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6" f/5.6 Newtonian

 

Introduction

 

 

This article actually begins at the end of my "first" mirror making experience. 

Like most "first" mirrors, this 6" f/5.6 mirror took months of mistakes, re-grinds, re-polishes and at least a half dozen failed figuring attempts before it was completed. 

Despite the fact that this was a "first" mirror, its quality is actually better than 1/10th wave.  I really did not have a great need for a 6" f/5.6 mirror at the time I started it, however, it has turned out to be a real work-horse for me.  All of my first deep sky astrophotos have been imaged using this little "first" mirror.

One of the best reasons to make your own mirror is that it will be better than almost any mirror that comes with a store bought Newtonian.

I had plenty of help making my first mirror.  The gang at Star taught me how to make my first mirror step by step.

 

Image of Final Ronchi 
 Side by Side Images of the home made Ronchi and Foucault Tester.  The image to the left shows the Ronchi head.  The image to the right the Foucault head.

I learned how to measure the figure of a mirror using both Ronchi and Foucault methods as part of the "first mirror experience. 

After using several different home made mirror test rigs, I made my own.  I used 1/8" Masonite and some scraps of wood hot glued together.  It wasn't very pretty, but it worked. 

This tester had two inter-changeable Ronchi and Foucault test heads.  Each head had its own LED light source.  The Ronchi grating was purchased for Willmann-Bell, Inc.  It had 85 lines per inch.  The Foucault head used a razor blade as the straight edge. 

 A 1/3 Center over Center [1/3 CoC] stroke was used to create a concave spheroid shaped mirror face during the grinding and polishing process.  Final mirror figuring was completed using a "W" stroke instead of the 1/3 CoC.  The "W" motion of the pitch lap over the mirror face deepens the central zones of the mirror and rapidly transforms the spheroid into a paraboloid shape. 

The Foucault test is used to measure the Radius of Curvature [ROC] at several different locations on the mirror.  These measurements are used to compare the actual mirror shape to that of a perfect paraboloid. 

A paper mask is used to divide the mirror into uniform zones.  CouderMask can used to print the paper mask for the Foucault test.

 

 Final Foucault test result using Figure XP.  This mirror is better than 1/10 wave.
 Finish mirror coated by Majestic.

Typically, three readings should be taken at each mask zone during the Foucault test. 

I like to start at the center zone working my way outward one zone at a time.  I return to the center zone two more times repeating each prior reading.  

This data is input into a computer program that does all the number crunching for you.  I like FigureXP, but there are other free programs available that will perform similar calculations for you.  Not all are compatible with Windows 7 though.  An example of Figure XP results is provided above.

As mentioned above this mirror is better than 1/10th wave.  A good target for the amateur mirror maker is 1/8 wave or better. 

The completed mirror was coated at Majestic Coating in Clark, NJ.   

  Final Telescope Plans

 Plans were prepared using A9CAD.  I like to have a good set of plans before starting the shop work.  This saves me time in the shop.  I am not figuring it out as I go. 

The bottom ring of the OTA supports the mirror cell and protects the bottom edge of the cardboard Sonotube.  It has a central hole that allows air to get to the back of the mirror.   

The bottom ring was cut from 3/4" plywood using a drill press and router bit.  The rough cut plywood part was rotated around a 3/8" diameter central hole and bolt affixed to the drill press table top.  Several passes were completed under the spinning router bit to make each full depth cut.  My drill press has a belt and pulleys than can be used to change the cutting speed.  I used a fairly high cutting speed for the router bit.  The outer edge of the ring was countersunk so 2/3 of the ring fit the inside diameter of the Sonotube. 

 Bottom Ring of the OTA.  Cut using a router bit and drill press.
 Top Ring of OTA.

 

The OTA tube was cut from a 8" diameter Sonotube purchased at a local lumber yard. The end of the Sonotube was trimmed square using a utility knife. 

I learned that 8" diameter Sonotubes comes in a least three different diameters, all close to 8".  I think they do this so they can nest several tubes within each other for shipment and storage at the lumber yard.

The top OTA ring was also made using 3/4" plywood.  It was cut using the same techniques used to make the bottom ring. 

Both the top and bottom OTA rings were held in place using (8) pan head screws equally spaced around the end of the Sonotube.  The screws were installed through the Sonatube and into the lip of the ring inside the Sonatube.

Like the bottom of the Sonatube, the top of the OTA was trimmed using a utility knife to the length shown on the plans.

 

 Next the triangular cell was cut from 1/2" plywood.  It too has a central hole to allow air to reach to the back side of the mirror.  Three cell support locations are pre-drilled with a smaller pilot drill bit near each of the points of the triangle. 

The secondary mirror support parts are also fabricated from plywood.  The secondary mirror holder is made of several layers of plywood clamped and glued together forming a cylinder of wood.  The center of this wood cylinder was cored out on the drill press using a spade drill bit.  The cylinder was then cut on a 45 degree angle on the table saw.  The 1/4" central hole was chamfered to accept a spherical nut.

