• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.
  • PLEASE SUPPORT OUR FORUM - UPGRADE YOUR ACCOUNT HERE!
[4]

13" F/3 telescope build

[3]
[10] Like what you see?
Click here to donate to this forum and upgrade your account!

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#1
18 19 years ago I built myself a 20" F/4 telescope:

All SetUp.jpg

This telescope has served me well, but as I was approaching retirement the year previous to last
I wanted to build another set of telescopes to last me the rest of my life. I had several criterion
a) I wanted a telescope with a shorter focal length--in particular 1/2 the focal length of the 20".
b) I wanted as large an aperture as could be made to work well with the optics available today.
c) I wanted to do more and better machining

As to (c) above, the above telescope was built with nothing more than a hack saw, files, drill
press, sandpaper, and I farmed out the woodwork to a friend with a cabinet making shop.

Being an engineer by profession, an amateur physicist, amateur mechanical engineer, and
a all round driven person, I invented a new mirror cell architecture which one can puruse at

https://www.cloudynights.com /topic/547689-mitchs-mirror-cell-architecture/

The optical train in a Newtonian telescope is well established. With both (a) and (b) requirements
that largest primary mirror and pin-point star images with coma corrector is in the F/2.75 range
enabling a 14.5" mirror. F/2.75 is the recommended "fastest" Newtonian one should make with
todays optics (in particular a Paracorr 2 coma corrector). At this "speed" not very many kinds
of eyepieces will put up the the <now>F/2.75*1.15 = F/3.15 light cones. Luckily (or on purpose)
I had only been collecting EPs that would put up with such fastoptics.

But I backed down to a primary of F/3 which left me with a 13" aperture. To build the scope
around. At F/3 this telescope has 2× the Field of View as the current 20" F/4 enabling wide
field viewing of <say> the andromeda Galaxy and with 13" of aperture a bright image of same.

The EPs mentioned above all weight in in the 1-3 pound range, the Paracorr 2 is just over 1
pound, and if a Barlow is used for further magnification, we have 6+ pounds of weeight out
cantilevered 6+ inches from the typical focuser mounting position. So, instead of following
conventional reasoning (some might say any reasoning whatsoever) I designed a mounting
system that minimizes the cantilevering of forces, greatly stiffening the ability to hold an EP
at just the correct spot. A summary of this can be found at:

https://www.cloudynights.com/topic/545307-mips

In Jan of 2015 the mirrors were ordered, with a proposed ship data 9-10 months later and
ended up arriving in May '16 for the 13".
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#2
So, with my destiny set, I proceeded to fret about machinery--I should have also been fretting
about running electricity to the machines, but we will get to that in time.

In April of last year, I ordered a 12-36 lathe, an 8*32 knee mill, a 14" bandsaw and a 10" table
saw. About a week later they arrived. On the day they arrives the friends who said they would
be there and help me move the machines into shop would become the shop wigged out. The
delivery guy was kind enough to allow me to place 2 sheets of 4*8*3/4 plywood alternatingly
so we could maneuver the 1000+ pound machines towards the shop.

The next day I had some real friends show up, one with an engine hoist. If I had filmed this
It could have been in contention for the funniest home video ever--but alas. If we knew
at the beginning what we knew at the end of the move it would have only taken about 2 hours
to get everything inside. as it was it took 5+hours and all we did was get them inside. Success.

A few weeks earlier I had made arrangements with an electrician to drop 6 220V services to
the shop. He kept putting me off so long I finally had to fire him and hire someone who could
get to the job at hand.

The shop is located in the back of a Duplex we have owned for 20-odd years. It was originally
built out as the 3rd apartment in a triplex, but the cityu said NO, and the previousowner sold
the property to my wife and the back area had been used as simple storage for 18 years.

The new electrician was faced with the daunting task of bringing the electrical service drop at
the duplex "up to code". So we started with a service drop for the 2 apartments and the dead
back room as:

shopimg03.JPG

None of this meeting current code. After a week of negotiaion with the city we had an
acceptable plan and began the electrical work. And 5 weeks later the outside looked like:

shopElect03.JPG

And I had electricity to run the machine, and LED lights in the shop and all was well with the
world. I got six (6) 30A 200V circuits into the shop, 5 of which were consumed immediately.
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#3
With the shop now operational, a quick view is in order:

cell-stuff63.JPG

A long time ago my wife had a tenant who did stained concrete as a living, adn these
floors came in a trade fro rent money. All of the machines are on adjustable machine
feet, and over the next 6 months I ahve had to adjust them a couple of times as the floor
"takes a set".

But is is time to actually use the machine and start building stuff.............

