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The Mapstar 22" build


mapstar

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37 minutes ago, Physopto said:

Damian did you actually try a friction test on  the formica before you bought it?

I didn't Derek but the author of the book I use has all the testing of laminate and teflon combinations. Ebony star quarry effect was the ideal.

 

I knew what texture I needed, and I took a piece of teflon with me. The guy in the shop obviously thought I was a bit barmy!

 

I spent months looking for the best match it works really well and is very smooth 

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Hi Damian,whats this about more scope`s,great write up,easier said than done but try not to worry to much about employment,you do a dedicated job in whatever you turn your hand too,quality is always in demand.

Take care,

Regards 

Mike

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Just now, Demon Barber said:

Hi Damian,whats this about more scope`s,great write up,easier said than done but try not to worry to much about employment,you do a dedicated job in whatever you turn your hand too,quality is always in demand.

Take care,

Regards 

Mike

Cheers Mike and thanks for taking a look and commenting. Take's nearly as much time to write it all up as make the scope :eek:

Trying to get detail in that people may find useful is difficult, as some are more accomplished tinkerers than others :wink:. Glad you're enjoying it though

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I knew the focal length of the mirror and so it was onto calculating the truss pole length, also adding  6mm above the end of the focuser tube where the longest length eyepiece should come to focus.

The scope would be using a Feathertouch Focuser with the SIPS system. I calculated the truss poles and because they were going to be welded ends I made a frame up from wood to test that the images came to focus.  The weather had other Ideas and it was another 3 weeks before I got a clear enough night to test the assembly. Here's the feather touch before fitting

SDC12737.JPG

The SIPS unit has a specific set up method detailed on the instruction CD that comes with it. It is adjustable in/out on a thread (under the part with Televue on it) by the range of about 1".Having set this thread half way I found I needed to add length to the tune of 45mm between the Lower optical assembly and the UTA before I could focus using my longest length eyepiece 21mm. With the thread being set halfway I still had a range of adjustment once the truss poles were welded up.

With this measurement set I cut the truss poles and had the ends welded on. Here's a couple of the machined ends before welding. The smaller machined diameter would fit inside the tube

Clevis.JPG

I made a jig with the correct hole centres for the M5 bolts to go through. I took them off to the same company in Sheffield (JNC) that welded the cell up.

SDC15604.JPG

I spent a couple of hours then covering them in heat shrink. I bought 41mm flat to shrink onto the 22mm poles. The poles would be joined at the top and bottom. At the top I used bicycle skewers but on all eight poles fixing. Here's how the top fixing looked with the link bar in between. Underneath the two springs are two 8mm diameter spacers these would rest against the inner cup of the top mount copying the way an actual bicycle hub fixes into the forks. It should not rest on the spring and the thickness of the spacer should be smaller (approx 4mm) than the thickness of the top mount (5mm) attached to the Upper Tube Assembly

Trusstop.JPG

The lower would be a triangular plate to locate on the LOA corner mounts. An M5 allen cap bolt on each would provide the pivot point but also allow the truss to be locked up when the scope was finally set up creating a solid assembly. Below is how they looked once all fitted together complete

truss.JPG

I was finally on the finishing straight. There were a couple more things to finish off like the secondary holder, formica gluing for the bearings and the final finish.

 

Edited by mapstar
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41 minutes ago, Uplooker said:

Beautiful to read and observe this thread. Well done big man. You have a new career all planned out already ?

 

9 minutes ago, estwing said:

i agree Ian plenty of folks want big quality dobs building...

Thanks for the comments and compliments guys. I'm still refining bits but it's been another learning curve in many ways.

I am really glad you're all enjoying the read, as I don't want to bore anyone with too much detail. 

Just a few more bits to do and then the shots of the complete item. Gotta think of a good location this time for some arty back drops :headbang:

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Just now, estwing said:

yes mate its been a while....think it was you who had my slice of cake at the last PSP...!looking forward to your first light report, any planned night?,location?...

