In April 2001 I realized a dream that had been many years in the dreaming and a year in the making, a large aperture dobsonian.
The decision process that eventually settled on the 18″ f/4.5 design was a long one. As a very active amateur I had had many opportunities to examine other scopes. To see where they excelled or where they fell short. With this experience I eventually decided on a list of requirements.
- The scope had to have sufficient aperture to take advantage of the dark skies available near Tucson. I wanted to see spiral arms in galaxies.
- The design was to be visual only. No drives, but provisions for an equatorial platform at a later date.
- The mount would be a no compromise rigid structure, capable of allowing good optics to perform at their best.
- The scope had to fit through a standard doorway.
- The scope had to fit in the cargo compartment of a Ford Explorer Sport without dropping the seat for safety during transport.
- The eyepiece must not be an excessive distance above the ground, allowing use while standing on the ground much of the time. (But then, I’m 6’2″ tall)
Over a decade of engineering experience has taught me that a well defined set of specifications can make all the difference at the end of a project. With these design goals in mind the plan then progressed rapidly.
The heart of any telescope is the primary mirror. The primary started life as a cast pyrex blank from Newport Glass, ordered for $550. Purchased with f/4.5 curve generated and a blanchard ground backside. A polarized light test showed no visible stresses.
I wanted a primary better than the average mirror. So instead of purchasing the mirror a deal was struck. I commissioned Tom Peck of Tucson to grind and figure the mirror. Tom was at the time a fabrication technician at the University of Arizona optical sciences department and regularly works on much larger and more demanding projects. Just before starting my mirror he was finishing a 72″ lens! With a hologram etched on this impressive piece of glass will test a secondary for the 6.5m Whipple MMT telescope.
His price for the mirror… a second 18″ frame.
I delivered the blank to Tom and received a finished mirror six months later. Under an interferometer it showed a smooth 1/16th wave surface. Star testing shows no obvious defects. A coating applied by Spectrum Coatings and I have a beautiful primary. The second frame was completed a couple years later when Tom purchased an 18″ blank of his own.
The Mirror Cell
The mirror cell is an 18 point flotation design built to the specifications from Berry and Kreige. The only departure is in the design of the strap.
The spreaders and triangles of the floating cell are machined from 316 stainless steel. Something I will not do again by choice. The alloy was extremely tough and in the process I ruined a carbide end mill. But in the end I managed two sets of parts.
The 1″ wide spreaders were done to the dimensions given in Berry and Kriege. I have decided that they are too narrow. Mine had begun to bend under the weight of the primary, especially during transport. These have been replaced with wide aluminum spreaders.
I had to shorten the middle/lower collimation bolt. The bolt brushed the top of the azimuth encoder and I just didn’t need that much adjustment range.
The collimation bolt interference could be easily solved by arranging the bolts with the two bolts on the lower rung, this would allow the rocker box to be roughly 2cm (1 inch) shorter and more compact. This would also lower the eyepiece an equal distance closer to the ground. The spacing of the rungs and other dimensions would have to be changed to do this.
With the cell turned over you can see the cooling fan and the collomation bolts. With more of the violet anodized aluminum that gives the scope its name.
The strap is a piece of beryllium copper alloy surplused from an engineering project I worked on. It was intended for stamp forming of battery charger contacts. It is very flexible, has little thermal mass as it is only 0.020in. thick and does not stretch.
The strap is held in place with two machined stainless steel posts with a 1/4-20 adjustment screw holding the end of the strap. To hold the thin metal of the strap I folded it around a thicker piece of aluminum and put two machine screws through both. This arrangement allows adjustment of the strap length to center the mirror in the cell.
The end result is a mirror cell that holds the mirror with no visible distortion. I have star tested the telescope and any flexing of the mirror should have been visible, but none is seen. I am going to attempt to photograph the defocused star images. The plan is to use polaris to overcome the lack of a drive.
As of Sep 2002 I had solved (at least for now) the problem of the bending spreader bars by taking the set from the second mirror cell and doubling them up with the old ones. At the same time flipping the old ones over so that the bend was upwards. New wide aluminum spreaders have been manufactured for both scopes.
