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ATM: Building a 28" Dob


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How to spend all your money building a telescope                      

Howard Banich

Portland, Oregon

 

28 inch f4 alt-az Newtonian

 

Prologue

Since 2002 I'd been closely examining and admiring the all metal scopes made by Tom Osypowski (http://www201.pair.com/resource/astro.html/regular/products/eq_platforms/spicaeyes.html),

and then in 2003 an intriguing all metal string scope by Dan Gray (http://www.siderealtechnology.com/28inch/) got my attention.

 

An all metal (i.e. aluminum) scope is appealing because of the Pacific Northwest climate I live and observe in. The weather here is tough on wood scopes and I was tired of re-varnishing my 12 year old 20" Obsession, which I otherwise loved. At the 2003 Table Mountain Star Party I was impressed by the 24" f4.1 Steve Kennedy (http://www.kennedy-optics.com/) mirror in Tom O's metal scope. Tom mentioned that Steve was going to make 28" f4's in the near future, and that a 28" has about twice the light gathering ability as a 20" does - and wouldn't that be a nice move up for me?

 

Man, I suddenly had a raging case of aperture fever!

 

My primary observing interest the past few years has been galaxy clusters, so a larger scope would make this pursuit considerably easier and certainly more aesthetically pleasing. Being a life long star hopper I enjoy moving my new scope manually, and of course any new scope would have to also be easily moved by hand and accurately track the sky because I'd been spoiled with equatorial platforms for about 11 years. But my checkbook was depressingly empty so this was only pleasant daydreaming at first.

 

As fate would have it - and by the way fate, thanks - just a month after being impressed by the Kennedy mirror in Tom O's scope I unexpectedly had the money to buy a 28" f4. A stunning turn of events really, and most importantly my Dear, Blessed Wife Judy was all for it - she has surely secured a favored place in heaven for her encouragement - so I threw caution to the wind and placed my order. Oh yeah!

 

After I stopped shaking and could breathe normally again, my thoughts on the design began to gel. This took about three months to complete and was helped considerably by using a simple CAD program, TurboCad. My good friend Chuck Dethloff showed me how to use it and after trying it for a couple of days I was off and running. My original thinking included an equatorial platform, but good timing intervened as Dan Gray had just finished his new alt-az drive system prototype that included slip clutches. This really got my attention since it allows the scope to be moved by hand without having to disengage anything, and the scope continues tracking the whole time.

 

It becomes a goto drive by plugging in an Argo Navis, but I'm a dyed-in-the-wool star hopper so at least for now, no thanks. The slip clutches essentially promised that I would be able to use the 28" as if the scope were on an equatorial platform, only it would never need to be reset and will work anywhere on the planet equally well. Also, I would only have to install the drive to the rocker instead of building a new equatorial platform, so construction and ultimately set up would be considerably easier. Sold! I quickly revised my Turbo Cad plans and in January 2004 fabrication began.

 

Basically, my scope is a blend of Tom O's and Dan Gray's designs, borrowing on their strengths with a few of my own ideas blended in to achieve my functional goals. I think I stole some really great ideas.

28 inch f/4 TurboCad file

 

Functional goals

Ø    One person transportation and setup, no more difficult than a 20" f5 Obsession with equatorial platform.

Ø    All metal construction, rust and warp free.

Ø    Scope to have no more than a 2 second wobble time after a sharp hit to the cage.

Ø    Fully baffled.

Ø    Must have an accurate drive system that naturally and easily allows manual movement of the scope for star hopping.

Ø    Easily installed front surface fans with an exhaust door to cool the entire front surface of the mirror.

Ø    A bug proof mirror box while the scope is not being used.

Ø    A solid focuser board that won't flex with a binoviewer, and helps baffle the focuser.

Ø    Easy maintenance.

Ø    Still be able to use my 8 foot tall aluminum orchard ladder.

Ø    Looks cool. Not a functional goal, but what the heck.

