Astro Sky structure

My plan to begin construction on this scope hung fire for nearly a year, victim of too many competing priorities and demands on my time.  I toyed around with the notion of buying a completed Dob structure from Night Sky Scopes or Tscopes, but that wouldn't have allowed me to do the kinds of customization I wanted during the construction process.  (Not to knock any of the fine vendors in the market; part of what I wanted to do was modify the scope's design interactively during construction as various ideas played out.)  Therefore, when I saw an ad on Astromart for a Dob kit from a vendor I hadn't heard of-- Astro Sky  (http://astrosky.homestead.com/trussdobsonian.html)-- I was intrigued.  A bit of research revealed that, while Astro Sky was just getting into the Dob market, the owner James Grigar had been making well-regarded high end metal piers and tripods since ~2000. 

This particular Dob kit was one James had been working on for a 15" scope of his own, but he had in the meantime decided to go with a larger mirror. It featured Baltic birch wood construction throughout, and followed the Kriege & Berry (hereafter, K&B) design specs explicitly.  James was to send me the rocker box and mirror box already assembled and sanded, and the trunions/altitude bearings, secondary cage rings, and secondary cage struts precut.  He threw in the associated strut hardware, homemade wooden Astrosystems style truss tube upper and lower connectors, and agreed to make me an 18 point tailgate/mirror cell of welded and powder coated steel for a remarkably low price.   I found James to be extraordinarily easy to deal with (he voluntarily re-made some parts to the size appropriate to a 16" scope, even though 15" dimensioned versions would have been 'close') and he was responsive to requests for modification (such as going with spoked-style trunions with a central hub to accommodate DSC's).  The Christmas holidays, some production backlog issues, and a serious illness on his part (stay away from spiders, James!) caused some delivery slippage from the probably overly optimistic initial six week timeline.  As for the price... suffice it to say this the only time have ever rounded a payment up to the nearest hundred dollars, since I was concerned James was losing his shirt on the deal!

The construction quality of the scope structure I received was very good, with the joints on the assembled parts nicely puttied and sanded smooth.  One trunion was 1/8th inch larger in diameter than the other; I don't know if this is "within spec" for 22" arcs or not.  There were a few definite problems, though:

1.  The structure arrived as a 68 pound single box shipment, and didn't fare well in transit.  All of the parts had been nested inside each other with minimal padding, and the ends of the rocker box had been smashed off by the weight jostling within them.  The top end of the mirror box also had a couple of dents in it when the box must have ended up half on and half off some hard-edged surface.  James was willing to replace the rocker box, but I opted to try to re-glue this one myself.  It is not exactly 90 degrees "true" at its corners as a result of the damage; I will have to see how this affects pointing accuracy with the DSC's.  The wood putty I bought was *supposed* to be a good color match to the Baltic birch, but it turned out to be less than perfect.  My original plan to stain the scope a "natural blonde" color turned into a decision to go with a darker/redder cedar stain to minimize these tone differences.   I had already decided that the focuser board and upper cage rings would be dark in finish (to minimize light/glare near the eyepiece), and decided to finish the trunions off in this color to offset the scope base.  I used Walnut for the upper cage, and mixed it with a bit of Mahogany stain for the trunions, since straight Walnut turned out to be too "black" when placed against the cedar and needed reddish hues added.  All surfaces were treated with 4-5 coats of Marine Spar Varnish, and the edges of any holes drilled into the wood were given at least one coating of Spar Varnish to seal the exposed wood fibers.

2.  When I got the structure assembled and the tailgate (which arrived later, due to delays in powder coating) installed for the first time, I found that the mirror box bottomed out in the rocker assembly by 1"+.  I believe that what happened is that the rocker side dimensions were not increased to those needed for a 16" vice 15" scope.  James again offered to make me new rocker box components, but I undertook to fix these.  My first attempt to fix these --see the "Saga of the Arcs" to the right-- was to simply make a 1"+ spacer arc to affix atop the rocker sides where the teflon pads lie.  This didn't work (see the "first try" images), because this meant trying to fit a 22" arc (the trunions) onto a now 21" circle radius-- the original  rocker box chord/diameter minus the inch of stand-off.  After another abortive try I finally got my geometry right, and opted to make the full-arc crescent moon "slices" you see labeled as "third try".  They are make of the 3/4" Oak plywood I had on hand, and hence are thinner than the rocker sides, but this doesn't seem to cause any stability issues.