I also cut the lower collimation bolt 1cm shorter while I had everything apart. It no longer interferes with the azimuth encoder.
The Mirror Box
Shown to the right is the mirror box just after assembly, with some of the clamps removed. All of the wooden assemblies are manufactured from Appleply. Expensive and hard to find, this wood is great to work with.
One of the cage rings is sitting atop the mirror box allowing you to see how the secondary cage will nest in the box.
Shown here is the gusset gluing operation. The gussets are made from scraps of the same wood as the box. They were carefully mitered on a table saw with a custom made jig to get the 45 degree angle to the edges. The hope is that using gussets to reinforce the corners, there would be no racking of the mirror box, that it will remain perfectly square under load.
Gluing the gussets in required buying a new clamp that could operate in a reverse fashion, pushing outwards. (Another excuse for a new toy). The clamps and a couple aluminum blocks I had allowed high pressure to be applied while the glue was setting.
All of the glue used was a waterproof wood glue. This is in keeping with the goal of making a telescope immune to dew and maybe even rain. Exposure to moisture is all too possible with a telescope used outdoors. All glues waterproof, almost all the hardware is stainless steel, the machined aluminum parts were anodized, all of the wood was liberally treated with an oil stain/preservative before being coated with multiple layers of polyurethane.
Update: After ten years of use, being heavily covered in dew many times to the point of drips running down the side, even one thunderstorm downpour, there is no moisture damage apparent.
The Rocker Box
When I reviewed the design of the rocker box I realized two attributes were important. The box must be accurately made, the four elevation pads held squarely to the lower surface, and the assembly must be very stiff.
This takes care in planning how to fabricate each piece. With an eye to the order of operations and very careful measurement.
The sides of the rocker are two layers of 5/8″ appleply, the end pieces a single thickness. The bottom of the box is double thickness 1/2″ baltic birch. You can see in the photos that slots have been cut for the biscuits that will help create a very strong structure.
After many attempts to get a satisfactory finish with a brush applied polyurethane I tried a spray-on version. Somewhat more expensive, but it left a highly attractive satin finish with NO DRIPS. An unexpected benefit was that the surface was lightly textured, forming a “non skid” texture that is perfect for parts meant to be handled and carried.
The photo above shows the fully assembled rocker box with the last coat of polyurethane drying on the patio. Warm Arizona spring days made for quick drying times of the thin sprayed coats.
One aspect of the design I put some design effort into improving was the azimuth rotation pivot. I realized that if this heavy scope was to be used on an equatorial platform that a great deal of sideways load would be placed on the pivot. This called for a heavier assembly than some designs I had seen.
My solution was a 3/4″ stainless steel threaded rod, machined to a 1/2″ shaft along half of its length. A bearing would be mounted into an aluminum pillow block set into the bottom of the rocker box.
Above is the pivot in place, looking into the bottom of the rocker. All that remains to do is insert an E-clip to retain the ground board.
The end of the pivot is prepared to receive the azimuth encoder by drilling a hole in the end of the shaft to receive the encoder shaft.
The Ground Board
The completed ground board can be seen in the photo to the right. Teflon pads and the azimuth pivot are installed. Also seen are the large circular cutouts used to reduce the weight here. This is possible as the ground board serves only one purpose, to hold the azimuth pads and feet in the correct place.
The large hole in the center of the teflon pads is to allow access to the mounting screw for the foot. The feet are removable to allow experimentation with vibration dampening designs if this proves necessary. For now a simple wooden foot is in place.
Update: After many years it has never been desirable to improve the feet, these are stil the simple solid wood feet the telescope was originally equipped with.
The Secondary Cage
The business end of the scope, the secondary cage is built as light as possible to allow the scope to balance near the rear. I used the traditional design, two high grade plywood rings held apart by aluminum struts and a sheet of kydex plastic to block the light.
