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12" Dob Project


simon84
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My dob mount ('tis the mount what makes it a dob) for my SNT (tis a newtonian scope-just to add to the debacle) offers a maintenance position by turning the scope 180 degrees in the mount, i.e. the primary end rests in on the front cross member when I wish to remove the corrector plate, or conversely, the corrector plate end resting on the cross member should I wish to remove the primary cell. Normally, with the end cover removed and no dew shield it will adopt the latter position, thanks to good old gravity. The OTA can be lashed down fast using a bungee chord, should I anticipate any extra-vigourous wrestling to be occuring.......

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As I have a variety of EPs of varying masses, I chose to add tensioning discs to the sides of the mount.

Slew bearing is via about 3 million teflon squares within the base.

http://i174.photobucket.com/albums/w117/yeti-monster/Astro/DSCF6777.jpg

I do find that it requires a fair amount of torque to start motion, which leads to a little frustration, especially whilst attempting to track with a webcam......

Something I am dwelling on for my return to blight-y.

Edited by yeti monster
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Me forget photos, never.....lol.

Looks good Yeti and I want to get some more of those Sky Rover EP's but I cant find the website, not that I could read the website but its got to be worth a try.

I've got the 20mm SWA and it is brilliant, I am really really impressed by it. I just want a really nice low power one now and that will keep me happy with the other EP's I have.

I thought something to add tension would be required but from what I'm reading good balance is the key thing here, I'm adding a cooling fan to the bottom of the scope aswell.

So its all looking fairly good at the moment.

As soon as I have something worth photographing I will get the camera out.

I'm not convinced with the moonlite yet (sorry Earl) after using the dual speed on my Equinox I am thinking of going the SW dual speed route. But we'll see, lets get the thing mounted first.

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As regards balance (and also stability), large bearings such as those in truss tube designs, are excellent as they allow for a far greater range of weight at the eyepiece end because wider bearings allow for a greater variety of balance points. They are also more stable in wind. Mine has no wobble at all during or immediatley after focussing (unlike my tripod-mounted refractor). This is partly effected by the incredibly rigid nature of the truss tube-design, however there is much more work in making it than mounting a metal tube, I am sure. (and more setup time).

I made a light-shroud but found it wasn't worth putting on every observing session as I couldn't see any difference in contrast. Perhaps other stray light causing factors overwhelmed it. (eg light from above the secondary cage). (The shroud is now a hide for observing badgers !)

All this information and more is contained within the truss-tube-builders bible by Kriege and Berry.:- http://www.amazon.co.uk/Dobsonian-Telescope-Practical-Building-Telescopes/dp/0943396557/ref=sr_1_1?ie=UTF8&s=books&qid=1288612598&sr=8-1

Alan

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Thanks Yeti, I'm struggling with that website bigtime....lol. Cant I just send you the money......I'll get there, it looks like the 20m WA that I already have is the widest in that range, I have a 4mm and 7mm Nirvana which is from the UWA range and the price of the biggun in that set is very tempting indeed, just worried about having it shipped to the uk......is there much risk involved do you think?

Thanks for the tips. I was thinking of using 6" diameter bearings but I might go bigger than that now, I will have a look for that book though, sounds like a good read.

For now I'm keeping it as simple as possible and then hopefully I might even try making my own truss dob and use the mirrors from my skywatcher, that would be a great project to have.

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The best bearing material for the Azimuth is EbonyStar laminate with Teflon pads. Telescope Service sells pre cut EbonyStar rings in different sizes that are perfect for the job.

http://www.teleskop-express.de/shop/product_info.php/info/p3875_Ebony-Star-Ring-D-50cm-zum-Selbstbau-von-Gleitlagern.html

John

great link John - nice one! I am going to make a 16" truss dob like John's eventually.

Yeti Monster - I think I read somewhere that Teflon works better under more pressure than less. maybe just adding some weight in the bottom on the mount and on top of the azimuth bearing might help with smoothness (and add even more stability to boot?)

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All looks good, I cant read metric either....lol.

I'm hoping to pick the materials to make the base at the weekend and get the ol' jigsaw out......I dont think I'll be doing the cutting though, my best mate will be helping me with that.

Its all rather exciting and I really want to get it finished asap now that my new EP is on the way.....going to look odd though, black and green EP, red and black focuser, blue tube......hmmmmm. Maybe time for a respray on the tube.

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Definitely have a look at the thread that Rusty Strings did on his dob project Simon, it's one of the best I've seen tbh! I borrowed a few of his ideas to make my own dob mount for my little 200P and I' very pleased how it turned out! Link: http://stargazerslounge.com/diy-astronomer/110537-dob-has-landed.html

I used a Val Doonican LP and three teflon pads for the main bearing and it seems to move with just the right amount of friction needed. For the altitude bearings I used two drain pipe end stops attached to the ota via the original tube rings, which sit in simple 'V' cut outs on the base with teflon pads screwed in. I am going to add a rotating ring to the ota at some point and also some kind of balance system, probably involving magnetic weights!!

Good luck with your project Simon!

Doc

Edited by Space Beagle
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Cheers Doc, blew me away when I saw the quality of the base he made, that really was something else.

I love the LP idea, putting something in there that you truely loathe....lol

I'm still looking at this lazy susan idea, most of them turn freely but I'm looking to see if there are any that are under tension.

I was going to use timber bearings but the PVC end stops do look much nicer.

I've got some magnetic weights which should work really well when I put a heavier EP in the focuser.

Fingers crossed I can do this project justice.

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  • 3 months later...
Just finished one of these.used the ota mount rings and attached round bread boards to it for alt bearings, covered them in kitchen worktop edging (stippled one) tyen teflon strips on the cradle for bearing surface. Used LP and teflon furnitaure moving pads for the azimuth bearings.

I looked for those 'furniture-moving pads' all over the place when making my dob but never found any. Where did you get them ?

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I looked for those 'furniture-moving pads' all over the place when making my dob but never found any. Where did you get them ?

I am just about to embark on building a dob base for my SN10, the pads I bought are called "Magic Sliders" they come in several different shapes. The blurb on the packet says they can carry a load of 1,500 KG :) we will see! Magic Sliders

I am going for the v sape cut out and 6" diameter alt bearings with rectangular 2 x 10 cm rectangular "sliders'. Then eight 2cm dia sliders for the main az bearing, (probably need some more though)

I have tested the sliders and they are very "slidey" so I am hopeful they will do the job.

I will post some pics then I get a round tuit.

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