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AdeKing

Restoration of a 12" Dob - Advice

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I've just acquired a 12" Dob from the local Astro group.  I'll post images later as I can't resize them properly on my phone.

The scope is of the Heath Robinson ilk with the main tube comprising 14" diameter plastic pipe. The primary mirror cell looks like it's made of wood and the Base is a fairly conventional DIY dob base with plumbing tube for that Alt bearings.

The secondary mirror is chipped around the circumference but the coating looks OK.  The primary looks mostly OK but has a nasty scratch abut 1/3 of the way in at the 9 o clock position as you look down the tube 

The focuser is a fairly agricultural looking helical job but feels reasonably smooth. 

Everything needs a really good clean and the long term resident arachnids need evicting.

I'm viewing this as a long term project to be completed as funds allow.  However, I'm in two minds as to whether I update the scope with some more modern parts or whether to restore it largely as it is.  

Someone has obviously put a lot of time and effort in to the scope and part of me thinks I should try to keep as much of the original as possible.

The tube measures 1800mm end to end so I'm guessing the focal length is maybe 1600mm though I haven't measured it yet.

I'd like to add a Crayford focuser as I have a spare Skywatcher one but would need a suitable spacer to fit the tube.

I have no sentimental attachment to the scope but am interested in views regarding upgrading vs restoring.

The scope was free so as yet has cost nothing. The mirrors will need recoating at some point but I want to see what they're like once cleaned first.

How would you approach this?

 

 

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looks like a very interesting project, as you got it free personally I wouldn't worry about keeping as much of the original as poss unless you know the original builder. It's probably had plenty of use in that format so for me it would be a total rebuild.

I would start with the base, but it all depends on your skills and personal wants.

you can totally clean the mirror off and do some tests on planets and the moon with no coating.

 However having loked at the paint job on the tube it looks quite nice.. but I would go for a truss tube rebuild after changing the base to make it lighter, stiffer and smoother in it's action.

 

Have fun.

 

 

 

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Thanks for replying to the thread.  Things have moved on a little on another thread (https://stargazerslounge.com/topic/292817-monster-new-12-dob-new-to-me-anyway/), and I have since discovered that the telescope was not a DIY job in someone's shed (well at least not the last owner) but that the scope was made commercially by Dark Star telescopes sometime in the 1980s or 1990s, albeit made in a barn near Market Drayton.

To start with, I'm looking at improving the primary and secondary mirror assemblies and replacing the agricultural focuser with a modern Crayford of which I have a couple of spares laying around.

I've just been offered a replacement secondary mirror of a suitable size which is in good condition (thanks @andrew s).

I'm intending to test the primary mirror before getting it recoated, as the mirror has been cleaned and is still very hazy. The mirror could be a David Hinds mirror (if I'm really lucky).  I'm not so worried about the damage to the primary now as its pretty close to, if not within the shadow of the secondary, so although it looks bad, I don't think it will cause too much of a problem.

After reading other DIY threads by @Moonshane and @mapstar, I would really like to do what you've suggested and convert the scope to a Truss Tube Dob for the reasons you list, but in the meantime I'll be happy with a useable 12" Dob with refurbished mirrors.

The base needs some work to make things balance properly and I will probably make a new base with larger bearings.

Edited by AdeKing
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Just picked up on this and what a lovely project.

You sound like you are heading down the right tracks to me woth getting a working scope with whats already there.

Hopefully a hinds mirror in there but no way of telling. Fingers crossed the testing will go well. Star testing will tell you lots and this can be performed with a poor coating. Plenty of literature on the net about it Mike Lockwoods site is a good one.

I'll keep an eye on the thread and chip in if I can 

Damian 

 

 

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3 hours ago, mapstar said:

Just picked up on this and what a lovely project.

You sound like you are heading down the right tracks to me woth getting a working scope with whats already there.

Hopefully a hinds mirror in there but no way of telling. Fingers crossed the testing will go well. Star testing will tell you lots and this can be performed with a poor coating. Plenty of literature on the net about it Mike Lockwoods site is a good one.

I'll keep an eye on the thread and chip in if I can 

Damian 

 

 

Hi Damian,

When I picked the scope up, I was completely daunted by the task that lay ahead, but after reading other self-build threads, yours included, I've been inspired to embrace the project and do things myself rather than pay someone to do them for me.

I thoroughly enjoyed reading your thread about the 16" build and am incredibly impressed by the build process and the incredible end result.  If I can achieve something only 10% as good I'll be very happy.

I'm currently sorting out tools and raiding the carpentry toolkit that I inherited from my grandfather, though for the life of me, I cannot find the electric router that he had.  Its in the barn somewhere, but its a big barn and the $64,000 question is where exactly?

