Finished a little project for a school's telescope my wife teaches at & I'ld like to share it for those of you who may have the same scope, even if you have limited tools & skills.
Scenario: 4.5" scope on an equatorial mount belonging to a school. It had not been used in the years my wife has been at this school, mainly because nobody knows how to use it, & the eq. mount scared anyone who thought of using it.
My solution: Give it a new dobsonian mount.
The 4.5" scope is probably one of the most common first telescopes bought. However, most come with an equatorial mount, which is usually wobbly & intimidating & frustrating to use, especially as a first telescope.
This dob mount is inexpensive & easy to make. All the hardware can be found at a local hardware store, except for the teflon bearings (more on this later), and the larger hardware stores can cut the pieces for you. You can cut the pieces yourself to save money if you have at least a saw. A jigsaw makes it quicker & helps with the altitude bearing cut-out. You can also use a coping saw to cut out the bearing face, which is cheaper than buying or hiring a jigsaw just for this cut.
Because this scope & mount set-up is quite light, teflon is not necessary to use for the bearings. A PVC end cap can be cut up to obtain the bearing material. It will be nearly as smooth in action as using teflon, & much cheaper. Teflon can be obtained through specialist plastic suppliers listed in the yellow pages. 3mm thick sheet with the size cut to 15mm x 15mm is sufficient (you will need 7 such pieces). You can drill the hole through it, but don't forget to counter sink it for the screw head to remain tucked away.
Here I used two sheets of melamine shelving, 445mm x 1200mm. Ply wood of 12mm or 15mm can also be used. Just make sure that the bearing surface in contact with the teflon on the azimuth bearing is textured laminate like on the melamine board. Smooth, shiny laminate does not work quite as well.
For the altitube bearings I used two 100mm PVC flanges screwed onto the scope craddle.
The craddle is oversized to the tude diameter by no more than 1mm either side. This will allow the use of thin foam to line the craddle to hold the tude in place without crushing the scope. It should be firm enough to allow the craddle to side up or down the tude, just, but with some effort. If too loose, well, the tube will be unstable. This tiny amount of play will allow the tube to be balanced in the rocker box, so that it stays put no matter were the tube is pointed. Once the balance is sorted, either tighten the craddle a little more, or insert a small strip of double sided foam tape between the tube & craddle.
I hope this has served to inspire some of you to dust off your 4.5" scopes & give them a new lease on life.
Last edited by mental4astro; 25-08-2009 at 08:48 PM.
Reason: clarification
Don't have time to read your whole entry at the moment, but the photo's are inspiring me to do the same for my 130mm eq mounted scope.
Will read properly when I've got the time.
Thanks for posting that.
Really nice work. That scope should be very user friendly now. It is such a shame that these instruments often get sold with mounts that are often nearly unusable. A mount like yours is so much better for people entering into this hobby.
Thanks for that, a brilliant, simple little project. I have a 114mm skyrover on an equatorial mount hidden in the roof space that I never really got along with. This would be a great use for it, and when my kids are a bit older they would have their first scope
I was so enthused with this project that I have told a friend about it who has a similar scope she will be looking to do something along the same lines in the future
Hi Alex,
I'm currently having a go at making my own dob mount for a 130mm OTA reflector, as inspired by your great job.
I would just like to know - what is the bearing material in the cut out for the flanges?
I want to get it right first off, so that I know what exact size to make the cut outs.
Thanks,
Molly.
I used Teflon here, though in the case of a 4.5", or a 5" like yours, a piece of PVC cut from a flat surface from a PVC pipe cover would do just as well because the scopes are quite light.
For a scope like a 4.5" or 5", if you buy a strip of Teflon 15mm X 75mm, 3mm thick, you can then cut the necessary tabs you will need. I'd suggest you measure, drill and countersink the tabs before you cut them out. Use a hacksaw to cut and a stanley knife to tidy up the burs.
For the altitude bearings, four pieces, 15mm X 12mm is ample. The scope/brace box is so light that the 15lb rule will be difficult to aquire, as the these parts will struggle to top 10lb (4.5kg)! I'd suggest spacing the tabs at 90 degrees to 100 degrees to each other along the arc of the bearing cut-out. The orthodox 70 degrees done on big dobs makes the altitube movement too light and will be too sensitive.
For the azimuth bearing, three pieces 15mm X 15mm is enough.
I'm finalising an article for the iceman to go over to hopefully publish in the Projects and Articles section on this topic.
I haven't used felt pads, someone else may have had used them. I've only seen them used as the bearing members of mirror cells holding the mirror.
I got my Teflon via phone order from a mob called 'Cut to Size Plastics'. I found them through the net. As I just got the strip I wanted, and cut and machined myself the stuff, it was way, way cheaper than getting it done by them or anyone else.
One more point on the felt pads, see what size scope they are being used on. Reason being that the effectiveness of any bearing material is a balance between the weight of the components, materials used, angular separation, and the such. A heavy scope responds different than a light one for the same materials used.
A question for mental4astro (Alexander)
