In 1998 was published "The Dobsonian Telescope" (David Kriege, Richard Berry. Ed. Wilmann-Bell), an authentic handbook for making, step by step, large amateur telescopes, based on well-known Obsession's design. Information contained in the book is very complete and really useful, rigorously studying the different design alternatives this kind of telescope has had last years, having allowed to amateur astronomers accessing to large apertures that were inaccessible years back.

I suggest to any people interested in making a large Dobson telescope to buy this book (in Spain, you can buy it at ORYX bookstore, phone 93 4185511).   Obviously, the book is English written, but it has a lot of schemes, plans and photos that make it very comprehensible, even for people with basic English  knowledge.

Given the multitude of aspects that there are to consider for an appropiate design of a telescope of this characteristics, as well as this web page limited purpose, it has been written assuming that interested people has actually the book, so that I will only consider those aspects that are not sufficiently considered in the book.
 
 
 

1.- PREVIOUS CONSIDERATIONS

Primary mirror diameter election

Large Dobsonian’s best benefits are obtained on deep sky objects, so that we could always say that bigger is better; however, it’s necessary to consider some aspects ( in addition to purely budget-concerning ones that, because they are so obvious, I don’t discuss):
 

- Telescope frame: This design is a truss tube telescope, that allows telescope disassembling for traveling to observation sites.  This kind of structure is very rigid, although I consider that its advantages become evident for diameters from 300 mm (12”); for smaller mirrors it is usually better to use a simple tube.
 

- Dimensions and weight: Although this kind of design allows making “Big Ones” up to 36”, it’s necessary to notice that:

Diameters up to 350mm (14”): The telescope can be carried by yourself in a normal car, being able to be transported through a building stairs (although with certain difficulty).

Diameters up to 450mm (18”): You need a big car (wagon station, pick-up, …), with flat charge surface , or a tow.  Its handling in building elevators or stairs is almost impossible.

Diameters of 500mm (20”) and bigger: I adddition to the transport problems (tow use is imperative), it must be warned that these telescopes can not get through normal 70cm wide doors, so that it can not be kept, nor constructed, in a house, but in a garage, shed or similar; moreover, it’s necessary to use a big ladder for observing.
 
 
 

Telescope design

Once you have selected the primary mirror diameter, we have to carry out the design of all elements of the telescope; using Autocad (or similar CAD software) will be things easier.  You have to be very careful at this phase, verifying many times all the dimensions and checking if the telescope will be able to get through home’s doors and transported in your car.
 
 

Ordering optics

At this moment we can order optics. You have to realize that delivery times on large mirror are about one year; so, we will be able to make telescope parts calmly, leaving only final adjustments for being made when optics arrive.
 
 
 

2. - MATERIALS AND TOOLS

Almost all materials used have been Kriege & Berry’s suggested.  A very important issue is the correct election of laminated wood that are going to be used in critical telescope parts.

I have used Baltic (Finnish) Birch, 15mm (11 layers) and 5mm (9 layers) thick; the last one is really a wonderful material, although very expensive (about 60€/m2).  When buying wood you have to consider that cuts made by warehouse supplier are not exactly perpendicular, so that you have to ask for wood pieces with certain margin, for carefully cutting them later.

I have to advise you that I don’t like the procedure described in the book for sticking large wood pieces in order to obtain a greater thickness ones (raising a vehicle for supporting a wheel over pieces), because there are wheel displacements when the car is being leaned on wood, and pressure is only done over contact zone with wheel. I have preferred to use clamps for it.
 

Necessary  tools, in adittion to small basic ones (screwdrivers, hammer, file, …) of a small home workshop, woul be:

Precision framing square, essential for obtaining perpendicularity on wood cuts.
Tube cutter (it is worth to use a quality one)
Column drill (however,  you could use carefully a manual one)
Forstner drills
Router
Welding machine
 
 
 

3.-  TELESCOPE MAKING

I have basically followed the methodology described in the book, although I have introduced small modifications, that I believe slightly improve the telescope’s observing perfomance.  Next, I will describe step by step suggestions for each telescope part.
 

Mirror cell

Book’s description for its accomplishment is very complete. I have use a vehicle safe belt as mirror belt, since its streching under small loads is practically null.  Its is very advisable using a wood model for positioning and attachment of the mirror cell steel structure for welding.
 
 
 
 
 
 
 

Mirror box

Cover attachment has been made by small trunk closings, although I have had to increase the hoops with wire; thus it is avoided a possible falling on the primary of inner attachment systems (magnetic or similar).  There is a level bubble stuck (assured with foam) on the box cover, for an easier telescope positioning.
 

I have put an inner Kydex baffle, covered with flat black velour paper, that is fastened with small nails on the inner box corners’ reinforcements. On the box bottom, there is also a Kydex baffle (velcro fastened) that avoids light passing whereas allowing air current for mirror cooling.
 