28 inch cad file detail

 

Major component materials

Ø    28" f4 mirror and 5" diagonal purchased from Kennedy Optics (http://www.kennedy-optics.com/).

Ø    The mirror and rocker are rectangular 1/8" thick wall rectangular T 6061 aluminum tubing from Metal Supermarkets (http://www.metalsupermarkets.com/ ). They cut to length so there's little waste.

Ø    Truss tubes are 1.5" diameter round thin wall tubing from Tube Service (http://www.tubeservice.com/). They don't cut to length so there could be a lot of waste. I was able to use the leftovers for the altitude trunnion spacers.

Ø    Cage rings are Alucobond (http://www.alucobondusa.com/), a composite consisting of a plastic core with thin outer skins of aluminum. Alucobond is made to face large buildings so it's very flat, impervious to the weather and comes in 6mm thickness that's perfect for cage rings. It's as easy to cut as wood with a sharp router bit.

Ø    I used mild steel 1.5 inch square tubing for the mirror cell primarily for the extra weight to correctly balance the scope, with aluminum flotation triangles.

Ø    All bolts are stainless steel, and thank goodness for McMaster-Carr (http://www.mcmaster.com/).

 

Purchased components

Ø    Focuser is a Starlight Instruments (http://www.starlightinstruments.com/) Feathertouch V4.

Ø    Spider, diagonal holder, diagonal dew heater and truss shroud from AstroSystems (http://www.astrosystems.biz/)

Ø    Alt-Az controller from Sidereal Technologies / Dan Gray (http://www.siderealtechnology.com/ )

Ø    Mirror box tube clamps and cage tube brackets from Aurora Precision (Nate Currier, email nathan@aurorap.com)

Ø    Mirror fans are from Radio Shack (muffin fans on the mirror cell, (http://www.radioshack.com/) and Sundial Micro (front surface cross flow fans, http://www.sundialmicro.com/cooler_master_cross_flow_fan_stfb01e1_1745_360.html)

Ø    Azimuth drive belt from Belt Corporation of America (http://www.beltcorp.com/)

Ø    Encoders from US Digital (http://www.usdigital.com/products/h6s/ )

Ø    Servo Motors from Servo Systems (http://www.servosystems.com/servo_motors_surplus.htm )

Ø    Slip clutch "gear" timing belts from Grainger (http://www.grainger.com/Grainger/wwg/start.shtml )

Ø    Azimuth bearings are skate board wheels from a local skate shop.

 

Minor component materials

Ø    Back of the mirror box baffle is Coroplast (plastic "cardboard" http://www.coroplast.com/about.htm). It can be found at most sign shops, I got a 4'x8' sheet for about $20 at a local sign shop.

Ø    Mirror box back baffle cut-out is covered with window screen from Home Depot. Keeps out the bugs during storage very nicely.

Ø    Groundboard feet are hockey pucks from Garts, a local sporting good store. Hockey pucks are hard rubber discs about three inches in diameter that drill nicely with sharp drill bits.

Ø    The mirror box exhaust door is held shut with a magnetic lock from Home Depot.

Ø    The truss tubes are covered with heat shrink tubing from KVA in Denver (303-214-500). Ask for Steve.

Ø    The cage is lined with .040 inch thick ABS plastic. I used this same material for the top end baffle and the bottom end of the cage baffle. Call Garden State Plastics at 1-800-456-7945 and ask for Debbie. A 4'x8' sheet costs about $21.00 not including shipping.

Ø    7.5 amp hour batteries from Batteries Plus (http://www.batteriesplus.com/)

Ø    Assorted wires, connectors and switches from Radio Shack.

Ø    Altitude trunnion black wrinkle paint is Hammerite spray paint from Home Depot. It has a shiny, flaky but smooth texture when dry.

Ø    Mirror box and focuser board black paint is 3M automotive under body spray. It has a flat black wrinkled texture when dry, I found it at a local auto parts store. I also sprayed this on the bottom of the ground board (the top of the ground board was sprayed with Hammerite).

