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The Mapstar Scope build


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Hello everyone,

Firstly a little back ground on why I started this build (and the other two).

Having been into astronomy for quite some time the scope I have wasn't exactly what I wanted. Currently I use a  200pds on an HEQ5 mount, which is a great scope but not brilliant for visual. I'd seen quite a few with dobsonian type scope's and star charts so wanted to go down that road.

So with these thoughts along with Crashtestdummy (Rich) I attended a  mirror making seminar by John Nichol. Thoroughly engaged by the idea  and discussing with Rich we decided on a 22" grind and proceeded to order two blanks from Spectraglass. More on the grind in another thread.

Not long after I attended an event, held by Greenwich Birstall, where various societies including our own were promoting astronomy to the general public. So here the story begins way back in November last year.

A guy came to the stall and started to ask about the society and what I was into. After I said I was going to build my own scope and grind my own primary he asked if I'd done it before? thinking I was totally mad!

After talking some more, then came the question "sounds good can you build me one as I have a finished 24" primary and secondary and no one to build it!" A gulp later and always up for a challenge I said yes.

So I was lined up to make my own, the 24" and also a scope for Rich too. After watching so many people on here progress and build their scopes (Alan, Steve, Simon, Dave and others ) I finally can collate everything and here is my build thread.

A lot of reading, help, head scratching and about 1000 cups of tea  and here it is

Hope you all enjoy and feel free to pass comments.

Damian

To start with I made the cell.

It would be aluminium with edge supports at 4 points along the bottom. I wanted the collimation to adjust from the front and instead of adjusting three points just two.

Material was purchased from a local supply company in Sheffield and I set out on a board and cut the pieces.

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1"1/4 square 16gauge box section and 1" 1/4 x 1" solid bar aluminium.

I designed the cell points using PLOP, the mirror details being 600mm dia and 60mm thick F3.97 (huge and 33KG). After working out that it was going to be an 18 point cell all the measurements were then transferred to a lay up board and the aluminium cut to form the basic cell triangle. This took quite a bit of head scratching and tea!!

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To fit the wobble bars I needed to machine out the box section to fit some 1" square aluminium inside, also to drill all the holes to fit the pivot bolts, edge support bolts and collimation bolts which would be spherical bearings at the 3 triangle tips.Many thanks to my machinist friend Dave for the loan of his machine shop to mill, drill and Tap everything accurately.

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Next onto getting it welded up by a firm in Sheffield who did a superb job of keeping the cell aligned and flat.

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Next the wobble bars were machined to the plop dimensions and I did a trial fit with nylon bushing to centralise the bars. The photo below shows the bars fitted with mild steel allen cap 8mm bolts, these would be stainless on the final build. I had also trial fitted the spherical track rod end collimation bolts and also the top mount bracket that the cell would hang off.

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More later 

Damian

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Very impressive Damian. Looking forward to seeing the rest of the thread and hopefully the fruits of your labour at Galloway - not far off now.

Ian

Cheers Ian

Unfortunately this has taken me that long unless I borrow this scope mine isn't going to be ready :sad:  so it will be next years spring Galloway meeting if I can make it :smiley: . Looking forward to the Galloway event again though superb last year

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Best to tell the end user that you require the scope for some quality control activities and need to do a bit of further testing (viewing) with it and it will be delivered to them at the end of November :evil:

If that doesn't work what scope(s) are you taking to Galloway?

Ian

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Good idea although the owner is already testing it so will have to beg but I'm a bit loathe to borrow it which I doubt he'll let me as it might make my 22" seem a bit modest he he

I'll be taking the 200pds on HEQ5 which found so many galaxies up there I was staggered!

Can't wait to get up there again

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Good idea although the owner is already testing it so will have to beg but I'm a bit loathe to borrow it which I doubt he'll let me as it might make my 22" seem a bit modest he he

I'll be taking the 200pds on HEQ5 which found so many galaxies up there I was staggered!

Can't wait to get up there again

any pictures of the completed scope, thats if the new owner doesnt mind

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Looking very impressive so far Damian. Nice to see a different approach. Did you use the Albert Highe book?

Cheers Alan

Yes I have the Albert Highe book which some of the thought's came from and a lot of the calculations too.

