Sunday, March 8, 2015

Star Honor Series: Part 7 - Mounting Your Reflector Telescope on a Dobsononian Mount

In Part 5 and 6 of this series we built a Dobsonian Mount (right) and an unmounted reflector telescope. Now we get to put them together.   Here's what that process is going to look like.

First set the tilt box in the cradle on top of the pedestal. The mounting cradle should be lined with some kind of felt strips.  The pedestal is open under the tilt box which will allow the tilt box to sing all the way down so that the scope can point across a full range of motion from horizon to straight over head. 

Next, slide the tube into the open-ended tilt-pod mounting box. Move it back and forth so that it balances at the ears of the mounting box when the telescope is horizontal. If the scope feels loose, glue felt padding to the inside of the tilt box so the scope tube fits more tightly in the tilt-pod. The scope, if balanced properly should sit at any angle and rotate smoothly, without slipping once it's in place. that's really all there is to it. It should look like the picture below when you're done.  Finish by drilling a couple of holes through the top and bottom of the mount and through the scope. Measure the distance and drive a couple of screws through the holes to secure the scope. Be careful not to drive any screws through the optics inside the scope.


Next you need to collimate the telescope.  Adjust the primary mirror first by using the collimating screws on the bottom of the mirror mounting cell.  Look down the sonotube from the top the mirror and you'll see the image of the open end of the tube and your face looking back at you. You'll see the four spyder legs that hold the secondary mirror. criss-crossing the inside of the tube between you and the primary mirror. 

You want to center that image of the open top of the tube so that it will line up to give you a roughly centered image in the open telescope tube.  You do this by carefully loosening and tightening the screws below the primary mirror. If you force the screws you can crack your very expensive mirror.  

Once it looks close to centered, you need to adjust the secondary mirror to line up with the primary.  Look down the focuser tube. Further adjust the collimating screws until the secondary mirror is centered in the focuser tube. If you’ve got it right, your eye will appear in the center of the secondary mirror.
There are 3 screws through the primary
mounting plate which support the 
primary mirror as shown

Now, place the eyepiece in the focuser and test the optics by focusing on a distant object. You may need to fiddle with the collimating screws or the screws on the spyder legs of the secondary mirror. Once adjusted the telescope is ready for viewing. If you haven't mounted, the finder scope, it should be 90 degrees around the tube from the focuser, so that the finder is on top of the telescope tube. 
You'll need to line up the finder scope next.  It's best to do that in daylight. Roughly, line up the tube on a distant object and then look through the eyepiece with the lenses in place. Once you get centered on your target, check the lineup on the finder scope. Adjust the mounting screws so the cross-hairs on the finder line up precisely with your target.
Don't try to do this at night. Stars move and it's too hard to match the finder to the scope when the target is moving.
Be careful when you clean your lenses. Brush dust off the optics with a fine camel's hair brush or blower. Wiping your lenses with your shirt tail or even a hankie will scratch and dull lenses. 
Last Word: 
If you're building your own telescope, spend the bulk of your money on optics - a big mirror, eyepiece lenses and big focuser that will take 1 ¼ inch or 2 inch eyepieces. The larger lenses are way more comfortable for viewing and you can make your own eyepiece lenses. I made mine out of copper pipe and mounted the lenses in a PVC pipe cap slipped over a 2 inch pipe.  Works great and I got the lenses from an optical surplus. I got enough lenses to build 5 eyepieces for $25 and it came with the lens combinations for the various lenses packaged separately.

If you love star-watching, I highly recommend building your own telescope. It is a seriously cool project!

(c) 2015 by Tom King

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