Ø    Red trim paint is automotive "anodized" metallic red spray paint from a local auto parts store.

Ø    Bare metal is sanded to 220 and protected with Turtle Wax automotive wax. This includes the cage rings and mirror box cover.

 

Building the scope

The last thing I'd made from metal was a small tool box in junior high shop class, so I was a little nervous about the prospect of making an all metal 28 inch scope. Fortunately there was nothing to worry about as cutting aluminum and steel was about as easy as cutting a hardwood. The sparks were a cool bonus, but the smell was terrible so I confess that I still prefer cutting wood.

 

Welding was really a breeze - a friend of a friend who builds custom motorcycles offered to do the welding for free because he thought a home built telescope was cool. I used up a ton of lucky points on this one. In two evenings - separated by about very long two weeks - all the welding was completed. No complaints when it was not only free but also top notch.

 

The cage rings were cut with a plunge router using a straight bit, just like wood rings would have been. The s**** was used to make the focuser board and a separate piece became the bottom of the rocker box. A thinner, less expensive version of Alucobond, called Dibond, was used for the mirror cover. The straight cuts were made on my table saw using a fine tooth wood blade.

 

The altitude trunnions seemed like they would be a challenge but a suggestion by Dan Gray made this a snap too - they were cut by a water jet machine. The friend (Chris Rizzo) of a friend (Jim Thorne) both regularly use a machine shop with a water jet cutter so they explained all I had to do was send a fully dimensioned CAD file to the shop. I was surprised just how fully dimensioned the file needed to be, and after three tries the file was usable and the trunnions were cut.

Altitude trunnion cad file

 

The cutting charge was nominal compared to the cost of the aluminum so this turned out to be good deal. I also had the shop water jet the triangles for the mirror cell because my manual attempts to cut them were not as accurate as PLOP indicated I needed. They also jetted out the edge support brackets for the mirror cell. Another good deal.

 

After making smaller parts like the brackets for the azimuth wheels, cutting the tubes to reinforce the trunnions and various other fittings I was ready to start the initial assembly. My plan from the beginning was to put the scope together, use it for several months and when it was finally working the way I wanted it to, I'd take it apart and paint it. So I didn't worry about the outward finish of any of the parts at first, I just wanted everything to fit together and work well. This made the initial assembly a lot easier.

 

The cage was first, then the mirror box and mirror cell. The cell took the longest simply because it has the most parts and it had to be precise. PLOP showed that an 18 point cell for a 2 inch thick 28 inch f4 mirror is right on the edge of proper support, so I paid close attention to all dimensions and went slowly to make sure I hit them all. One of the cool things about the mirror cell is that the collimation knobs move the entire cell and mirror as one. The collimation bolts are at the ends of the cell where they fit into brackets welded to the mirror box. The collimation knobs are accessed from the back. The mirror edge supports are two wiffle tree brackets.

 

Unfinished mirror cell in the unfinished mirror box (left). The photo on the right shows a close up of a collimation knob and the mirror cell. All

three corners of the mirror cell have a collimation knob, and note how each collimation knob moves the mirror cell, not just the mirror.

 

Assembling the cell, mirror box, rocker and trunnions involved lots of drilling and tapping, which I enjoyed but it was a little nerve racking because the holes had to be in exactly the right spots. Measure 10 times, drill and tap once.

 

Bolting the large assemblies together was gratifying as the scope sprang together over several weekends with no major snafus. The cell went into the mirror box, the altitude trunnions were bolted to the mirror box, the altitude roller assemblies were bolted to the rocker, and the rocker was placed on the ground board.

 

Rocker and mirror box assembly.

 

The ground board is an interesting anomaly in that it represents were I began to run out of money. Originally slated to be aluminum, it became fiber board as the budget began to tighten up. It's quite functional, and now paint obscures the material so maybe it will just stay that way. It certainly works fine.

 

The lower clamp blocks were bolted to the mirror box corners, the truss tubes were inserted and suddenly I had "first assembly". A few nights later I trimmed the truss tubes, and with the help of a clear sky I had a - barely - functional scope. No drive yet, but it was exciting to have a usable 28" scope all of sudden.