The Sumerian scope's also inspired me so I put a few ideas together for my slightly different approach.

I'll post a few more pic's of the cell coming together.

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After the wobble bars were trial fitted I needed the triangles which I'd actually decided on trefoils which looked that bit different. So onto the CAD and I drew them out. Passing them onto the laser cutter to cut from 6mm ally, he also cut in the wobble bar mounting point and the mirror contact holes ready for tapping at 5mm

Here they are just laid on top of the cell wobble bars

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I had decided to attach them to the wobble bars using spherical bearings so it was a trip back to the machine shop as the bearings needed a counter bore at 4mm deep 14mm dia to accept the outer race. The central hole also needed opening out to 11mm to allow the head of the wobble bar mounting screw and some movement.

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The three holes on each arm were also tapped M6 to accept nylon screws for the mirror contact points. The central mirror contact point (of which there is 6 on the first pic) is a slightly different dimension to the two outer contact points as plotted by PLOP.

Heres the counter bore again close up which is actually the underside (wobble bar side) of the trefoil.

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And a pic with the spherical bearing glued in with bearing retainer on the outer race.

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I then mounted them with a turned brass screw onto the wobble bars with a nut as a spacer. The heads of the screws were also turned flush to the top of the ally 

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The photo above shows how freely the trefoils move. I'd also screwed in a nylon screw the wrong way up as a tester

Phew it had now taken almost a month to get to this point and my shares in Yorkshire tea were getting much bigger!!!!!

More soon 

Damian

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Onto the mirror side supports.

First the two posts which would hold the side support bars. So more machining

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Another session with the CAD drawing and laser cutters to get the aluminium edge supports cut out. Here's how they came back still in the aircraft ally I had them cut from 1/2" thick

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You may notice there are 6 the other 4 a different shape are for the two 'smaller' 22" scopes 

Here's one seperated from the material ready for its clean up and machining.

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This took quite a while as I also wanted to lighten them a little. Here they are finished and fitted to the cell with the 4 nylon mirror contact points fitted

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The height of the contact points was set by the Sagitta of the mirror. From the back of the mirror they needed to be 27mm forward so M10 tapped spacers were made to suit and the whole thing and the posts locked onto the cell. The edge supports were then fitted and locked down just tight enough with a nyloc nut to allow movement so the mirror could settle.

Hope you like the pics

Damian 

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The cell was coming together nicely and the last couple of bits to fit were the ring to hold the trefoils in the correct orientation, an adjustment for the mirror edge support nylon, and the nylon mirror contact screws on the trefoils

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I machined the top hanging bracket (first two photos below) and also the side adjustment brackets from 1"1/4 x 1" aluminium bar on a milling machine (fantastic bit of kit every home should have one)

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The top bracket was made so the spherical track rod end bearing would slide in and bolt up. The two adjuster brackets would be mounted on the inside of the mirror box with the 8mm countersunk allen caps fixing from the outside.They would attach to the cell with 10mm stainless threaded rod with knobs on the front of the box for adjustment.

 At this point it was pretty much hope all this works but confidence was high. So another cup of tea  :grin:

The cell was now complete, I'd also been doing the other two cells for the 22" scopes for myself and Rich as time at my friends machine shop was at a premium so got as much done as I could. 

Here they are together on my kitchen table 

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By this time I think it was nearing the end of February 

Next onto making the spider

Damian

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Thanks for that Steve 

I ordered more material to make the spider which was 2" aluminium round bar to make the spider. I wanted to use another spherical bearing at the top so that the secondary would pivot.

So back to the engineering workshop.

A milling machine rotary table was used to mill most of the bits.

First pic is of the spider central boss which the vanes would mount to.

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Next was the collimation plate and bolts 

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Lastly the pivot point at the bottom which would have the secondary mounting plate screwed to it with the spherical bearing in. I modded this several times to save weight.

The pic shows it unfinished needing a bit of fettling 

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With the secondary mounting plate at 110mm  (secondary was 120mm )

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And finally ll together with the vanes which had been cut from 16 gauge galvanised steel

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So next step was cutting the spider vanes to length and painting once I'd made the UTA

Damian

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