different perspective for ATMs to consider when planning simple, versatile, and very low cost refractors.  Since ALL of us know much more than any one of us, please share YOUR ideas and suggestions for improved (better, cheaper, easier, faster-to-build?) ATM focusers. Also, thanks to "Arnold3" and "Mirzam" for their 2010 CN posts showing refractor diagonals within the tube.     -Bill


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#11 TOMDEY

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Posted 02 April 2020 - 01:44 AM

Nice! Some Newtonians comprise similar, where the flat Secondary Mirror moves along the axis of the OTA with the eyepiece.    Tom

 

~click on~ >>>

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#12 m. allan noah

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Posted 02 April 2020 - 06:31 AM

An interesting project, and an excellent write up. Thank you for your efforts.

 

allan



#13 John Rogers

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Posted 02 April 2020 - 10:34 AM

Thank you Bill.  Your detailed instructions and photographs live up to the spirit of Lloyd Mallan!



#14 tim53

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Posted 02 April 2020 - 10:51 AM

I have a 5" f/5 refractor with a similar configuration that I got from a late friend many years ago, when he was cleaning out his parts collection.  It was built into a PVC pipe.  The diagonal mirror was mounted to a machined aluminum plate with a lead weight (unfortunately for thermal purposes, inside the tube) and screwed onto the end of the tube.  The eyepiece was one of those military erfles with its own helical focus.  It isn't interchangeable, but the diagonal doesn't need to move as a result, and the low power views are quite nice.  I've never used it, but have thought about mounting it as a finder on a larger instrument.  I should scare it up and take pics of it, though if you didn't know what it was, you'd think you were looking at a Newtonian OTA.

 

-Tim.



#15 Optics Patent

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Posted 02 April 2020 - 02:38 PM

If you're trying to be economical, why did you buy a new toilet flush valve?



#16 kb58

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Posted 02 April 2020 - 06:51 PM

Ahahaha, adds a whole new dimension to "sticky focuser"... ewww


Edited by kb58, 02 April 2020 - 08:16 PM.


#17 sunrag

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Posted 12 April 2020 - 08:57 PM

All I can is: Wow! 

Amazing idea executed with amazing craftmanship!




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