As mentioned in the mirror cell architecture thread, the goal of the mirror supports are:
a) low friction
b) robust support
c) dynamic balance.

Most cells support their mirror on plastic pads of some low friction stiff plastic material. The mirror
slide around on the pads. But when sticktion arises, the mirror can stick and get its surface distorted.
Pads also have the property that the point so accurately located by PLOP is distributed over a fairly
large area. I wanted lower friction, lower sticktion, and stiffer connection to the backside of the mirror
while maintaining accurate support locations.

The 13" mirror is supported by 6 ball transfers on 3 beams at points (3.902" apart) determined by
PLOP. The mirror is free to slide over the ball transfers with about 10% of the friction of the lowest
friction pads one can find. This ensures that the mirror is not held artificially in any orientation.
Similarly, the edge supports will utilize the same ball transfers, this time set at the center of gravity
of the mirror.

Each beam rotates on an axle supported by a pair of 1/4-1/8-3/32 ball bearings. Thus, the beam rotation
is light and delicate and has low friction. ultimately the beams will be dynamically balanced so that as the
mirror cell moves in altitude, the beams cannot impart any forces onto the back of the mirror, other than
the support the mirror requires.

Three <ahem> perfectly machined beams for the 6-point cell in my 13" DOB project:

13beams01.JPG

The ball transfers are 0.470 in diameter and I have them set 0.300 deep. The ball seats are 0.468
in diameter created with a boring head on the mill. There is a 0.440 hole in the back for push out
access. The ball transfers are pressed in with a thumb, then can be pressed out with a screw driver.

As seen above, I still have to machine a tool to cut a bearing-fit on the 1/4-1/8-3/32 ball bearings
the axle pivots upon. The bearings measure 0.2498 in diameter. A good seat was created with a
D-sized drill bit.

Once the bearings are fit, I will machine (skim) the bottoms to achieve balance. Right now it should
be bottom heavy if all measurements have been done correctly. Balancing the beams means that the
beams will not push on the back side of the mirror as the scope moves in altitude (or azimuth.)
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#4
The holder for the secondary mirror requires a 45º angle on the nose of the support plate and
means to attach the spider vanes. It is desired that this component be light but also stiff.

The secondary mirror is attached to the secondary holder in such a way as to minimize most
cantilevering forces . In the following figure, the spider vanes are attached to the left side of the
spider holder with 4-40 socket head screws, and the plane where the vanes are attached runs
directly through the CoG of the secondary mirror.

secondaryCoG.jpg

Likewise, the secondary is attached to the spider assembly with three socket head collimation
screws and one tension adjustment rod with a thumb screw bolt. The bolt applies a pulling force
directed at the CoG of the secondary mirror, while the three socket head collimation screws apply
pushing forces defining the plane in which the secondary mirror is pointed.

SecondaryPull.jpg

Only the offset weight of the secondary itself applies cantilevering forces to the spider assembly.
The spider vanes attach to the spider holder close to the backside of the secondary mirror reducing
cantilevering forces.

Notice that the optical axis (black cross) is closer to the focuser than the CoG (green cross) of the
secondary, thus the secondary holder is offset away from the focuser. In order that the four spider
vanes create 4 diffraction spikes the opposing vanes must remain parallel, which causes the vane
load to be articulated into the upper assembly offset away from the focuser.

An F/3 Newtonian is essentially unusable without a Paracorr 2 (or similar coma corrector) in the
light path coming to focus. If one inserted a Paracorr 2 with tunable top into the focuser, and then
a heavy 2" eyepiece into the tunable top of the Paracorr 2, large cantilever forces are applied to the
tunable top, to the focuser, and to the base plate. To top it all off, the moment of inertial of the upper
assembly is lower due to the small radius of the upper assembly round plywood tube. as illustrated
in the following figure:

SIPS1.jpg

Instead, knowing that the Paracorr 2 is an integral part of the upper assembly, it is designed into
the optical path in order to achieve the stiffest possible design at the light weight an upper assembly
desires. The tunable top of the Paracorr is removed, and the Paracorr 2 itself is inserted into a tube
held concentric with the focuser. The Paracorr does not move during focusing.

Thus the weight of the Paracorr is on one side of the focuser and the weight of the eyepiece on the
other side canceling out much of any cantilevering force. Furthermore, the moment of inertia of the
upper assembly is dramatically improved because the round tube on which it is built has a 20" radius
instead of a 15" radius. Finally, the truss poles articulate directly into this tube, further stiffening the
upper assembly.

MIPS.jpg

Seen in plan view, we have:

UTAside01.JPG
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#5
Now that the upper assembly architecture is understood, let is proceed to the fabrication of
the secondary holder and spider system.