 

First assembly in my workshop.

 

 

Dan Gray was an enormous help with his drive system. Just a few months earlier he had installed the first version of his drive on his brand new 28" f4.5 scope, and he not only gave me access to his shop but showed me how to use his lathe to turn several of the parts needed for the slip clutches. He gave a lot of his time and expertise, for which I'm most grateful. Thank you Dan!

 

We spent an afternoon and evening installing the drive to the rocker in his shop, and the next night I tested it from my driveway. A day later I drove to the 2004 Shingletown Star Party and had real first light. Woohoo, it worked! Well, mostly.

 

Dan graciously helping to install the drive (left) - I'm fortunate his shop is only 15 minutes from my house. The slip clutches are the large round discs, and use Ebony Star and Teflon. The finished drive system (right). The fenders over the altitude slip clutch (forward) and altitude encoder (back) are dew shields. You can also see the front surface cross flow fans on the mirror box.

 

The azimuth drive belt configuration needed some re-working because the drive belt kept skipping off the drive shaft spur gear, but by the next month I fixed that and had a fully functioning drive and scope. This involved turning the drive belt inside out so it worked more like a timing belt. By the way, the altitude axis is a friction system, with the altitude trunnions riding on two long rollers. The altitude slip clutch is attached to one of these rollers. This arrangement drives both trunnions and provides plenty of friction. I made a few more improvements by the Oregon Star Party, and even more since then.

 

Finally, in March 2005 I had the scope were I wanted it and promptly took it apart for painting. I used the spray paints listed in "minor component materials" above, following directions on the cans. All the parts that look like bare aluminum were sanded down to 220 grit and then protected with Turtle Wax.

 

The finished scope

And now it's "finished", or at least as finished as a scope ever gets. Sure, I already have several modifications I want to try, but they'll have to wait until I make some progress on some projects I promised Judy. But really, even if I didn't have these other projects lined up I'd probably leave the scope as is for awhile simply because I want to observe with it - enough building already! After all, I spent all this time and money to have a big scope to observe with, so I'll modify it later.

 

 

Without shroud (left) and with shroud (right) at the 2005 Shingletown Star Party.

 

Moving the scope around and getting it into, and out of, the back of my van is relatively straightforward with a wheelbarrow system. The wheels lever down into position to give enough clearance for the ground board to clear the top of the ramps. This is a heavier lift than the 20 inch Obsession, but not too bad. I designed the rocker to accept 2x4's as handles and the wheel assembly. They slide in, are held by friction and then slide out when the scope is parked. Note that the cage sits on the mirror box cover and is held in place with two bungee loops. There's plenty of space in the van for everything else, too.

 

 

Wheels are in the "up" position (left), and fully engaged and ready to roll in the lowered position (right).

 

 

Ramps make getting the scope into the vamp pretty easy (left) and there's plenty of room left for everything else (right).

 

Set up

Setting up the scope takes about 20 minutes unless a parade of people are walking up asking "how long does it take to set up the scope, and how many people does it take?". Then I need about an hour. Set up is almost exactly like a regular Dob with the exception that I take care to level the base. This is make sure the azimuth drive isn't working uphill at any point of its rotation. I use a series of wood blocks as needed under each ground board foot to achieve level, and use a bubble level attached to the rocker as my gauge.

 

Set up from here is very much like a regular Dob - truss poles are inserted into the mirror box clamps, the scope is rotated to horizontal and the cage is attached to the top of the poles. I then attach the top and bottom cage baffles, which are just clipped into place. The shroud goes on next, then the Telrad and finally the eyepiece observing shade.

 

Collimation stays pretty close from one set up to another but I've needed to touch it up every time I've set up the scope. Collimation using a laser and then a Cheshire usually takes about 5 minutes.

