Kutter telescope building log

This page is a description of how the Kutter was built. For the mirror set, go to the mirror making page.

Design review

The design overview of the realized telescope:

Final design

The two finished mirrors turned out with the following parameters:

  • Primary: D=155mm, RoCs=4465mm, RoCl=4479mm, k=0
  • Secondary: D=72mm, RoC=-4520mm, k=0
These values require some design changes in the telescope design, in the tilt angles and the distances:
  • Primary tilt: 2.7°
  • Secondary tilt: 10.82°
  • Primary to Secondary: 1370mm
  • Secondary to focus: 1404mm
The resulting spot diagram then looks like this:

Final spot diagram

The Strehl ratio in the field center is 99%, at the edge the ratio is still 94%.

Building the OTA

Some shifting around with design parameters finally resulted in a barely fitting 80mm PVC tube for the final optical leg and a matching focuser. The tube for the primary is taken from the now dismantled Gregorian (probably to be remade at a later stage...).

Mirror cells

Knowing the tubes, the first thing was to make the mirror cells and adapter for the focuser. The vertex of the secondary should ideally remain fixed when it is adjusted for collimation. There are various ways to achieve this, I tried two.

Secondary bearing 1 Seconday bearing 2

The center of rotation should be at the mirror vertex, and hence the required bearing has the shape of a sphere with its center at this vertex. I purchased an acrylic sphere of teh right diameter, and tried to ride in on a PE circular base. The big advantage of this method is that the tube can be cut-off straight. The big disadvantage is that the bearing didn't work very well, and I decided to go the other way...

Secondary cell 3 Seconday cell 4

The solution was the proven spring-loaded three set screw design. It requires a bit more engineering, like cutting off under an angle (10.82°) and offsetting the cell with the proper amount, but in the end it proves less cumbersome.

Seconday drill 5

To produce the edges of the secondary disk I made a jig to hold the disk in the drill press. Coarse sandpaper cuts away the surplus, until it fits the OTA.

Precisely the same strategy was used for the primary cell. However, where the secondary is glued to the cell, the primary rests on felt patches and needs to be secured with clamps.

Primary cell 1 Primary cell 2

For both cells, the feeds through the wood base were made with aluminium tubes. These prevent wear, and also provide better positional definition of the collimation bolts. For smooth operation some lubricant can be inserted, or otherwise a plastic tube would probably have been better (hard to find though...).

Secondary tube

The secondary tube requires two more things, the adapter for the focuser and a funny hole.

Focuser adapter 1 Focuser adapter 2

The focuser fits on a 74mm outer diameter ridge, which is probably a standard size for factory OTAs. This Kutter is based on a standard 80mm PVC rainpipe, with a wall thickness of 1.5mm. To get to the 74mm an additional 2mm has to be taken from the radius, and this was accomplished with another standar PVC attribute, the pipe extender. Inside the pipe extender another PVC attribute was modified to fit. To this purpose, a slit was cut to enable squeezing it in.

The funny hole is required to allow the light from the primary to get in. The shape of this hole is dependent on angle of incidence and ratio of beam and tube diameters. For convenience, the beam is taken as a cylinder and the shape of the hole is therefore the intersection of two cylinders as shown below.

Pipe saddle calculation

The angle of the intersection line at some point (x,y) is given by:
Pipe saddle calculation
For calulation, the value of x can be parameterized from the angle inside the incoming lightbeam, measured from the top:
Pipe saddle calculation
The corresponding value of y is then given by:
Pipe saddle calculation
For the template the length s along the tube wall is important:
Pipe saddle calculation
The template is then given by the points (y,s).

The angle α is twice the secondary tilt, so in my case 21.64°.

Using this spreadsheet the point set for the template for my hole was calculated, and then fed into a CAD program to generate a scale drawing. You can also use millimeter paper instead. I reduced the angle of incidence somewhat, to compensate for the conic shape of the beam.

Hole template On tube
Cut out View on sec

The hole was drawn on the tube, cut out and further sanded to size. The secondary cell is nicely visible from the primary direction.

Center piece

The construction used to connect the primary and secondary tubes is based on aluminium U-profiles. A U-profile can be shape-locked with the curved wall of a tube and hence ensure good alignment. The profiles are screwed on with M6 bolts, and as such form the basis for mounting the 9mm birch ply panels that define the mutual orientation of the tubes.
The shaping of these panels needs to be quite precise, since it largely defines the mirror locations inside the system. After this, only tweaking with the collimation bolts is possible. To achieve this, the two pieces are always worked together. The holes are 8mm, filled with an 8x1mm aluminium tube which precisely matches an M6 bolt. The alu tubes are sanded flush with the wood surface, like in case of the collimation bolts of the mirror cells.

Center piece Tubelets
Sanding Result

The shape match with the tube is sanded in; the required amount has obviously first been added to the panel size. The reslt is a very rigid and well defined construction. Only some support for the alt/dec axis should be added, but this can only be done after the balance point is determined. For this, the full construction needs to be made, including mirrors and eyepiece or camera. But before that, the system will receive a nice paint job.


The painted OTA was placed on the Polaris mount in order to align and try out the optics.

Test setup First light

On Oct 26 the telescope was roughly aligned and directed to the sun. The sunspots for that day can be seen clearly, even considering bad collimation and suboptimal contrast of the uncoated primary. One defect became clear, the distance Primary-Secondary appeared to be a few cm too large. This could be corrected quite easily, by shifting the primary tube forward.

Test overview

Another defect is that the size of the OTA is incompatible with the GEM on a tripod, since the primary bumps to the legs. A solution would be to place the Eq head on a pier, or better yet to make a fork mount.

A new fork mount

So the next step before taking the Kutter into operation is to make a fork mount. The fork is an ideal mounting for Kutter telescopes, because of their mirror-symmetry. The declination axis can conveniently be mounted to the flat part connecting primary and secondary tubes. This is where the telescopes' center of gravity normally will be. The fork must be made long enough to allow the primary to pass through. Mounting the scope "upside down" has the advantage of a higher eyepiece position, and a natural roof over the secondary mirror.
The fork is supported on three ball bearings and has a large wormwheel for driving the RA axis. This wormwheel is made from sheet aluminium, using a jig and a spiral tap. The jig pulls the aluminium disk towards the rotating tap, which cuts the thread and at the same time forwards the disk.

worm cutting worm cutting

The fork mount is driven by a unipolar stepper from an old floppy. This stepper is coupled to an M6 threaded rod, that matches the teeth cut in the big worm wheel. The coupling provides an additional 4:1 reduction. The total reduction rate is then approximately 3000.

fork fork detail

To drive the stepper I have upgraded my Polaris driver by making a decent PCB and improving the software. Now it can handle some speed control buttons (faster/slower) as well as configurable step interval and direction. The latter means that this can be used for any unipolar stepper driven RA axis.