The secondary holder would have been easy to construct if someone made 135º angle aluminum
like they make 90º angle aluminum. 6061 T6 is just about the ideal substance to construct ATM
telescope parts, strong, light, easy to cut, file, machine, and corrosion resistant.

Creating the 45º nose on the 1/4" 6061T6 aluminum was a bit difficult as I had to make a bracket
to hold the spider-plate to the machining vise while clamping the nose piece in the vise and have
the ability to mill a flat at 45º on the plate before drilling a 0.086" hole through the plate at 45º and
directly into the nose piece. Before moving the mill head, the hole in the plate was enlarged to 0.111.
The holes drilled into the nose piece are threaded 4-40. This took 3 tried to get it right, the second
one would have been alright, except the spacing on the holes caused one hole to intersect a 4-40
tapped hole in the nose piece, which snapped the drill bit--Grrrrr.

This support plate was also the first time I had used the ability to set the vise at and angle other than
perpendicular to the table movements. I lightened the center of the support plate with a 5/8" end mill
ide cutting the 4 triangular holes. Various holes are 0.086 drilled and 4-40 tapped.

The nose piece has a 0.257 hole (F-drill bit) on the line which will pas through the center of gravity of
the secondary mirror to provide clearance to the 1/4-20 attachment bolt. There are three 4-40 tapped
holes on the 40% elliptical radius. Into these holes will be 4-40 socket head screws with their ends cut
into 60º points. These points will fit into points on the plate silicone glued to the back of the secondary
mirror. These 4-40 bolts are manipulated by hand during collimation. I have used a similar arrangement
for 18 years in my 20" DOB and it works well.

Every surface is cut by machine.

secondaryframe01.JPG

This first view shows how I used 4-40 screws to join the spider plate and the 45º secondary nose.

I just wish somebody extruded 135º 6061T6 angle aluminum 1/4 thick 4-5" on each side. Would
have saved me days of setup for two simple 2 minute cuts.

secondaryframe02.JPG

This second image shows the miter angle between the nose piece and the body. Forces exerted
by hand on this nose piece give the impression that the nose piece attachment is at least as
strong as the plate to which it is attached.

{For those more easily amused, this took 3 tries, the first two got ruined when the machining
forces were greater than the clamping forces leading to bad cuts and mispositioned holes.}

The secondary mirror is silicone glued to a collimation plate. In order to avoid thermal problems,
the plate is machined in a Y form and each leg of the Y is cut twice 75% of the way through creating
a long spring between the place where a pulling force is exerted on the plate and the place where the
pushing forces are imposed onto the plate. This long force path will reduce the forces due thermal
difference in expansion with temperature. The silicone pads are directly over the coned 4-40 socket
head cap screws. The seat is drilled with a #1 center drill so that each is fully supported but allowed
to tip/tilt in collimation.

cell-stuff77.JPG

I did decide to leave the flanking sides where the vanes attach on this iteration, unlike the drawing.

Here is a picture of the collimation plate sitting on top of its collimation screws and tensioned with
a 4-40 nut. The 1/4-20 threaded rod is secured into the collimation plate with a set screw.

cell-stuff76.JPG
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#6
Now, with the router radius attachment, I routed three rings, 2 for the upper assembly, one for
the lower assembly:

rings01.JPG

The lower ring and the upper top ring are 20.5" with 14.5" hole in the middle.

The lower top ring is 20.5" in diameter with 16.5" hole in the middle.

This allow the missing disk from the lower top ring to be used as the dust cover in transit and
storage.

I thought I would take some time and explain a few architectural things about truss poles that
make for good design and good engineering. These concepts are from Albert Highe's book
"Portable Newtonian Telescopes".

The first item on the list is eccentric loadings--you don't want any. When the load carried by
the truss pole enters the pole on the axis of the pole, there are no bending moments imposed
by the application of the force, otherwise there are. We don't want bending forces imposed on
the poles, the poles bend under compression anyway. We do this by applying the load forces
onto the poles through a <spherical> ball end.

The second item on the list is bending loads on pairs of truss poles. A pair of truss poles comes
together at a bracket. When one extends the axis of the poles to where they both meet, and this
virtual meeting point is at the center of gravity of the assembly being supported, then there are
no bending moments imposed by the bracket onto the pair of poles.

The third item is that we want the system of poles to be stress free prior to the clamps being clamped,
the unclamped truss allows the assemblies to find the point of least stress. If the clamping action can
be applied without moving from this balanced position, the whole truss will be stress free.

