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ATM: 16" Ultra-compact Dob
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AKA: The 27 lb. laptop(?)!
I've dabbled in amateur astronomy over the last 20 years utilizing commercially available 4 and 5 inch refractors and a 6" maksutov cassegrain, but only became frustrated with the image content and lack of portability (particularly the mount/tripod), and, in the case of the mak, cool-down time. Then one day in late 2003, I received the latest addition of my (former) club (Sacramento Valley Astronomical Society) newsletter and it featured an article on an airline carry-on, 8" dob, based on a design that Gary Seronik of Sky and Telescope magazine had published earlier. It was a nested design that resulted in a very compact and portable telescope. I found this very intriguing. I had always been wondering what I would build with the carbon fiber panels a friend of mine, who used to work at Boeing, had given me a few years earlier. (Yes, I am EXTREMELY fortunate, this stuff is awesome!) This was a perfect project for the carbon fiber panels!
Airline carry-on was always an attractive feature for me, so I checked the United website to find out what the carry-on dimensional limits were and it turned out to be 45 linear inches. So, I set about trying to figure out just how big a scope I could fit into that dimensional requirement; there has to be some design limit, right? In addition, I wanted the biggest scope I could use without a ladder that offered great planetary/lunar and DSO views.
Well, my original calculations showed that I could fit a 14" scope into
those limits. I began researching, designing and building a 14" dob. I checked
out all the different scopes on Mel Bartels' website and came across Dan
Gray's string scope and decided that would be the design I pursued. After
many "tweaks" I finally finished the 14" project (which is now for sale)
and even though I am very pleased with the result, I had learned along
the way that I could build an even better one (engineer's remorse?). The
specs on the 14: 35 pounds, 17" x 17" x 10 3/8" (stored). (Disclaimer: I
have not attempted to take the scope on a plane, so I don't know if it will
actually fit in the overhead compartment, or under the seat, which of course
is the true airline carry-on test.)
On Christmas Eve 2004 I came across a classified ad (on that other website that starts with an "A") for a 16" quartz mirror and I had heard so many great things about quartz. This would allow me the opportunity to build an even better scope with a quartz mirror! My improved calculations (and previous experience) showed I could still meet the 45 linear inch maximum for airline carry-on (so long as the secondary mirror was removable). How could I pass this up? Well, I didn't, I went for it.
This time I went with a fully nested design, which, in hindsight, I should have done in the 14"; live and learn! (I have my excuses.) Going with the Kineoptics HC-2 focuser from the onset allowed for a 3.5" tall secondary cage, which meant I could have a 4" tall mirror box (the carbon fiber panel is 0.4" thick). This results in a rocker box that is 5.5" tall, allowing room for the base/azimuth bearing and altitude bearings to be stored "inside". I used the secondary cage ID formula that resulted in an ID of 16.875", which results in a rocker box OD of 19.75" square. 2(19.75) + 5.5 = 45 linear inches!
The carbon fiber panel is extremely strong and light so I was able to go with an octagonal cage constructed of carbon fiber panel sides. This works very nicely in a nested design, since it results in a very "thin", strong and light cage. The octagonal design also allowed for having supports and hold downs for the primary mirror that ended up being "tucked" in each corner of the cage when nested.
As mentioned earlier, I went with a Kineoptics HC-2 focuser, which is a very light, precision focuser with zero image shift. I do intend to use a binoviewer with this scope and I know it's going to be a bit of a hassle with a helical focuser, but the much lower weight of the HC-2 was more important to me. I also have the Burgess Optical binoviewer, which is much lighter than the others and provides wonderful views. (These are an incredible deal.)
I had my local sheet metal shop make the curved, single arch (180 degree) secondary mount for me (for free!) I used a piece of ABS pipe cut at a 45 degree angle for the secondary holder. It's a fixed and removable secondary. Setting the position the first time was quite a hassle, but now it installs easily and can be collimated quickly.
The secondary mirror is a 2.6" pyrex mirror from Bryan Greer at Protostar. (He convinced me that pyrex would do just as well as quartz.)
The attachment points for the truss tubes and strings are made out of 7/16" aluminum u-channel that I got from my local shower door installer (1' for free!) The forces of the strings and truss tubes cancel each other out and they are confined to the u-channel, so there isn't any stress on the secondary cage; it just keeps the poles positioned in the right place. Each truss tube has a 1/4" tent pole tip on the top end (free from Texsport!) which inserts into a hole drilled in the bottom of the u-channel. I splurged and went for carbon fiber truss tubes, too. They are pretty expensive, but again, they are incredibly strong and light.
I covered the interior of the secondary cage, mirror box, rocker box and altitude bearings with flocking paper from John Duchek. Not only will it improve contrast, it will protect the paint when stored!
Since I'll be using a Lumicon Sky Vector, I decided to just go wih a "red-dot" finder to do the initial setup of the SkyVector. The finder is the new MRF (Multi-Reticle Finder) from Burgess Optical, and it is much nicer than the other "typical" red-dot finders. It also remains attached to the secondary when nested.
The light shroud is made of a "Little Foamie" that I found at my local
craft store. I covered the "in-side" with flocking paper, too.
