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Newastro

Building an 8.75" Newt - Update. (Long)

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Hi Folk's, me again.

A bit more on the 8.75" Newt, I thought I'd share with you my proposed OTA plans which I guess are maybe as important as the mounting :).

I initially looked into the cost of buying a carbon fibre tube until i realised I'd have to sell my new car to pay for it :shock: and so my second point of call was to talk too my friend who makes fibreglass boats and we came up with a way to make the uniform 10" i.d, 6.5ft long and 4mm thick glass fibre tube :) shown below,

image.jpg

the tube is drilled to accept a 2" dual speed crayford focuser as shown here,

image.jpg]

the primary mirror cell was based on a design found on the web that had been used in a Lourie-Houghton system and was both rugged and simple to construct, the original design was made from aluminium but for my purpose I wanted something lighter to keep the tube balance point nearer the centre of the tube, also I didn't have the resources to fabricate the ring and backplate from aluminium. The cell is shown below and is constructed from scratch using? , yes you guessed it 'fibre glass'.

image.jpg

The mirror is attached to the backplate using silicone (mirror sits on 3 raised points on a radius of 0.4, calculated from the PLOP design software to ensure least mirror deformation) and then the back plate attaches to the tube ring mounting via 3 push/pull sets of screw adjusters.

I guess this rather long post has gone on long enough so for now I wish you all clear skies.

Malcolm.

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Hi,

I think I slipped up with the photos somewhere ( an understatement, damn new technology!), I hope these are better.

image.jpg

image.jpg

Sorry folk's for the confusion.

Malcolm.

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It's not gone on nearly long enough if you ask me. Keep us posted, and I want to know how you make a fibreglass tube properly please.

Captain Chaos

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Last time, I guess I should mention the secondary whilst I'm at it. I wanted to remove the diffraction spikes often seen in images of stars taken with a normal 4 vane spider, so I ordered a curved vane spider from the US and attached it to my secondary as shown (hopefully) below,

image.jpg

I hope that after the assembly of the whole OTA I will have an optical system that was worth the waiting.

Malcolm.

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Hi CC,

After prolonged discussions over a pint :occasion5: we came up with a couple of possible ways of making this glass fibre (GRP) tube, both depended on having a form on which to 'layup' the GRP. I was lucky enough to be around at work when some ducting was being installed in one of the lab areas and there was this 6->7 ft piece of 10" id grey PVC ducting about to be thrown in the skip, why not use this for the tube? - because the wall was only 2.5mm thick and definitely not rigid enough to prevent bending/flexing during use. So we could;

a) Add a few layers of GRP to strengthen the PVC........... wasn't keen on this really as it would give an even smaller tube id for the primary etc but might be the only option if method :) didn't work

:) Use the PVC tube as a form to create a completely new GRP one of 10" id.

Now I'm no GRP expert so I'll try to explain it as it was explained to me.

The tube was made by initially coating the outside of the PVC tube with a layer of releasing agent ( you have to bear in mind that for a 6-7 ft 2 layer tube the resistance against getting the inner one out might be considerable if not impossible ) followed by a black gel coat layer ( to make the internal surface permanently black ) and then applying a couple of layers of GRP matt and resin. Once dry a layer of E-Glass was added ( a linear stranded GF material) to add longitudinal strength and then another couple of layers of GRP matt and resin were applied to give a final outer tube thickness of 4mm.

In order that the tube didn't bend or warp during curing of the resin the tube was Hung vertically from my mates workshop ceiling rafters for 1 month( "let gravity do the work" , unquote).

Now the moment of truth arrives, can we get the inner form tube out??? The guys at my mates workshop were having a sweepstake on how long it would take him to get the inner tube out and how many !@&% words he'd use doing it! ( all guesses gratefully received).

Any way he did get the inner tube out and we drilled the hole for the 2" focuser and the job was finished.

This sounds easy but my mates very good at his job and spent some time arranging for a smooth operation and what to do if things didn't go to plan (e.g. - like hanging me from the rafters for a month for giving him the problem in the first place!).

Hope this gives you some ideas.

Malcolm.

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Hi again folks.

I thought I'd add a photo of the aluminium drive worm and wheels I intend to use to motorise the Newt.

image.jpg

I have a pair of each of these which were made for me as specials, the worm wheels are 8.75" o.d with 360 teeth and a 1" bore for fitting to each shaft, the worm itself is a single start and together they should give me a reduction ratio of 360. I haven't yet decided on the rest of the drive mechanism so any suggestions would be helpful to give me a range of options.

Cheers for now.

Malcolm.

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Hi again folks.

I thought I'd add a photo of the aluminium drive worm and wheels I intend to use to motorise the Newt.

image.jpg

I have a pair of each of these which were made for me as specials, the worm wheels are 8.75" o.d with 360 teeth and a 1" bore for fitting to each shaft, the worm itself is a single start and together they should give me a reduction ratio of 360. I haven't yet decided on the rest of the drive mechanism so any suggestions would be helpful to give me a range of options.

Cheers for now.

Malcolm.

Well Malcolm, you appear to have everything well in hand with your project. I do like the worm and wheel.

I was looking at a Byers one on Astromart a while back, I think it was 12" dia. made by the Master himself. The last one I believe as the man is pretty old now. Anyway, I think the asking price was about $6.000 That isif my memory serves me well, I may have the figure wrong, but it was expensive, that I am certain of.

I only know your final polar shaft rotational speed siderial rate that is, is .000696rpm. A stepper at a rate of 1rpm driving your wheel would give a rate of .00285rpm so you will need to reduce that by roughly a factor of 4. A stepper controller will give plenty of leeway for varying the final rates. Although probably servo motors are a better option.

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Hi Barkis,

I've done a bit more digging around on the subject of the drive for this beast and there seems to be a couple of good bets;

a) There is the Mel Bartels system from the US which was my first choice initially - I believe that Arthur had a complete system up for sale recently but someone beat me to it - d*mn it! :x

or

:D There's the AWR Technology system which is relatively close to me in Deal (Kent).

Either system with the correct gearing should allow me to both motorise the mount and hopefully make it 'goto' with the aid of a laptop and Skymap Pro software. My real problem's going to be attaching the drive mechanism to the actual mount as I no longer have a friendly engineer around to advise me and make any mounting plates etc.

Malcolm.

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Malcolm,

I may have missed an earlier an earlier post on your project including your proposed mount for your Newtonian.

Are you going Dobsonian, or equatorial, and in either case, have you done any preparity drawings?.

Like a few other members, I am always interested in a build project from the ground up.

Ron. :D

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Hi Barkis,

You can find the beginning of this project at

http://stargazerslounge.co.uk/index.php?topic=11596.0

Unfortunately I don't have any drawings for the mount, but I guess it wouldn't be too difficult to measure a few things for work to progress.

Malcolm.

Hi again Malcolm.

I do apologize, I do remember that first post on your project. I asked you about shaft diameter.

Your mount is remarkably similar to one I made for my 12" Newtonian many years ago. Except for the rotational tube arrangement you have incorporated into yours. I have post a scanned image of the mount I copied from N.E. Howard's Handbook for Telescope Making, which I have found to be very helpful to me down the years. It was surprising how little it cost me to produce that mount. Most of the parts I needed were donated by people who expressed great interest in astronomy. Of course I made promises I couldn't keep, like "If I find a comet, I will will give it your name" Needless to say, they are still waiting. :D :D

Ron.

By the way, your mount lends itself well to fitting large diameter setting circles. If you know someone with a dividing head that would do them for you, you could dispense with the goto cost. If you go the circle route, contact me again before proceeding with the RA circle.

image.jpg

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