Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

J.T.'s 12.5" F/4.3 Hexapod Dob

  • Please log in to reply
244 replies to this topic

#76 ckh

ckh

    Surveyor 1

  • -----
  • Posts: 1,938
  • Joined: 21 Feb 2015
  • Loc: Arizona

Posted 20 January 2019 - 12:30 PM

Off topic:  

 

When I was a kid, my father had a muffle furnace (kiln).  I put a crucible with lead in it in the furnace and then forgot about it.  Later when I opened the furnace the lead had completely evaporated.

 

It could be why I'm a bit dyslexic, although I wasn't in the room when it was boiling.



#77 starcanoe

starcanoe

    Vanguard

  • *****
  • Posts: 2,488
  • Joined: 07 Nov 2013
  • Loc: Gulf Coast, Panhandle of Florida

Posted 21 January 2019 - 10:43 AM

Yeah....that doesn't sound like an OSHA approved procedure to put it mildy....though it was probably less unpleasant than the time I drank fiberglass resin as a kid...


  • Earthbound1 likes this

#78 ckh

ckh

    Surveyor 1

  • -----
  • Posts: 1,938
  • Joined: 21 Feb 2015
  • Loc: Arizona

Posted 21 January 2019 - 12:41 PM

Didn't notice a thing, but then I do have some problems. Looking forward to J.T.'s next installment. 



#79 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 21 January 2019 - 04:09 PM

 ...Looking forward to J.T.'s next installment. 

Well, I don't have too much to report on from the weekend.  Saturday I had to work at our company's sister plant on a water jet programming emergency, Sunday I spent most of the day digging out lots of windblown snow in single digit (F) weather.

 

...oh!.. there was that minor matter of lavishing attention on my bride and taking her out to dinner to commemorate our thirty orbits around Sol together....

 

There was also a fascinating large diameter shadow that crossed Luna last night.  We ended our day by enjoying quick views of that from 10:30 -11:30 pm Eastern Time in subzero weather in which snow squeaked and snot froze.

 

This morning work came too early. I envy those who had today off as a holiday.

 

I'll try to post in the next couple of days portions of the project that I've done and photographed but not yet shared, except briefly in "What did you work on Today?" earlier this month.  This thread is expected to slow somewhat as most of the work shown was performed during the December holidays and presented in January. 


  • Dave O likes this

#80 ckh

ckh

    Surveyor 1

  • -----
  • Posts: 1,938
  • Joined: 21 Feb 2015
  • Loc: Arizona

Posted 21 January 2019 - 05:56 PM

No rush. Just looking forward to it since I'm learning a lot. Lots of good ideas and methods for working with wood.  (I went to bed. I guess I'm not much of an astronomer.)



#81 Jeff Morgan

Jeff Morgan

    Voyager 1

  • *****
  • Posts: 11,617
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 21 January 2019 - 06:47 PM

No problem! 

 

Get low melting point alloys here:

https://www.rotometa...fusible-alloys/

 

You can get dense metals that will melt in a cup of not-too-hot tea!

 

The classic, cheap alloy is Wood's Metal that melts at 157 F and they are currently selling for $10/lb:

https://www.rotometa...ot-woods-metal/

 

It contains lead and cadmium (but if you were planning on melting plain lead anyway that should not be a concern) but since you can melt it underwater, and handle the melted metal with plastic ware, it is easy to arrange to handle all of the manipulation under water so that no contact with fumes or particles is possible. I made telescope weights by melting the ingot and pouring it into a plastic tube, from which I could dispense convenient sized pieces. Never had to touch the metal.

 

That's really neat! 



#82 careysub

careysub

    Mercury-Atlas

  • *****
  • Posts: 2,573
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 22 January 2019 - 12:29 AM

When I built R/C planes i had one that was very nose heavy. I used  lead shot that is  used for reloading. Mixed up some epoxy thinned with isopropyl alcohol and poured over lead shot in a built up receptical in tail.  Lot easier and safer than melting lead. Never moved.