 

Secondary cage

Most focusers' design implies that drawtube gets inside the telescope tube; I have put a wood supplement for avoiding that.  Into this supplement there is guides for a filter sheet, with an Lumicon OIII 2” filter. I find this accesory very useful, because it avoids to remove the eyepiece for filter threading; moreover, by this way the filter is not directly exposed to the outside ambient, so that dewing is delayed.

 
 

Wood rings have been bored for decreasing weight (always a critical aspect of the secondary box); I have had a 300g (0.66 pound) saving, although rings varnishing has been very hard.
 

On secondary box’s inner surface I have placed flat black velour paper, whit superb characteristics for avoiding any light reflection.  There is also wires for powering heaters on eyepieces, finder and secondary mirror.  Power is obtained from battery using two truss tubes like current conductors.
 
 
 
 

Bearings

Altitude bearings design has been basically made taking care of their aesthetic, although without stiffness reduction; however, cutting and varnishing work has been considerably  increased.

Regarded formica, in Spain there are not Stardust neither Ebony Star commercial brands, so that I chose a medium roughness one, although I think that this is not a critical design issue. You are advised that formica is only sold by large dimension sheets (about 3 x 1.5 m).  You must take care cutting formica, because it is an extremely fragile material;  I suggest to cut greater formica pieces, sticking them to wood with carpenter’s glue and then trimming them carefully with a router.

 I have used 3mm thick Teflon, fastened by screws.  In  order to assure an uniform contact between bearing and teflon surfaces, teflon pads have to be sandpapered until getting a perfect contact; for that I have used a bearing as a tool, sticking sandpaper on its surface with double-side sticky tape.
 
 

Truss Tubes

Several ways for the secondary cage attachment to truss tubes are described in the book.  I have chosen that is observed in the photography, with two aluminium 3mm thick sheets sticked; by this way, truss tubes are connected by pairs on a single point, so that there is a great flexure stiffness.

An important issue is the aluminium sheets union to tubes; the best way is to use threaded inserts, as described in the book: ingenious, simple and very rigid.  Nevertheless, I haven’t been able to buy them in Spain, so that I had to order them to USA (material cost $6, shipping costs $30!).

Tubes are 30mm outer diameter, 1.5mm thick.   I ordered them with flat black anodizing (cost increment is very low).  Covers are flat black Armaflex, that is an air conditioning pipes isolating; their main utility is to protect truss tubes during transporting as well as to make easier the cover adjustment.
 
 
 
 

Two of the tubes are used as electrical current conductors (+ and -) from mirror box until secondary cage; for that I have instaled, by threaded inserts, steel straps into tube’s bottom, that are supported into tube’s cages by thin steel sheets connected to the batery.

Before cutting the tubes, it’s necessary to leave a safety margin of about 50mm, so that you can cut it carefully later. At this moment it’s convenient to have the eyepieces set that you will use with the telescope; you can make preliminar calculations knowing that eyepiece’s focus approximately corresponds with its stop field position. However, some eyepieces (as TeleVue’s 31 and 22mm Nagler) have their focus position over their focuser support, well inside the main body.
 
 
 

Tubes’s split-block sockets

Several types of these supports are described in the book; I have modified one of them for an easier construction with my tools.  The photography shows one of them; I have made it with three pieces of 15mm thick Baltic Birch that, once sticked ant drilled, conform the support.  I think that the result is both practical and elegant.
 
 
 
 
 
 
 
 
 
 
 
 
 

Rocker box

I have installed teflon sheets on the box inner sides, in order to avoid hard rubbing between rocker and mirror box. The photography also shows the rocker box handles to make easier its handling during transport.

In order to obtain the azimut bolt bore, since box dimensions prevents the use of a column drill, I have used carefully my router, with good results.
 
 
 
 
 
 

Ground board

I have installed steel/rubber supports under azimuth teflon bearing; they are threaded for telescope leveling.
 
 

Handles

These are really a useful accessory: I had to make them folded, so that they could be transported in my tow.  I use one of the rocker threaded inserts for fixing the battery support, so that its weight does not affect the telescope balance.
 
 
 
 
 
 
 
 
 
 

Telescope cover

Ripstop nylon is known in Spanish tapestries as plasticized blak nylon.  I close the cover by means of metalic buttons, instead of a closed one, so that it is not necessary to disassemble the finder to put the cover .
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Other items

I always use a laser collimator; reaching a perfect collimation is a two minutes task. Furthermore, mine is a cross-reticle one, that makes easier to see the return laser beam on the collimator base.

Another useful accessory is eyepiece heathers, that will allow us observing on wet nights (approximately half of them, by my own experience).  They are very simple to construct, using resistors and foam.  I use 3W power, in continuous DC mode, for 2” eyepieces, and 1.3W for 1.25” ones.

Finally, I strongly advice you a Spanish old saying: “Make all experiments with soda water” (instead of good wine); whenever we are not totally sure about following steps, it is better to stop and meditate about its consequences.  For example, it is very useful to make a wood model (ballasted with lead) of the primary mirror, so that you will be able to practice the movements sequence for introducing it into the mirror box.

Clear skies.