I'm sure that there will be plenty of questions along the way, but I'm looking forward to the ride and am eagerly awaiting the arrival of the secondary mirror so that I can at least get the scope collimated as it is.

Ade.

Edited by AdeKing
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hope it goes well buddy....any probs just ask..

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There is plenty of help here so you should be fine Ade and good on you that your having a go at it.

14 hours ago, AdeKing said:

I thoroughly enjoyed reading your thread about the 16" build

I just hope my scope hasn't shrunk as I've not used it for a while :eek:

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Hi,

Do you have, or do you know someone who has a 3D-printer..?
You could print a Crayford focuser, If you're interested I can send you the files of my designs. I have two 2" focusers I designed myself. Well printed and assembled they work VERY smooth...

btw, a friend of mine has exactly the same scope, it has superb optics. To speed up cooling of the optics we removed the bubble wrap from under the primary mirror. It is  a very basic but a very good scope. Good luck with the makeover...!

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 Hi AdeKing

Only just seen the thread , Project looks great! and having built my own 14" Dob its really not as hard as you first think, and once its up and running i just added new things and made better parts as time went by.

Mine may not look great but its left outside under a tarp, and i use it 80% more than any of my other scopes (C9.25/MN190/MN152) as with these 100deg or 82deg eps out these days its easy to use and gives great views.

As Chriske said above 3d printers now make the process of making one off parts very easy, wish i had one when i built mine 8 years ago,  I also added a Crayford focuser! and now having a 3D printer i made the base plate myself

for the MN152.

One of the best mods i made was adding black velvet flocking inside the tube , used spray carpet glue and its held for 8 years, makes a big difference.

Added a pic of my scope as its much the same Plastic duct tubing ect ect, and a pic of the test focusing plate 3d printed and designed by myself (saved £120)  

I wish you all the best mate and keep posting 

 

592065e1ea9ce_dob1.JPG.61cd19ca139e81a3db397f607e29d463.JPG

 

592066d0e0c7e_Dob3.JPG.94f585c15ad3e5f554b6a252e61e2a34.JPG

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On Thursday, May 18, 2017 at 14:29, Chriske said:

Hi,

Do you have, or do you know someone who has a 3D-printer..?
You could print a Crayford focuser, If you're interested I can send you the files of my designs. I have two 2" focusers I designed myself. Well printed and assembled they work VERY smooth...

btw, a friend of mine has exactly the same scope, it has superb optics. To speed up cooling of the optics we removed the bubble wrap from under the primary mirror. It is  a very basic but a very good scope. Good luck with the makeover...!

I'm very interested in you'd focuser design and though I don't have a 3d printer myself I'm looking into where I can get 3d printing done locally.

It's good to hear the these scopes had good optics.  I'm really looking forward to progressing with the renovation.

 

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9 hours ago, AdeKing said:

I'm very interested in you'd focuser design and though I don't have a 3d printer myself I'm looking into where I can get 3d printing done locally.

It's good to hear the these scopes had good optics.  I'm really looking forward to progressing with the renovation.

 

Hi,

Maybe I should start a new thread about my focusers(I don't want to hijack your thread)

 

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@p14b thanks for posting the image of your scope, I absolutely love the marbled black and silver paint job.

I can now see the benefits of a 3d printer looking at the printed Base plate for your focuser.

I will keep posting, but as I don't have a lot of time at the moment posts will be relatively infrequent.

Really looking forward to working on this though.

Ade

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@AdeKing

Ade, I had some bits 3D printed by an operation I found on 3DHubs.com

The first few times I used them, I had the design from Thingiverse. However, subsequently I needed someone to turn my ideas into a design before printing. 

For a few extra shekels the lady who did the first print jobs was happy to sit down and turn my sketches into the correct designs to be printed. 

If you don't know anyone locally you might find someone via 3DHubs.com But I can vouch for Semra at PrintHub London. 

She and I have never met. We've done everything by email and she's posted stuff to me in Hertfordshire with no issues...

Simon

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Hi

These telescopes have a Grade A David hines Mirror in them and are well worth doing up. I restored a 8.25 inch a few years ago,  How bad is the secondary mirror?

Jim

 

 

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@James Vincent the coating on the secondary is not too bad but it's badly chipped and is an elliptical shape with a definite flat instead of a curve on the end of the secondary closest to the Primary.

The primary needs recoating and has some damage, but the worst damage is pretty close to the shadow of the central obstruction if not within the shadow area.

The primary has a small number etched into the back but no indication  of a maker and lacks the A or B after the code that I've been led to believe the David Hinds mirrors had.

Will carry out some testing to see how good it is before recoating.

Edited by AdeKing
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@Racey thanks for the info on 3d printing, I'll look into this soon as I think I'll need an adaptor making so that my Skywatcher Crayford will fit the 12" scope as all I have is the focuser with no adaptor plate.