Without knowing up front, one can guess that the center of gravity for the lower assembly will be
close to the back face of the primary mirror, and similarly, one can guess that the CoG of the upper
assembly will be at the CoG of the secondary mirror. The following figure illustrates these CoGs and
the archetypal pole spans:

poles1.jpg

One wants the convergence point of the poles to coincide with the plane of the CoG at the upper
assembly and the lower assembly. Slight inaccuracies (as much as a whole inch) do not effect the
efficiency of the truss poles much, so a guess on the CoG is accurate enough for ATM builds.

One needs the poles to avoid vignetting the light column that reflects off the primary and reaches
the focal plane. In addition, as the radius of the truss increases, its stiffness increases cubically. I
followed Albert Highe's recommendation to use Drum Shells as round plywood for the upper assembly.
Secondarily, I wanted the cantilever moment of the focuser on the upper assembly to be small, so by
choosing a 20" drum shell for the 13" and having the Paracorr apply weight inside the shell and the
focuser and EP applying similar forces on the outside of the shell, the majority of the cantilevering
force cancels. Thus, the poles are aimed at the 20" shell at the upper assembly. This prevents any
reasonable sized pole from vignetting the light path.

One needs the poles separated by enough distance at the pole clamping bracket such that a single
simple clamping can be applied which is equal to both poles. A simple single bracket guarantees that
both balls are clamped with the same force. The following figure illustrates where one would desire the
pole <ball> ends to be located:

pole2.jpg

As the pole balls increase in distance from the primary mirror the mirror box grows in height. As the
pole balls decrease in distance from the primary the separation between the two balls decreases, until
at some point a single simple lamp will not fit between the pole balls.

The single clamping mechanism has been decided to be a 1/4-20 thumb screw and thus one needs
enough distance the human hand can get between the poles and tighten this thumb screw.

Now the bracket can be attached to the mirror box from below or from above, as illustrated in the
following figure:

poles3.jpg

This ATM decided to place the bracket below the <plate> ring on the mirror box, and to use the upper
clamping bracket to capture the poles in transit. Furthermore, the clamping thumb screw is semi-captured
on the upper bracket <plate>.

The same logic holds at the upper assembly, as illustrated in the following figure:

UTAside204.JPG
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#7
The second thing I machined were the seats for the truss poles. The next picture shows the components:

poleclamp02.JPG

The requirements here are a firm clamping action on the balls on the ends of the poles, a strong
attachment to the upper or lower assembly, fast, easy, and simple clamping mechanism. In addition,
for geometric reasons, the upper bracket and the lower bracket must have the ball seats accurately
positioned to properly apply tension forces to the balls, converting the pinned joint (pre clamp) to a
rigid joint (post clamp) which doubles the stiffness of the truss.

Towards the right are the pole-captive attachment brackets with semi-attached thumb nut. A nice ball
seat was milled into the captive bracket to hold the balls on center as a clamp.

Towards the left are the frame brackets. These screw onto the upper and lower assemblies of the scope
giving a firm location for the truss pole ends. Ball seats were also milled to properly locate the truss ball
ends. The frame brackets have countersunk wood screw holes located so that the bracket can be held
firmly to the wooden rings on the upper and lower scope assemblies.

At the top is the thumb screw with 1/4" of threads removed. This tightening device threads through the
captive bracket and then turns freely.

Now, to be fair, it took me several tries to get these things made precisely enough--all part of learning
how to use the machines I put in my shop, how to measure accurately, and in what order does one
machine things to avoid tolerance stackup problems.


poleclamp03.JPG
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#8
Today, I got a mirror cell support plate <jig> made.

cell-stuff01.JPG

This support plate has 6 accurately positioned accurate diameter and height stand-offs to hold the
mirror beams in <near> perfect position so that the beam pivot supports can be attached to the
mirror cell frame. As machined, the tolerance on the positioning of any given point at the end of the
beam is on the order of 0.002".

The Blue tape is just to hold the beam pivot supports in line so the mirror frame (Y) can be positioned.
I was intending to use the brackets shown above and using a Muggy-Weld product to solder aluminum
to steel. Ultimately I could not figure out how to make sure that the two axles from each side were
sufficiently co-axial that I made different brackets (see 2 paragraphs later.)

This was the third try at making such a jig. The previous ones failed in the accuracy department and in
the usability department.

Basically over the last 3 weeks I have tried a number of ways to mount my 6-point cell beams to the Y-frame
such that the points at the ends of the beams are at exactly the right positions (within 0.003"). In addition to
this, the beams themselves are balanced so that they can be oriented in any orientation and remain balanced.
This requires the CoG of the beam is on the axis of rotation.