Overengineered and overbuilt

I hate flexure in the optical tube of a scope; watch the spot from a laser collimator wander across the primary as you move a scope in altitude from the horizontal to the vertical if you want to see what I mean, and imagine what it does to your collimation!  I therefore opted to overbuild Natasha by using 1.25" truss tubes and upper cage struts, even though K&B only call for 1" for this aperture scope.  I realized as I got into cutting them that my truss tubes (recycled from Boris) were 0.035" wall thickness rather than 0.058", which makes them ~1/3 more flexible but still stronger a third more rigid than standard 1" tubes.  (Guess that accounts for part of why Boris flexed so much in use!) 

Although Astro Sky had given me perfectly serviceable wooden Astrosystems style truss connectors, I opted to re-use the hardware from Boris, since I had recently sold its' optics.  This allowed me to use Moonlite ABS lower truss connectors and a set of beefy aluminum Astrosystems style upper connectors.  In order to avoid crushing or deforming the ends of the truss tubes when they are in the Moonlite connectors, I inserted wooden plugs-- of Oak plywood-- into each (overkill?!).

Designed around the Clement Focuser

I built Natasha around the Clement focuser, which is an unusual (and patented) design available only from its inventor, Don Clement.  It features a unique cantilevered arm design that offers a number of advantages over a Crayford (see Don's website, http://www.clementfocuser.com/pages/1/index.htm for a complete explanation), but the most important to me was the ability to have a full three inches of focuser travel-- without any drawtube to intrude into the light path at minimum extension!  (see images at right)

This focus range was important to me since my favorite planetary observing aid, the Sirius Variable Optics filter systems (http://www.siriusoptics.com/) requires nearly 1.5" of extra in-travel to come to focus without a barlow.  I like the Sirius since, thanks to its "tunable" three peak interference filter, you can literally dial in various settings that bring out different features on your target while you watch. (In my experience it works will with planets but does not take the place of a nebular filter for Deep Sky Objects.)  My favorite planetary eyepiece is the Nagler 3-6mm zoom, since I like being able to vary the magnification dynamically with the seeing conditions.  Coupling these two produces a highly customizable observing experience, yet having to put even the lowest powered Barlow I have (1.5X) into the mix means that the lowest magnification possible is 450X, which is frequently too much for my skies.  I designed Natasha's truss tubes to allow the Sirius filter to be used unbarlowed, producing a more realistic lowest magnification of 300X.  I included an extra 1/8" of in-travel for observers whose eyes focus differently than my own... At the same time, I was able to accommodate the eyepiece I have that needs the most out-travel (a Meade 8.8mm Ultrawide, which I prefer to a Nagler 9 T1) with a comparable 1/8th" of spare travel for other observers.  That means I am using 2 3/4" of focuser travel range in my observing!

I had a JMI DX-1 dual speed focuser on Frankenscope, but found in practice that I rarely used the fine speed knob, even on planetary observing.  The Clement is single speed, but at 10 turns per inch thread, it is extremely smooth (with no backlash or slippage under load!) and fine enough for me.  There is only one plastic part on the Clement--part of guide/housing for the lead screw that moves the focuser in and out.  Unfortunately, mine broke the second night I had the focuser-- which I initially installed on my 20" f/6.7 Boris to test it out.  Don fixed it, but the replacement broke after only an hour's use on Natasha!  This time, Don re-designed the part.  I installed it myself and from what I learned in the process, am confident that it will be fully reliable in the future.  (In any event, there is a lifetime warranty on Clement focusers, barring abuse.)

UPDATE:  The Clement, which I believe had a manufacturing problem and never performed as advertised , has been replaced by the trusty JMI DX-1 from Frankenscope.  This means I can no longer use my Sirius Variable Filter on Natasha without a Barlow-- but I can do so on Brutus (Brutus Mods Rd 3), and in the mean time Natasha now holds collimation.