Lead shot and epoxy are certainly the cheapest and easiest solution, and should be the "go to" approach unless there is a specific reason (cosmetics is one if the weight is ever visible, a cast piece of metal looks nice). The other likely situation is not having much volume where you need to put in weight.

 

For amusement here are some density values:

Lead   11.34

Bismuth 9.8

Wood's Metal 9.7

Copper 8.96

Mild Steel 7.85

 

Typical sphere random packing density: 0.64

 

So lead shot with epoxy is about 0.64*11.34 + 0.36*1.0 = 7.6

 

So a chunk of steel would be slightly denser, if it fits into the necessary space.

 

Lead is about $3/lb, all the other metals (but steel) are about $10/lb.

 

If you wanted to get wild and crazy and get a custom shape dense weight but don't want to melt lead, you could get heat lead shot in your kitchen oven, then pour Wood's alloy on it.

 

0.64*11.34 + 0.36*9.7 = 10.75

also

0.64*$3 + 0.36*$10 = $5.50/lb.


Edited by careysub, 22 January 2019 - 12:33 AM.

  • piaras, jtsenghas and Earthbound1 like this

#83 careysub

careysub

    Mercury-Atlas

  • *****
  • Posts: 2,573
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 22 January 2019 - 12:31 AM

That's really neat! 

Thanks!

 

You may ask, why not just use nitrile gloves, etc? Doing it underwater was more fun.



#84 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9,543
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 22 January 2019 - 12:56 AM

Use tungsten.  It'll do you proud.  But don't try to melt it.  Get tungsten shot from AliBaba.

 

https://www.alibaba.....7596672bPM5EXh

 

https://www.alibaba.....7596672bPM5EXh

 

$10 a kg it seems!


  • Dave O and Earthbound1 like this

#85 Oberon

Oberon

    Gemini

  • -----
  • Posts: 3,172
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 22 January 2019 - 07:23 AM

Copper and iron...old transformers, or flourescent lamp ballasts. If hidden.


  • jtsenghas and Earthbound1 like this

#86 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 25 January 2019 - 01:06 PM

Leveling Feet:

 

Because this scope will use an inclinometer and a base azimuth circle, I want it to have a provision for leveling.  Even though the inclinometer will display correct altitude within its resolution and accuracy even if the scope base is not level, the azimuth angle may be a bit off if the base is a bit tipped.  The error in azimuth would vary with angle and the location of the object used as a reference for the scale at setup.  There is an excellent thread in the Equipment forum on manual setting circles for dobs entitled "Degree Circles" that has been running since May 2006.  Recent improvements both on inclinometers and astronomy apps for phones (such as my favorite, Sky Safari) have made these manual setting circles even easier to use.  With up-to-the-second updates on the alt-az coordinates of objects one can both set the setting circles to a reference object and look up the current alt-az coordinates of objects for one's time and location.  It is not necessary for one to read the entire (now nearly 13-year-old) thread to get some useful information.  Just the last several pages will do.  In those last pages I share my setup on the Tardiscope and my preferred inclinometer starting here.  A couple posts down from that starting point I share the PDF file of the setting circle I drew and will also use on this scope, just scaled up a bit.  I'm pleased that at least a few people have found my drawing useful when scaled to match their own scopes over the last two years.

 

Since my scope in progress will have a pretty large diameter on the base I didn't want leveling feet to stick out much beyond the disk of the base.  I want it to be easy for me to scoot my observing chair quite close to the scope for observing high altitude objects in any direction.  Additionally, I wouldn't mind if these feet add a couple of inches to the scope height since it is a very squat design and has only a 54" focal length.  I chose to embed 7/16"-20 nuts into the disks of the base for the threaded rods of my leveling feet to screw into.  I wanted feet that I could spin with my finger tips and could be placed directly on firm ground or on wooden pads on softer ground.

