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Telescope Hardware

   Although it can done on an ordinary tripod, most astrophotography (photos of the night sky and the objects within) require a lot of extra equipment. A friend and I operate an 8" Newtonian on a Celestron motorized mount, a Nexguide autoguider, and two Canon DSLRs. A lot of the commercially available hardware and mounting accessories are expensive, and so we have decided to make some of the stuff for ourselves.


   The object in the center is an adapter needed in order to step down from the telescope's 2.5" ID eyepiece tube to the camera's "bayonet" (a plain tube where a lens usually is). By tightening the set screws, the camera is firmly attached to the telescope. The 8" Newtonian becomes the camera's 'lens'!

   The object on the right is an ordinary mount for attaching any camera to the telescope's exterior via the traditional 1/4-20 mounting screw. This part has made for a lot of fun, and will be remade to be lighter in the future.

  

   This is an improved replacement part for a aluminum 'True 2' adapter sold through a major distributor. It is also a piece that goes between your camera and the telescope during mounting. The lip on the back accommodates and traps the set screws from a Canon lens ring. The inside was painted flat black to minimize reflection.




Making a Guidescope Mount

   We often rely entirely on the telescope's motorized mount for tracking the starry skies as the Earth rotates. Error between the tracking motor's motion and the actual motion of the sky limits our camera's exposures to approximately 2 minutes length, or until 'trailing' or blurriness is present.


   My friend purchased an auto-guider system, which is a small computer made to regulate the main motor's tracking. It requires a small auxiliary telescope (called a 'guidescope') to which it is attached. It was my job to find a way to attach this auxiliary telescope, which has a 4 inch diameter, to the tube of the main telescope.

   I followed the basic design of most commercially available mounts: two large rings through which the guidescope would be enclosed and which are mounted on a support rail spanning the available holes on the main telescope. I drew up a concept in Solidworks and settled on some final dimensions.
 
   The rings were made from 4" nominal diameter aluminum pipe. Cheap and easy. I tapped some hole for nylon set screw to capture the guidescope. With that done, I cut up some 1.5" x .5" aluminum bar for the beam and the two dovetail 'carts' which would ride along its length.

  
 
   All I could find was a 60 deg. dovetail cutter, which is a little extreme for this purpose, so I tilted the head in order to bring the angle to 15 deg. off the vertical.

   After making the two 'carts', the rail was easy. I adjusted the cutter's angle to compliment the dovetails on the carts, and started cutting toward the finish dimension. The picture above shows the moment when one of the carts just barely slipped on.


   With the rail nearly finished, I set it right-side-up in the vice and milled a long slot down the middle to accommodate all the fasteners needed for assembly. It also lightens it up, which is important.

  

   It was important that the carts be able to slide up and down the rail because I did not have the guidescope on site during machining. Nearly everything on this design was made to be easily adjustable on site.


   The pieces assembled together quite nicely. Some nylon set screws with knobs instead of ordinary heads would be a major ergonomics improvement.



   Overall, the project was a complete success, and probably a bit nicer than the store-bought model. Improvements for the future include thinner rings, a lighter and slightly thinner rail, and more 'knobbed' fasteners.


   I don't have a picture of our entire setup arrangement during day-hours, but I can offer a couple night pictures below. Enjoy.





X-Y Guider Locator

   Coming soon...



Bahtinov Mask

   A Bahtinov Mask is a device used to help focus a camera which is taking pictures through a telescope. Focusing on very dark night-sky objects can be difficult. There seems to be three ways to focus a camera under these conditions:

 1. Try to do your best, and by using trial and error.
 2. Using 'Live View,' a digital real-time display available on most modern DSLR's which seems to pickup dark subject quite well.
 3. Using a Bahtinov Mask.

   One of our cameras, the Canon 450d has 'Live View,' but for the other, the Bahtinov Mask is helpful. By fitting this 'screen' over the end of the telescope and taking a picture, a diffraction pattern is evident on the final picture. A minute adjustment in focus has a large effect on the diffraction 'spikes' about a star, and the process of taking a picture is repeated until the diffraction pattern is centered. If out of focus, the diffraction 'spikes' will be lopsided, and if it is in focus, the middle 'spikes' will seem the thickest. Here is a well-focused picture:

   The design for the mask was created using the 'Bahtinov Mask' generator on Astrojargon.net and with knowledge of the telescope's focal length and aperture. This .SVG file was converted to .DXF for use with CAD programs, and was imparted some extra features to allow for quick attachment to the telescope tube. The finished design was programmed to be cut on an Omax waterjet out of 1/16" aluminum.
   This fairly complex design was cut in approximately 6 minutes. The waterjet operates at a pressure of approximately 45,000 psi, and with the addition of abrasive garnet sand, is able to cut inches of steel. This Bahtinov project was hardly a challenge for the machine.




   Some small feet were made to accept a nylon 10-32 screw and would serve as the fastening device after slipping the mask over the end of the telescope tube.


   Here is the final product, painted a flat black to minimize reflections, and without the nylon screws shown. It is a useful tool, although, experience has taught us that the 'Live View' function, when available, is preferred. For very dark subjects however, this remains to be a trusted Plan B.


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