Thursday, 14 December 2017
New dobbie mount for an existing telescope.
With the horrid run of astro weather we have had here in Sydney, I've had to keep myself busy with other astro projects to keep my hands occupied. So, it became the perfect opportunity to revamp a flawed little scope, and transform it into a brilliant performer!
I’ve had this 130mm Celestron Astromaster scope for a little while. The single biggest problem with it is the poor equatorial mount it comes with. The mount is too wobbly, and really more of a pain in the neck than what it offers. And for beginners, this mount is the single biggest frustration that leads to this and similar scopes being put away and never used again.
The finder bracket on this scope had also broken off. I am not too sorry about this as the original finder it came with was rubbish too. Too close set to the tube made using it difficult, and its design is not easy to use.
So, the solution to this scope to give it a new lease on life is to ditch the eq mount and make a new table top dobbie mount for it, and install a better red dot finder to it.
I went to town on this little scope! I had some fine materials left over from some other DIY projects, so I was able to use the following stuffs:
· * 15mm marine grade plywood
· * Ebony Star laminate
· * Teflon bearing pads
· * Marine grade varnish
· * Stainless steel azimuth pivot bolt and washers
· * And a brand new 20mm red dot finder on a dovetail block.
Many dobbie mounts, including table top designs, do not have a balanced Optical Tube Assembly (OTA). This means that if you switch between a wee little eyepiece to a very heavy eyepiece, the tube become top heavy or tail heavy when you switch back. So these mount have a friction mechanism by which the altitude bearing is tightened, and there by making action of the altitude bearing stiffer so the tube does not drop under a heavy load. Biggest problem with this being the quality of the action of raising and dropping the scope not only becomes harder/stiffer, but it makes tracking a target at high magnification as the action becomes jerky and very difficult to control.
Not with this little scope!
I designed the mount without any friction mechanism, but the OTA is perfectly balanced and the mount allows for the OTA to be loaded with any size of eyepiece, and the scope does not drop or rise – it stays put and the quality of the action not only remains exactly the same all the time, but it is silky smooth ALL the time!
This little scope now is a red hot, stable and very user friendly instrument. I can swap eyepieces and the tube does not shift, even with no eyepiece in the focuser. I can more easily locate targets because of the higher set red dot finder, and I can easily keep track of targets no matter the magnification I am using!