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Stub Mandrel

New Scope - Darkstar 10" - Restoration Project

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2 hours ago, NGC 1502 said:

I’d probably feel a bit naffed off myself if a freshly coated mirror didn’t look perfect.

Yes, but I appreciate that they range me up to explain the situation.

The mirror was held in place (on the MDF cell) by three generous blobs of silicon.

I've sanded down to remove every trace of the silicon. I've bought a tube of Evo Stik mirror adhesive, is this the best thing for fixing it in place?

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NO. It will not give you two important properties.

1. space between the MDF and mirror to remove the mirror easily next time.

2. prevention of astigmatism by contacting the mirror at only 3 points ( small areas, about 1" dia ) only.

If you have a raw MDF surface then prime it with a thin coating of epoxy and roughen it. Apply three blobs of Silicone ( I use the clear type that mentions use for glass applications ) , enough to give a blob about 1"dia and 3mm thick and place three spacers about 3mm thick on the cell before presenting the mirror to the cell and pressing down to the touch the spacers. Wait 24 hours before removing the spacers to let the Silicone cure.

Job done.

Nigel

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1 hour ago, Astrobits said:

NO. It will not give you two important properties.

1. space between the MDF and mirror to remove the mirror easily next time.

2. prevention of astigmatism by contacting the mirror at only 3 points ( small areas, about 1" dia ) only.

If you have a raw MDF surface then prime it with a thin coating of epoxy and roughen it. Apply three blobs of Silicone ( I use the clear type that mentions use for glass applications ) , enough to give a blob about 1"dia and 3mm thick and place three spacers about 3mm thick on the cell before presenting the mirror to the cell and pressing down to the touch the spacers. Wait 24 hours before removing the spacers to let the Silicone cure.

Thanks Nigel,

That's how I'd expected to do it - I removed the mirror with a guitar string to cut the blobs, and I was planning to use spacers as you suggest. Your priming idea is good, especially as this will key into the surface.

The Evo Stick mirror adhesive is silicon based and is permanently elastic as well as having high adhesive strength so it should work at least as well as ordinary silicone adhesive. You can use it to fix mirrors to ceilings if you are so inclined (sic).

www.farnell.com/datasheets/1489819.pdf

https://www.anglo-adhesives.co.uk/wp-content/uploads/2018/.../mirror_adhesive.pdf

 

 

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The 3mm ( at least ) thickness for the silicon is important to allow the probable difference in expansion and contraction for the glass and cell.  If you get it wrong, then the mirror can be stressed, easy to see with a triangular shape to a defocused star.  The “triangle” can be quite subtle and easily not noticeable......

And yet, back a few decades Astro-systems of Luton glued the mirror to the aluminium using a circular piece of what looked like green snooker table cloth, to make a “sandwich” of mirror/glue/cloth/glue/aluminium - this was over the whole of the back surface of the mirror, it worked very well indeed. That’s probably because the glue didn’t penetrate the thick cloth and so allowed the glass and aluminium to expand and contract independently.  At least that’s how I understand it.

Peter Drew on here could comment I’m sure, because he was one of the top people at Astro-systems. Apologies if my understanding is off the mark......

And Neil, if I were you, I wouldn’t fix your mirror to the ceiling  😁

Ed.

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I'm don't know if the Evo Stick adhesive will do the job. Ordinary mirrors are silvered on the back and this is then overpainted so the Evo Stick is intended to stick to paint. You're welcome to try it but I have used the greenhouse silicone for over 20 years without problems and don't see any reason to try something different the next time I stick a mirror to MDF.

I found priming the MDF was essential as without it the failure was always the silicone to MDF bond.  In the 90's I sent an 8" F/6 Newtonian tube via couriers who proceeded to drop it with such force that the 19mm MDF in the cell broke sending the mirror down the tube, with the remains of the cell still siliconed to it, to hit the secondary :eek: That proved to me that the procedure that I have adopted was good ( apart from the use of couriers that is ).

I have come across the felt option mentioned in Ed's post and I found it difficult to separate the mirror from the cell when needed as the felt blocks up the saw teeth and, being wool, doesn't surrender to wire very easily.

It is also very important to use only three blobs of silicone and not too large, 1" dia for an 8-10" mirror is sufficient. . Use 4 or more, or go too large in area and you are immediately in astigmatism territory. My 16" mirror has been on three blobs of less than 2" dia for at least the last 10-12 years.