A few weeks ago, I had beams that were perfectly balanced, but it turns out I read the wrong dimension and
while the beams balanced perfect, the points were no where close (about 0.2" off) to where PLOP wanted them
(3.902"). So, drat, I whacked out another set. At each end of each beam a 0.420 hole was drilled, then each hole
was bored to 0.468 so that the ball transfer bearings could be pushed in with thumb pressure and not fall out.
This set of 3 came out as; 1 was 0.000,5 too long, one was 0.000,5 too short, and one was 0.003 too short. I
decided this was good enough.

The axle was precisely located and cross drilled, each side was then drilled 0.110 deep with a 0.246 (D) drill so
the 1/4-1/8-3/32 ball bearings could be pushed in under thumb pressure.

Woe:: It turns out that drills wander as they drill holes, and the axle was not exactly perpendicular to the beams
even though the head on the mill is accurately trammed to the table of the mill. I found this out on the first set
of beams, and although I tried drilling slower, at lower speeds, at higher speeds, at lower pressures, and as much
as I could bear to put on the 0.89 drill bit, all of the holes wandered enough to be seen with dial calipers. I did
some research and found out that it is not easy or inexpensive to bore holes this small, either.

The beams were balanced by first shaving material from the back side of the beam until the CoG was at the height
of the axle (defined by the ball bearings sitting in their races on an axle.) Then the heavy end was hand filed until
balance was achieved horizontally left. Then the beam was balanced again vertically, and then the beam was balanced
in the horizontally right.

If the beams are not balanced, then as the mirror moves in altitude, the imbalance on the beams can apply forces
to the back of the mirror and distort its surface. I definitely did not want this.

The trick is to make something that connects the axle pick up points to the Y-frame. I won't bother you with the
number of things I tried (6) before finally getting one that ALMOST worked. I machines some u-brackets out of
aluminum and drilled a 0.089 (minor diameter of 4-40 threads), drilling through both ends of the bracket in a single
drilling operation. This is how one gets two holes drilled so that both are absolutely coincident--but this is only
straight, not completely true (perpendicular.)

The holes were then threaded with a 4-40 tap 0.400" deep. The width of the threads allow bending forces to be
picked up by the bracket efficiently. in the final assembly a 4-40 nut will be used to lock the axle in place.

I bought some precision ground rod for the axles. As it arrived, I miced it and it read 0.1251 and would not fit into
the ball bearings. Grrrrr. So I chucked the axels up in the lathe sanded it down with 1000 grit and WD-40. At 0.2499
(on my uncalibrated micrometers) it would fit through the bearing with a stiff hold on the inner race. I tried for a
couple of hours getting 2 bearings on the axle and the bearings in the beam races and never could get the beam
to pivot freely. So, back to the lathe, and I took the axles down to 0.2496 and could get the beams to pivot with
low friction. But now the lateral friction was so low, the beams would move on the axle. Grrrrr.

In each end of the axles I drilled a #1 center drill hole, and then I took six 4-40 socket head screws and put points
(cones) on them to fit in the end of the axle. Now, finally, I have beams, properly balanced, in low friction pivots.
Success like this deserves a couple of pictures::

cell-stuff02.JPG

Here we can see the beam with ball transfers at both ends, and small ball bearings in the middle.
An axle runs through the bearings and is "caught" by two 4-40 screws. The beam will move when
about 1/3rd of a grain (0.02 grams) is placed on either end--that is: low friction and good balance.
The imbalance is also the maximum amount of force the beam cam impart into the back of the
mirror at any elevation.

The beam is surprisingly stiff in deflection and in twist being supported only on the 1/8" axle and
located by the 4-40 socket head screws.

In order to properly locate the beams, brass tubing was cut and filed to thickness. This gets rid of
the lateral movement of the beams due to the sloppy fit of the axle to the inner races of the bearings,
but hampers the ability to move the beams inward or outward finding the perfect support radius.

At the top of the image are two little brass tubes used to space the beams on the axle:

cell-stuff03.JPG

Originally the axle was supposed to hold the bearings stiffly enough that no spaces were required,
and that the beams could be moved laterally with the 4-40 screws. But this is where this build ended
up.

In order to attach the brackets to the Y-frame, I machined in some relief slots at precise angles on
the bottom of the brackets so the brackets would practically grab the metal on the Y-frame. each
slot was carefully positioned to pass through the center of the bracket. As to the brackets grabbing
onto the Y-frame--yes they did.

With all of this machining and tight tolerances, at some point or another one has to question whether
this whole thing can be assembled and have any fidelity to the original design point. In order to address
the tolerance stackup, I built a wooden support plate with precisely located holes and machined up some
collars to hold the beam ends at exactly the right positions. This plate eliminates tolerance stackup, only
to expose your machining flaws.

