Back in March I picked up a cheap GSO 8" OTA (f5) to use as a "knockabout" telescope. After a bit of contemplating (and a few beers) in the workshop over a few nights, I decided to play around with converting it to a small Dob.
I am determined not to
(a) spend too much or
(b) over engineer the build. I decided to apply many of the principles outlined in Kriege and Berry's Dobsonian book. It will not be a minimalist-type but a smaller version of the big Dobs. So here it is....
Secondary Cage Rings
The first task was to construct the secondary cage rings. Pretty easy to make really, I've made a few in my time. Like most of the structural timber in this project, these are made from 20mm AA grade hoop pine left over from previous Dob build. Their dimensions are 285mm (OD) and 220mm (ID). Two rings were cut out using a plunge router. Four holes in each were drilled for the cage struts and recesses made using a forstner bit.
Secondary Cage Struts
I decided to make the struts out of 20mm diameter solid aluminium rod, 180mm in length and recessed 10mm into each secondary cage ring. I decided on solid rod instead of tube because I do not have access to the threaded tube inserts to fit inside a tube this small. I chose an 'anodising grade' aluminium rod, making it suitable to have an anodised finish applied at a later date if I wish. A 1/4" hole was drilled in each strut for the attachment of the secondary mirror spider.
Focuser Board
The focuser board was made from laminated layers of 2mm 3-ply plywood. Again this was left over scrap I had laying around. I clamped and glued together four small sheets using the rolled metal tube of the original OTA to create the correct curvature. Then it was trimmed to size and the focuser hole cut.
Secondary Mirror, Holder and Spider
I used the original 2" (50mm) mirror and mirror holder for this project as it functioned well. One issue was one of the spider vanes. A pin holding the threaded fastener to one of the vanes had been pulled too tight at some stage in its previous life, and elongated the hole in the vane. It didn't seem like it would be secure in the long term, so I purchased a Novak secondary spider from BinTel and made a hybrid spider/holder from parts from each one. Then gave it all a re-spray of matt black paint. It came up pretty spiffy.
I gave it all a brief "test fit" to ensure all my dimensions and measurements worked.