6 inch dob telescope
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Chuck Fellows
Newt Software
Siebert Optics
Orion Telescopes

Mounting the Focuser and Finder Scope

homemade telescope mirror cell I used the Newt software to calculate the focuser hole location from the face of the mirror. The face of the mirror distance from the bottom of the tube was calculated using the assembled dimension of the mirror cell, mounted to the tailgate with the springs compressed and then opened back up 1/4" to allow for alignment added to the the thickness of the mirror added to the distance the tailgate rear face was from the end of the tube. All I can say is I am very happy to have had the help of an experienced telescope maker on this part.

Look at the photo of the focuser hole cut in the tube. Inside you will see a small circle. With just an 1/8" pilot hole drilled in the focuser hole location I used one of the spokes and a square to mark the precise spot oposite the focuser to assist alignment. I glued a very short piece of dowel rod on that spot to give a permanent referrence point. The dowel will not interfere with light entry as it is outside the baffled area anyway.

The focuser hole must be large enough for the focuser tube to slide into, including the gear rack on the side. The focuser stage was cut very tight with a notch for the rack to keep out as much stray light at possible. Probably not necessary but I'm a nerd about things like that.

I chose to put a stage between the focuser and the tube and use an allen head cap screw and spring arrangement to facilitate adjusting the focuser instead of shimming it during alignment. If you choose to do the same this stage height must be considered in the focal length calculation when determining the location of the focuser hole.

Simply put, I screwed up and caused myself a lot of pain by making the stage fixed and focuser base adjustable instead of the other way around. I don't know what the "proper" way to do this is, but intend to find out before the next scope. I put threaded brass inserts in the stage for mounting the focuser and matched the angle of the holes in the focuser base when doing so. The angle of the screws through the focuser base is the problem. Because they are on an angle, the only time they line up with the inserts is when the base is sitting directly on the stage. Add springs and flat washers on either side of them and the focuser moves up which moves the screw alignment wider. I had to drill the focuser base holes oversized to allow mounting and movement of the base. Once adjusted it has stayed put well but was extremely difficult to install and adjust the first time.

Instead of spending $34.95 plus shipping on Orions EZ Scope I went to Wal-Mart and bought a Daisy BB gun red dot sight for $8.88. A piece of 3/8" x 1/8" aluminum bar stock, 10-32 x 3" machine screws, #10 nuts and lock washers at Lowe's and I had a red dot finder scope for about $15.00 along with the satisfaction of fabricating it myself. I cut three pieces of the bar stock 3 1/4" long and drilled clearance holes for the #10 screws in all three pieces, locating the holes in relation to the various pieces on the scope mount, they are not symetrical to both ends. I then ground a dove tail on one of the aluminum bars to match the mount on the scope. Paint all hardware black and assemble with one of the bars inside the tube, sandwiching the tube wall to stabilize the mount.

The Orion finder has a reostat adjustable brightness control for the red LED where the Daisy scope has a three position switch. One position is bright for daylight use, one is dimmer for night use and the third is the off position. It is still fairly bright but I got used to it quickly. The field of view of the scope allows you to use it with both eyes open, keeping both the dot and your target in sight. With a little practice and a good alignment and it works very well.

The other difference is the scope adjustments. The orion finder has thumb screws for adjustment but on the Daisy you must use a small screw driver.

Finally, I chose the finder scope to focuser mounting as shown in the photos because we are right eye dominant and it seemed logical to sit on the left side of the scope and keep the finder on the side away from the focuser out of the way. Be sure and don't mount the finder scope too close to the tube or you will not have enough room for your head to get down to use the finder. The three inch screws I used seem to be fine but I would not go any shorter.

secondary mirror spider
homemade telescope mirror cell
secondary mirror spider
homemade telescope mirror cell