Two Simple "Grab-and-Go" Telescopes

Mark IV 8" f/6 and 10" f/5

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Introduction

Although I use larger scopes for serious observing from remote, dark skies, I prefer a small "grab-and-go" scope when I don't want to invest a lot of time setting up and tearing down. There are three types of observing where speed is important to me:

1. For a quick peek of the stars from my driveway or backyard. Usually I want to watch a particular short-lived event, or unwind for an hour before heading off to bed. I want to carry the scope outside and start observing immediately, and carry it back inside quickly when I'm done.
2. Public star parties. These usually start shortly after dinner and end early in the evening. They may be during the week. I want to arrive just before the public does, and usually make a quick getaway after they leave. I want to take the scope out of the car and be set up within a couple of minutes, and put everything away even quicker.
3. Observing at one of our closer near-town sites. These aren't truly dark sites, but they usually have better horizons and there are no streetlights or cars passing by. These observing sessions are often midweek. Short tear-down time means I can observe just that much longer before driving home. 

 The "grab-and-go scopes must meet the following goals:

1. For simplicity of construction, use the basic Mark IV design for attaching struts (Mark IV 12-1/2" f/5 and Mark IV 13" Travel Scope).

2. Protect the upper and lower optical components during transport.

3. Shield the upper and lower optical components from Moonlight and neighbors' lights when in use.

4. The OTA must fit across the back seat of a moderate-sized sedan (<50").

5. The rocker must fit within the trunk of a sedan or on one of the passenger seats.

6. The telescope must be assembled in less than one minute.

7. Telescope must be ergonomic

   a. have 1X and RACI finders

   b. have comfortable eyepiece position when seated at adjustable-height chair.

8. Heaviest piece must be less than 25 lbs, and prefer heaviest piece to be less than 20 lbs.

9. Components should be good to very good:

   a. Crayford focuser

   b. Mirror < 1/6 wave P-V at the wave front

10. Telescope should not move when changing eyepieces, yet motions must be smooth.

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

Two Grab-and-Go Telescopes

Figure 1 8" f6	Figure 2 10" f/5

Each uses three 1-1/2"-diameter parallel struts similar to the Mark IV 13" f/4.5 travel scope. Follow that link for basic construction details. 

Like the larger Mark IV 13" travel scope, the OTAs use no clamps. The struts are attached to the bottom ring of the lower tube assembly and the lower ring of the upper tube assembly using 1/4"-20 screws which thread into tube inserts recessed in the ends of the struts. The spacing between the struts in the upper tube assembly is closer than in the lower tube assembly. As the struts bend inwards, they are forced against the through-holes in the top ring of the lower tube assembly, holding everything rigidly in place. 

Upper tube assemblies

Unlike the 13" travel scope, these grab-and-go scopes use a full upper tube (Figures 3 and 4) to house and support the optical components. The upper tube better protects the secondary mirror during transport, and better shields neighboring lights than a single baffle opposite the focuser. Like in previous scopes, I used drum shells for the upper and lower tube assemblies. The tubes are attractive and relatively stiff (5-ply Birch on the 8" f/6 and 5-ply Maple on the 10" f/5). End rings on top and bottom tube assemblies are made from 1/2" thick Finland Birch plywood.

Figure 3 Underside view of the upper optical assembly for the 8" f6. Note the struts lie in three corners.	Figure 4 Side view of the upper tube assembly for the 10" f/5. Note the fine 2" FeatherTouch focuser.

Like the 13" travel scope, for the 8" f/6, struts lie at three corners of the OTA (Figure 3). However, for the 10" f/5, the top strut is placed along the centerline (Figure 6) like in my Mark III scopes. This is more ergonomic for transport. It is very each to lift and carry the OTA from the balance point along the top strut. The tradeoff is that the OTA is a little taller. The focusers and finders are placed in ergonomic positions for observers who are left-eye dominant.

Figure 5 Front view of the 8" f/6.	Figure 6 Front view of the 10" f/5

Altitude bearings

Unlike the Mark IV travel scope, clamps attach the large diameter bearings to the struts (Figures 7 and 8). This reduces weight because the bearings no longer require side panels for attachment to the OTA, and the lower tube assembly is shorter. In addition, removing the side panels shows off the attractive drum shells and permits easy adjustment of OTA balance. 

As in past scopes, the bearings surfaces are 3/4" wide and have a 1/4" thick, 1/4" high raised lip on the inside, keeping the OTA centered in the rocker box. The C-shaped bearings are oversized: 14" diameter for the 8" scope and 16" diameter for the 10" scope. This provides smooth motions while maintaining scope position for most eyepiece changes.

Figure 7 Detail of altitude bearings for the 8" f/6, showing clamps and Walnut inserts.	Figure 8 Detail of altitude bearings for the 10" f/5, showing clamps with clamping knobs. Note the top strut lies along the centerline of the OTA.

These two scopes demonstrate two methods for attaching the clamp bolts. 1/4"-20 T-nuts are recessed in 3/4"-diameter blind holes drilled partway through the bearings. A 5/16" hole is drilled all the way through. Sections of 3/4"-diameter walnut dowel are glued in afterwards and sanded flat. Long 1/4"-20 button-head cap screws pass through the clamps and thread into the hidden T-nuts. Since balance is rarely adjusted using the clamps, this approach creates a nice finished appearance.

In the 10" f/5, 1/4"-20 lag bolts pass through the bearing from the outside, through the clamps. Clamp knobs threaded onto the ends allow tightening of the clamps by hand and present a different look.