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I've been acquiring various materials to make some mods to this scope for the last few weeks and have obtained some thick walled HDPE pipe which I'm looking to use for the secondary mirror holder.  The original secondary holder uses 63mm OD plumbing pipe with the wall thickness being about 1.5mm, which seems a bit on the thin side to me.  The pipe offcut that I have is 63mm OD and 50mm ID so the wall thickness is approximately 6.5mm, which I figured would give more area of adhesion when the secondary is attached.

However, my next question regards the central hole in the pipe, is it necessary to fill the void with anything?  The original holder did not have anything inside the hole, but I've seen various designs which either use a solid secondary holder or pack the void with cotton wool or similar.  Could anyone with experience of making your own secondary holder give any pointers?

Next up is a question regarding the spider assembly.  The original spider simply had 6mm nuts spot welded or soldered to the ends of the vanes, but one of the joints has failed and the others are showing signs of cracking, so I'm looking for an alternative.  I have a cheap 130p that I got from Astroboot for about £25 which I've never been 100% happy with and I'm willing to cannibalise it for the spider tension attachments that pass through the tube and also the centre part of the spider.  Given that the original spider is all spot welded, I was looking at making new vanes to fit, but am not 100% sure what thickness or indeed what metal to use.

I have seen aluminium kick plates for doors which are large enough and are 0.5mm thick, and have also found stainless steel and mild steel in the same thickness.  Given that the vanes will need bending to attach the the central part of the cannibalised skywatcher spider assembly, I figured that aluminium might be easier to work with, but then I wondered whether the Ally would be less forgiving with vibrations than steel.  

I centainly know from riding steel and aluminium framed bikes that the aluminium frames give a harsher ride than the steel frames as they don't seem to absorb the bumps and vibrations as well as the steel, I know its a completely different application but will this be true with the scope, or am I over thinking it a bit.

Another idea was to use the whole secondary spider and holder assembly in the 12" scope, but I'm not sure that the secondary holder will be big enough.  I haven't measured the size of the holders on my Skywatcher scopes, but to my eye they all look a similar size, and I have 130, 150 and 200mm tubes.  Does anyone out there have a 12" Skywatcher scope that they can measure the size of the holder that the mirror is stuck to so that I can compare?

Ade

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Don't trust gluing to HDPE unless you have checked the bond strength with your chosen glue.  PVC plumbing pipe is much easier to bond to, Polyolefins ( LDPE, LLDPE HDPE ) are notoriously difficult to bond with adhesives. Personally, I would never trust an adhesive bond to polythenes, and I have spent a number of years in Plastics research.

With a wall thickness of 6mm you could screw an aluminium plate to the angled end of your HDPE pipe and glue to that.

Nigel

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Thanks for the warning about the HDPE pipe @Astrobits I had no idea that gluing to HDPE was a problem, I was actually going to use silicone sealant, but may just not use the HDPE, as I really don't want an incident with the secondary mirror very kindly donated to me by @andrew s.

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Hi Ade

After today i have a few days off, if you would like me to Design and 3D-Print you a focuser adapter ? id be happy to help if i can.

As long as you cover postage of the item and materials! my time is free.

But you need to get in touch with me ASAP, some pictures of the actual focuser to be used would be needed as well as some measurements.

If your interested. (i'l be back home about 4pm today)

Thx Paul

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Hi Ade,

Looks like a lovely project you have found yourself.

After close looking at the pics of the secondary it looks as if the flat is rotated 180 from how it should be fitted as this will mean the flat sides of the secondary will then be parallel to the main tube. This would suggest that in its lifetime someone has had the secondary off and refitted it upside down which gives it a huge offset .

Others will be able to check my thinking on this though aand give their thoughts.

Cheers

 

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1 hour ago, p14b said:

Hi Ade

After today i have a few days off, if you would like me to Design and 3D-Print you a focuser adapter ? id be happy to help if i can.

As long as you cover postage of the item and materials! my time is free.

But you need to get in touch with me ASAP, some pictures of the actual focuser to be used would be needed as well as some measurements.

If your interested. (i'l be back home about 4pm today)

Thx Paul

Hi Paul,

Thats a very kind offer, and I've already sent you a PM.

Regards,

Ade

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@astronymonkey you are correct, the secondary had been re-fitted the wrong way around, and was 180 degrees out as you pointed out.

I have been kindly donated a secondary mirror by @andrew s which is due to be fitted shortly, but I haven'd yet had time to remove all of the previous glue from the secondary holder.  As the holder is hollow, I'm also toying with the idea of sticking an aluminium plate to the secondary holder so that I can use three larger contact areas to attach the secondary to the holder.

Given that the secondary appears to have previously parted company with the the holder, I'll also be employing a fall-arrest system so that if the secondary becomes un-stuck then it doesn't fall and hit the primary.

Ade     

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