JEFF HINELINE'S TELESCOPE PAGES

MY TRIBUTE TO THE EARLY COULTER TELESCOPES

Being a planetary observer, I had a slightly different set of design goals in mind when building my new telescope:

1. I wanted the scope to be a reflector, eliminating any chromatic aberrations to poison my views.
2. I wanted the scope to have a large primary mirror as to support high power viewing, at least to 1000x if the seeing permitted.
3. I wanted a long focal ratio so that I would have the smallest central obstruction possible.
4. I wanted a full thickness mirror to avoid "potato chipping" when viewing close to the horizon (here in Michigan, we are pretty far north)
5. I wanted the scope to fit into my Dodge Stratus.

My final decision turned out to be a 14.5" Dobsonian telescope. I was certainly not looking to build a flimsy ultra-light Dob. I needed to have a durable scope that I could pack up in the morning, dead tired, without worry.

I was fortunate in finding an original Coulter mirror that had been hand re-figured to a very fine and extremely smooth finish. It has a strehl ration of greater than .980. This mirror is a full 2.25 inches thick and weighs thirty-one pounds. I used PLOP to design a 9-point mirror cell. This mirror is perfect for this project, because the weight offsets the length of the truss poles. The very best part of this mirror is the focal ratio. With a focal ratio of f/7, the mirror allows for a very small secondary mirror of 2.38" and an obstruction by area of less than 1.5%.
 

The rocker box and mirror box for this scope are made from Appleply. Appleply is plywood made with fifteen layers of wood instead of the usual five. It stays flat and does not warp easily. I ordered it with exterior glues between the plies to ward off the dew (Did I mention I live in Michigan?). In order to have the strongest, flex free joints, I used all dovetail construction. No screws or nails are used in any wood-to-wood joint. I often read online that you cannot cut dovetail joints in plywood. After a few test cuts, I found a way to do it with no tear-out at all. Dovetail joints are the strongest type of joint one can use in wood joinery. They have 12x the strength of the box joints often seen on CNC telescope parts.  All the wood parts received 5 coats of polyurethane as further protection from dew.

Once I started building the project, I had an idea concerning the humble origins of the mirror for this scope. For those of you that have been astronomers long enough to be around when Coulter telescopes were quite popular, you will remember the distinctive Coulter blue scopes. I felt that since this mirror was one of the rarer full thickness Coulter mirrors, I would honor the original maker by making this telescope a nice Coulter blue. I did not want anything gaudy, like an electric blue. Once I started the project, the blue theme seemed to take on a life of its own.  I started out by staining the mirror box and rocker box blue.

 Once I saw how good the blue looked and how close it was to the original Coulter Blue, I decided to go all of the way and make the entire scope blue. As it turns out the truss poles are now anodized blue, the UTA assembly is all blue. Even the Feather Touch focuser is anodized blue.

Most Dobsonian telescopes that I see use Kydex for the UTA. Unfortunately, Kydex tends to come off the UTA in the sunlight at a star-party. For this project, and in keeping with my original parameters of keeping this project as light as possible, I decided to make the UTA entirely out of aluminum except for the rings, which are Appleply.  The aluminum light shield becomes a strength adding part of the structure, not just a shield held on with double sided tape.

Almost all Dobsonian telescope makers paint the inside of the mirror boxes and UTA a flat black, in order to keep reflections down to a minimum. I found the results of a great reflectance test online, that showed the "blackest stuff on earth" is black velvet called Fidelio. It has been measured to be twice as dark as commercial flocking paper. I lined the mirror box and the UTA with this double black velvet. It is so black, that even in bright sunlight, there is virtually no light reflected.  I  replaced the screws on the secondary with thumbscrews,  so that collimation can be completed with no tools at all (and thus no tools to drop on the primary mirror!).