My experience with this focuser has been bittersweet, to say the least.  I designed "Natasha" around its unique focus travel capability, but was plagued with equipment breakage from literally the first hour of use.  This was compounded by what seemed to me excessive shift of the focal point as the focuser was racked in and out.  On Natasha (focal length 72") this was nearly 3/4" that a laser collimator spot would shift across the primary.  With Brutus (focal length 96") this shift was a full inch.  This played hob with collimating the scope (wherever you had the focuser racked in its travel could cause you to erroneously re-collimate the secondary mirror, primary, and focuser tilt.)  When the focuser came back from rebuilding by Don Clement, the maker, I confess I didn't even think to test for this, since he'd replaced some stressed and mis-installed hinges.  Much to my surprise, the problem remained.  Don, who I believe was as frustrated by this (or dealing with me!) as I was to have wasted countless hours pursuing false leads, graciously offered to refund my purchase price even though I'd had the focuser for far longer than the 30 day return period. 

Left image: The Kendrick laser spot with the secondary collimated and the focuser racked out.

Middle image:  All that has been changed is the focuser is racked in (~2.5" of its 3" travel, although most of the shift you see above occurred in the first inch of travel traverse).  The laser aim point has moved a full inch across the primary.  Since even a 1mm mis-collimation of the secondary to the primary can reduce an optically excellent mirror to below 'diffraction limited' performance, the implications are obvious.

Right image:  a close-up of this image shift

Designed to be moved while set up

I know that K&B caution against moving a fully assembled Dob, but that was one of the design requirements for my project.  The solution was simple-- on each of the two upper back corners of the rocker box I put a 1/4" eyebolt (for ease of turning) that mated into threaded inserts in the mirror box.  This meant that the mirror box could be securely fastened to the rocker box, even when the trusses and upper cage were attached.  I have not had any qualms about the security of this in practice, and frankly find that even attaching just the screw on one side gives enough piece of mind when wheeling the scope around on a relatively smooth surface.

Upper Cage

I made my upper cage relatively "long" to provide enough distance above the focuser hole so as to avoid the necessity of dealing with a removable baffle over the cage.  Most of my observing is done on my sidewalk with streetlights in direct line of sight, so this would have meant manually installing and removing a baffle at every observing session.  I already knew that I was going to have counterweight the bottom of the scope, so I was not concerned about the extra ounces.

My spider and secondary mirror mount are from Protostar.  I've always used Astrosystems hardware and really like their tool-free secondary adjustments, but Rob Teeter of Teeter's Telescopes turned me on to the fact that the vanes on the Protostar spider are ~40% thinner than standard, which should translate into reduced diffraction spikes.  I have to admit that the "clutch" system on the Protostar spider hub makes it very easy to use, since you can adjust a single screw without having to take up or release tension on the others when you are done.  (This is good for folks like me who are obsessive about their collimation!)  I opted to get the secondary mirror dew heater with power drawn through the spider vanes rather than via diffraction-inducing separate wires.  The Protostar also has two screws on each vane for a more secure fit-- something unusual in this diameter of telescope.  (I think I prefer a single screw, though, since it makes it easier to "twist" a vane to the minimal diffraction position if you somehow end up "off" in your installation holes.)  The Protostar is also a bit large for the B&K dimensions, with the vane screw hubs protruding slightly into the light path, especially if you off-set.  I'd blame B&K for this-- I think the upper cage rings should be spec'd 1/2" larger in diameter.

I decided not to go with Kydex for the walls of my upper cage, preferring to try the roll of aluminum roof flashing I had already purchased for use with the Dob Driver II installation on this scope.  Roof flashing took paint readily (I used some Royal Blue enamel Krylon spray paint I had lying around), looks nice, and turned out to be lighter than Kydex.  What I had forgotten, though, was that since two of my spider vanes could be carrying electrical current, using a metal cage shroud was going to require extra care in insulation.  I went overboard at insuring that there was plentiful insulation-- even going so far as to try to minimize the effects of dew coating the outside of the cage-- only to learn in the "powered ground board" saga below that this aluminum flashing was non-conductive!!!

I have always found it inelegant that Dob users are expected to reach out and grab the upper cage rim to slew their scope.  (This also helps to introduce an unnecessary thermal plume into the optical path, too!)  I installed a garage door handle on Frankenscope below the focuser, and opted to do the same thing for Natasha.  Since the mounting options are considerably more limited on a truss Dob's cage than a solid tube, I'm not as happy with the placement of the handle on Natasha in terms of ergonomics and comfort, though.

