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A 24-incher - and be quick! Cobbling up an f3.4 in the depths of the shed

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#1 Aperturefever

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Posted 29 July 2019 - 10:12 PM

I guess this all began about six years ago when I happened upon a 24" blank and decided it was a good opportunity to build the big, fast scope I had been dreaming of since I was a starry-eyed kid chasing Halley's Comet with my trusty 3" Tasco reflector. You'll forgive me if I don't go into the mirror-making process: such threads tend to end in very animated discussions, but I will say one thing - if you want to bounce off someone during the process, just pick one person, preferably with a similar sense of humour, and stick with them.

Suffice to say, after working around kids and plain old working, the big glass soup bowl is ready for some star testing, so it's time to bang up the scope.

I've always like Webster telescopes - they just seem made to observe with - so this is loosely based on that kind of design. Bob Schilling talked me out of carpeting the rocker box, so that will be a little different. The clamping system will be Webster clamps, but the mirror cell and spider are creations of John Pratte. Mirror support was crucial and I was very lucky John helped my out in this regard.

So it was up to me and the old man to get the woodworking done with some wise counsel from Peter Read of SDM Telescopes.

And it all began with the UTA as I needed to figure the weights to come up with a depth for the mirror box. A couple of hoops a few bits of straight plywood later, and I was in business.

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#2 Aperturefever

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Posted 29 July 2019 - 10:17 PM

I started by attaching the brace woodwork to my focuser board, and accompanying boards that will carry the Telrad and 80mm finder. This shot is pre-sanding: as well as a clean-up, the braces had to be shaped to match the inside curve of the UTA hoops. The focuser will be the bright red lightweight Feathertouch that is on my 16" at the moment. A SIPS system sure would be nice, but the Barn Science Fund will only stretch so far, and I am sure this will all work fine in any case.

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#3 TOMDEY

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Posted 29 July 2019 - 10:23 PM

So far... so good!    Tom



#4 Aperturefever

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Posted 29 July 2019 - 10:52 PM

Thanks Tom!

So in a big, fast scope alignment is everything. One of the first things I did was clamp the UTA rings together and mark and label everything so the struts would align and the spider would sit precisely with no twist. One of the things I have noticed in this project is how I spend about 90 percent of the time thinking things through, planning and checking maths, and only 10 percent is the fun stuff, like making sawdust and swearing at power tools!

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#5 Aperturefever

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Posted 29 July 2019 - 10:57 PM

This was the first mock-up. Adjustments will be made to put the focuser up a little higher than three o'clock for better ergonomics, but again, I find myself checking a rechecking everything as I go along. Honestly, you wouldn't think a mirror swinging in a big box could get so complicated ...

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#6 Aperturefever

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Posted 29 July 2019 - 11:03 PM

I used these fasteners on my UTA pole struts. The best thing to do is use the drill press to drop these in vertically into the poles. Trust me; once these are in ... they're in. Forever. And if you get things a little on the crooked side, you're cactus. But they are lightweight, simple, and a fantastic way of bolting things together. A steadying beer and everything went according to plan.

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#7 Ian Robinson

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Posted 29 July 2019 - 11:15 PM

I used these fasteners on my UTA pole struts. The best thing to do is use the drill press to drop these in vertically into the poles. Trust me; once these are in ... they're in. Forever. And if you get things a little on the crooked side, you're cactus. But they are lightweight, simple, and a fantastic way of bolting things together. A steadying beer and everything went according to plan.


Do you have a link to those in tube fasteners ? https://furniturelev...r-round-square/ these ?

Edited by Ian Robinson, 29 July 2019 - 11:18 PM.


#8 PirateMike

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Posted 30 July 2019 - 01:01 AM

One of the things I have noticed in this project is how I spend about 90 percent of the time thinking things through, planning and checking maths, and only 10 percent is the fun stuff, like making sawdust and swearing at power tools!

You will thanks yourself for wasting "90%" of your time. Believe me!

 

 

Miguel   8-)

 

.


Edited by PirateMike, 30 July 2019 - 01:04 AM.

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#9 macdonjh

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Posted 30 July 2019 - 08:12 AM

You will thanks yourself for wasting "90%" of your time. Believe me!

