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.
Fitting the Focuser board to the rings
I disassembled the cage and returned back to the secondary rings and routed a 10mm deep recess to accommodate the focuser board. fits good.
The internal distance of the focuser board is 160mm, making a BinTel low-profile 10:0 focuser just fit. The fine adjustment knob clears the inside of the ring by about 7mm. I could have offset the focuser on the focuser board to allow more clearance, but this would have meant moving the secondary mirror, the mirror holder, the spider, and the vane attachment point on the struts.
More work on the struts
Tomorrow with some help of an IIS member, I'll have the solid struts drilled and complete.
Test assemble (again)
As mentioned above, I fitted an ol' faithful BinTel low profile 10:1 focuser, right slack bang in the middle of the focuser board, then put it all together again to find the focuser is in pretty good alignment with the secondary mirror. I also fitted some Bob's Knobs collimation screws to the secondary.
Thanks Mick, I have this vision of getting them anodised, and the 'anodising-grade' rod wasn't all that much more expensive anyway. I know some may be thinking that the solid rod will add weight but in the scheme of things, I am happy with it so far. The secondary cage weighs in at just under 2kg. Fine for me.
Mirror Box
Finished off gluing the mirror box tonight, made from four pieces of 20mm A-A grade Hoop pine to form a box with outer dimensions of 270mm square and 250mm deep. Now waiting for it to dry and then next week, in goes the mirror holder. I am going to reuse the existing mirror holder but make my own collimation screws and locking screws from fasteners I have lying around the workshop. If that doesn't come up satisfactory, I'll buy some Bob's knobs.
I sourced some thin 'kydex' type material today from, of all places, an arts supplies.
I'll be watching this with interest Stephen, I have the same OTA which I'm thinking of doing the same to.
Phil, if you choose to go doen this path, you certainly will be able to refine on my design. I am pretty much copying from Kriege & Berry's book, so it is by no means ultra compact or minimalist. I am finding a lot of projects like this for 8" Newt on the internet. I'll try to post a few links when I get a chance.
Last night, with the help of Trevor H., we drilled the ends of the secondary cage struts and tapped them.
Mirror Box cont.. Today, I made the rear panel for the mirror box, cut out a central hole for the cooling fan, then on the inside of the box, I routed a square hole for the 100mm thick mirror to seat into, making it flush with the inside face of the box. Then fitted the rear panel into the rest of the box, marked the holes for the collimation screws and test-fitted the mirror into the box.
A Quick Test...
I set up the secondary cage and the mirror box onto a flat platform, threw in my trusty TeleVue 25mm Plossl, did a quick collimation, and determined the distance between the to main parts, which is 510mm. Focussed on a distant communications tower perfectly!
Rob, I am using the mirror cell that came with the original OTA. It is similar, if not the almost identical to what is in a current model GSO solid tube Dob. It is a round ring, slightly larger than the mirror, with three rubber mirror clamps on the outer edge. I think it is made from a cast metal material, but I am unsure what type. It does the job and I could not justify building an entire new cell, and it certainly a mirror this small did not warrant a flotation cell.
At present I only have some 5mm x 30mm RH screws to hold it secure in the mirror box. The week I will investigate 3 Bob's Knobs-type collimation screws, 3 locking screws and 3 springs. Also how I am going to mount a low profile power toggle switch for the fan.
Thanks for that. Not sure I would worry about the locking screws. It you install the Bobs Knobs springs they have plenty of strength to hold the 8" mirror without the hassle of using the locking screws, I find them horrible as they actually tend to de-collimate the mirror when used so its another back and forward operation.
Thanks for that. Not sure I would worry about the locking screws. It you install the Bobs Knobs springs they have plenty of strength to hold the 8" mirror without the hassle of using the locking screws, I find them horrible as they actually tend to de-collimate the mirror when used so its another back and forward operation.
I tend to agree with you on the locking screws, a bit of an over kill and yes the to tend to de-collimate when you tension them.
Attached the 4-AA cell battery compartment and a power toggle switch and LED to the rear of the mirror box. All the wiring in recessed into routed channels inside the box. I will remove the parts when it comes time to stain and varnish the timber.