Features moveable laptop/eyepiece tray

For Frankenscope I built a removable laptop tray, and found that it was an indispensable perch for laying eyepieces, observing notebook, etc. on, usually to the exclusion of the laptop computer!  Therefore, for Natasha I made the tray larger and with a higher rim (wouldn't want the Nagler 31 taking a nosedive onto the pavement!).  I opted to for a Cedar finish on the multi-ply Oak scrap plywood that blends nicely with the scope.  After much deliberation, I ended up securing the table to a simple 1" dia. oak pole that secures by wingnuts to 1/4" bolts protruding from the rocker box.  At the top, there is a 1/4" bolt through the pole, a disc resting atop this, and the table inserted through a 1" dia. hole in the corner to rest atop this.  By not securing the table into place in the horizontal axis, I can slew the table to whatever position I want.  Vertically it is secured by the ~4" of excess pole that protrudes above the table.  (I may cut this down to 2", since the fit is tight enough not to need 4", and this makes it tougher to load the scope into my van without hitting the roof.) 

Saga of the Powered Ground Board

It sounded so easy!  After having put up with a heavy 75 amp hour marine battery mounted on Frankenscope's rocker box, I decided to get this same independence of rotation on Natasha without mounting the battery on board.  In principle making a powered ground board has only one tricky part-- getting a consistent flow of power from the ground board to the rocker box despite rotation.  In practice, it took me 30 40 hours and three four tries to come up with something I was happy with.  Even now I'm not truly satisfied with the electrical performance, though the "Mk IV" is the most traditional approach (featuring carbon brushes typically used in small motors).  Still, I think it's past the point of "diminishing returns" for the time being, in terms of time and effort invested.  Heck, as a practical matter I could just as readily have an external power cord running straight to an off-scope battery from the rocker box!  ;-)

My fellow NOVAC member Ed Karch had good luck making the connection to the rocker box with old wiring brushings, but I couldn't get my hands on any when I started out.  I dismantled two defunct small appliance motors in vain (they had coils instead of brushes), and Radio Shack looked at me like I was asking for Dilithium crystals for my star cruiser.  I already planned to make the two concentric circles on the top of the ground board out of aluminum flashing, and it occurred to me that a strip of flashing secured with a screw at each end could be forced to retain a "bow" under pressure.  If one of these screws runs all the way through the rocker box, it can be used to carry the power in.  For this first attempt, I was racing the clock and trying to get the scope in useable shape prior to my Club's star party ("Almost Heaven Star Party") in early June, so in my last day of spare time prior to leaving I tried to blitz this to completion.  That meant taking shortcuts, in the sense of not checking for electrical continuity at every step of the way.  When I was done, I had a base that was very noisy thanks to the grating of aluminum-on-aluminum, and failed to conduct electricity through to the rocker board.  I chalked it up to a short and left the scope at home.  After the star party when I had the leisure to tear things apart and test, I found that the aluminum flashing itself was non-conductive.  ARGHHH!!!  

Further testing showed that, while I had countersunk the screw heads as much as possible, the vertical clearances between the rocker box power screws and the ground board sheet metal rings were so tight that the sheet metal was getting gouged and eventually torn by the screw heads.  The solution turned out to be to increase the vertical separation between the two boards, by adding shims of plexiglass to the three outer Teflon pads and a second Teflon pad to the central pivot.

If at first you don't succeed...

For my second try, I decided to use some heavy gauge sheet metal I had and which I empirically verified was conductive.  <g>  It was so heavy and hard to cut, especially for the smaller diameter circle, that I ended up using a drill press to punch a series of circles in it that I could connect with the heavy duty metal shears.  It made for a ragged edged circle, but I knew it wouldn't be visible, and it seemed like the thing to do at the time.  <g>  For the link to the rocker board, I went with short lengths of the same steel strapping or banding tape I had picked up gratis at Home Depot when they cut a pallet of 2x4's apart.  When bent at an angle, this metal acts like spring steel.  I used two strips of this and the same screw principle as in the previous trial, and ended up with something that worked! 