 

Inspired by the leveling feet on my kitchen oven, I made a similar version on steroids:

 

leveling feet complete.jpg

 

To make these I simply drilled out three 5/8" thick disks of my Red Stuff with a 3 1/2" hole saw, and bored rosette scallops in eight places in each with a Forstner bit as shown here:

 

leveling feet drilling.jpg

 

Precision wasn't required, so I didn't need a fancy rotary table.  I drilled a 15/64" hole into a scrap piece of wood, pounded a 1/4" dowel into it to serve as a pivot, and clamped the board to my drill press setup. I marked radial lines on each disk every 45 degrees for a visual reference and clamped each in eight positions for drilling as shown.  The hole saw has a 1/4" pilot bit, so the disks turned easily to each position.  When the drilling was done I sanded the corners lightly on my disk sander and the faces with 240 grit on my random orbit sander.  7/16" through holes were drilled to accommodate the threaded rods. Right now there is a nut tight against each surface of the disk.  I might add a fancier foot to each later, or just use them as is.  A few disks of wood of a couple different thicknesses could be used in the field to accommodate soft ground or significant out of level conditions.  In general, I don't anticipate needing much more than an inch of adjustment in my usual observing locations.

 

In use, just a couple of those scallops will extend beyond the disk of the base for adjustment.  That should be accessible, but not in my way.  Like the Tardiscope I expect to set up by bringing several pieces out one at a time and plan to rough-level the base prior to placing the OTA onto it. 

 

Fine leveling will be done with the inclinometer with the scope aimed close to the horizon.  I'll tweak these feet to show the same altitude reading with the scope aiming in each direction.  The trick I like to use for a rapid tune-in is to place one foot to the north and to adjust the north foot only until I show the same reading facing north or south.  I then aim the scope east and west and tweak the other two feet in equal and opposite amounts until I get the same altitude reading facing east and west.  Finally, I verify that north to south is still good, or tweak it for any amount level in that direction may have inadvertently shifted.  This method of adjusting the second two feet in equal and opposite directions preserves the leveling in the perpendicular direction.  The same method can be used with adjustable legs on tripods, or on a three point leveling head on a mount.


Edited by jtsenghas, 25 January 2019 - 07:15 PM.

  • PrestonE and dyode like this

#87 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 25 January 2019 - 04:21 PM

Tube inserts:

 

When it comes to the truss tube inserts and the adjustment of collimation on a hexapod, I'm participating in some some relatively new experimentation.  There are several ways the ends of the tubes can be fastened, and it appears the most popular method so far on this forum is to use heim (rod end) joints.  On the hexapod thread Mark Cowan has shown he'll be using heim joints, but with two hold down knobs and bolts per pair for a robust way to keep the spheres of the heim joints seated firmly.  Other promising methods shown on that thread are variations on using spheres, and a couple of options Carl has suggested would eliminate Jonathan's concerns about the spheres themselves turning, but may require more custom machining than most can accomplish at home.  As Jonathan demonstrated with Merope, looseness and slop must be pretty much eliminated, but adjustments must be possible with moderate torques on the poles so that collimation can be tweaked by hand.

 

I'm going to attempt, at least on a first pass, to use simple eye bolts salvaged from turnbuckles in place of the heim joints.  I'll be trapping the eye bolt heads in the radial direction of the optical tube assembly with angle aluminum "v-blocks", and I'm experimenting with a couple of methods to trap them consistently in the perpendicular or tangential direction around the base.  I need to be careful to use a method that, like Merope, allows for the tubes to be bundled when the scope is broken down.

 

Regardless of the actual ends of the trusses--be they heim joints, spheres, or turnbuckle eyes--the threaded sections of the assembly must be tight, but not too tight in the tube ends.  Too loose and collimation may shift dramatically as trusses go from compression to tension, whether from altitude adjustments or from the torque induced by the user in aiming and tracking the scope.  Jonathan offered one solution on the first page of the hexapod thread, in which Nyloc nuts are potted with epoxy into the tubes.  This is a very reasonable way of eliminating the slop, however it does require using both left hand and right hand Nyloc nuts.  These appear to be much more available in metric rather than inch sizes.  Of course, coarse and fine right hand threads are another option to keep everything right handed, although for me I think the adjustment would be way too fine.  In this case the effective pitch is the difference of the thread pitches rather than the sum.  In my case of using left and right handed threads, 1/4"-20 threads would each give 0.050" per turn, or a tenth of an inch total per revolution.  Across the diameter of the scope I believe that will be plenty fine enough for small angular adjustments. 

