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AdeKing

Monster New 12" Dob (new to me anyway)

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Yesterday I collected a new toy, but am slightly daunted by the size of it.

Its a 12" Dob which has was someone's DIY job.  The tube is 1800mm from end to end and is just less than 14" in diameter with a 12" mirror.  The OTA is a one man lift as it the tube is actually plastic, which is a bit of a relief.

The mirrors are in a worse state than I originally thought with the secondary badly chipped on the end closest to the aperture and two large areas on the primary where the coating has come off.  Holding the mirror up to the light shows many small holes in the coating.

This is going to be a long term project, and I'd be lying if I said that I wasn't more than a little daunted by both the amount of work required and the sheer size of the thing.

More pics on this thread.

Ade

20170506_121843.jpg

New Dobsonian.jpg

Edited by AdeKing
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Ironically enough, I ordered my 12" Dob from FLO exactly 12 months ago today.  Fantastic scope, you'll love it once you've done the restoration.  Looks like a really nice project, best wishes for its speedy and successful completion. ??

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That looks decent :icon_biggrin:

I believe it was made by Dark Star Telescopes. I have a catalogue from the mid-1980's which shows their various dobsonians and they look identical to yours.

 

Edited by John
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4 minutes ago, John said:

That looks decent :icon_biggrin:

I believe it was made by Dark Star Telescopes. I have a catalogue from the mid-1980's which shows their varios dobsonians and they look identical to yours.

 

Ah, that's interesting, I've googled them and you're right the images I've found look just like it.

I assumed that it was a DIY job because it looks a bit Heath Robinson.  The primary cell is mostly wood with quite sharp aluminium holding the primary in place.  The secondary mirror assembly is unfortunately beginning to fall to pieces, so I'm looking at getting a secondary assembly from Teleskop Express at some point.

I'll have a bit more of a google and see what else I can find out.

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In those days some of the dobs produced commercially were a bit "Heath Robinson" !. The USA and Japanese ones (no Chinese or Russian back then) were better engineered but very much more expensive.

In my mid 1980's Dark Star catalogue the 12" F/6 custom dobsonian (which I think yours is) is listed at £495. Today, 30+ years on, you can buy the Revelation 12" dobsonian for about the same price (when it's in stock).

The Chinese in particular have really changed (for the better IMHO) what is available to us now and at what price :icon_biggrin:

Your scope should show some good views though, when it's collimated etc, etc.

 

Edited by John
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They were a company based in Wales if memory serves me and they used David Hinds mirrors.

Have seen a few of their 16 inch dobs about.

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@John I'm looking forward to restoring it, but it will be a long term project.

The aftermarket wheels and makeshift metalwork added to hold them has already been removed.  I've no doubt that they made life easier for the last owner at some point, but as I rarely set up on a patio, they just foul on the lawn.

I'm pretty sure that it is the f/6 version as the tube on its own is as long as I am tall.

Thanks for the information, I now know a lot more about the scope than I did before I posted.

Ade 

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Ade I agree with John this looks like a Dob made by Dark Star. I visited the site in the late 1990s looking for a new scope. The showroom was in a farm building and there was a mixture of Newts and Fracs. Bit hazy on the exact location but somewhere in the Welsh Hills.

Good luck with restoring the Dob.

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I believe Keith Lowe was the proprietor and the Dobsonians were of the "cheap and cheerful", for the time, end of the market. Optics were sourced from wherever available so some will have first class ones. Hope you will be lucky with yours.  :icon_biggrin:

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Welcome to the Dob mob....clear skies!

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@estwing thanks for the welcome.

Here are some images of the mirrors that I took when I dismantled it yesterday.

As you can see the secondary is fairly damaged, with the coating on the primary in need of some attention.  I have cleaned both mirrors and the primary is now less hazy than this image, but is still fairly hazy.

@Peter Drewthanks for the info on the mirrors, here's hoping that I've got a good one.  Any ideas whether there will be any identifying marks on the mirror to give an indication of its origin?

Although I've cleaned it, I haven't had a good look at the back as I was to worried about damaging it.

The scope has a clip on wooden cap at the mirror end, but is missing one for the open end.  I did try putting the cap on the open end, but it throws the balance off and the tube becomes secondary end heavy.  With the cap attached to the mirror end, the tube is perfectly balanced with lightweight eyepieces, but would struggle with my ES 24mm 68 degree or the 14mm Baader Morpheus, but it seems to just about cope with my range of BST Starguiders, but its a close thing.

The focuser is a basic helical affair, not a R & P like I've seen on most of the images of Darkstar telescopes that I've found.

The finder is........interesting.  Its basically the objective of a 50mm binocular attached to a length of plumbing pipe.  The odd looking eyepiece is attached to some rubber bellows which you use to focus the crosshairs and then secure with a small brass screw.  

Many thanks for all of the information provided, this makes the whole scope renovation more interesting.

Ade 

20170506_164455.jpg

20170506_164507.jpg

20170506_164625 (1).jpg

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I wonder if there is a link between the chip out of the bottom of the secondary and the dark mark on the primary near the centre spot which looks sort of the same shape as the missing bit of the secondary :icon_scratch:

You might get some views even with the mirrors in their current state.