Later I had an intermittent loss of power, and discovered that when I'd epoxied the two metal rings to the ground board, the epoxy had actually surrounded the copper wire coming up through the ground board on the positive connection.  While I had the base apart to fix this, I decided to address the fact that, according to my voltimeter, the power would sometimes drop as the base was rotated, due to unevenness in the mounting of the metal rings on the ground board.  I decided to add a short second "pickup" to each terminal, reducing the odds of power drop out to virtually nothing, since both pickups are less likely to hit a low spot on the ground board ring at the same time.

Third try's the charm?

After a couple of nights of successful use, this "Mark III" groundboard suddenly stopped carrying enough current to power its circuits.  Coincidentally my old Makita 1/2" drill had died, and I was able to salvage two heavy duty spring-loaded carbon brushes from it.  I opted to do a total rebuild of the powered ground board system, judging that the relatively coarse sheet metal circles on the ground board itself would chew up the relatively soft carbon brushes in short order.  I therefore made new circles out of flashing-- this time playing a successful hunch that galvanized flashing would be conductive, unlike its aluminum counterpart.  I opted to load the brushes vertically via a simple system that will allow me to move them closer to the ground board as the carbon wears away.  Since this new system didn't have the screws protruding from the underside of the rocker box, I opted to take away the shims on the teflon pads that I'd resorted to, so the gap between the rocker bottom and ground board is now back to the K&B/Astrosystems default.  I took advantage of this time to increase the loading on the central teflon pad (via three milk carton shims) to make the scope turn even easier in azimuth.

I am plagued by voltage drops once again (as with "mark II"), and the new system doesn't have anywhere close to the current carrying capabilities of Mk II, since it is unable to power even the voltage inverter for the laptop at the same time the fans are running.  After watching my DSC's reset themselves a few time running off of the main power lead, I put the 9 volt battery back into the Sky Commander to act as a form of on-board UPS.  Oh well, at least the new system is quieter-- the old spring steel on sheet metal versions made a quiet scraping noise as the scope was rotated. 

FINAL UPDATE: suspecting that a single pair of carbon brushes simply wasn't carrying enough current and was subject to voltage drop-outs from irregularities in the height of the sheet metal, I opted to add another two pair of brushes, spaced approximately 120 degrees apart around the circumference of circle scribed by the ground board's metal ring.  The total current transmission still isn't quite what I'd like-- not enough to run the Kendrick dew heater, for instance-- but voltage drops are a thing of the past, and (knock on wood-- or carbon and steel!) reliability has been 100% for dozens of observing sessions thus far.

Wiring on board the Scope:

I opted to put a terminal block into the rocker box to make for orderly routing of power within the scope, and covered the terminal block with a clear plastic cover to avoid shorting from dew.  I have a circuit for the seven fans, one for a power adapter for the Sky Commander Flash 4 DSC's, one for the Kendrick Dew Controller, one for a voltage inverter for the laptop, and one for the Dob Driver II.  Three of these are actually 12v cigarette lighter style pickups, which makes them versatile.  Several are independently fused, especially for those circuits like the fans or Dob Driver that don't have internal fusing. 

Lubrication and form-fitting Teflon

"Natasha" nearly complete without shroud

Completed scope without laptop tray or wiring

My field set up

---------------

Damage to the rocker box suffered in transit

------"Saga of the Arcs" ----

First try at fixing inadequate side clearance

Close-up of why this attempt failed

Third try: Tapered arc attached

Tapered arc close-up

Completed arcs!

My low-tech "arc workshop", aka my front porch, and homemade router jig

----Clement Focuser in action---

Clement Focuser racked in

Clement Focuser racked out

The completed upper cage, showing the power leads for the spider vanes (note the wires going through the two cage struts)

Oak pole for laptop tray.  The clear rubberized plastic to protect the scope walls can be faintly seen.  The right two holes are mounting holes; the leftmost is for one of the two eyebolts that secure the mirror box while moving the assembled scope

----- Powered Groundboard ------

Power cord into Ground Board

Covered and ready for caulking

First successful rocker box contacts

Second generation rocker box contacts

The scope with circuits and DSC azimuth encoder in place

Close-up of terminal block with clear plastic cover

"Mk IV" ground board's carbon brushes as seen from the underside of the rocker box

Carbon brushes as seen from inside the rocker box