 

I experimented with making tightly threaded inserts with my Red Stuff by partially tapping through with a taper tap so that the threaded eye bolts would cut their own threads somewhat.  I discovered that there is a very fine line on tap depth between producing inserts that are either too loose or too tight.  Because of that, I'm concerned that with the slightest wear a formerly tight fit may loosen fairly quickly over time.  This Red Stuff tooling board is similar to Delrin for hardness, but I don't have any Delrin to experiment with to see if it would do any better.  Nylon compresses much more, and I can see why it is used in Nyloc nuts for such a purpose.  I fear that if nylon were used for the entire insert, though, that it may be a bit too flexible and yield too much (elastically) under the varying loads in use.

 

I'm choosing to make inserts out of Red Stuff that is tapped completely through with 1/4"-20 threads, and I have obtained left handed taps for the six inserts that will be left handed. Surprisingly, I got a set of three taps, taper, plug and bottom for less than $15 delivered. Cool!  To make the bolts tight, and to give me the ability to maintain this tightness over time I'm cross-drilling my inserts and installing some nylon set screws into them for the bolt threads to cut into their tips, essentially making my own Nyloc nuts.  I have a bag of 100 1/4"-20 nylon set screws that I bought cheap online a few years ago as hardware for a possible lightweight string telescope.  If these wear significantly they could be either turned in deeper against the bolts or simply replaced.

 

I've already turned my inserts using woodworking equipment as shown here:

 

 insert turning.jpg

 

The only critical dimension is the diameter that fits into the tube.  This was easy to sneak up on using the outside caliper in the above photo.  Note that this minor diameter has angled undercuts similar to, but reversed in direction of, eyepiece undercuts.  The purpose of these is to provide an anchor for epoxy to adhere to within the tube.  I know from experience that this urethane material epoxies very well, and this may be a significant material advantage compared to Delrin for this use as an insert.  Carl also pointed out earlier in this thread that this material is also slightly less dense than Delrin, and less than half as dense as aluminum.

 

A dry fit of the inserts satisfied me:

 

insert fitted.jpg

 

The cross hole is through the insert and was made by a number 7 drill in anticipation of being tapped for the nylon set screw.  Because it is outside of the aluminum tube I could service it by extracting the set screw if needed and replacing it.  I doubt I'd simply tighten it because the eye bolt tends to really mutilate the tip of the screw and it would have to be advanced a full turn for the threads to realign with the eyebolt.  A bit of experimentation has me satisfied that if the two nylon set screws 180 degrees apart are threaded just inside the minor thread diameter of the axial hole then the result is just enough friction.

 

I tapered the ends of the inserts to allow the eye bolts heads to be closer together, better approximating a true truss with triangles on the scope.  One disadvantage of using turnbuckle eye bolts as compared to left handed and right handed rods is that I'm limited by their 2" of threaded length.  I can't have them extend about 2" out of the inserts or they would unscrew completely.  On my scope the included angle between adjacent trusses will be almost exactly 20 degrees, so I tapered the conical section about 10 degrees per side. This allows for screw eyes to be inserted almost completely into the inserts. 

 

The following photo shows my sequence of operations for making these inserts:

 

insert progression.jpg

 

From left to right you see I table sawed the board into square stock, then table sawed corners to make them octagonal, then drilled axial holes, then drilled cross holes for the set screws and saw cut the ends for the lathe spur to engage into, and then finally turned them complete with a skew chisel and a parting tool (see first photo on this post).

 

The two shorter inserts in the top of the above photo have already been installed into a cross brace tube between my altitude bearings (see first photo on this thread).  These will have thumbscrews to hold them in place in use, but for now simply have 1/4" bolts.

