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astrolunartick

20inch Dobsonian Construction

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Fantastic work Pete and a truly nice construction. Nice photos too thanks for sharing them.

That central boss for the spider looks like the one I made for my last scope build

Works really well.

Where did you get the formica from as I prefer the black to the green I have. How do you intend to finish it? Varnish or painted? Or possibly stain?

Damian

Cheers Damian

Ohh nice bit of Aluminium work there mate. I wanted to make it from Aluminium but I haven't the tools for that. 

I got the Formica from Ebay The formica under the rocker box is mat black.. http://www.ebay.co.uk/itm/FORMICA-1200-x-1200-Sheet-of-Matt-Black-Formica-/261896319187?

As for finish. Im a big fan of wood. I really like the pale light birch Plywood and Ash wood look that the scope has now. I dont like the yellowing that most vanishes give so i will try to find a none Yellowing exterior vanish. Either brush on or in a spray can. I dont mind. 

Pete ;-)

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Cheers Damian

Ohh nice bit of Aluminium work there mate. I wanted to make it from Aluminium but I haven't the tools for that.

I got the Formica from Ebay The formica under the rocker box is mat black.. http://www.ebay.co.uk/itm/FORMICA-1200-x-1200-Sheet-of-Matt-Black-Formica-/261896319187?

As for finish. Im a big fan of wood. I really like the pale light birch Plywood and Ash wood look that the scope has now. I dont like the yellowing that most vanishes give so i will try to find a none Yellowing exterior vanish. Either brush on or in a spray can. I dont mind.

Pete ;-)

Great pics of your build. You may want to look at Osmo Oil. It is a liquid wax finish. It can be bought in various colours and clear. I used it on Ash in my kitchen and on the utility bench (Oak). It is easilly cleaned and although not waterproof it allows the wood to breath. It can be easilly re coated at any time. Don't get it on your hands when painting it on as it be comes sticky like a varnish. Once dry it is a very hard wax finish. Takes a few coats but fantastic finish.

Derek

Edited by Physopto
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Thanks, I had three to make and had to borrow some time on my friends milling machine for a couple of days.

I too like the pale birch and as you say it's difficult to find a varnish that doesn't add the yellowing as most say clear but they're not.

Thanks for the ebay link I'll check it out and keep up the good work fella as it's been a great thread up to now

Damian

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Great pics of your build. You may want to look at Osmo Oil. It is a liquid wax finish. It can be bought in various colours and clear. I used it on Ash in my kitchen and on the utility bench (Oak). It is easilly cleaned and although not waterproof it allows the wood to breath. It can be easilly re coated at any time. Don't get it on your hands when painting it on as it be comes sticky like a varnish. Once dry it is a very hard wax finish. Takes a few coats but fantastic finish.

Derek

Cheers Derek i will give that a look. 

Pete

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Pete,

I forgot to say if you do use Osmo oil it is best applied with a lint free rag. Just use by vynil type gloves. A little goes a long way. Doing it this way you can get a finish that just looks like a nice waxed finish. I'll try to put a pic of the finish on for you.

Derek

Edited by Physopto

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Cheers Steve.  

Some things on this build are experimental for me. .Im testing out the pole attachments to the secondary mirror cage.  These semicircular pieces of plywood that the poles are attached to may change. As i'm thinking the poles being attached together at the top may make it difficult to get the pair out of the split blocks as the holes in the split blocks are not parallel, creating a dovetail effect. One solution may be to just loosen the pole from the semicircular piece of plywood. but i'm not to happy doing that because i want to keep the truss as solid as i can. The split blocks are not fixed to the mirror box yet as the poles are not cut to length yet so the angle of the split block will change. 

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I like the slip on quick release mechanism and want to keep that but i may design the semicircular piece of plywood attached to the poles to come apart in some way. Unless somehow the pole come in and out of the split with ease. 