 

Aligning the alt-az drive

Aligning the drive is also simple and takes a minute or two. If the correct latitude has been entered into the controller all I need to do is point the scope at the NCP and turn on the power. Alignment is complete - start pushing the scope, line up an object through the Telrad, look in the wide angle eyepiece, find the object and observe. Pile on as much magnification as needed.

 

It's a two step process if I'm somewhere other than the latitude that's entered in the controller. With the aid of a good level, I point the scope at the zenith and push the top right button on the wireless hand pad for 5 seconds. Then I point the scope at the NCP and hold down the top left button on the wireless handpad for 5 seconds. Alignment complete again, start observing.

 

Or I can plug my laptop into the controller and change the latitude directly.

 

 

Cooling

The front surface fans are primarily to cool the entire front surface of the mirror. The cross flow fans are sometimes called squirrel cage fans, but whatever they're called they put out a wonderfully laminar flow of air that takes care of the boundary layer. But this does little to improve the image at the eyepiece, and I suspect the reason is that a mirror this size is limited by atmospheric conditions. Unless the boundary layer is significantly more turbulent than the sky, the fans won't create an improvement in image sharpness. But then cooling the entire front surface of the mirror altogether seems like a good thing since this will at least avoid the potential of adding zones to the mirrors' figure as it cools.

 

The two muffin fans on the back of mirror cell are meant to draw air out of the closed mirror box on hot days so the temperature doesn't build up too much higher than ambient.

 

 

Front surface cross flow fans (on the far side of the mirror box) and the closed exhaust door (left). Opened exhaust door (right).

 

The two back muffin fans can be seen through the screen, and are meant to pull hot air out of the mirror box. The three collimation knobs can also be seen in the cut out areas of the back baffle. The mirror cell from the front showing the two muffin fans - and too bad the cell is covered up so well, this is the coolest looking part of the scope.

 

Baffles

The most effective light baffle on the scope is the rotating observing shade. It acts as a peripheral vision baffle by blocking almost all light coming into the observing eye from the side, and adds obvious contrast to the fov even in dark skies. People who have used it for the first time often describe the experience like putting their "head into a black hole" and often have to shine their red flashlight into the observing shade to see where the eyepiece and focusing knobs are.

 

This does what the rubber eye guards on eyepieces are meant to do, but without the potential to fog over the eyepiece eye lens if you screw your eye in too close. Plus, this really does block light from the side. The observing shade is manually rotated to be in the most effective position for any observer, and there's enough room in there for my big head covered with a thick wool cap.

 

The observing shade consists of three parts - the turntable, the backstop and the shade itself. The rotating turntable is made from a ring of Dibond and is held in place by four posts that are attached to the side of the focuser board. These little posts have a slot cut into their top ends for the ring to fit into and slide around in, plus they fold down out of the way when the observing shade is taken off. The ring is backed by a separate piece made from ABS that fits over the posts and acts as a backstop to the ring to prevent any light from coming in the back. The flexible shade is made out of synthetic leather, but any black, fairly stiff material would work. The observing shade components are the last things I put on the scope and the first thing I take off because otherwise they would be in the way of putting on/pulling off the shroud.

 

There's probably an easier way to make an observing shade, but so far this is the best I've been able to come up with for this scope. Fortunately the results are worth the effort.

 

By the way, in some of these photos you'll see a wool cap over the Telrad. That's to prevent the sun from burning the reticule during the day, plus at night I leave it on unless I need to look through the Telrad. This slows down dew formation quite a lot, and most nights this is all I need to do to keep the Telrad dew free. Low tech rocks.

 

Eyepiece shade "turn table". With the flexible observing shade attached (with Velcro).

 

Through the eyepiece

The views through the scope are awesome, The Kennedy mirror provides superb images and gives a really, really nice star test. Actually, it gives the equal of the best star test I've ever seen in any size scope, which is amazing for a mirror this size. At the 2005 Shingletown Star Party last week I had the mirror up to 1422x on NGC 7026, a nifty little bi-polar planetary nebula in Cygnus, even though the seeing wasn't quite that spectacular. Nonetheless, the image was quite sharp given the conditions. I was interested to see how the overall image would fare after the star images started to lose total sharpness, and interestingly it seemed that once past about 820x there was no further break down. Maybe the seeing gradually improved as I upped the magnification but whatever the reason it was remarkable to see. NGC 7026 certainly looked great at 1422x, certainly my best view of by far.

