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A Custom SBS Saddle Plate

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#1 GJJim


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Posted 24 July 2014 - 11:07 AM

Three years ago I started a project to design a dual-channel, astrograph for wide field imaging. My goal was to dramatically reduce the time needed to acquire data, and work within resolution constraints imposed by local seeing conditions. Last month Intes-Micro delivered the two OTAs , they are 25 cm aperture, fast f/3, and employ a flat-field, Medial design with a Mangin primary similar to the Astro-Physics RHA and Officina Stellare Veloce astrographs. The tubes are closed, with no spider vanes to create diffraction spikes, and the 44% central obstruction is smaller than the >50% CO of the competing Honders designs.

The combined weight of the OTAs is about 80 pounds and this presented a challenge for mounting them side-by-side. Existing SBS saddle plates with the standard Losmandy dovetail seemed to fit the bill and I purchased one at a sale price assuming it would work well enough. Then the OTAs arrived and after seeing and handling them I realized the ADM SBS might not be up to the task. The problem with dovetails is their inherent tendency to move and loosen under heavy loads, and the SBS saddle I bought had three of them! Each OTA had a dovetail, and then the saddle bracket itself had a third dovetail that fits in the Paramount's Versaplate. All three of them in the stack had to remain absolutely tight through slews and meridian flips.

Another issue was aligning the two fields of view. My dovetail SBS saddle had no provision for adjusting the relative angle between the OTAs. Yes, I could shim the dovetail to adjust the angle, but that would make an already marginal attachment point even less secure. I reluctantly concluded the store-bought SBS saddle was not adequate for supporting or aligning the two heavy OTAs.

My solution was to make a custom saddle plate that would attach directly to the mount with twelve bolts and hold both OTAs without any dovetails. The pointing angle of one OTA is adjustable over +/- 3 degrees. Its FOV can be boresighted with the other, or alternately, it can be offset to increase the effective FOV in the sky. A guidescope can be mounted between the two astrographs, and there is plenty of space for other accessories like USB hubs and voltage regulators.

This photo shows a 31-pound piece of cast aluminum tooling plate, dimensions: 22" x 11.5" x 1.25". After a few hours in the VMC, it now weighs 12 pounds. Next steps are finishing to match the color of the mount and test fits of the OTAs and mount plate. Custom camera adapters are also in the works and turned out to be quite a challenge since the available back focus is only 56 mm.

Top of saddle after machining

Bottom side after machining

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#2 choran



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Posted 24 July 2014 - 11:45 AM

Beautiful work!!

#3 Hilmi



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Posted 24 July 2014 - 12:03 PM

Truly beautiful. It is when I see work like this that I get tempted to purchase a mill and learn how to work with it.

#4 GJJim


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Posted 24 July 2014 - 02:18 PM

Choran, Hilmi, thanks. The machine I'm using is now ten years old, my accountant says it's a "fully depreciated asset". My wife jokes that it takes metal worth $5/pound and turns it into chips that sell at the scrap dealer for 35 cents a pound. :grin:

#5 BKBrown



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Posted 24 July 2014 - 06:52 PM

Absolutely beautiful work Jim, and it looks rigid as all get out :waytogo: I am using the ADM SBS solution on my AP1200 but the loads are 30 and 28 pounds...rather less then your astrographs, but I check all knobs before each session. Can't wait to see the whole kit come together for you. :grin:

Clear Skies,

#6 fetoma



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Posted 24 July 2014 - 09:49 PM

I really miss having a lathe and mill around. I have to do all my machining on a stupid drill press. I feel like a caveman when I have to make something. :bangbangbang:

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