Guess what? Remember that drills do not drill straight, well the beam axes are not exact, nor are the 4-40
screws in the bracket exact. Grrrr.

cell-stuff11.JPG

Here we see all 3 brackets face forward on the alignment plate which <all but> eliminates tolerance
stackup. The beam point positions are entirely determined by the support plate bushing locations.
These can be measured (and have been) and are within about 0.002 or correct.

We can also see that the beams are in balance and that none are touching the support plate nor their
bracket mid-section.

Now, I remove the ball transfers, flip the brackets over and press them onto the guide bushings:

cell-stuff12.JPG

At this point, the Y-frame can be dropped into the bracket slots which have been relieved so that
the both the bracket and Y-frame can jiggle about 0.001-0.003". Just enough to prevent stress from
<ahem> repositioning the support points on the ends of the beams.

When the upper part of the mirror cell is ready, the Y-frame will be trimmed to length, sanded, and
painted (gloss black) and when dry, the brackets will be epoxyed onto the Y-frame while being held
on the support plate. Now, the ball transfer roller balls will be precisely positioned.

You might notice that brackets and beams are labeled. Once the brackets are epoxyed to the Y-frame,
each beam goes in exactly one direction, one other thing is that the ball transfers are also "indexed"
nto a particular hole in a particular beam (they weigh differently--at the scale these beams can judge
weight 0.3 grains).

So, a tale of woe is finally solved by a bit of cleverness.

Some observers might say "Why go to so much trouble". There are 2 responses to this: 1) It's my telescope,
2) I am trying to build it more like a Swiss watch than a John Dobson Dobsonian. There is also a third reason:
Mike Lockwood has been writing about telescope mirror cells stating that low friction is key to making thin
light mirrors work.

This build is an experiment in how low a friction a mirror cell can have (thus ball transfers supporting the back
and sides) and ball bearings supporting the beams. In the scale amateurs can achieve, this is pretty close to a
little friction as reasonably achievable.

Some might ask if this is Overkill--Absolutely! it is!

But as an experiment in low friction cell design, in a low (wind) profile frame design that allow essentially unfettered
air to the back and sides of the mirror itself. See later.

The mirror cell is a square with two 1/2×1 mild steel tubes 14.5" long and two 1.25×1/8 flat steel silver brazed into a
square. It was a conscious decision to position this frame fully surrounding the mirror itself. Each edge supports
transfer their loads to a frame stiffening member. The frame stiffening member is then silver brazed to the mirror
cell and the mirror cell is screwed directly onto the altitude bearings.

The Y-frame has been calculated to heave on-the-order of 0.001" from zenith to the horizon--well within the
focusing tolerance of an F/3 scope. The frame stiffeners are considerably stiffer considering the 10.6 pound weight
of the mirror.

The mirror cell will be directly attached to the altitude bearings leaving no intermediary way to lose stiffness.

The edge supports are ball transfers sitting in a fixture. On the top of the fixture is a threaded hole which will be
used to hold the top edge restraint (so the mirror cannot fall forward out of the mirror cell.) At the back of the fixture
is a 1/4-20 thread so the fixture can be bolted to the frame stiffener. The height of the fixture above the ball transfer
was sized so that when the mirror is touching the edge restraint the ball transfer should be exactly at the CoG of the
mirror.

Here we see the bolt holding the fixture to the frame stiffener holding a ball transfer.

cell-stuff23.JPG
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#9
The ball transfer will be positioned at 0.481" from the back side of the mirror (CoG) in a future
take (hopefully not one with so much woe involved.)

Two of these frame stiffeners are fitted to the mirror frame.

cell-stuff21.JPG

We see two of the stiffeners being held up on spacers touching the actual 13" mirror. This picture
was taken before the stiffener tubes have been fitted. We see the square frame as described earlier.
In the flat plates on the side, we can see some 1/4-20 holes in the plate where the altitude bearings
will be bolted.

Any time I need to touch/move the actual mirror, my hands get washed, dried, and then I wait until
the skin has no residual water remaining on it. The vast majority of the time the top cap (cardboard
above) is left on the mirror to protect the surface. Only when the top edge of the surface is critical is
that layer of protection removed, the top tissue paper remains.

Fitting of the stiffeners to the frame involves cutting material off of the ends of the stiffener such that
the ball transfer remains in the center of the stiffener. This took a number of tries.