 

 Cell and secondary mirror support spider parts roughed out of plywood.
 Trial Fit of Cell

 The triangular cell was supported on three medium stiffness springs and 1/4" bolts.  The heads of these bolts were  counter sunk into the face of the cell to provide a smooth surface for mirror mounting later.

The mirror was affixed to the cell with three generous blobs of silicon caulk after the cell was sanded and finished.

 The secondary mirror support and spider clamping system are shown in the photo to the right.

There is a central 1/4" bolt used to clamp the entire assembly to the spider arms.  This bolt provides the "pull" of the push-pull collimation system. 

The "push" is provided using three long #10 bolts.  These bolts react against top of the mirror support and the tee nuts seen on the underside of the center plywood disc. 

  

 10 Secondary Support Assembly
 Spacer base for focuser  A 3/4" thick focuser base was shaped from white pine.  This spacer is needed for the "racked-in" focuser condition.  The large central hole was cut using a saber saw after the correct saddle shape was achieved. 

 As can be seen in the plans, the OTA is supported using a clamping support box.  The altitude bearings are mounted on each side of this box.

The box was assembled, wood glued and nailed together as single unit.  It was then cut in two halves to form the OTA clamp.

The inside edges of the clamp that contacts the OTA was lined with a soft material to hold the OTA snugly without damaging it once clamped.

 

 OTA support and altitude bearing box.

Black coating inside Sonotube

The Sonotube was coated on the inside with two coats of ebony polyurethane.  This was done to increase the telescopes contrast, reduce stray light finding its way into the eyepiece and to improve the OTA's resistance to the night's dew.  
 The ground board was made in a triangular shape much like the cell.  The ground board does not need to be thick because the weight of the telescope passes through the three teflon bearings mounted directly over each of the three support feet. The feet were cut from 3/4" plywood.   Top side of the ground board.  Three teflon bearing are placed directly over the support feet.
 Ground board trial fit.

 The ground board is held to the bottom of the telescope support using a 3/8" bolt.  The bolt is only snug, not tight.  A self locking nut is used to maintain the snug only bolt tension.

The bottom of the telescope support was laminated using formica later are final sanding and finishing.  The combination of telfon slider bearing on the formica laminate provides a nice smooth feel.

 Before final sanding and finishing all the major parts were assembled for a trial fit.

The OTA clamping box with altitude bearings holds the OTA snugly.  The clamping action is created using a window sash latch.  The back side of the clamping box was held together using light weight hinges.  The hinges are shown later in this article.

 

 

 Final trial fit.
 Stiffeners added to support side panels  After the trial fit, a friend suggested I add a gusset stiffener to each support side panel.  These were cut from 3/4" plywood.  The other parts of the telescope support were made using 1/2" plywood. 
 Polyshade finish

 

Internal parts coated with black. Exterior color a rich bombay shade.

All the completed parts were given two coats of a Polyshade urethane finish.  Parts internal to the OTA were made black, external parts were done in a rich bombay color.

The top and bottom rings are anchored to the OTA using (8) stainless steel screws equally spaced around the tube's circumference. TOP RING ANCHOR
Spider Details

The spider was made using galvanized perforated pipe hanger strap stock found at the hardware store.  Later painted black.

The four spider arms are actually made of (2) two "|_" shaped pieces as shown in the image to the left.  Each end of the spider arm is anchored to the inside of the OTA.  The center corners of each "|_" piece are tightely clamped between the upper and lower plywood discs.  The central 1/4" thread rod, from which the secondary holder is hung, provides the clamping force using a pair of clamping nuts above and below the two clamping discs.

The altitude bearings are a typical "dob" type bearing.  Two teflon bearing pads are fixed to the lower wood arc cut out.  A matching plywood disc is anchored to each side of the split box OTA clamp.  The outside face of each plywood disc is covered with Formica laminate. Altitude Bearing
SPLIT BOX HINGES

Two light duty hinges are used on the underside of the clamping altitude box.  A window latch [shown earlier] provides  gentle clamping pressure on the OTA so it does not slide.

 Something Santa's helpers made?  First Light 12 29 2010
 Another First Light photo.

 The outside of the OTA was coated using MonoKote.  MonoKote is normally used to cover model airplanes.  It has good resistance to the fuels used by model airplane flyers and is therefore waterproof too.  It is ironed on using a hobby iron which activates its sticky backside glue.  The heat of ironing also shrinks the MonoKote plastic covering providing a smooth wrinkle free cover, nice looking  and dew-proof too.

 

 

First light for the completed 6" f/5.6 Dobsonian was just after Christmas of 2010.  

 

This little OTA has become my main Deep Sky Imaging telescope.  When imaging I remove the OTA from the Dobsonian support system and mount it on an Orion medium duty Sirius GEM using a set of plywood rings. The OTA is very light, less than 10 lbs. with a stock Canon XS DSLR camera.   There is another aritcle on the webiste that discusses the hardware and software used to image Astrophoto's here