Mark IV 13.1" f/4.5 Travel Scope

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Introduction

I had planned to take to Australia the Mark IV 12-1/2" f/5 I built last year. However, there are some drawbacks using it as a travel scope. 

1. The mahogany rocker box looks too nice to bang around.  
2. The long struts would have to be packed separately from scope and other luggage, requiring a third checked bag.
3. The fan and exhaust holes in the mirror box are difficult to seal, allowing more dust and debris to enter while traveling.  

I wanted to keep the basic Mark IV design. The travel scope had to meet the following goals:

1. Modify design of mirror box, substituting other lightweight materials for the paper honeycomb mahogany panels.

2. The mirror box must contain and protect the mirror during transport.

3. Most of the scope components must pack within a cube 18" on a side. These components will be checked as bag #1.

4. The struts and altitude bearings must fit within my second checked bag carrying clothes, charts, etc.

5. Lower eyepiece height so I can observe more of the sky while seated.

6. The scope must be easy to assemble (fast assembly of minimum number of parts, requiring one or no tools).
7. The assembled scope must perform well. Designing it for travel should not compromise performance.

Figures 1 and 2 shows the fully assembled telescope that met all of the above goals. 

Figures 1 and 2 Mark IV 13"  f/4.5 travel scope made with segmented struts. The struts are joined at the intermediate ring via 1/4"-20 threaded posts screwed into threaded inserts. The weight of the fully assembled scope, including finders, is 45 lbs.

Each of the three segmented struts disassembles into two pieces (total of six) approximately 26-1/2" long. Struts have a diameter of 1-1/2" with a wall thickness of 0.049".

Mirror "box"

The mirror "box" is essentially a fiberglass tube with wooden end rings. It weighs six pounds without optics or cell, and is very strong and stiff. A layer of fiberglass epoxied within an outer shell of Ebony Star™ laminate makes up the tube. 

I cut the top and bottom tube rings from ½" Finnish Birch plywood. The top ring has an opening large enough to insert the mirror through it. Because the lower ring supports the mirror cell, I cut a smaller opening in the bottom ring to permit airflow through the mirror box. On the inside surface of each ring, I used a router to cut a 3/16" circular groove about 3/16" deep with a diameter the same as the fiberglass tube. I then epoxied the end rings to the fiberglass tube.

Note the four small feet on the bottom of the mirror box. These feet keep the mirror box from resting on the adjustment knobs while I assemble the telescope and while it is sitting in the rocker box during transport. The rubber feet slide over 5/8" diameter dowel rods that are glued into holes drilled approximately three-fourths of the way through the bottom of the mirror box. The four feet are longer than the adjustment screws, but short enough in position so they don’t touch the bottom of the rocker box when the telescope is in use.

Tongues cut into top and bottom of the two side panels are glued into the slots along the sides of upper and lower rings. The altitude bearings will be attached to these side panels using two 1/4"-20 "T" nuts.

Figure  Details of mirror box construction, showing fiberglassed Ebony Star tube, wooden end rings, and side panels.	Figure  The fiberglassed Ebony Star tube will be epoxied into the grooves of upper and lower rings. The tongues of the mirror box side panels will be glued into the side grooves.

Figure  Bottom ring of mirror box with mirror cell installed to test fit.	Figure  Close up of cell attached to the inside of the mirror box bottom.

The end of each strut contains a threaded insert. Like the Mark IV 12-1/2" f/5, struts screw onto 1/4"-20 screws in three corners of the bottom of the mirror box. However, the struts would leave visible gouges where they contact the wood. So I installed large stainless steel washers to protect the finish. A pair of #8 stainless steel wood screws hold each large washer in place. I had to drill and countersink the holes so that the screw heads were below the surface of the washer.

The completed empty mirror "box", without primary, mirror cell, lid, or altitude bearings, weighs 6 lbs. The completed mirror box with primary mirror, mirror cell, attached lid and altitude bearings, weighs 25 lbs.

Mirror cell

To hold the primary mirror more securely during transport, the mirror cell pads are considerably larger than in previous cells. The diameter was increased from 3/4" to 1-1/2", providing four times the contact area to the back of the mirror.

Figure  Top view of mirror cell.	Figure  Bottom view of mirror cell. The central triangular support is cast aluminum.

Figure  Detail of rocker bar hardware. Each rocker bar is supported by a 1/4" diameter stainless steel shoulder bolt. Washers position the bar away from support triangle. The curved spring allows the cell and mirror to expand and contract without stressing/distorting the mirror.	Figure  The height of each 1-1/2" diameter stainless steel pad is adjusted by screwing its post into a threaded hole at each end of the rocker bar. A jam nut locks it into place. The rocker bars were cut from 3/8" X 1/2"  aluminum stock.

I enjoy viewing through excellent optics. This 13" was refigured by Woden Optics and now has a Strehl Ratio of 0.988!  It is 1-5/16" thick and weighs 13.2 lbs. I was concerned whether gluing the mirror cell with larger pads to the back of the mirror would affect its figure. It appears the mirror cell introduces no visible distortions.

Hinged lid

Figure  The attached hinged lid prevents it from coming off the rocker box during transport.	Figure  In use, the black plastic clip holds the lid up.

Joining struts at the middle ring

After screwing the lower set of struts into the mirror box, I screw threaded rods into the threaded inserts on the tops of the struts and lock them in place with a wing nut. The middle ring (1/2" thick) then just slips over the ends of the struts. I counterbored 1-1/2" diameter holes 1/8" deep on both sides to locate the struts.  The through hole has a diameter of approximately 1-1/8" to allow the wing nut to nest within it. Next, I screw the top three struts onto the threaded rods. Finally, I attach the top ring.

A 1/4"-20 rod, screwed into the threaded insert, is locked in place with a wing nut.	The intermediate ring slips over the lower set of struts. The hardware on top of the ring in the image is there to show the pieces in the joint.

The rocker box and altitude bearings are the ones built for the 12-1/2" Mark IV. Due to the lightweight honeycomb panels, the rocker box weighs less than 10 lbs.

Packed for transport

The mirror box, top ring, intermediate ring (under mirror box in the bottom of the rocker box), and rocker box, packed for transport, fit within an 18" cube. The combined weight is 37 lbs. The scope will be double-boxed (still meeting checked baggage size restrictions) and weigh less than 50 lbs when checked at the airport. The weight limit for checked bags on international flights is 75 lbs.	The six strut segments and altitude bearings (dimensions 10" X 26-1/2") will fit easily into my second piece of checked luggage (15" x 16" X 30").

I decided to remove the secondary mirror and holder. Along with the finder, they are held securely by foam padding within the mirror box.
The intermediate ring sits on the bottom of the rocker box.	I also removed the ground board, wrapped it in bubble wrap, and inserted it between upper ring and mirror box.
Protected by the two nested boxes, the scope made it to Australia and back without damage. The box was opened and inspected on the way there.

All images and text copyright © 2005 by Albert Highe, unless otherwise noted.