As seen in the photo I mount a Telrad, an AstroSystems Phase-4 2″ crayford focuser and the electronics unit for the digital setting circles all in very convenient positions. Als used is an AstroSystems spider and secondary holder. I like the four screw arrangement on the secondary holder. Setup in a push-pull design the four screws allow really easy collimation of the secondary.
I added a machined aluminum ring over the knob of the focuser to expand the knob size and make it much more finger friendly.
Update: The Astrosystems phase IV was a decent focuser, but the Feathertouch is better! The Phase IV was replaced in 2004 with a Feathertouch, a very nice improvement.
Digital Setting Circles
Finding one’s way about the sky can be a challenge. I am a bit of a purist when it comes to this issue. I believe that to learn the sky properly no finding aids should be used. I often star hop and for a Messier Marathon I use nothing but charts and a Telrad.
But for this scope I decided to install DSC’s from the start, I would be using this scope to hunt very dim fuzzies and DSC’s allow confident identification of your target. For public star parties the DSC’s allow me to find targets quickly when a line of people is waiting and I have tracked an object simply by watching the readout, not having to climb the ladder between viewers to insure the object is still in the eyepiece.
After talking to may other large dob users I decided to get the Sky Commander unit. Good choice! The unit is easy to use and very accurate. On my first night using the unit to track down Herschel 400 objects the sky commander would routinely place an object dead center in a 20 arcmin field allowing me to find and record over 35 objects in a single short summer night. No time was spent hunting for the objects. This says a great deal about the accuracy of the telescope mount. If you have a well build dobsonian with truly orthogonal azimuth and elevation axis a set of DSC’s will work well.
I did order the 2048 step encoders from US Digital, with 4 phase quadrature these encoders give 8192 steps of resolution, or 2.6 arcmin. Easily enough to put the target in the field of a high power eyepiece.
Transporting a big dob and setting it up is where the use of a telescope this size becomes a serious commitment. Setup and takedown are about 10-15min, add in the need to collimate the scope each time. This, of course, becomes smoother with practice and the development of routine. Finding where to put everything, how to pack the scope and ancillary equipment in the vehicle. All of the trivia that we who regularly use portable scopes require to make this task reasonable.
Final setup clamping the secondary cage in place
The reward is well worth the effort. Seeing images in the eyepiece comparable to the Digitized Sky Survey. The spiral arms of M51 are readily apparent. My favorites, the planetary nebulae shown the green colors of ionized oxygen and detail in galaxies, globular clusters are awesome.
Setup begins by removing the scope from the vehicle. This scope is HEAVY but I can just do this as a one person lift, moving the mirror box, then the rocker box to the rug laid on the ground. I set them together and then set aside the secondary cage.
The truss tubes are then clamped into the mirror box and the secondary cage is set on top and clamped into position. The scope is stable through this operation as it is back heavy and tends to point straight up. Once the secondary cage is in place the scope is close enough to balance to be moved to any needed position for installation of the shroud, encoders and finder.
The mirror cover is left in place until everything is assembled to protect the primary from the many things that could easily fall on it.
The final touches are pulling the shroud into place and setting up the encoders. The arm for the elevation encoder fits quickly into place with two 1/4″-20 cap screws that thread into brass inserts into the side of the rocker box.
These photos were taken while setting up at Las Cienegas for a wonderful night with many others of the TAAA on 12 Jan 2002.
As mentioned above part of the challenge is to pack it all neatly in the vehicle. Part of this was by design and part just worked out with experience. One design specification was for the scope was to fit into the back of a two door Ford Explorer without dropping the rear seats, this was achieved. Some of the chance elements; finding a ladder that fits along the side of the vehicle neatly, that the other basic gear, eyepiece case, accessory case and library, stack neatly in the remainder of the cargo compartment.
First light was achieved atop Kitt Peak, Arizona. TAAA’s spring Star-B-Que atop the mountain gave me an opportunity to set up the scope under the stars for the first time. My traditional first light target is M42 and this was no exception. It was tough waiting for it to get dark enough with the covers on. When I inserted an eyepiece and focused the first thing that appeared from a hazy blob was the Trapezuim. As it got darker we were all amazed by the image and the extended detail the nebula offered despite being low in the sky. As my wife remarked “my dark adaptation is ruined!” looking at the core of the nebula.