After some work all 4 frame stiffeners holding their ball transfers bolted to their tubes have been fitted
to the mirror and to the frame.

cell-stuff22.JPG

At this point there is less than a piece of paper thickness between the mirror and any of the ball
transfer--call this zero clearance. Later, I will adjust the clearance to more than 0.003 and less than
0.010. 0.003 is the amount of dimensional change between the frame and the mirror between 100ºF
and 30ºF. ) 0.010 is a small enough clearance that collimation will not shift as the scope moves in
altitude.

Overnight I decided to use 0.008" thick card stock for my spacers--as it was easily available (at hands
reach). and that I would space the ball transfers such that each was spaced 0.008" away from the edge
of the mirror; 0.016" in total clearance. This is vastly more than thermal requirements, and still small
enough that the mirror is accurately located. 0.008" clearance was obtained by machining 0.006" off
the each end of each stiffening member and depending on a SQRT(2) = 1.414 multiplier due to the
45º angle of the stiffener.

Having achieved this clearance, the frame stiffeners were silver brazed and test fitted against the actual
mirror and a diametrical clearance of 0.016". These card stock shims fit with significant force required to
push them between the mirror and the ball transfer. In order to achieve this < ¿perfect?> fit I had to
shave 0.003 off the back of the ball transfer fixture. Apparently, the silver brazing allowed the stiffeners
to end up 0.003 farther out on the mirror cell than the fitting procedure did. Oh well.

The mirror cell was then turned upside down with the mirror retainers attached at the bottom of the edge
support fixture. The mirror was then carefully placed upside down touching it only by the edge and only on
soft aluminum. The ball transfers were verified to be positioned at 0.481" from the bottom edge of the mirror;
that is, at the CoG of the primary. Later on, the mirror cell retainers will be shaved 0.010" to provide the
vertical clearance the mirror deserves and requires. (i.e., not and never touching the mirror)

cell-stuff40.JPG

The beams and brackets were taped onto the Y-frame and positioned over the inverted mirror
and manipulated until the beams and balls were at their proper geometric point on the back side
of the mirror. In effect, the Y-frame is holding itself in the perfect position by using the actual
beams, actual brackets, and actual ball transfers.

It is now a process of measuring and fitting some small posts to the mirror cell, then cutting and
drilling a hole in the Y-frame so it can be semi-permanently attached to the mirror cell.

The astute observer will see 2 machinist triangles in the image. These are being used to prevent the
Y-frame from moving under the slightest touch or vibration so that some measurements could be
taken! Yes, Virginia, there is low friction, here.

cell-stuff41.JPG
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#10
At this point, measurements of the vertical space between the mirror cell and the Y-frame were taken, and card stock used to verify the fit prior to cutting steel.

Steel posts were cut and fitted between the mirror cell and the Y-frame. During the measurement and fitting one can measure that the right hand side of the Y-frame is about 0.002 taller from the back of the mirror than the left side of the y-Frame. The nose (0.850) to tail (1.115) dimensions are just about spot on the original drawings.

The mirror put back into safe keeping, and replaced by the support. The support plate is accurately centered and the Y-frame marked for cutting. Here we see that the support plate has been fitted to the edge supports on the mirror cell and sunk down to the same height that the Y-frame was found to be when supported on the back side of the mirror.

cell-stuff42.JPG

The Y-frame was cut and fitted to the posts. The posts had a press fit nut inserted and silver
brazed into its top flush with the steel.

After cutting, the Y-frame is assembled on the support plate and the location of some holes determined
for 1/4-20 socket head screws. I drilled the holes 7/32" to be smaller than 1/4" and used a round file to
make the center of the hole match the center of the press in nut on the post. This fitment ensures low
stress is induced bolting the cell to the frame.

After cleaning up the welding detritus, the mirror cell was fully assembled for the first time.

cell-stuff50.JPG

In this form, the astute reader will recognize the almost unfettered access of air flow across the
mirror when in use. The back side of the telescope will have nothing to prevent airflow from occurring
naturally, and will include a fan to help airflow early in the night.

Here is a look at the 30º angle cut on the Y-Frame giving unfettered access to the socked head screw,
and a pleasing look.

cell-stuff51.JPG

A bit of painting and presto:

cell-stuff60.JPG
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#11
The assembled strut system is now fitted to the mirror cell:

cell-stuff61.JPG

The two back truss pole attachments will couple to the mirror cell through the altitude
bearing, giving a stiff vibration dampening attachment. The single front truss pole attachment
couple the mirror cell through the stiffener directly to the top of the mirror cell where it is
attached to the Y-Frame. By these means the <removable> truss is stiffly attached to the
mirror cell without a mirror box getting in the way.