 

Here's a photo showing how I drilled the axial holes.  They came out nicely close to on center, but the holes were used to center the inserts on the lathe anyway, so the holes became concentric with the exterior profile when turned:

 

insert drilling.jpg

 

(Continued...)

 


Edited by jtsenghas, 26 January 2019 - 10:13 AM.

  • Oberon and dyode like this

#88 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 25 January 2019 - 04:37 PM

Tube Inserts (continued):

 

This photo shows approximately how the inserts will be oriented on the scope top and bottom, at least for my initial attempt:

 

insert orientation.jpg

 

The aluminum angle is sitting in my test cut shown in a previous post. Those troughs have already been cut in the UTA ring, but not the mirror box ring.  This insert has the nylon set screws installed.  I chose to only partially tap the set screw holes to retain the set screws firmly.  Otherwise my set screws may have needed set screws! I know that nylon does not epoxy well and I wanted them to be tight. I found it hard to avoid stripping the tiny 1/8" Allen wrench hole in the set screw, so I used pliers to install them and cut them flush with a saw.  In the unlikely event they need servicing I can drill and extract them.  

 

Not shown yet are the cross bolts and hold down bolts, although I think I've worked out the details.  That hold down block is tapered about 6 degrees to accommodate the splay angle of the trusses and get those screw eyes VERY close together.  I should be able to run that screw eye out about 3/4" from the position shown, giving a full 1 1/2" range of adjustment per pole.

 

So far it looks encouraging!  Even if I end up changing the way I manage the tube ends I think these inserts will do their job very well.  I'll probably epoxy and rivet them in place for insurance, as Jonathan did with Merope.


Edited by jtsenghas, 25 January 2019 - 06:52 PM.

  • Oberon, Kipper-Feet and Earthbound1 like this

#89 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9,543
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 25 January 2019 - 04:41 PM

On the hexapod thread Mark Cowan has shown he'll be using heim joints, but with two hold down knobs and bolts per pair for a robust way to keep the spheres of the heim joints seated firmly.


Small interruption to J.T.'s most excellent thread, but it looks like I'll be using something simpler, if the prototype pans out - Delrin balls in machined retainers holding threaded rods feeding your sort of tube inserts.  Lower cost, less snaggy bits on the ends when disassembled, flat folding, lighter weight, and easier to build.  My evaluation is they will reach the same level of replicating the collimation when assembly, which is the vital part for fast mirror structures.  Sorry no pics yet. ;)



#90 CrazyPanda

CrazyPanda

    Vanguard

  • *****
  • Posts: 2,112
  • Joined: 30 Sep 2012

Posted 25 January 2019 - 05:21 PM

J.T. what is that red material made of? Is it DIY or can you buy it off the shelf?


  • Earthbound1 likes this

#91 ckh

ckh

    Surveyor 1

  • -----
  • Posts: 1,938
  • Joined: 21 Feb 2015
  • Loc: Arizona

Posted 25 January 2019 - 06:31 PM

Google "Red Stuff".


  • Earthbound1 likes this

#92 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 25 January 2019 - 07:01 PM

J.T. what is that red material made of? Is it DIY or can you buy it off the shelf?

Yes, it's a Cass Polymers product that isn't really expensive, but is probably still available only in 12" x 60" sheets in various thicknesses. See my second to last paragraph in my post on material choices on this project on page one of this thread here.

 

Edit - it appears you may have to be more specific when googling it than Carl suggests. Try PP-1052 or include "tooling board" in the query  


Edited by jtsenghas, 25 January 2019 - 07:06 PM.

  • PrestonE, Augustus and Earthbound1 like this

#93 dyode

dyode

    Sputnik

  • -----
  • Posts: 38
  • Joined: 20 May 2018

Posted 31 January 2019 - 05:37 AM

...thanks for great thread and great read jtsenghas - your woodworking skills are clearly excellent and your trunnions look great. 

