Why I Love Building Telescopes.

By John Reed

Woodworking:

I have always enjoyed wood working since I was a teenager.  There is something about taking a once living material -- irregular, non-homogeneous and filled with beautiful grains following a pattern of its own -- and shaping it to a particular need, giving it a smooth finish that doesn’t hide this natural beauty, but rather enhances it.  I have always felt that a painted surface is a shame and should be avoided, nature is far more appealing than any artificial finish.  Woodworking is a labor of love that I do for pleasure rather than work that I am required to complete.

Telescope Making:

It is not surprising that my interest in amateur astronomy, my other passion, would cross paths with my wood working.  I first built a totally wooden telescope in 1979 and have been making them ever since.  To the best of my memory I have designed and built something like ten complete instruments and made uncounted modifications, adaptations and mounts.  In addition I am not sure how many scopes for myself and others that I have performed complete renovations.  Sometimes my amateur telescope making (ATM) encroaches on my time spent observing, and this doesn’t bother me!  I have built several 10" dobsonians, a 4 ¼",  a binocular, a 10" dob, and an 18" dob.  I use either solid oak or oak plywood in my construction. I reciently won an award for my 18" dob at the 1999 Okie-Tex Star Party for best wood working.

Teflon:

During my years of practicing the art of ATM I have noticed several aspects of dobsonian construction which I prefer from the view point of an end user.  I have also used a variety of commercial instruments and have found that there were problems with a mass produced instrument that greatly affected performance.  Because of this experience the scopes I build today are much better than the original redwood open frame tube I built in ’79.  One of the most desirable traits I feel a scope should have after optical quality (I do not make my own optics) is mechanical function.  A dobsonian should move smoothly.  When John Dobson first came up with this design he used Teflon plastic as a bearing surface.  Most commercial dobsonians do not use Teflon, instead using other types of plastic.  Teflon is the only material I know of that the sticking friction is actually lower than the sliding friction.  This is not a good trait for an automobile tire, but it is an excellent one for a telescope mounting system.  When a dob is being used it is moved with a lot of small shifts in position.  If the mounting surface sticks, the scope, no matter how rigidly made, will distort under the user’s effort to move the instrument.  This will spring load the system so that when the user lets go of the scope it releases this stored energy and the scope will return part way to its original position.    The operator must learn to over shoot in hopes that the scope will land in the right spot of sky!  This is commonly called backlash.  My first goal was to eliminate this problem as much as possible from my designs. Teflon and good, rigid construction does this. It is also important to place the Teflon pads on the ground board directly over the feet touching the ground, which also decreases backlash.

Focusers:

Another mechanical problem I have noticed in all but the most expensive commercial instruments is focuser quality.  I have observed image shift, sticking and slop in many commercial focusers.   With a fast system it is very difficult to get an accurate focus, especially at high power.  I have always been unhappy with the various focusers I have used.  However Astrosystems Crayford design seems to be excellent.  It is the best one I have ever used.  Once I tried one I never wanted any other type.

Diagonal Size:

Finally optical quality as I mentioned is mostly a function of getting an excellent primary mirror.  I have used several brands and have my favorites.  Right now I am using an 18" mirror made by Pegasus Optics right here in Arkansas.   Another problem is diagonal size.  On most commercial units the secondary mirror (or diagonal) is either too large or too small.  Both are bad.  If it is too small, not all the light from the expensive primary mirror reaches the eye.  If it is too large then contrast suffers and star images are larger than they need to be due to larger diffraction rings.   The cure for this is first to use a low profile focusers that get the eyepiece as close to the side of the tube as possible.  Secondly the tube should not be overly large to keep the focuser as close the center of the tube as possible without blocking the actual light entering the tube.   My designs use a low profile focuser mounted on a piece of ½" plywood (Baltic Birch) which cuts a chord across the circle of the upper tube assembly, getting the focuser closer to the center axis.  This allows the smallest secondary possible without losing any light either in the center of the field or off axis to about 3/16" on either side of center.  Thus a circle of 3/8" diameter has a 100% light from the primary.  Outside this circle there is less light (called vignetting). This effect makes stars at the edge of a low power eyepiece somewhat less bright.  Vignetting does not affect the quality of the image, just the brightness.

Storage:

When a typical scope is stored the ends of the tube are covered, the focuser is plugged and the tube is hopefully kept in a horizontal position.  However a lot of the dust that is trapped in the tube will find its way to the optics.  Because optics are so smooth they attract charged dust particles.  Lint, aerosols and dirt will collect on the coatings which will have to eventually be removed.  Removal invariably causes scratches and sleeks no matter how carefully the cleaning is performed.   The best approach is to store the optics in a smaller volume of air.  If an open truss design is used then a cover can be fabricated which fits over the primary mirror.  A lot of high quality dobs have this feature.  A problem with these removable covers is that if the scope is placed in a horizontal position the cover falls off.   Another problem with the covers is that if they are not placed carefully over the mirror the edge can scrape the delicate mirror’s coating.  I have actually heard of this happening.  The solution is to hinge the cover with spring loaded hinges which hold the cover closed similar to kitchen cabinet doors.  A magnet is then installed which holds the cover open during observing.  I have also fabricated a small cover to fit over the secondary holder to keep this surface as clean as possible.  Completing the system is a wood plug which seals the hole in the back of the scope to keep insects and dust from entering through the bottom.