Nigel

 

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Agreed Nigel, the felt option is not one I personally would use, the lack of air flow around the back of the mirror would make cooldown longer, and you’re right, when I wanted to remove my Astro-systems mirror for a recoat, it took me ages.

The silicon I’ve successfully used is one specifically labled for building aquariums. That may have been an expensive option, but I’d sooner spend a bit more and get it right.

And I was daft enough to sell that wonderful Astro-systems 8.5” F5 scope, complete with a fabulously engineered alt-az mount.......

Ed.

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1 minute ago, NGC 1502 said:

The silicon I’ve successfully used is one specifically labled for building aquariums.

Sounds like Aquafix silicone....It's very firm when set doesn't set rock hard but it's very firm!

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26 minutes ago, Astrobits said:

I'm don't know if the Evo Stick adhesive will do the job. Ordinary mirrors are silvered on the back and this is then overpainted so the Evo Stick is intended to stick to paint. You're welcome to try it but I have used the greenhouse silicone for over 20 years without problems and don't see any reason to try something different the next time I stick a mirror to MDF.

"It is solvent free and bonds to timber, concrete, brick, glass, plaster etc."

The ingredients are the same silanes as used in other silicone adhesives.

14 minutes ago, NGC 1502 said:

The silicon I’ve successfully used is one specifically labled for building aquariums. That may have been an expensive option, but I’d sooner spend a bit more and get it right. 

It's more expensive than aquarium silicone - and it comes in a smaller tube!

What I like is that this is specifically a flexible adhesive, aquarium silicone may not be as rubbery.

31 minutes ago, Astrobits said:

In the 90's I sent an 8" F/6 Newtonian tube via couriers who proceeded to drop it with such force that the 19mm MDF in the cell broke

The 19mm MDF of the cell had spilt horizontally where the screws go in, probably because the screws were fitted with too-small pilot holes. I filled the cracks with aliphatic adhesive under pressure (Super Phatic - I once repaired a plate with it and it survived a year of use including going through the dishwasher).

To ensure no further split risk I have drilled out 6.5 mm holes, then cross drill 10mm for the same sort of 'barrel nuts' used to assemble MDF and chipboard kitchen furniture. These will hugely reduce the stress on the MDF.

Oddly, the three fixing holes in the tube were very poorly positioned so fitting the cell was distorting it quite noticeable. This was probably why the original focuser had a tick washer under one corner...  Some simple maths and three equi-spaced holes and the mirror cell now sits centrally.

I was advised to move the mirror up by an inch to allow use of a filter wheel. I've achieved the same effect by moving the new focuser down a bit, also I have a low-profile 2" to 1" adaptor so I should have no focus problems.

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The cell that failed for me didn't have any horizontal screw holes and it failed diagonally through the sheet from a surface screw hole. A very difficult way to break MDF.

I expect that Aquarium Silicone will be O.K. I have just looked at my supply, it is Ever Build Everflex 225 and it particularly mentions glass to glass applications. Other products are available😁

Nigel

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Mirrors have arrived!

Secondary looks OK despite an attempted labrador attack!

DSCN1265.thumb.JPG.424b1bc97857439cec9b75fd9492ecb2.JPG

Primary, better than I expected but I can see one large mark south of centre and a few smaller blemishes. This must be compared to the 'before' image!

DSCN1266.thumb.JPG.b024eca9fb63e23b58a9bc2402a401c4.JPG

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I left the mirror in place for two days before removing the spacers and three days before moving it (call me paranoid...) but the scope is now all in one piece.

The marks on the mirror, on close inspection, are wiggly knots of lines, so I am pretty sure it is the equivalent of 'lens fungus' etched into the glass. I hope they will prove cosmetic.

Instead of moving the mirror up, as suggested, I moved the  focuser down by about 3/4" (it needed a much bigger hole so I just placed the top of the new hole at the top of the old one. This meant I needed a new 'stalk' for the secondary. I replaced the rusty 1/4" one with a stainless steel 6mm one which fits fine and secures with the existing brass screw. I made a much more  robust steel clip to replace the twist of wire as the safety mechanism to stop a loose secondary dropping on the primary.

Looking though my makeshift cheshire (film canister with a  hole in it) the secondary almost perfectly matches the mirror. I haven't collimated yet. If it is clear later I will do this on the base, using the laser and follow up with a star test.

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I have the darkstar 12” I’ve had it a few years now, just used it for moon and Jupiter really, I need a finder scope as takes me ages to find anything, base was rotten when I got it, so made another and not that’s rotten, can’t use it now until I get another base sorted, was looking at buying another Dob or goto, but thinking I may as well sort this one out. 