 

 

Miguel   8-)

 

.

Inventing (or building successfully) is 10% inspiration and 90% perspiration.  I think THOMASDEY said that...



#10 Aperturefever

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Posted 30 July 2019 - 07:06 PM

I think the 90 percent perspiration is nervous sweat every time I cut some wood or drill a hole!

 

Ian - I don't have a link and have had these things sitting around for ages, but the link you provided shows something awfully similar and would be worth a go I reckon.


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#11 Aperturefever

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Posted 30 July 2019 - 07:16 PM

Happy days ... the UTA looks about right for the moment. I have cut the 1mm plywood skin and can now weight everything up to get some calculations done for the mirror box. For the moment I am not going to cut my focuser hole (more maths!) or drill the boards in position. I'd really like to mock everything up with truss tubes in place and just make sure I have the focuser exactly where I want it. The secondary holder is screwed up tight against the spider to reduce any potential for sag, as per John Pratte and Mike Lockwood's recommendations. I will be following Mike's advice on mounting the secondary to the letter - this features on his website. He does this stuff for a living; I don't!

I am hoping he gets around to writing up cooling strategies sometime soon, as this is about the only part of the scope I haven't yet planned.

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#12 Aperturefever

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Posted 30 July 2019 - 07:24 PM

Out of interest, here is the unboxing of my Antares optics secondary. I was nervous as hell about the thing coming halfway around the planet, but there was no problems at all. I think I could have thrown the box at the wall and it still would have been fine. Not that I would have tried! So now I have a secondary bigger than my old Tasco primary lol.gif

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#13 Aperturefever

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Posted 30 July 2019 - 07:24 PM

The security of cardboard! Lots of cardboard!

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#14 Aperturefever

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Posted 30 July 2019 - 07:25 PM

Almost there ...

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#15 Aperturefever

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Posted 30 July 2019 - 07:27 PM

And there it is. I checked it when it arrived and I am not going to tempt fate by unwrapping it again until I need to! It came with an optical report ... I prefer looking at stars to optical reports ... suffice to say it's flatter than a lizard on his last legs!

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#16 stargazer193857

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Posted 30 July 2019 - 11:55 PM

If you can move a 24" mirror around easily, even a 1" thick one, you are probably a lot stronger than me. I can move one, but I would not look forward to doing so often. Hopefully I can happen upon your star party and just look through the eyepiece after you have it all set up.
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#17 PirateMike

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Posted 31 July 2019 - 12:01 AM

Very Nice. I have always wanted to build my own telescope, I still may do it, but I will wait to see how yours turns out first.

 

I could imagine that If something does not work out exactly as planned, you just need to send it back to the manufacturer for repair!  Free shipping both ways of course. lol.gif

 

 

Miguel   8-)

 

.


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#18 Aperturefever

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Posted 31 July 2019 - 10:29 PM

I can only blame the builder, Miguel lol.gif

 

And a 24 mirror is not the easiest to move, especially lifting it in and out of a test stand, that's for sure. But this is my backyard - I only need some wheelbarrow handles and pneumatic tyres and it can trundle the few metres out of the barn. I could build a roll-off roof observatory, but a roll-out scope seems easier ... and the bar fridge is already in the barn! Maybe I will take it to Coolah next year for a spot of observing ... apparently Allan can throw around a 24 incher like it's a toy as he loves wrestling the 32 ... jawdrop.gif

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#19 Aperturefever

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Posted 03 August 2019 - 05:02 AM

So after weighing up my UTA assembly, clamps, focuser, Telrad, 80mm finder, Paracorr II, average eyepiece weight (13 Ethos!) and averaging out pole weights - and then crunching some maths - we came up with a shade under nine inches deep for the mirror box, allowing for 6.5mm ply on the front of the box. The other consideration was the mirror cell and the fact that it has a tailgate system that allows you easy access to the mirror. This was something that I wanted as a (very) amateur mirror maker. I made up some cardboard templates to check my maths, added the Obsession side bearings and calculated the mirror box swing. I needed to scope everything out to get a bunch of wood cut at once. This was one of the most brainpower intensive parts of the exercise but I really needed the inch or so clearance to get the Stellarcat system in place.