 

...class job on the inserts and your big red biscuit feet are excellent - I remember merope had bolts only which has good points and I'm almost up to that stage now so I'm wondering what to use. Good thinking on the shape, it'll be easy to level and adjust them reaching in and below the base with minimum fuss.   

 

(My hexapod is sitting in pieces behind me as I work out the final attachment method between the mirror box and trunnions  - I've sunk some threaded inserts into the inside surface of the trunnions and the bottom of the mirror donut and I'm going to use 90 degreee aluminium brackets to provide some reinforcement to some vertical through bolts - ny 'scope is going to be a bit clunky but it's fun having a crack at it!)


  • jtsenghas likes this

#94 dyode

dyode

    Sputnik

  • -----
  • Posts: 38
  • Joined: 20 May 2018

Posted 01 February 2019 - 02:44 AM

...hey jtsenghas just to show that the pieces actually are sitting behind me here they are...

 

....the parallelogram is from Larry in Ohio and I notice from his website that he's winding down - I ordered this from him a few years ago as a novice in this area with a plan to star hop and just gaze with a set of Nikonon 10x50's I bought. The parallelogram is beautifully constructed and has provided hours of pleasure and I feel honoured to have it quite honestly.  The tripod is a K-Pod and is a virtual russian tank... bought it from a rock video producer here in Melbourne Oz who didn't want it any more and it's a rock solid base for the parallelogram - I leave it on wheels but it has screw in feet similar in form/function to your feet but smaller diameter... it is my grab and go for an impulsive gaze and there is skyful of stepwise wonder available for binocular gazers...

 

I only committed to build a scope after taking my son on a road trip to every astronomy site on the Australian Eastern seaboard (optical + radio) and then discovering this website.... thanks again for your build thread!

Cheers.

 

the bits before fusion + parallelogram

  • Dave O, jtsenghas and Allanbarth1 like this

#95 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 16 February 2019 - 04:18 PM

Secondary Holder and Wire Spider Work:

 

Well, it's been a slow couple of weeks on this project due to cold and outdoor weekend work clearing snow and fallen branches at home giving me little time the last few weekends.  I did, however, get pretty far on the secondary mirror holder and the wire spider setup for it a week ago. 

 

After tinkering with a prototype and some craft wire on a board that can be used as a temporary jig for staging the mirror, I made the following:

 

secondary1.jpg

 

Wires will come off each of those screws in two directions, using a spider design of Mark Cowan's that he has prototyped with dental floss, but reportedly not used yet on a scope.  The angles of plywood edges and of the screw attach surfaces keep the wood and screws entirely hidden from the primary mirror.  The wires also will not touch the wood next to the screws due to the screw angles chosen, so they should be good and straight and not cutting into the soft wood surface.  Note how very little the center of gravity of the secondary assembly will be cantilevered from the lines of force.  There is room on the mirror mounting surface for three blobs of RTV adhesive around those holes, which serve only to lighten it a little.  Two blobs will go under the screws to the left in the above photo, and the third one will fall between the screws to the right.

 

First, I made a couple of Baltic birch holders for the 2.6" minor axis secondary.  I traced the outline of the mirror onto the plywood for each, offset the profile inside that about 3 mm, rough sawed it oversized a bit, and then sanded it to the undersized profile on my belt sander with the piece attached with two-sided tape to an oak 45 degree block:

 

 secondary sanding.jpg

 

This conveniently kept all edges parallel to that of the mirror and allowed for as large a attachment configuration as possible that would still remain hidden from diffraction effects.

 

To stage the mirror for a focuser mounted exactly at the center of the UTA (upper tube assembly) for height, I made a template jig from a scrap piece of melamine-coated particle board shelf stock:

 

secondary jig.jpg

 

The board was cut precisely the inside diameter of the UTA for width and the height of the UTA for height.  The notch for the mirror is at 45 degrees and is offset both down and away from the focuser from the center the 0.180" calculated by the Mel Bartels secondary calculator.  If you look carefully you can see a knife mark showing where the plane of the mirror should fall.  The blocks for temporarily attaching the secondary holder are offset from that line by the combined thickness of the mirror and plywood holder when the mirror is held with temporary two-sided tape.The 1/4" dowel at the lower edge of the mirror is a safety peg just below the mirror in case it sags during staging.  Six of the eight wire attach points of the secondary are accessible from the UTA anchor locations.


