5" f/4,2  Schwarzer Reiter

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Figures:

Primary Mirror (self made):
Material: Plate
Apterture:   5" (127 mm)
Thickness at the edge: 0.6" (15 mm)
Focal length:    13,9"  (535 mm) = f/4,2

Secondary mirror:
1.52" minor axis, 96% dielectric coating, imported from von Antares, USA

Weight: 
Teleskop: 57.5 Ounce(1630g)  - without eyepiece


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At the moment my 14" and my 20" newtonians have to share one finderscope. This is not a real problem, as it is very seldom that I'm using both telescopes at the same time (in fact I haven't done it yet). This drawback give me a good arguement to build another telescope. As I would like to start working with CFK this gaves the opportunity to make first experiences at the same time.
"Reiterlein", the existing 4.1" (106mm ) newton finder uses an 1 1/4" focusser (in fact it is just a tube with 1 1/4 " free aperture). The new one should get a 2" focusser to allow maximum field of view. I would like to admire widefield objects as a whole in future with this instrument.


Optics:

The "blank" is second hand. It was working in a cheap chinese telescope befor a guy had the idea to clean it  rubing down with a normal towel. I got it for few Euros.  As it had f/7 focal ratio I'd to start with rough grinding. I build a sandwich tool made of stoneware tiles as I did it in the past. You can find a detailed description building this kind of tools (unfortunately only in German at the moment)  here . As the blank did had a  precurve (and the tool did not) I  "preginded" the tool with an angle grinder at the outer radius to save some time. It took me aprox. 100 minutes to finalise rought grinding. After another 250min (net) I declared the mirror ready. Due to realtively "soft" glass (plate) I made fast progess, both with grinding and with polishing.

As with all my mirrors (it was No. 10) I made some notes (in German) that can be fould here.


Mechanic:

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Mirror Cell:

I built a 6-point cell for storage of the mirror. The frame is made of 0.4"*0.4" aluminium square tubes, 0.04" thick  The bars are made of  0.1 aluminium. Coliminating will be done with screwing the bolts. At the moment the mirror is fixed with adhesive tape. Later (after coating) I will fix it gluing with smal silicone blobbs.


Support points are calculated with PLOPP The 6-point design gave better results than a 9 point design. Why 6 points for a 5" mirror? Well, that's a bit overstated, but I like it to built mirror cells.


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Secondary Unit:

I took  0.03" thick sheet metal for the "arms". I the middle you can see aluminium square tube piece (length 0.6"). How to fix the arms at the upper cage? I glued scrue nuts at the side of the arms with some epoxi resin. As there are only minor forces with this smal unit, it is stable enough.

The 38mm (1,52) diagonal is silicone glued with 3 blobs of 2 mm (1/10") thickness directly onto to the diagonal al-sheet, that I cut with a simple mechanical fret saw out of a aluminium metal sheet and bended it to a 45 degrees angle.


Linear obstructtion is 30% - thats a lot, but as this telescope should primarly show wide fields at low power I gave priority on large iluminated field of view. Nethertheless I'm sure that I can enjoy magnification up to 150*; not bad for a finderscope


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To protect the secondary mirror against dust and touching, I cut a eyepiece capsule 4 times (one for each arm of the spider). At the top of the cuts I drilled small holes. It is very easy to screw and unscrew the "mirror protector".

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Secondary Cage and Focusser:

Basis for building the secondary cage are two rings made of baltic birch plywood, inner diameter aprox. 6", 0.4" thickness. I coverd it with 0.03" airplane plywood before it was laminated with Carbon fibre. From stiffness point of view laminating was not neccessary, but I think it looks nice.

I did my first experiences with carbon fibre and laminating. It took much longer as expected until epoxi resin soldifies; I turend the unit more than two hours to avoid that the liquid epoxi resin will drop from the cage down to earth.
The focusser board is made of CFK-plate that got very cheap via ebay. 

I use the same foucusser as for most of my telescopes, it's a Kineoptics HC-2 helical  bougth from Joe LaCour. As all the others it performs very well. It is one of the most lightweight focusser I know, this for reasonable cost.  

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Upper truss clamping:

Same principle as with URSUS, my 20" Newton. The tubes (0.4" carbon fibre tubes) will be fixed by two with smal  brakets made of aluminium. Key-Hole houses the skrew bolt head. Clamping will be reached with srewing the flat knurled nuts.


I cut a hole for each nut in the cage, this enables easy screwing, at the same time the nuts are outside the lightbeams to the primary mirror.
My plans are to take this smal telescope with me on holidays. Therefore I built it in a way that allows to mount and unmount it in less than 3 minutes without any kind of tools. The disadvantage: It took me a lot of time to find solutions for realising this approach.


 

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Lower truss clamping and mirror cell:
Similar built to  PHOENIX and URSUS. The split block sockets will be clamped with flat knurled nuts. These are fixed at the outer side of the mirror box. It is easier to screw and unscrew at the outer side, but the main advantage is not shadowing the lightpath from the stars to the primary mirror.

The mirror cell is made of 0.018" carbon fibre plates, cutted and glued with epoxi resin. Sawing was very difficult, as my saw blades were not stiff enough for this material. At the end it was more melting and rubbing than sawing.

 

 

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Stand-alone-Use:

It is possible to use this newton with a simple photo-tripod. Mounting will be realised with a bolt already fixed at the mirror cell. this bolt will be screwed at the basis plate of the tripod. With some smaler adjustments it is possible to use this combination up to mid-power magnification (100X), for high-power a "real" mount will become neccessary.
At the 17th Jan 2005 I had the chance to have a first view to the moon with the uncoated mirror. Unfortunately I was most of the time clowdy with only very view broken regions, but it was enough to test the focal plane with all my eyepieces.
With my 27 Panoptic I will have real 3.3 degrees field of view at 19x, not bad for a finderscope in my opinion.

 

As I still have a 13.3" (340mm) selfmade mirror in the basement, my next plans are to build a telescope around - CFK should become the favourite basic material for it.

 

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