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6" f/12 Newtonian



As I became more interested in webcam imaging, I decided to build a longer and slower Newtonian just for imaging planets and double stars. [Some folks refer to slower Newtonians as "Planet Killers."]

I choose f/12 as my target f-number because I could achieve f/24 and f/36 with this telescope using either a 2x or 3x barlow.  This is right in the "sweet stop" for an Imaging Source monochrome camera that I purchased a NEAF.  

 MIrror Grinding with 120 Grit  

I acquired a 6” glass blank for this project and started grinding with 120# grit after making a matching grinding tool of plaster and floor tiles.  [Go here for a grinding tool making tutorial.] 

In no time I reached my target depth.

In less than an hour I had ground to a nice spherical shape with the 120# grit.

I spend approximately one hour for each finer grit to complete the grinding process.

I polished for approximately 8 hours, in one hour sessions using a pitch lap.  [Go here for a pitch lap making tutorial.]

No figuring was needed for the Planet Killer mirror.  A 6” f/12 mirror can be left a sphere. 

 Pitch Lap Polising
 Focuser Spacer  Orion Focuser

The details used for this OTA were basically the same as were used for the 6”f/5.6 telescope I had already completed.   I made a base for the Orion focuser first.

 Cell, Bottom and Top OTA Rings  

Next I completed the bottom ring, mirror cell and top OTA ring. 

I used two layers of 1/2" plywood to form top and bottom rings.

The smaller ring fits nicely inside the end of the Sonotube.  The larger ring covers and protects the end of the Sonotube during transport and storage.


The spider was made with old hacksaw blades and plywood.

I used the same push-pull details again.  Why mess with a successful detail?

 Hacksaw spider and plywood secondary support
6 f 12 Spider Details The arms of the spider are made of hacksaw blades.  The saw teeth were grind off on the stone wheel grinder.   The cut end of the blade was inserted and epoxy glued into a slot cut in the plywood disc.  A steel washer was also epoxy glued to the bottom of the plywood disc.  This washer was drilled and tapped to recieve the collimation bolts.

All wood parts were coated with polyurethane Polyshade.  The mirror was silicone caulked-glued onto the wooded cell.

 Finished Parts
 Sonatube coated insdie with black polyurethane  

I used Sonotube for this OTA too.

It was coated inside using black polyurethane to improve image contrast. The polyurethane also helps water proof the Sonotube on the inside.

The spider is supported on the inside wall of the OTA using an "L" shaped bracket at each end.  These brackets have slotted holes that allow some minor adjustments and centering of the secondary assembly.  Spider Detial
 Planet Killer  

The outside of the OTA was coated with MonoKote to waterproof the Sonotube on the outside and give it that nice finished look.

Since the f/12 and f/5.6 OTA’s have the same outside diameter, the plywood rings I made to mount the f/5.6 on my Orion Sirius GEM also fit this f/12.

Below is a photo imaged with this 6” f/12 telescope, Imaging Source camera and 2x barlow.


 Saturn using Imaging Source Camera and 2x barlow.