 

Bright objects take on new dimensions in a dark sky, and faint objects I could barely see - or not see - in the 20" have personality. For example:

Ø    The Orion Nebula is totally 3D even through a single eyepiece, and hints of color are everywhere.

Ø    Seeing three of the component galaxies in Hickson 50 and suspecting a fourth was pretty amazing.

Ø    The Eskimo Nebula had internal detail much like the better ccd images.

Ø    Two of the faint background galaxies near M51 are rather easy with averted vision, to say nothing of all the spiral arm detail in the Whirlpool itself…

Ø    Saturn has a more solid personality than in the 20" and the colors are more saturated.

Ø    The Trifid looks more interesting than any image I've seen and there are dark nebulae all around it.

Ø    Seyfert's Sextet (Hickson 79) shows all 4 galaxies and both tidal tails.

Ø    And on and on… It's all great stuff!

 

Alas, still no Technicolor views, but there are hints of color in more of the brighter objects like M42.

 

So, would I change anything?

Ø    The 18 point mirror cell sometimes puts a little astigmatism into the image, which is alleviated by giving the primary a lift off the floatation points for a second. I've seen this mild astigmatism develop about a third of the time after driving the scope to an observing site. I'm not sure if the 18 points store up stress from the road and I should have made a 27 point cell, or if I just didn't make the current cell accurately enough. For now this hasn't become an issue big enough to rebuild anything, but I may put some lubrication on all the cell pivot points to see if that makes any difference.

Ø    I'll make the wheelbarrow handles about a foot longer so the lift force needed to pick up the scope will be reduced a bit. I'll also add an ergonomic shape at the end of each handle that fits my hand - a regular 2x4 isn't all that comfortable to grab as is. Although quite manageable now, in 15 years maybe it won't be. Perhaps by then I'll have made remote controlled motorized wheel assemblies so I don't have to lift anything.

Ø    At altitude orientations below 30 degrees, the scope takes about 3 seconds to stop vibrating from a sideways jolt to the focuser board. At 30 to 60 degrees it takes about 2 seconds and above 60 degrees about 1 second. That averages out to 2 seconds, but I'd much rather the scope dampen out in 1 second in all orientations. This seems to be caused by the azimuth drive belt slowly losing its original tension and adding some rebound to the azimuth motion. I need to find an azimuth drive belt that doesn't stretch.

Ø    The .040 inch thick ABS used in the cage deforms in the hot sun, only not as much as .040 inch thick Kydex. I'll probably end up using a thicker ABS, but for now I'm not sure.

Ø    I'd like to come up with a simpler observing shade system.

 

Final words

My functional goals have been 99% met, plus I think the scope looks pretty slick. All the work has been so very much worth the trouble and expense, and I'm quite pleased with how straightforward the scope is to set up and use. It really does takes about the same amount of time and effort to set up as my old 20" and equatorial platform. A critical component of the success of this scope is having the right vehicle to transport it in, and I'm fortunate to have a VW Eurovan that's a perfect fit. I like to think of the van as the TTS - the Telescope Transportation System. It makes taking the scope out for an observing run a pleasure rather than a chore, and so the 28 inch will likely remain my most used scope for a long time.

 

Appreciation

I could go on and on at this point about fortunate I am to have Judy supporting me with this expensive and time consuming project, but suffice to say that without her support and encouragement I'd be nowhere with this scope, and recognizing that makes the time, effort and money I've put into it all the more precious. She's the greatest and I'm lucky enough to know it.

 

Created by AccuSoft Corp.

The lucky guy having fun at the 2005 Shingletown Star Party.

(photo by Chuck Dethloff)




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