Sliding counterweight

While the 8" f/6 uses a JMI 1-1/4" Crayford focuser, the 10" f/5 uses a 2" FeatherTouch focuser. Because the 10" accepts heavy 2" eyepieces, it is convenient to have an adjustable counterweight. Rather than create another component, I use the primary mirror cover (Figures 9 and 10) from the lower tube assembly as a sliding counterweight. Two plastic tool clamps attach the cover securely to either the top or a side strut. Yet, it is easy to slide the cover down to accommodate heavy eyepieces or accessories. Using the cover as a sliding counterweight puts it to good use and prevents one from accidentally leaving a critical component at home.

Figure 9 Side view of 10" f/5 showing mirror cover attached to top strut.	Figure 10 Two plastic clamps securely hold the cover in place, yet allow it to slide as an adjustable counterweight.

Lower tube assemblies

Figures 11 and 12 show the underside of the lower tube assembles for the 8" and 10" scopes, respectively. Like in previous scopes, when removed from the rocker box, each OTA rests on four feet to protect the collimation knobs and bottom of the lower tube assemblies.

Rapid cooling of the primary mirror often is necessary when observing immediately after taking a warm telescope into cooler night air. With a little fancy cutting and sanding, I left area to mount a primary fan on the 10" f/5 (Figures 12 and 13). 

Figure 11 Bottom view of the 8" f/6 lower tube assembly. Note the large opening, collimation knobs, and four feet.	Figure 12 Bottom view of the 10" f/5 lower tube assembly. Note the fancy cutouts, leaving support for the cooling fan. Power connection and switch can be seen left and right of the fan, respectively.

The 8" f/6 mirror is full thickness Pyrex from Parks Optical. Testing shows it is approximately 1/6 wave P-V at the wave front (Strehl Ratio = 0.92). The 10" f/5 is a Pyrex mirror custom made by Woden Optics. It has an edge thickness of 1" and reported accuracy of 1/13 wave P-V at the wave front (Strehl Ratio = 0.97). Both give excellent views.

Figure 13 Bottom view of the 10" f/5 lower tube assembly before assembling the OTA. You can see the pattern of holes and cutouts and the four wooden dowels that support the rubber feet.

Mirror cells

A simple three point cell (Figures 14 and 15) more than adequately supports a full-thickness 8" mirror. Location of support points was determined using PLOP, although placement isn't critical. The triangular frame is made from 1/2" Finland Birch plywood. Three blobs of silicone can be attached directly to the wood. I like to use 3/4" diameter stainless steel weld nuts as the support pads. They can be unscrewed individually from the rear of the cell during disassembly. Each one can be scraped off the back of the mirror more easily.

Collimation bolts (1/4"-20) and all other hardware are made from stainless steel. 

Figure 14 Top view of the mirror cell for the 8" f/6.	Figure 15 Bottom view of the mirror cell for the 8" f/6.

A six-point floatation cell (Figures 16 and 17) is necessary to support the thin 10" mirror.  Note that the design is different than my previous six point floatation cells. (For example, see the mirror cell for the 13" travel scope). The current design was inspired by the mirror cell built by Achim Strnad, one of many outstanding German ATMers. The triangular frame is made out 1/2" Finland Birch plywood. Because most of the flex occurs mid-way across each span, I glued a small trapezoidal-shaped plywood piece there. This thickened area also provides enough material to support the pivot shaft for the rocker bars.  The rocker bars are cut from aluminum C-channel. Total weight of the cell is 0.8lbs.

Figure 16 Top view of the mirror cell for the 10" f/5.	Figure 17 Bottom view of the mirror cell for the 10" f/5.

Rocker box and ground board

Rocker box and ground board are similar to those I've made previously. They are shorter and stiffer than most scopes this size due to the large altitude bearings. Sides and bottom are made from 3/4" thick Finland Birch plywood. Figure 18 shows how the rocker box for a 8" scope fits in a passenger seat with room to spare.

Figure 18  Rocker box for 8" scope comfortably fits in passenger seat.

OTA in transit

Figure 19 8" f/6 lying across the back seat of Acura Integra.

Performance

These two telescopes meet or exceed all goals. They have been a joy to build and use. They are attractive, easy to transport, provide excellent high contrast views, and allow comfortable viewing. 

The length of each OTA is less than 50", fitting across the back seat of most sedans. Keeping the OTA assembled and short is one of the keys to a scope that sets up quickly. Placing the OTA on the rocker box is measured in seconds.  It only takes a few more seconds to remove it from the car vs. setting up outside the house. The limiting factor is how far I walk with it before I begin to observe.

Although I didn't design these scopes to be ultra lights, they are noticeably lighter than most commercial scopes this size. Including finders and covers, total weights of the 8" f/6 and 10" f/5 are 30 lbs and 40 lbs, respectively. The OTAs are heavier than the rocker boxes.  The 8" f/6 and 10" f/5 OTAs weigh 18.2 lbs and 23.8 lbs, respectively. It is straightforward to modify the designs to make these telescopes lighter and/or collapsible for travel. 

I spend a lot of time achieving a professional-looking finish. The appearance of these scopes should not deter a novice ATMer from building one similar or copying it exactly. The first scopes built by a number of people have been variations of these and the travel scope Mark IV designs. For example, take a look at Mark Bracewell's "Little Dipper." 

Unfortunately, I don't own either one of these scopes. I'd have a garage full of scopes if I didn't sell my old ones and find people who were willing to buy one of my "experiments." But given the combination of performance and portability, I'm going to have to build one for myself.

All images and text copyright © 2006 by Albert Highe, unless otherwise noted.