Areas of Interest

4" Schiefspiegler

 

 

 

Gary Fuchs presenting at the STAR meeting 

Several years ago I attended the monthly Star meeting in Monmouth County, NJ. There were several great Amateur Telescope Making [ATM] projects presented that night. Gary Fuchs came with his award winning Schiefspiegler.

 

Shortly after that meeting, I was given an old Edmund Scientific mirror making kit.   It had two glass blanks, one of Pyrex and other plate glass, just what I needed to make a Schiefspiegler telescope like Gary’s.

 Old Edmund Scientific Mirror Making Kit
  

When I posted that I had the two glass blanks on the STAR ATM Forum, Gary, Dave Groski and other ATMers at STAR suggested I try making my first Schiefspiegler.  Here is a link to the STAR ATM thread for this telescope build.   Warining...  It is a very long thread.  It starts in 2009 and ends in 2012 when I finally finished the Scheif.

I learned more and more about Schiefspiegler telescopes through this small group of guys . They referred me to the yahoo group for Schiefspiegler telescopes and this article found on the CloudyNight's website.

  Delmarva Schiefspiegler Plans

Dave has optimized a 4” Schiefspiegler design, now known as a DelMarva Schief shown above

The interesting part about making a Schiefspiegler is that you grind glass on glass to shape both the primary mirror and secondary mirrors at the same time. At the end of grinding, both primary and secondary mirrors have a matching Radius of Curvature. [ROC] The primary mirror becomes a concave spheroid while the secondary mirror becomes an exact opposite convex shape during the grinding process. After the primary and secondary have been polished a 2” secondary is cored out of the convex 4” diameter glass tool.    Sounds simple right?

  
 Blanks after 220 Grit was completed  

The first step was to grind the primary and secondary to a target depth for the given ROC. This was done using 120 grit.

 

After initial shaping with 120 grit, the two mirrors were finish ground using finer and finer grits just like any other handmade mirror. An hour per grit was more than enough time to achieve a uniform surface texture for these small 4” glass blanks.

 

 

Dental plaster backs were poured to make pitch laps. I made three laps. You can use plywood backs instead of plaster. I had the dental plaster so I use that for my lap backs.

 Dental Plaster Lap Tool backs formed over glass blanks.
 Normal Pitch Tool with square facets  

Two laps were made with a normal square facet patterns. The first was used for the concave primary and second was used for the convex secondary. The third lap was cut with a star shaped pattern at Dave’s suggestion. This one was used on the secondary only.

 

The concave primary was polished using the square facet lap using standard 1/3 CoC strokes. The shape of the primary is checked using standard Ronchi and Foucault tests. The ROC of the primary is checked and double checked to verify is it within ¼” of the design ROC during is Foucault test. Its length was controlled by polishing either Mirror on Top [MOT] or Tool on Top [TOT].

 

 

The star lap was used only on the secondary mirror. As the secondary mirror was polished using the square facetted lap, its ROC flattened.

The star lap polishes outer zones of the secondary mirror more than the center. This slowly brought the flattened secondary mirror back into shape matching the primary.

 Star Pattern cut into pitch lap
 Modified Lap - Starfish  

At the advice of Jerry from Dave’s Yahoo group, I later modified the star lap to be more starfish shaped. This small change in the lap seemed to speed up the ROC recovery of the secondary working the outer zones more than the center.  

 

The shape of the secondary cannot be checked using either Ronchi or Foucault tests. It is verified only by the number of “fringes” it makes when placed against the primary mirror illuminated with a single wave length light source.  

 

As the shapes of the polished primary and secondary begin to matched, the fringes show as straight lines. For me this actually took approximately 12 hours of polishing to achieve. Dave, Gary, Jerry, Cliff and others encouraged me not to give up or stop before the job was properly completed. [I would have if not for them.]