The mirror cell is not under the mirror as it is in most Dobsonian designs, only the Y-frame is.
The mirror box has absolutely no structural loads, the mirror box is present merely for weather
protection of the mirror. The slightly higher placement of the mirror frame enables the altitude
bearings to be slightly smaller and thereby slightly less bulky in this F/3 design.

And I found out that the nose of the mirror frame needed a stiffener to pick up the loads from
the isolated truss attachment at the top of the truss system. I knew this stiffener was needed all
along. I was originally going to make this out of aluminum and bolt it to the wooden (ahem)
mirror box, but I decided to braze it to the steel frame and prevent the compression of wood
from altering the stiffness of this critical strut attachment.

cell-stuff62.JPG

The stiffener is carefully fitted to the mirror cell and to the truss pick up point. Inside the stiffener is a
fitted press in nut which is also silver brazed. This nut is rotationally positioned such that the thumb
screw going through the truss pick up point threads right into the stiffener, joining them as if one.
Later on I will use some Muggy-Weld stuff that can solder aluminum (truss pick up) to the steel stiffener.

The stiffener is silver brazed onto the mirror cell and also onto the post holding the Y-frame. During the
brazing, a vertical c-clamp is used to make sure the post does not move and the stiffener is clamped to
the post.

Yes this did cause me to have to repaint the mirror cell. That is one of the costs of getting your telescope
build out of order.
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#12
Here I am cutting out the material that will become the vanes of the spider.

cell-stuff70.JPG

The material is 0.008" thick galvanized roof flashing that comes in a roll at the local Home Despot.
$10.00 worth will provide for a lifetime of spider vanes of the typical ATM.
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#13
The strip is folded in the middle and then the middle is bent back out to 90º.

cell-stuff71.JPG

Before painting the seam will be silver soldered to retain a nice crisp 90º bend under tension.

Finally, the taper on the vanes is cut, the adjustment ends fitted and this is placed on a setup jig
for measurements and sanity checks. As imaged here, the vanes need to be shortened by about
0.500.

cell-stuff73.JPG

And this is about where the whole kit and caboodle is right now.
 

savarin

Active User
H-M Supporter-Premium Member
Joined
Aug 22, 2012
Messages
1,805
Likes
2,709
#14

jrkorman

H-M Supporter - Premium Member
H-M Supporter-Premium Member
Joined
Jan 22, 2017
Messages
50
Likes
44
#15
Fantastic - Going to have to reread this a couple of times to get it all in - Love the 'scope - I've only a small refractor and binoculars to use here! And since we have quite dark skies here, I have been thinking about buying/building something a bit better!
 

Bob Korves

H-M Supporter - Sustaining Member
H-M Supporter - Sustaining Member
Joined
Jul 2, 2014
Messages
4,997
Likes
5,333
#16
as an ex chef I only use rough guestimates.
I am an ex chef as well. That is the ultimate improvise, use your imagination, and git-er-done job, more so than machining. It is also a good way to work strange hours, and every weekend and holiday.
 

john.oliver35

H-M Supporter - Premium Member
H-M Supporter-Premium Member
Joined
Apr 26, 2014
Messages
114
Likes
62
#17
Mitch - Very nice work! I am not a telescope guy, but I bet your record of this build will be valuable to a lot of people!
 

petertha

H-M Supporter - Premium Member
H-M Supporter-Premium Member
Joined
Jan 15, 2016
Messages
506
Likes
339
#18
Very impressive. I can see that hobby makes a lot of demands to many aspects of machining, materials types & especially fitting.
Do you integrate a digital camera into the assembly to record images?
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#19
Do you integrate a digital camera into the assembly to record images?
At my current level of machining, I typically make a part 3 or more times before it is good enough to go on my telescope.
So, under these circumstances, I just take pictures when I have something to show.
 

petertha

H-M Supporter - Premium Member
H-M Supporter-Premium Member
Joined
Jan 15, 2016
Messages
506
Likes
339
#20
No, your build pictures are excellent! I mean do you have provisions for a camera hooked up to the telescope to take pictures of distant galaxies? :)
 

Mitch Alsup

Active Member
Active Member
Joined
Nov 17, 2017
Messages
162
Likes
112
#21
No, your build pictures are excellent! I mean do you have provisions for a camera hooked up to the telescope to take pictures of distant galaxies? :)
Sort of:: I have an adapter and t-ring that attached to my Canon dSLR; that goes in the focuser and enabled astrophoto.
But 99.9994% of my telescope use is visual.

I thought about building a fork mount for the 13 (maybe after I get the other telescopes done) that would be good enough for long exposure astrophoto. But its in a not yet category.
 
[6]
[5] [7]
Top