Admittedly it's already had it Calvin but that will be included at the end 

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27 minutes ago, estwing said:

yes mate its been a while....think it was you who had my slice of cake at the last PSP...!looking forward to your first light report, any planned night?,location?...

What's that I hear you say "Damian eating someone else's last piece of cake". I have never heard such truth!!!!! ?

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28 minutes ago, estwing said:

yes mate its been a while....think it was you who had my slice of cake at the last PSP...!looking forward to your first light report, any planned night?,location?...

Oh no Ian will be on that comment like a rat on a bag of scraps 

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O.k. so moving on from all that light hearted banter I shall continue the thread.

The most troublesome part of the build was trying to keep the Upper Tube Assembly weight down for balance purposes. My secondary weighed 650g on it's own, so having all that weight to start with would cause problems. The original design I had chosen was an aluminium plate and to use silicone but I went off the Idea as it can cause astigmatism in larger secondaries especially when bonded to something with differing thermal properties to that of low expansion glass. So I opted for a design like the Astro systems holders which are considered by many to be the best way of holding a large secondary. Here is Mike Lockwood's thoughts on the design

http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html

Unfortunately my actual minor axis is 131mm (130mm coated area) and Astro systems do not do a holder to fit. With the space required around the glass for expansion and contraction (.5mm) the size was 5.177" and the maximum size the 5" astro systems holder would expand to was 5.05". In metric I was 3mm oversize.

So I set about trying to make a holder myself from .5mm thick steel. I'm no fabricator and the lip proved just too difficult to fold over and keep flat (to be fair it looked like next doors dog had chewed it on:eek:). I also took the drawings to two local fabrication firms but came away with a 9 week wait from one and 7 weeks from the other!:angry7:

I had a thought and contacted one of our fellow SGL colleagues. After a few emails and message exchanges I sent a Wood template of the secondary across country for someone else to also take up the challenge. During the wait I started to also experiment with a different method/material which was a complete first for me. Carbon Fibre

A couple of weeks later and I received a package with the steel secondary holder from the helpful SGL member. It was a quality crafted steel holder and beautifully finished in matt powder coat. There was just one problem it weighed half a kilo. Unfortunately it was going to be too much. Half a kilo at the top would mean I needed to redesign the bearings and rocker or find some other way to reduce weight up top.

I had four attempts with carbon fibre until I struck on a technique to make it work as I wanted. Apologies for all the writing as I know you guys will be wanting more pictures, so here it is not quite perfect cosmetically but it works brilliant

SDC15858.JPG

SDC15859.JPG

Reflecting the SIPS unit in the focuser (orange cover fitted)

SDC15870.JPG

SDC15871.JPG

The secondary sits inside on the flat lip supporting the full edge. The carbon fibre holder weighs a scant 74g and its thermal dynamics are much closer to glass than steel/aluminium. It is also a solid tube and front ring so much stronger than a split tube. My intention is to replace the countersunk allen cap screws with dome head ones but I've not got around to it yet. The dew heater inside is packed behind with foam to stop the secondary moving. 

I could take a little more out of the lip around the edge but the minor axis now measures 128mm so I have lost 1mm around the edge which wouldn't make any difference. All this had taken a long time but progress is never simple on something you've never done before :thumbsup:

I had also been doing a little painting too and in the background on one of the shots the dew heater vane wiring can be seen clipped to the inside of the UTA.

More soon 

 

Edited by mapstar
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Wow looking really nice. I really like the carbon secondary holder, similar to my ones I make.

Have you thought about motorizing the scope. I'm doing that atm with my 20" dob. I'm going down the Onstep road as any other option just costs way too much.

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39 minutes ago, RAC said:

Wow looking really nice. I really like the carbon secondary holder, similar to my ones I make.

Have you thought about motorizing the scope. I'm doing that atm with my 20" dob. I'm going down the Onstep road as any other option just costs way too much.

Thanks Raymond.

I'm not planning on motorising the scope at present although I think if it were to happen it would be the stepper motor and raspberry pie/encoder route? 

I will probably experiment with the carbon fibre a lot more in future builds as I've learnt quite a bit already and would like to work a lot more with the different techniques.

More updates soon 

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