 

 Final Fringe Pattern
 Coring Bits  

I purchased two different diamond coated coring bits from an eBay store. It was not clear to me from the store's website information if the diameters quoted for these bits were outside or inside diameters. So I purchased two different sizes. I used the larger one advertised as 60 mm on the website. The final secondary OD was 2.1”. After chamfering it was almost exactly 2”.  

Coring the Secondary  
  

Dave gave me very detailed instructions for coring the secondary. I followed them carefully as follows:

1) Make a wood fixture to center secondary blank under the coring bit chucked into a drill press.
2) Tape the mirror face for protection
3) Center fixture/mirror under the coring bit face down
4) Core half way from back side first keeping the bit wet
5) Remove the mirror without moving the fixture.
6) Clean the back of the mirror just cored. Gorilla tape the back of mirror you just cored. This keeps the core from spinning later.
7) Put the mirror back into the fixture face up.
8) Core through the front half. At break through, stop drill motor before lifting bit out.
9) Remove all tape, clean up, bevel edges

 

I had the primary and secondary mirrors coated at Majestic Coatings in Clark NJ. Jeff did them together with two other small mirrors I had made during one autoclave run.

 Final Mirrors
 Sketch  

Several different Schiefspiegler configurations were sketched. It is likely that any of these would have worked. In fact several look just like the Schiefspiegler Dave and Gary already completed.

 Sketch  Sketch
 Selected Arrangement  

I choose the arragement shown to the left for my Schiefspiegler. I used 3mm Luanne plywood for the outer skin reinforced in the corners with ¾” square pine. Its construction reminds me of my model airplane building days. It was like building a model airplane fuselage, only bigger. It finished up very stiff but light weight too.

 

As is my preference, I made a detailed set of plans before building. The side panels were cut first using a saber saw. The Luanne is easy to cut. I cut both sides together so they would match perfectly.

 Sides Cut Together
 Pine Strips Added  

The internal ¾” square pine corner reinforcement was wood glued and finish nailed to the side panels.

The end plate at the front of the Schiefspiegler was cut from Luanne too, but it was reinforced using ¼” plywood to stiffen it because it would later carry the secondary mirror and its cell. A plywood secondary cell was added. This cell had a central shaft to provide the “pull” and a set of three #10 bolts to provide the “push” of the “push-pull” collimation system.  

Secondary Cell Outside
Secondary Cell

The distance between the primary and secondary mirrors is critical to obtaining the correct focal length of the system. Both the secondary and primary cells are designed to hold their mirrors in an exact position without moving in or out while being collimated. [This cell design was improved later.  See notes and photos at the end of this article.]

 

 

The end plate that supported the primary mirror and cell was made using details very similar to the secondary holder. The cell was triangular shaped and had a central “pull” bolt. Three ¼” pan head bolts were used for the “push.” During final assembly the primary and secondary mirrors were silicon caulked-glued onto their respective cells.  [Like the secondary cell, the primary cell was improved later.  See notes and photos at the end of this aritcle.]

 

 Primary Cell Parts
 Ends Fitted  

All end plates were trial fit.  [The focuser shown here was replaced later with a new Moonlite focuser.  See notes and photos at the end of this article.]

 

 Focuser Mounted

 Finished Schief on at PPO  

The completed Schiefspiegler was coated inside using black polyurethane. The thin internal baffles were spray painted black too. The outside of the OTA was coated with a Bombay color.   The completed telescope is shown to the left.  It is mounted on a  medium duty Orion GEM which I control with my laptop, SkyTools and ASCOM drivers.   This image was taken at my Parsippany Patio Observatory. [PPO]

The Lunar image shown below was captured with the Schief and an Imaging Source monochrome camera.

 

 Moon

 

The double star Izar was imaged with the Schief and Imaging Source camera too.  Izar was easily split with a separation of only 3 arc-seconds from PPO.  The seeing in New Jersey is never that good and often limits imaging of close double stars. 

The Scheif has become my favorite telescope for double star imaging and measurement. 