The Finish:

As I noted above I have always admired the beauty of wood and don’t like to use paint as a finish.  I believe a telescope should be finished like a piece of furniture.  A finish should be hard and resist scratches.  It should also resist moisture.  Unlike older types of varnishes, one of the best space age finishes is polyurethane.  This dries to a hard transparent finish.  The gloss finish seems to be more durable than satin  or semi gloss.  Before applying the polyurethane I apply a light stain which enhances the grain of the wood and adds to the beauty of the surface.    The upper tube is covered with plastic laminate, as are the side bearings.  I have developed a method to wrap disks of wood with laminate.  The side bearings are ¾" thick oak plywood with the edge covered by thin strips of laminate.  This not only looks attractive but makes a much better side bearing than the metal or sections of PVC pipe.  It can also be made as large as required.  Finally all the hardware is brass to give the instrument a old world class look.

Balance and Stability:

Any telescope magnifies an image.  Power is defined as the numbers of times an image is made larger than when viewed with the naked eye.  Unfortunately any movement that the telescope makes is also magnified.  As I noted above backlash and vibrations are a problem.  The telescope should settle down in less than a second after being moved.  In any structural system there are properties which affect this.  One is the rigidity of the system.  The other is how low the center of mass is so that the fundamental vibration frequency is at least twenty or more times a second.  The higher the frequency the faster it damps out.  I once built a very heavy wood tube that though rigid was too heavy at the top end.  The fundamental frequency was only about five hertz.  It took ten to fifteen seconds for vibrations to damp out every time I moved it.  This taught me that any telescope needs to more than just rigid.   As much of the weight as possible needs to be near the bottom of the scope.  This is facilitated by the weight of the mirror.  However because the front end is so long it must also be as light as possible.   This is done by using trusses made out of ¾" x ¾" oak struts and an upper cage made of thin rings of ½" Baltic birch plywood wrapped by plastic laminate.  The upper cage is made only of an upper ring, a lower ring and a small ½" thick plate for the focuser.  The entire cage weighs only about a pound without hardware attached.  Even on the 18" I have built I have very good results.  The vibrations damp out well within a second.  A low center of gravity also helps with stability and keeps the telescope from tipping when being moved to a new position.  Large side bearings keep the rocker box more squat which contributes to smooth movement.  Putting the plastic laminate on the outside of the rings creates a light trap. Especially when the inside of the cage is painted flat black.  Most designs I have seen put a black plastic light shield on the inside of the rings.

Collimation:

After the scope is set up it needs to be collimated.  Usually just a touch up is required.  With a fast system this is a must to get good images.  Because of this collimation needs to be as painless as possible.  I have bought a variety of secondary holders from several manufacturers.  Most are difficult to adjust, using a push pull arrangement of three small bolts.  ProtoStar Astronomy Products uses a flexible plastic shaft that allows a very positive movement when an allen  screw is turned.  This is the best system I have found for accurately collimating a secondary mirror.  The adjustment of the primary is easy, the miror cell is mounted on three car valve springs with knobs to adjust the tilt, requiring no tools.  The Astrosystems Crayford focuser has small allen screws at each corner which allow you to tilt the focuser.  This is a must for accurate collimation.

Setup:

Setup should be fast and easy even for an 18" Newtonian.  This is no small task for a piece of equipment weighing 150 pounds!  The mirror itself weighs about 50 lbs so weight is a definite consideration.  I have built a hand cart that bolts to the side of the mirror and rocker box that allows the instrument to be moved around without having to carry it.  The hand cart rides on two heavy duty steel lawn mower wheels.  Once the scope is in place, it is tilted down on the turn table (or ground board) which is held captive to the rocker box by a ½" bolt.  The hand cart is then unbolted from the rocker box and removed.  The secondary cage is removed from the mirror box where it is stored.  After this assembly can begin.  The wood trusses are bolted to the mirror box, followed by the secondary cage which is bolted to the tops of the triangular trusses.  Because I use pre-made triangles less bolts are required than a standard dob.  It requires eight bolts for the mirror box and four for cage.  They are tool-free knob-type ¼-20 bolts purchased from Reed Tool Supply (no relation).   At this point a plug is removed from the rear mirror box opening and the mirror box door is opened.  Then collimation can begin.

The View:

I have seen galaxies as faint as 15th magnitude, but usually from Arkansas skies 13th to 14th is a more reasonable expectation.  Stephen’s Quintet  is visible without averted vision when the sky is transparent enough.  My favorite view however, is of the Veil Nebula with a Lumicon UHC nebula filter in place.  The striations and cirrus like wisps are remarkable! Surprisingly this telescope gives excellent planetary views. Saturn and even Mars are detailed, sharp and vivid. It appears I have excellent optics I hope to get many years of enjoyment out of this instrument.

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