I again took advantage of the amazing strength of the carbon fiber panel in constructing the mirror box. The truss tubes are mounted using the same u-channel mentioned above slipped over the top edge of the mirror box side. The u-channel has a socket-head bolt mounted through the bottom. The head of the bolt inserts in a hole in the top edge of the mirror box side, which acts as an index and works along with the u-channel to keep the truss tube in the right place. The threads of the bolt extend upward and screw into a threaded insert in the bottom of the truss tube. This allows for the truss tube to be unscrewed to tighten up the strings. Each string "set" is attached to it's own 3/16" aluminum angle bracket (wrapped in flocking paper) via a standard screw eye and mounted to the mirror box side at each corner. The screw eye allows for adjusting the string tension during initial alignment. By having each string set and truss tube attached to a separate mirror box side, this confines those forces to that mirror box side. This prevents any twisting of the mirror box due to string tension. The carbon fiber panel is plenty strong enough to support these forces. (BTW: I used 450 plus bowstring for the strings.) This arrangement works very well.
As you can see in the photo, I used a somewhat "typical" 18 point mirror cell design, except I again took advantage of the strength of the carbon fiber bottom of the mirror box to support the mirror. The collimation bolt for each support (see picture) is a standard 1/4-20 bolt that has a slotted end which can be adjusted with a screwdriver from the under side of the mirror box. (Note: all of the grooves for the e-clips were cut using my cordless drill and a hack saw as a makeshift lathe.) The bolts are threaded into brass inserts, which are found at the hardware store. The brass inserts are epoxied into the carbon fiber mirror box bottom. This mirror cell design is extremely low-profile (3/8" - essential for this design) and light! I'm not concerned about not having a hole in the bottom of the mirror box for ventilation, since it's a quartz mirror (although there's no reason why I couldn't). I plan on adding fans adjacent to the mirror to eliminate the mirror surface thermal boundary layer.
I used 1/8" x 1 1/4" aluminum u-channel (wrapped in flocking paper) to act as primary mirror edge supports. Four of these double as primary mirror holddowns and four others double as mirror cover mounts and, as mentioned earlier, all tuck into each corner of the secondary cage when nested. These prevent the mirror from moving when the scope is carried in the vertical positon; like a laptop. (I'm having a custom carry "bag" made for transporting.)
As mentioned earlier, I purchased the (unfinished) mirror (16" f/4.8,
0.9" thick, 12.5 lbs.) from someone on Astromart and I had it finished by
Mark Cowan at Obsidian Optics, who did an excellent job! I had it coated
by Richard LaRue at L & L Optical, who also does excellent work.
Rocker Box/Azimuth Bearing/Base
The rocker box is a typical low-profile design. The azimuth bearing doubles as the base and I installed 3 sets of roller bearings in the base that the rocker box rolls on. (I glued an aluminum track to the bottom of the rocker box for the bearings to ride on.) I used the same bearings to support the altitude bearings. The carbon fiber panel worked great for supporting the roller bearings for the altitude bearings. I just notched out a hole for each of the bearings and the "sides" of the carbon fiber panel support the bearings very well.
Given that I currently don't have an interest in astrophotograhy, I decided I just wanted a simple tracking/slewing capability; one that would keep the image in the eyepiece for 10-15 minutes and assist in finding objects. I found Gary Wolanski's Visual Assist Drive System (VADS) and decided I would create a variation on his theme. It's comprised of two (one altitude and one azimuth) Hankscraft three volt (DC) 1/2 rpm motors ($12 shipped for 2 on eBay!) powered by a variable voltage controller (very simple). I added a solenoid to each motor to disengage it when I want to move the scope by hand. You can see from the photo above left that the solenoid simply pulls the worm gear away from the worm. The return spring also keeps pressure on the shaft to keep it riding on the alt/az bearing when the drives are engaged.
The drives are "connected" to the altitude and azimuth bearings by o-rings. (I used o-rings on the encoders also.) I was incredibly lucky to find the worm gear set at a surplus electronics place (Goldmine). They turned out to be the 10:1 ratio I needed and they cost $1 per set!
The hand controller provides up/down and left/right tracking and fast slew along with tracking speed control for both altitude and azimuth. The button at the top left (see photo above right) is for disengaging the drives.
As far as the encoders go, I wanted to keep the eyepiece height as low as possible so I mounted the Azimuth encoder in the corner of the rocker box (see above left photo). This allows the mirror box to swing past the rocker box bottom with a minimum clearance of 1/8". In order to get the encoder to reach the azimuth bearing, I had to put a longer shaft on the encoder. Fortunately U.S. Digital has longer replacement shafts (Note: this is only recommended for people with a fairly high technical aptitude; it's not easy and it's risky.). I ran into a problem with the longer shaft "locking" as a result of the pressure applied to keep it in contact with the azimuth bearing, so I added a roller bearing (see above rocker box center photo) in the bottom of the rocker box to keep the shaft aligned so it doesn't lock.
I must say that I'm thrilled with how this scope has turned out. The balance with a 2" eyepiece is almost perfect, it's a bit heavier in the mirror box. The motion is soooo smooth with the bearings. So far, everything looks like it works as planned, although I really need to put it through a real-life observing session, which will be as soon as possible.
I'm especially pleased with the fact that it is a very portable scope with a very serious aperture and it only weighs 27 pounds! (not including the truss tubes, drives and encoders, which adds about 3 lbs.) I don't need to nest it completely every time I use it, so I can leave the secondary attached for faster setup. I'm really looking forward to checking out the planets with it to see how the images compare to high-end refractors. I'm also anxious to see how fast the primary cools down!
As you can tell by looking very closely at certain parts, I don't have a machine shop. The only power tools I had access to were a table saw (for cutting the carbon fiber panel), a drill press, a RotoZip and a cordless drill.
I would first like to thank my wife and sons for their patience and support!
I would also like to thank the following people for their assistance in building this scope:
Richard La Rue
PS: Any suggestions anyone might have for improvements, please let me know!
- AZ_J3 likes this