  • Oberon and Earthbound1 like this

#96 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 16 February 2019 - 05:05 PM

Secondary Holder and Wire Spider Work (continued):

 

Here is that jig within the UTA with the mirror on the prototype holder and cheap 26 gauge craft wire attached for determining wire angles and attach locations.

 

Spider layout work.jpg

 

For this work I secured the jig at the plane of the optical path with a thin wedge inside of the UTA.  I also stood the UTA on a full sized drawing of the plan view that showed the wire locations.  This allowed me to drill the tops and bottoms of the UTA dowels with a 3/8"  brad point bit to make counterbores and with a #3 drill in line with the desired wire locations.  I did this by eye, angling the hand drill upwards or downwards to match as best I could with individually stretched wires the lines to the attach points on the secondary holder.  For the angle in plan view I referred more to the lines on the drawing as seen from above. In the above photo the counterbores have been drilled, but the through holes have not yet been made.  This method made it pretty easy for me to determine where the holes should enter the dowels as well as which direction to point.

 

To avoid what Mark Cowan calls "Sticky Outy bits" on the UTA I plan to anchor the wires using these aluminum 1" long "binding post nuts" I purchased at Menards for $0.69 each:

 

post nut.jpg

 

In the past I've also seen these called "Chicago Screws" or even "s e x screws" due to how male and female fasteners with heads attach. (spaces required to keep the automated bots from calling these "**** screws").  I won't be using the supplied aluminum screws, but do have other plans for these particular screws on the scope.

 

These binding post nuts have 8-32 internal threads for about 5/8" depth, and I plan to drill and tap them about another 1/4" to give me more adjustment.  I also need to saw-cut screwdriver slots into the heads, which are currently flat and smooth.

 

The screws that will engage into these from inside the UTA are simple 8-32 x 1" long socket head cap screws drilled at about 40 degrees angle with a 1/16" bit as shown here:

 

drilling spider screws.jpg

 

The holes enter the heads at the centers of the allen bolt holes and exit just below the heads in the threads of the bolts.

 

I saw this method for my first time recently on this forum when Tom Otvos (totvos) shared it with his Mel Bartels-inspired scope.  I chose cap screws so that I can easily hold the heads in pliers when tightening  the post nuts with a screwdriver.  The wires will enter the heads, wind around the bolts, and exit through the same hole on the head and be twisted a few turns.  The result is a wire that is right along the axis of the bolts and will not tend to bend or flex it.  Since the post nuts will be within 3/4" of the ends of the dowels near the stiff (hollow) UTA rings I expect the setup to be quite rigid. 

 

I also expect the approximately 1/2" of adjustment I'll have on each wire for length will be plenty if they are initially attached in the staging jig because of how carefully I placed that.  The screw heads will be 1/4"-3/4" from the insides of the dowels, so should nicely stay out of the wall of the UTA by at least 1/2".

 

I'm fairly confident of my staging because I mounted the focuser and put a laser collimator into it.  It projected the laser within a millimeter of the center of my drawing and with the focuser empty the view of the secondary through the focuser was encouragingly round and centered with respect to the draw tube:

 

view through focuser.jpg

 

I didn't quite get all of this work done last weekend but expect to finish it up with about 90 minutes of work, which I hope to do tomorrow afternoon (after cleaning up more LARGE fallen dead branches from my yard).  I unfortunately have more chainsaw work to do this weekend too.

 

Overall I'm pleased with how this bit is coming along.  I'm glad this UTA will have fairly sleek lines for the spider since the heads of the binding post nuts will be slightly recessed.  I'll fit the thin plywood last and mark and drill it outside of the UTA so that the clearance holes for the wires are nicely positioned. 