It is a keeper......

 14449 2704A STF1877A Izar 2012 07-29

After using the Schiefspiegler for several months, I found that the optics were very good when adjusted properly, however, I also found that obtaining good collimation consistently was not possible with the cell design I used.  Also, there was just too much "play" in the old used rack and pinion focuser I initially used.  After consulting with my good friend Cliff I made the changes shown below. 
25a improvement planned
 I ordered a new Moonlite focuser and built a spacer atop the backplate of the Schief maintaining the focus position I had before.  The old Meade rack and pinion focuser was taller than the new Moonlite.  The new Moonlite had a 2.75" travel to match the travel of the old unit.  The spacer and back plate were stained and coated with a one step polyshade polyuretahne finish matching the existing Schief's finish. As you can see this new focuser is a 2" model which will allow the use of my Canon DLSR camera too.  The Moonlite is "rock solid" with no discernable play.  25 old and new focuser

 Next, I needed to find light weight spherical bearings matching the dimensions I needed for both primary and secondary cells.  I looked in the local toy stores, hoping to find a hard plastic ball, or maybe even something made of wood in the right size.  I did not find anything suitable. I looked into casting a spherical shape myself and even made a mold, but I was not satified with the finished product. 

I found several sources for wooded spheres online, however, the price for the sizes I needed were more then I want to spend once they included the shipping.  

Then I remember an article had I read about cutting concave and convex shapes in glass using a drill press and simple device called a "Sine Table".  I decided to try it out using my drill press, some scape plywood and the hollow bit I had used to core out the secondary mirror.  

 26 cutting spherical bearings

 As stange as it might seem, you can cut a spherical shaped piece using a simple fixture and a drill press.  If D is the diameter of the coring bit or hole saw and Theta is the angle the sine table fixture makes with a horizontal plane, then the bit will cut a sphere with a radius of R according to this equation:

R = D/(2*sin(Theta))

 27 progress photo cutting bearing

 The photo to the right shows the two finished spherical bearings cut using the same bit.  I alterred the jig angle, Theta, to achieve the two different spherical radii.

A mock up of the PVC seat and the spherical bearing shown below illustrates how the mirror can tilt without movement of the mirror face in any lateral direction. This is because the center of the sphere's radius of curvature and the center of the mirror face coincide.

 28finished bearing
29 improved bearing mock up  
30pvc  seats and cells

The finished cells, both primary and secondary, are shown to the left.  The mirrors were silicone'd to the cells.  The central locking bolt protrudes from an oversized center hole that allows the cell to tilt within the seat without binding of the bolt in the central hole.  The white PVC seats receive the urethane coated wood spherical bearing providing a low friction bearing surface.  Three jack screw push against the backside of each cell.  The central locking bolt provides the "pull" that keeps the spherical bearing in contact with the PVC seat while the three jack screws are use to "push" the cell into the correct tilted position.

This system has proven to be reliable.  Collimation of the cells is very repeatable.  These small improvements have made this telescope a joy to use giving consistent performance for visual observations of the planets and close double stars. 

Here are a few images of Saturn captured using the Schief, an Imaging Source Video Camera and a filter wheel.  The Top Left was done using Red, Green and Blue filters. Three separate videos were recorded in quick succession. These were stacked separately using Registax. Each Red, Green and Blue image was combined within Photoshop to produce a color version of Saturn.  The Top Right and Lower Left images were captured using a IR filter only.  As can be seen the IR image is much sharper.

Saturn 3 IR

M13 4f28 Schief Medium This is a Deep Sky Object image of M13 captured using a stock XS Canon camera.  I used a small 50mm finder scope and the Imaging Source Camera to guide the GEM.  This image consists of 20 x 180 second shots capture at ISO 1600.  They were stacked using Deepskystacker along with 5 Darks.  No Flats were included.   The Schief provided a high contrast - high definition image of this object.

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