Another unique feature of this project is the mirror cell. To the normal features of a mirror cell, I have added two useful features. The first is oversized collimation knobs. One of the frustrations that many astronomers face, especially in cold weather is trying to collimate optics while wearing gloves. I chose to place oversized knobs on the collimations screws of the mirror cell.  The second unique feature is a stainless steel sling.   Most Dobsonian mirror cell slings are made from nylon webbing. Unfortunately, the nylon usually stretches and you are forced to do a "touch up" collimation during the night. To prevent such unnecessary collimation, I chose to use aircraft stainless steel cable. This cable has an extremely strong tensile strength, and must encounter a force of 2700 pounds before it will stretch one millimeter. No touch up collimation has been necessary after the initial collimation.

Those who have newer Feather Touch focusers are aware of one useful feature that has been added over the older focusers. That feature is collimation set screws to collimate the focuser to be perfectly perpendicular to the optical axis. Usually if your focuser does not have such setscrews to adjust the focuser plate, shims need to be place under the mounting plate to square the focuser. Since I felt that adjustment screws on the focuser mounting plate are so useful, I placed three setscrews in the focuser mounting plate to perfectly square focuser to the optical axis. I chose three setscrews, as that is far easier to adjust than the four setscrews that the new Feather Touch focusers presently use.  It just makes more sense, and saves time.

Trying to keep weight down on this scope, I decided to use aluminum side bearings, as they are both stronger and lighter per volume than wood side bearings. I used the Obsession Telescope cast aluminum side bearings. I tried to anodize these bearings, but cast aluminum is notoriously difficult to anodize, so these cast bearing ended up being powder coated to match the rest of the color scheme for the project.  The second reason I chose Obsession Telescope bearings is the unique slight camber on the bearing surfaces on the Obsession bearings. It you look closely at the bearing castings, you will note that they are not perfectly square. They have a slight camber of three degrees, opposites on both sides. The advantage of this slight camber is the mirror box will self-center. Using a router, I was able to match exactly the cambered surface of the cast bearings. This resulted in a mirror box with no side-to-side play.

The trusses for this project are 1.25-inch diameter, aircraft aluminum. Some had argued with me that I did not need to use such thick poles, and that 3/4" diameter poles would have been fine. That may be true, but on of the design parameters for this project was to make this assembly as stiff as possible. Tapping on the scope results in vibrations that damp out in less than half a second. I may latter fill the trusses with Great Stuff to reduce the vibrations to zero.  I did not want the problem of wood truss holders contracting at different rates than the truss poles (as I have seen too many times at star-parties), so the truss holders were machined from aluminum. This way the poles always slide right in without sticking or play.

First light with this project involved cutting the truss poles to the correct length. That is not an easy job in Michigan, with 90% of the nights being either cloudy or partly cloudy. I did manage to find Vega in a clear spot between the clouds. At first there was not enough in-focus, so I ended up shortening the truss poles. After several tries, the poles ended up being 69 inches long. With a rocker box height of 12 inches, and a mirror box length of 14 inches, the height of the telescope turned out to be 8' 6" tall. After finding the proper length of the truss poles, the next object to view was Jupiter. It was completely awesome. Contrast was excellent, and many nice details, clearly seeing four distinct cloud bands along with light blue and brown cloud tops. It was one of the best views of views of Jupiter I had ever seen.

Finally, I tried the scope on Epsilon Lyrae, which many know as Double-Double. The tight pair of binaries was easily split in this scope.

Pointing this telescope is a pleasure. Movement on both axis is buttery smooth. Once the telescope it pointed to the object, there is no backlash what so ever. Point it and it stays in place. Touch it with a deliberate action and it moves to where you want it to be with no effort.

The last thing I have left to do is add the Servo-Cat/Argo Navis GOTO system. I built in the encoder mounts during fabrication, so I will have little to do when my system arrives.

This scope has been a pleasure to use. It is not so big, that I cannot transport it in my car, but it is not so small, that I would have to miss the wonderful DSO's the night sky has to offer. For those of you attending this fall's Great Lakes Star Gaze, I cannot wait to show you guys some great views!

-Jeff Hineline

I would also like to thank my daughter Tanya, and her friends Kristi and Beth, for the hours spent on hair and makeup for this "modeling session".  I hope you girls had fun.