Edited by John
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Secondary's sound....primary won't be great but ok on DSOs ...sorry didn't see the chunk missing....give it a go as it is

Edited by estwing
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I agree. It looks like the secondary had dropped and hit the primary where the crescent is. If you see any visual flares etc, it might benefit you to paint the primary and secondary damage with matt black paint. Dies not look great but may help.

I'd just use it and enjoy it after ensuring everything is secure etc. One easy improvement could be to increase the side of the side bearings. Would help with balance. You can always improve it over time. Bear in mind that sometimes the costs of repair/replacement are more than the cost of a good used one.

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@John I'm pretty sure that the damage to the secondary and primary are linked as tge size and shape of the damage to the primary is similar to the secondary. Although the mirror coating is gone there do not seem to be any chips to the glass of the primary.

@estwingapart from the physical damage the secondary looks good except for a duller band running diagonally across the centre.

@Moonshanethanks for the tips regarding the black paint and bearing size. I'm intending to enjoy it as it is to start with and see where we go from there. Having looked at the price of recoating the mirrors and a new secondary assembly, I see what you mean about the cost of replacement parts.....gulp!!  As it was a freebie I don't mind spending some money on it, but I won't be throwing silly money at it.

 

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A friend has looked at the images on the other thread and has told me that from the attached image that the secondary seems to have been fitted upside down as it sticks out into the light path (see the attached).  

This sounds like it fits with the secondary being dropped and why the damage is currently on the top of the secondary rather than at the bottom.

I hadn't noticed that but did find that I had difficulty trying to collimate it at the weekend.  Looking through the Cheshire, it was as though the secondary was sitting too high in the tube and I had difficulty getting a circular view of the secondary through the Cheshire.

Could the secondary being fitted upside down cause that?

Ade

 

 

20170506_121946.jpg

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Certainly could cause collimation issues but not drastic. to correct you'd need to remove the mirror from the stalk and reglue with an appropriate offset. Looks like the previous owner took it off, turned it round but applied the offset the wrong way.

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Agree with all the comments made. I also think that it looks very much like a Dark Star product. I seem to remember that they also started to make their own mirrors later in the history of the company as well as using mirrors from other sources.  It might be interesting to test the mirror before getting it re-coated.

The secondary is installed incorrectly.

Nigel

 

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@Moonshane and @Astrobits thank you for the additional information.

I've learnt loads from this thread.  I have had an 8" Dob for a couple of years but have been so paranoid about wrecking something that I've never done anything except of collimating and swapping a single speed Crayford for a dual speed one.  Not that much more needs to be done really as I got the scope brand new.

However, after messing about with this one, I can see that things are not as intimidating as I thought.

@AstrobitsI intend to test the mirrors to see how they perform before I make a decision about recoating.  A friend of mine has taken a look and thinks that they are actually not all that bad. I'm also going to have a good look at the Primary to see whether there are any identifying marks on it, but I suspect not.

I'm also trying to get some additional information regarding when it was originally purchased.

Many thanks for your help, and I will post updates as the renovation progresses.

Ade

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Have a look at the back of the mirror, if it is inscribed with a number followed by an "A" or a "B" it will have been made by David Hinds. If "A" then you are very lucky and the mirror will be well worth recoating.   :icon_biggrin:

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@Peter Drew thank you for the information, I will pull the mirror out again tonight and have a good look.

Lets hope that I'm not disappointed :happy11:.

Ade

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I just finish to build my home-made dobsonian mount. Let me say that the altitude wheels looks very small, IMHO. Can you replace them?

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@Rick_It thanks for the tip about the Alt bearings.

As it stands the tube is just about balanced with a BST Explorer eyepiece in it, but its right on the limit.  

I will need to put larger Alt bearings onto it (also suggested by @Moonshane), as I'm looking at getting a proper mirror cell for the primary.  I think  proper primary cell will make it back end (primary end) heavy, whereas at the moment its a little front end (secondary end) heavy once an eyepiece is installed.

I've spoken to a chap at Beacon Hill Telescopes, who does still take commissions for making telescopes and associated parts (despite the price list on the website being dated 2010), and I shall be sending a load of measurements across for both a quote for a new Primary Cell and new secondary mirror assembly and quite possibly a new secondary.

This will no doubt need further mods to the tube, as it will change the distances between the mirrors slightly.

Once I've got the new parts fitted, I'll be looking to enlarge the Alt bearings accordingly.

Edited by AdeKing
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For a 12" you could easily make a cell out of decent ply if you can use a router.

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@Moonshane unfortunately I have to admit to being more than a little DIY challenged, my grandfather said that I couldn't even hammer a nail in straight (he was a very good carpenter and was always disappointed by my woodworking abilities).  The only Router I know about I'm afraid is of the wireless variety that handles my network connection :hiding:.

However, I'm not averse to being educated, and googling shows me that I probably have one in storage somewhere with my grandfathers woodworking tools that I inherited.

I'll have a rummage around and see what I can find.

 

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