Having the ‘scope work so well first time out was a tremendous relief. All of the work, hours of sweating over every little detail. To see the image for the first time was the culmination of all that effort.
An early outing of the telescope to a desert site sees the ‘scope set up among the Saguaros of the Tortillita Mts. A number of issues had not yet been resolved, but the process is well underway… The new elevation bearings are installed as well as the encoders. The Sky Commander had not yet arrived, thus operation is completely manual.
The process of gathering up the kit of gear to use around the ‘scope has begun. An appropriate ladder, a case for the accessories such as the finder cables and bits of the telescope removed during transport. Also shown well is one of the two afgani hand woven wool rugs I picked up for very little at the Tucson gem show. These rugs were originally designed by nomads for uses just like this. The rugs works well and adds a touch of class to the setup. No astroturf here.
Desert Star Party
At this point the scope is still seeing heavy use almost three years after completion. Deep Violet has been a joy to observe with, performance is as good or better than any scope I have ever seen in her class.
When planning star parties with the TAAA, folks often ask “Bringing the big ‘scope?” It is fun to setup at club events an immediately gather a crowd. Even more fun to share the views offered by large aperture to many who have not had a chance to see through anything this size. It is a whole new sky with a telescope this size.
On the night this photo was taken I stopped by M33 expecting to spend a minute or two looking at a galaxy I had seen so many times before. Surprised might be too pale a word to describe my reaction. Spiral arms, HII star forming regions, structure everywhere that smaller ‘scopes show a mere haze. I spent the next hour exploring the galaxy, matching features with the charts.
24 Oct 2003 Farnsworth Ranch, Pima Co., AZ
46cm f/4.5 Deep Violet
Fantastic! A large diffuse object with a large core, some structure visible at 60x with averted vision, at 175x much of the structure is obvious, particularly the main arms extending north-south from the core and several HII regions with separate designations NGC588, NGC592, NGC595, NGC604, as well as IC137 and IC143, on the whole a sublime and complex scene, a whole new galaxy with the 18
There have been some modifications at this point, a new diagonal mirror and the replacement of the first stage spreaders in the mirror cell when the originals started to bend. The replacement were manufactured from 0.35″ aluminum and anodized… violet.
A memorable outing with violet set up at Las Cienegas early Thanksgiving morning enjoying a crisp Arizonan fall sky. Violet is still regularly used five years after completion. A few dings from the wear and tear of use, but no real issues have cropped up, indeed I have continued to work on the scope and solving the few issues that remain.
I had finally completed a finishing touch too long ignored. I had taken the time to sand blast and anodize the truss tubes. This resulted in a very flat black finish without adding any weight to the front of the scope. No longer are there funny eight spiked patterns around a very bright object like Jupiter or Sirius.
Another addition has eliminated the balance issues. A sliding weight added to the mirror box allows tuning of the balance point as well as adding another violet anodized piece of aluminum. There is little five pounds of brass cannot solve.
In 2007 Violet left her native southern Arizonan skies to travel to the excellent skies of the Big Island, Hawai’i. Here she can use the dark skies available high on the slopes on Mauna Kea to enjoy starlight from objects previously too far south to enjoy before, Eta Carina, the Jewelbox and other southern sky sights are now reachable!
The normal observing site is at or near the Mauna Kea Visitor Information Station. Setting up at the VIS means that the first hours of the night will be classic public observing. A constant line of folk wanting to see the sky through a big ‘scope. It can be a lot of fun, the sky here can be perfect. After the crowd departs, the remainder of the night is just me, the ‘scope and a dark sky. Some times I do not want to deal with the crowd and Hale Pohaku lights. No matter, there are several nice hidey-holes nearby where there is no light to interfere with the perfect dark offered by Mauna Kea.