 

I'd appreciate it, MARK COWAN, if you could share your stereoscopic views of your spider prototype that I'm approximating on this thread to show the stringing method.  I can't find where you've put it in past threads and the TOS prohibit me from posting the images of them you PMed me.

 

 

 

 

 

 

 


Edited by jtsenghas, 16 February 2019 - 09:37 PM.

  • Earthbound1 likes this

#97 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 16 February 2019 - 05:11 PM

As a reminder, this scope will have hexapod adjustment for collimation, so I'm not worried about there being no adjustment at the secondary itself, and only minimal adjustment at the UTA ends of the wires.  I expect that only a few millimeters of tensioning will be required, although I may have to redo any that have too much slack when they are at, or nearly at, the limits of tensioning.  After initial setup I plan on aiming the secondary purely by adjusting pairs of poles that meet at the UTA.  As with the work I did on the mirror cell, this scope feels eerily like it is missing bits!


Edited by jtsenghas, 16 February 2019 - 05:12 PM.


#98 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9,543
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 21 February 2019 - 12:36 AM

lol.gif  J. T.   you should've PM'd me before since I missed most all of last week sick.  But you've got my permission to repost those images, though I'll just do it anyway:

 

[astereo crossed 1_.jpg

 

stereo crossed 2_.jpg

 

I pretty much like how you've done the spider, only if you're planning to silicon blob the mirror on be aware that silicone adhesive isn't strong on any wood with internal oil.  So providing a mechanical anchoring hole in the wood would be advisable.

 

I'm also slightly concerned about the spider wanting to shift after being fixed in space and then the wires tightened, due to the angles of the wires and how they interact, but I've not tested this.  To explain - it's possible that with the location hub in place, you can tighten the wires but get some slop once the hub is release.  

 

Other than that I see I've got to finish the prototype and string it with actual wires.  waytogo.gif


Edited by mark cowan, 21 February 2019 - 05:56 PM.

  • Earthbound1 likes this

#99 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 21 February 2019 - 08:18 AM

I pretty much like how you've done the spider, only if you're planning to silicon blob the mirror on be aware that silicone adhesive isn't strong on any wood with internal oil.  So providing a mechanical anchoring hole in the wood would be advisable.

 

Yes, I hadn't mentioned it, but I plan on drilling each of the three blob locations on the wood with angled  holes in a couple of directions with the same surface locations to make shallow holes that are wider at the bottom, essentially "toenailing" or "dovetailing" the adhesive.  I've done this in the past and prefer the extra insurance of that method.


Edited by jtsenghas, 21 February 2019 - 06:23 PM.


#100 jtsenghas

jtsenghas

    Aurora

  • *****
  • topic starter
  • Posts: 4,729
  • Joined: 14 Sep 2014
  • Loc: The flatlands of Northwest Ohio 41.11N --Bloomdale

Posted 21 February 2019 - 08:27 AM

I'm also slightly concerned about the spider wanting to shift after being fixed in space and then the wires tightened, due to the angles of the wires and how they interact, but I've not tested this.  To explain - it's possible that with the location hub in place, you can tighten the wires but get some slop once the hub is release.  

 

If you're talking about initial shift after the setup jig is removed, I'll no doubt have at least some.  That's why I'm planning on drilling and tapping the post nuts a bit deeper to give me more ability to tension each wire at that point.  In fact, two of the wires can't be attached and tightened until after the setup jig is removed.  I contemplated making a fancier setup jig with a removable piece to both allow all eight wires to be attached while it was in place and to have a way of removing the jig while all eight wires were attached. The alternative was holding the secondary from the first surface side of the secondary, which I'd prefer not to do for a couple of reasons.

 

In the end, I decided to keep it simple and not bother, figuring I could add those wires after removing the jig and setting them initially with slight tension. Fine tuning all wires on scope assembly would be done when the struts were initially adjusted near the middle of their ranges.


Edited by jtsenghas, 21 February 2019 - 06:24 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics