Just came across this forum after I bought my telescope a week ago. I'm just starting out again after a long absence from astronomy as a childhood hobby. I was in an op-shop when I saw an old Tasco 132T 76mm reflector in a box for $10. It had 6mm, 20mm eyepieces and a 2x Barlow, a T-piece for a camera, and a moon filter. After putting it together in the shop and testing it, checking that the mirror was good etc, I thought I couldn't go wrong for that price. The only catch was that the mount and tripod stand was missing. After googling for different types of mounts I decided that I could make my own, so I went back to the shop and bought it :-)
I had an old camera tripod, and with a few bits of brackets, I had a makeshift stand happening. A bit dodgy, but it worked. Now to search the sky hehe!! Damn, three days of total overcast skies and light rain.
Got really sick of looking at distant trees and buildings. But finally the stars came out. My first look was at venus and jupiter. Now, I hadn't looked through a telescope for 30 years, and even then, it was just a small, cheap refractor, probably 2.5" or thereabouts. I was blown away to see 4 moons of Jupiter again, but better than I ever remember!! And to see the Great nebula in Orion was really great! I'm thoroughly enjoying this little scope!
Now that I've got around the sky a bit I want to get the mount happening. I decided to build a little tabletop Dobson mount, off some of the designs i'd seen here and on other sites for larger telescopes. Living in Tassie here, I had access to some beautiful native timbers, and so I built myself a Dobson mount out of Tasmanian Blackheart Sassafras timber, and polished and oiled it. It looks gorgeous, and is MUCH steadier and smoother than the tripod. A total improvememnt and a pleasure to use! I pivoted the round base on a piece of Tassie Oak board, with some thin nylon sheet circles between the board and the mount for smooth slip when rotating - it works a treat! The tension can be loosened or tightened with a wingnut on the pivot bolt. If anyone is interested in this design, I could take photos and 3-view drawings and upload them here. You can simply scale them up for larger scopes than mine.
Now that I've got that happening, I started thinking about making an equatorial wedge for it and mount the Dobson on it. I searched out the basic designs and working theory for an equatorial mount and I think I understand the principle. I may even put a home made tracking motor in it later!
Here is my question:- given my lattitude for where I live (Hobart, tassie) which is:
Lat: 42 50 34 S
Long: 147 17 18 E
At what angle from level should I raise a wedge surface to sit the Dobson on to be correct for an equatorial mount? How would I find this info out? Or is it not as simple as that? Are there other things I should know? I've read up on the setup procedure for an equatorial mount for the southern hemi, and I think I understand how to do it. I got a rough approximation by trial and error just by inclining my mount and facing the incline roughly south and tracking a star with my hand.
Here is my question:- given my lattitude for where I live (Hobart, tassie) which is:
Lat: 42 50 34 S
Long: 147 17 18 E
At what angle from level should I raise a wedge surface to sit the Dobson on to be correct for an equatorial mount? How would I find this info out? Or is it not as simple as that? Are there other things I should know? I've read up on the setup procedure for an equatorial mount for the southern hemi, and I think I understand how to do it. I got a rough approximation by trial and error just by inclining my mount and facing the incline roughly south and tracking a star with my hand.
Any info would be greatly appreciated.
Cheers, Graham
Hi Graham,
I'm no expert and only a newby, but from what I've learned given your latitude at Hobart I would think that you would need an angle of 47 10' 26" for your wedge (assuming the mount is sitting completely level). If pointed south at this angle, with the scope at 90 degrees in relation to the mount, it should be pointing at the south celestial pole which would be 42 50' 34" above the horizon. A compass to get it pointing in the right direction might also be handy.
I've never thought of using a wedge to turn my dob into an equatorial mount, sounds like a good idea .
I've never thought of using a wedge to turn my dob into an equatorial mount, sounds like a good idea .
Except dobsonian mounts are designed to be operated in a horizontal position. I suggest you experiment a little first. How well do the Alt and Az bearings work at an angle, particularly Az? Will it easily overbalance? Will the tube unit stay in the altitude bearings - could it "pop out"? Etc.
Welcome Graham! Would you please show us some photos of what you have created. That will help us comment.
I had thought about those limitations while I was building mine, based on the problems with the larger ones that I'd seen. The scope body is secured to the mount with pivot bolts, so it doesn't merely rest on the pivots, but is locked in. Fortunately this scope is very light (being only a 3" reflector) so uneven pressure on the alt az bearings is less of an issue here, and the dob base will be secured to the wedge with a bolt, so no danger of popping out there. Some quick indoor tests have shown it to work reasonably smoothly propped up at an angle on a board, but the proof will be in the pudding when actually looking at the stars or something distant.
Thanks for your advice, I'll get some photos happening!
very nice work there mate! I always get excited when people go and do things for themself! even better when they do it on a shoe string budget. I know this being a uni student, and noodles are stapel... (you know your going up market when you get Maggie Pepper Mega Noodles) hehehe.
Once again Congrats and welcome to the fine establishment
Yes, and the "Suimin" noodles-in-a-cup are for only the elite students from wealthy backgrounds
Thanks for the praise! It was great fun building it, and it really wasn't that hard, and it has turned out significantly steadier at high magnification than a tripod would have been, with the nice solid hefty wood base its bolted to!
I think you would get a way with a wedge on this scope because of its extreme lightness. When I was playing with the same idea for my 8" I needed bearings a method to lock the OTA into the cradle so it didn't fall off. As the scopes get bigger the engineering gets exponentially bigger.
An adjustable wedge for this scope woudl be easily set up and great fun.
Thanks for your praise! I am quite rapt in how this mount has turned out. I think that mounting it on a heavy, solid wood board and platform has really helped its stability, its MUCH more steady than a tripod now. And, yes, being such a light scope makes this thing workable. I have found that whenever you want to build something mechanical, you have to over-engineer it by at least 2x if you want it to last. It's one thing to make something that works, the problem I've found is that it's much harder to make something that works AND lasts as well.
Next, I'm going to build a plywood box top that simply goes over the whole unit, like a sewing machine case, and you just clip it on to the base and pick it up by a handle on the top and walk away with it!
I'm also going to try to make a simple tracking motor setup. I wanted to make it out of easy to get, common bits and pieces. I'm thinking of using a $5 Kambrooks 240v timer as a drive motor - one of those ones that you plug into a power point, and then plug a heater or whatever into, and you can set it to switch on or off at a certain time. It has a dial on it that you insert plasic pins to set the on/off time. The dial takes 24 hours to do 1 rotation. I thought I could use the dial to drive a pulley wheel on my dobson mount. Provided it spins in the right direction hehe!! I might be able to rip the motor out and put a 12V motor in it for battery power out in the field, if I can get the motor speed right. Then I can use it for photography as well, since the scope has a T-piece camera adapter.
I think whenever I get a bigger scope (maybe a 10" or even a 12"), I'll look back on this scope with fondness because of how easy and portable it is, and with all these features I've built for it that I won't have on the big one!
hehehe you are doing the great astronomers dream building it from the ground up, and customizing to suite. I like your idea about putting a timer on the side do it like the old days !
As for the 12v timers why dont you get that for a start, saves having to change it all over. you can always get a 240 - 12v set up, ( they are easy to make a bridge rectifyer with a cap to smooth it out. I made one at uni a bit of solder diodes and what not and happy days where had a nice smoothed AC waveform to DC but get a bit of information on it before you build ... 240 @ 10amps can be hair raising! but at least that way you can just take it anywhere and plug it into what ever power source you want.!
I haven't seen any 12V timers around as yet. But that would be the way to go. What I DO have lying around, now that I remember, is a clothes dryer control timer - although it handles 240v, it's basically a mechanical, spring driven timer with a big dial knob that you turn to however many minutes you want to run (up to 3 hours, I think), and then it just ticks away and runs down. I could just mount that under my scope board with a pulley belt around the timer knob - forget about connecting it to any power at all! The timer does about 270 degrees rotation when fully wound, and does this in 3 hours. All I need to do is make the pulley wheel the correct size to make the dobson base turn once in 24 hours. A clockwork tracking motor - now THAT'S old skool hehe!
Easy to calculate! put a 1:1 pully on the timer and another pin mark the string (or what ever you have going around maby even a lakky band) let it run for 5 minutes then you can find out how quick its turning then its just a matter of ratios. youll have a fully self contained dob running on its own . who cares if theres only a 3 hr running time. it takes 2 seconds to get it going again
if i remember a lecture from one of my communications classes for engineering.
"Creativity is not just an idea that is made, it is not a single person, creativity stems from many people often over many meetings to form a single idea/vision that is of the best possible solution"
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building it from the ground up, and customizing to suite. I like your idea about putting a timer on the side do it like the old days ! 
but get a bit of information on it before you build ... 240 @ 10amps can be hair raising! but at least that way you can just take it anywhere and plug it into what ever power source you want.!