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#20 Aperturefever

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Posted 03 August 2019 - 05:12 AM

In the ageing sawdust of my dad's cluttered workshed, we prised a floor space among well-worn and well-loved tools and held our breath as we pieced together the mirror box to check for fit ,,, and bad calculations! With sighs of relief we saw that we were in the market for some decent wood glue ...

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#21 Aperturefever

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Posted 03 August 2019 - 05:17 AM

Outside in the fresh air and wide open spaces where a young rocker box can grow we breathed another sigh of relief ... I know the corner looks a little wonky in this shot but it's actually fine, I was more concerned with preliminary fit and not having wood topple over and dinging. Dings are far better earned clambering around in the dark, scrabbling for eyepieces and searching for faint fuzzies.

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#22 Aperturefever

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Posted 12 August 2019 - 05:44 AM

A bit of work done ... after all that thinking and planning, things have built up speed. So let me take it back a step in my mission engineering - I know I've posted some pics of the raw cut rocker box and mirror box, but I will say there was an awful lot of work done to get it right. The size of the mirror box went all the way back to the concept stage and design with the Webster clamps, and this measurement was then passed along to John Pratte for the creation of the mirror cell.

 

From there, as mentioned, we figured out all the weight to get the depth of the mirror box. But because I was getting my dad to cut a bunch of stuff at once, and a ways down the track from me, I had to sort out the rocker box as well.

 

Of course I wanted it as low profile as possible, but the swing of the mirror box had to clear the bottom bolt for Stellarcat considerations. The front board of the rocker box wasn't so critical, as I knew I would put cut-outs in to allow the bottom trusses to sit horizontally. That could be done in time. But the rest? I did the maths, but I wanted some tangible proof of my results, and this involved physically scribing out arcs, and eventually, making cardboard templates to convince myself I had it right.

 

And I was convinced, eventually ... sort of. What could possibly go wrong?! I told the missus excitedly that my jumbled of cardboard was stating to look like a telescope. She looked at me sympathetically and changed the subject! Go figure!

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Edited by Aperturefever, 12 August 2019 - 06:43 AM.

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#23 Aperturefever

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Posted 12 August 2019 - 06:02 AM

My concept of operations dictates structural integrity above all else, including ease of portability. So when it comes to wood joins, I just want what works best within my abilities. Dovetails are beautiful, but beyond my scope of fabrication, and possibly not necessarily the strongest option in the modern age (as I casually duck for cover!)

 

Modern adhesives offer incredible bonding capabilities. I used a two-part metal epoxy in the Lightbridge II build, and it is still going strong. So I opted for simple butt joints using this pictured woodworking adhesive. The strength rating is higher than wood; in other words, the wood will fail before the glue does. I have a mate who can vouch for this. So my process was simple: check for fit; sand; drill pilot holes for 50mm (2") 8G stainless steel self-tappers; glue; screw; clamp; retension the screws. Sweat nervously. Drink beer (or maybe I started with this bit?).

 

Again: what could possibly go wrong?!

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#24 Aperturefever

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Posted 12 August 2019 - 06:50 AM

Don't panic! This wobbly fit slid about as I took the shot, and I haven't accidentally built the thing backwards lol.gif but the point I was getting at was making sure of fit ... John Pratte kindly included a dummy mirror. I needed to check things. Again.

 

There's nothing like a big, fast, Dobsonian build to bring out your inner obsessive compulsive!

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#25 Aperturefever

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Posted 13 August 2019 - 05:20 AM

You know things are getting serious when it's time to crack open the glue. And given my glue is stronger than the wood, there is no turning back. I had a worried lump in the back of my throat when I started, but it had dissipated a few hours later and was replaced by sore knees from clambering around with numerous plastic right angles and set squares. Making an entirely square box seems such a simple task ... but it requires a great degree of concentration to make things stick (pun intended). In the background of this shot you will note almost half of my gym weighing down the double thickness rocker box bottom which I was sticking together. I will need all those weights back in the gym so I can train up to move the thing when it is finished! I think Kriege and Berry mentioned something about parking your car on the plywood. I simply wasn't game and chickened out ...

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