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1 hour ago, Tonyhaz said:

I have the darkstar 12” I’ve had it a few years now, just used it for moon and Jupiter really, I need a finder scope as takes me ages to find anything, base was rotten when I got it, so made another and not that’s rotten, can’t use it now until I get another base sorted, was looking at buying another Dob or goto, but thinking I may as well sort this one out. 

I had to fit a new board to the replace the rotten one at the bottom of the base. I used a surplus cupboard door and replaced the three tiny teflon squares with 25mm teflon furniture gliders.

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I used a lazy Susan on mine work pretty good, id like to make a plastic base so it don’t rot, but not sure where I’d get the plastic thick enough. Yours is looking amazon, I also had same problem with my mirror when had it done, but still works ok. 

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Mirrors look good and in terms of views you won't notice any difference.

First light is fast approaching 

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I've had a sneaky first light:

 

Not much achieved beyond showing my collimation is passable and  the focus range is right! As soon as I got anything in view a cloud popped up.

I've since added a synta finder shoe. I also need to think about counterweights.

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Always cloudy when you get ya scope out lol.

i had counter weight problems as well, I had to use a very big ring spanner to help 😂,  I need a couple of metal brackets to go round mine, had no luck finding any, might have to make some, also I have no finder scope so it very hard work, would my best option be a telrad for a Dob ? 

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35 minutes ago, Tonyhaz said:

Always cloudy when you get ya scope out lol.

i had counter weight problems as well, I had to use a very big ring spanner to help 😂,  I need a couple of metal brackets to go round mine, had no luck finding any, might have to make some, also I have no finder scope so it very hard work, would my best option be a telrad for a Dob ? 

I've seen a magnetic toolrack fitted to a dob allowing weights to be easily positioned, as a set of 3 from toolstation is a tenner (the cheaper screwfix one has bad reviews) taht will happen.

Telrad sounds good. With a possible laser pointed and dovetails for two different finderscopes I shoudl be able to find anything (not!)

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3 hours ago, Stub Mandrel said:

I've since added a synta finder shoe. I also need to think about counterweights.

A magnetic knife rack, seems to be a popular choice for attaching counterweights to.

That is what I'm adding to my 12" Dark Star now that my garage has been tidied and I can access it to finish the renovation I started 18 months ago.

Hopefully it'll see some use this season.

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2 minutes ago, Tonyhaz said:

Good ideas guys, and good to see I’m not alone with a darkstar Dob. 

No you're not alone, mine is in a worse state than @Stub Mandrel as the previous owner had managed to let the secondary fall off and strike the primary and had then refitted the damaged secondary the wrong way around.

I have a new secondary to fit, the secondary spider was knackered with several of the spot welds having failed so I now have an old Orion Optics two-vane spider to fit.  It also needs the hideous helical plumbing pipe focuser replacing with a Crayford.

Then I can test the mirror and see whether its worth re-coating.

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Sounds as bad as mine, the secondary fell onto mine too, I need a new secondary as well and me spider held in with welding wire 😂 not sure if that affects my viewing or not. Only good part is the tube. Need spares to fix mine up

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21 minutes ago, Tonyhaz said:

Sounds as bad as mine, the secondary fell onto mine too, I need a new secondary as well and me spider held in with welding wire 😂 not sure if that affects my viewing or not. Only good part is the tube. Need spares to fix mine up

It sounds like replacing the thin twist of wire stopping my secondary dropping with a thick clip was a very good idea!

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11 minutes ago, Stub Mandrel said:

It sounds like replacing the thin twist of wire stopping my secondary dropping with a thick clip was a very good idea!

I've added some thin aluminium plate with a hole drilled in it between the mirror and the 45 degree plumbing pipe holder.  I'm then attaching the secondary to the aluminium plate with silicone, and then siliconing the aluminium plate to the plumbing pipe holder.  I'm also attaching a length of string to the back of the secondary mirror, threading it through the hole in the ally plate and attaching it securely through the secondary holder to the spider so that if any part of the secondary assembly fails the secondary will be caught by the string and will hang a short way down the tube rather than striking the mirror.

As I've replaced the spider with an old Orion Optics one, I've replaced the secondary adjustment bar with a length of threaded rod with nuts to adjust the mirror up and down.

I'll resurrect my 12" Dob thread in a bit when I've made a start and got something to show for my efforts. 

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