Pete

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Pete,

Out of interest where did you get all the mechanical bits from the cam clamps look well made.

You have certainly put in some careful work.

Derek

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Pete,

Out of interest where did you get all the mechanical bits from the cam clamps look well made.

You have certainly put in some careful work.

Derek

Cheers Derek 

most of those parts are from Ebay my friend. bicycle Seat post Quick release clamps. Im testing those out too. if i have any problems with them i will revert back to Thumb screws or star knobs. 

Pete

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Hi Pete,

 A friend has very similar clamps on his DOB. (Demon Barber). Seems to work very well.  His has the same wood clamping  blocks at the bottom and aluminium end fittings at he top. (Obsession)  I think it gives a rock solid end placement for collimation. Not sure as don't use a DOB.  Damian knows him and has seen his scope.

Derek

Edited by Physopto
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Love the bicycle seat clamps, they look good and should work well. I'll be looking to use these on my build so will be interested on how they perform for you Pete.

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Cheers Derek 

most of those parts are from Ebay my friend. bicycle Seat post Quick release clamps. Im testing those out too. if i have any problems with them i will revert back to Thumb screws or star knobs. 

Pete

Hi Pete,

I'm sure as Derek says they will be fine. As far as I can remember the obsession style dobs have similar mountings at the bottom although they use a pin at the top that goes through the ply ring of the lower cage and then an R clip to secure them at 90 degrees through a hole in the pin. This is then tensioned by a quick release lever in between the truss tubes. It's a good system

A big help in the type of dob your building is the truss tubes don't have to be a finished length straight away and you can play with them a little to get focus. I went the hard route for the scope build I did and the other two will be the same so more head scratching.

There are plenty of others on here who use the split bottom blocks and I can see no reason why the truss poles wont fiddle in so to say. 

Keep up the good work

Damian

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Hello Scopers 

I managed to get the inside of the secondary cage and the mirror box Flocked. this was fun and tricky. I enjoy a challenge. I varnished all the surfaces to be covered with flock to ensure good adhesion. The flocking is very fine and so black. From some angles its difficult to focus on it. Which is really good. There will certainly be no reflections in this scope.  :grin: Difficult to take a photo of. the gussets and braces in the mirror box blend in to the sides. 

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I also got the secondary spider sprayed matt black. Im really chuffed with how this came out. Its far more ridged than any other secondary spider ive made or brought before. and the vanes are super thin at 0.6mm.  

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I had a brainwave while trying to figure a way of keeping the power cables tidy for the fan control and the dew system between the mirror box and the rocker box. For some reason I ended up watching some CNC machines on youtube and realised the cable chains used in cnc would work. I had to make some alterations to the chain. There is a small lug on each link that stops the chain from bending the other way. So i cut away a little of one side of the lugs on each chain, freeing up the movement a little. A combination of that and reversing the chain links at a certain point made it roll and retract in the way i wanted.

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Looking great Pete. 

Where's the dovetails gone on ya rocker box? They look suspiciously like wood screws to me tsk tsk  :police:  :grin:

Kidding mate, great job. :)

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Looking great Pete. 

Where's the dovetails gone on ya rocker box? They look suspiciously like wood screws to me tsk tsk  :police:  :grin:

Kidding mate, great job. :)

hehehe. Hi Steve

I did think about Dovetails for the rocker box. But i want the rocker box to come apart. as im thinking i might go Argo navis and servo cat next year. So it will be easier to work on it,  if it comes apart. I have got some nice Red aluminium anodized screw caps that are going in those screw holes. so should still look sweet.  :grin:

Pete

Edited by astrolunartick
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Great work again Pete and thanks for the photos

The drag chain looks a nice touch too and it's all coming together nicely.

Have you found any clear finishing varnish mentioning doing the inside of the box? Or was it just some you had lying around seeing as it won't be seen?

Damian

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Great work again Pete and thanks for the photos

The drag chain looks a nice touch too and it's all coming together nicely.

Have you found any clear finishing varnish mentioning doing the inside of the box? Or was it just some you had lying around seeing as it won't be seen?

Damian

Hey Damian Cheers mate.

Ive not had a chance to find the varnish yet. i might have a look tomorrow. That was some clear vanish i brought to do the whole scope but its to yellow. fingers crossed i will find some soon. im nearly at that stage of the build. 

Pete

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Didn't even register about the dove tails as it looks great either way Pete. Steve you are too observant! ;)

As Damian says the cable chain also looks amazing, no scrap that, the whole thing looks amazing!

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Hi Pete,

Any reason for off entering the fan, or was it just convenient?

Derek

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Hi Pete,

Any reason for off entering the fan, or was it just convenient?

Derek

The fan looks to be in the centre of the support triangles to me Derek as the horizontal bars aren't spaced equally down the box due to the 120degree spacing of the cell triangle supports if you get what I mean

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Ah! Must be just me then. It's a heck of a job, nicely done.

Derek

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Hello again Scopers. 

Almost finished scope and First light experience.

Ok so Ive push the project almost to the end now. Ive had to lash together my secondary mirror holder for now. Because Im still waiting to get my Lathe over to my new workshop, then i will turn up a former out of hardwood for shaping some sheet aluminium round it to enclose the secondary mirror.  I used Aluminium for the bodge for now and roughly shaped it and wrapped  the edges in electrical tape so it wouldn't damage the secondary mirror. I decided to go with Sikkens Clear satin vanish as I have a lot of experience with that. Its not an external varnish but it is hard waring and as we all know it will never get rained on just some dew. which i will wipe down at the end of each observing session. I got the Heat shrink on all the poles. and i decides to matt black formica the top of the mirror box. 

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I ran a 12v power cable up one of the poles and drilled a hole in the bottom of the connecting split block for the power plug to slip through and then wired in a socket just under the split blocks. I used a large m8 washer as a plate for the socket to mount in.

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All the Power is run off a 12v 10Ah Tracer Deben Lithium polymer battery. Im using a 12v DC Cigarette lighter socket splitter this has 12v sockets and one 5v usb output. The usb isn't needed but may come in handy to charge my phone or something at some time. I ended up taking the splitter and the 12v  plugs apart to remove the red LEDs in them as i was getting a red glow up the scope when pointed up at Zenith. The fan controllers are 2 Voltage regulators i got from Maplins many moons ago for an old scopes fan control. they work well. Up at the mirror cage the power cable running through the pole is plugged in to a Hitecastro 4 channel Dew Controller That i modified slightly by unsoldering the power cable inside and fitting a socket to match the cable for this scope.   

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I spent a day stripping the scope down and sanding it. I have given the scope 2 coats of varnish for now and will give it 2 more once i have tested it properly and when im happy with its Performance. The Vanish did darken the birch plywood and the Ash hardwood a little as i expected but it didnt yellow the wood which im very happy about. I knocked up a dust cover for the mirror box. im thinking i might make it so that it fits on the top of the secondary cage once the scope is set up so to cap the end off while im waiting for the optics to cool and for some protection. I also made some wheel barrow handles. this thing is heavy. I had purchased some Black lycra for the light shroud that i sewed up by hand but ive not taken any photos of that yet. The last thing i needed to do before first light was to cut the poles to there final length. The poles originally came in three 3 meter lengths and three 2 meter lengths so i cut eight 1.5meter lengths for the main poles and what was left was for the secondary cage and the handles for the mirror box.  So the poles being 1.5meter. I estimated  that i would need to cut 145mm of each pole to achieve  the correct focal length. I had already cut of 100mm in my workshop. so once the scope was outside i did a quick test and as i thought could not get the scope to focus  so i cut off 30mm and the result was i could achieve focus but the focuser was racked right in with only about 5-10mm left with most of my eyepieces. so i cut off a further 15mm as i have 50mm travel on the focuser. so im rather chuffed that my original estimate of 145mm was correct. Also in cutting this last amount off the poles brought the scope in to perfect balance. ;-)

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FIRSH LIGHT!!!!!!!!!

My location is rural but i am in between several towns and a small city so light pollution is there but on good night i can make out the milky way. but my horizon is bad up to about 25-30 degrees.

First target had to be Saturn.  despite it being so low down i could make out 6 moons. I could clearly see the Cassini division. and make out some surface features. that i had never seen with my 10" years ago when Saturn was better placed. At times the atmosphere became still and i got some really breathtaking views. So glad i choose Saturn as a first target.  I then swung round to Jupiter and Venus. both were very low. No more than 10 degree above the horizon. Kind of pointless trying, But i had to give it ago. At times when the atmosphere became still i could make out the red spot very clearly and some structure on the surface. this was such a tease. I will have to wait till later in the year to get good views of Jupiter. Venus was very disturbed  it was a nice crescent but no surface detail. 

I then thought i would try something i've never seen before and tried many times with my 10" but always failed.  Which was the Crescent Nebula (NGC6888) I was expecting to need my Oiii filter so i screwed that in to my 24mm ES and  swung scope round to Cygnus, centered the scope with the Telrad on the location of the Nebula and climbed the ladder. To my surprise there it was. straight away . As clear as i had seen the Veil nebula in my 10" maybe more so. i was also surprised buy the size of the Crescent nebula. I then moved the scope over to the Veil nebula. this was gorgeous. lots of structure like a ribbon twisting in space. i followed the veil round to the filamentary nebula this was even more detailed. lots of structure. I then moved on to The Ring Nebula (M57) and the Dumbbell (M27) These were very bright and clear. Ive always loved moving my 10" scope from Albireo through Cygnus to Deneb and beyond. such beautiful starfields. This was breathtaking with the 20" so many stars. little clusters popping out all over the place. 

I was egger to move on to some Galaxies. I went straight for The whirlpool galaxy (M51). I could just make out some spiral structure but it was faint. so was the detail in Bodes galaxy M81. M82 on the other had was bright and clear with structure clearly visible along its stretched out oval shape. with like dark patchy blotches along it. Im now egger to get the scope out to a good dark site and test it out properly. 

THE SCOPE

The scope moves beautiful, It is heavy and you can feel this when you move it but it doesn't stick. It stays were I point it with no back lash at all. It stayed balanced with all my eyepieces. in regards to set up. I was concerned that the poles would be difficult to get in the split blocks being attached at the top. creating a dovetail effect. which did happen  but i quickly resolved that by loosening of the nuts at the top of the poles. This did not effect the rigidity of the scope thankfully. I can have the scope out and set up in about 15 minutes. I find it takes about 45 minutes to 1 hour for the optics to acclimatise to the surroundings with the fans on.   

THE OPTICS

The Optics are beautiful. They perform very well from what i have seen over the last few nights observing. Stars are pin point. I may need a coma corrector at some point due to a 4.2 focal length but as yet im happy with out it for now. John Nichol is a real craftsman and i feel luck to have one of his mirrors.  Everyone that I have showed them to have oohhed and arrred.  I will give some more updates on the performance as i use the scope and when i get it to a dark site. I don't know to much about the construction, grinding, polishing and figuring of Mirrors but ive owned quite a few scopes over the years and these mirrors from John Nichol perform great.  I hope my scope construction performs up to a standard for the mirrors great performance. 

Many thanks for reading my posts 

Pete

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It looks very nice Peter. Great craftsmanship.

Thanks for sharing your build. :)

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It looks very nice Peter. Great craftsmanship.

Thanks for sharing your build. :)

Cheers Steve.  what with the scope being finished and Ive just received a 5mm 110 degree Myriad Eyepiece in the post. so no clear nights for me now.  hahaha

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