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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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#26 Aperturefever

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Posted 13 August 2019 - 05:39 AM

Gluing up the base board. This is single thickness (19mm) - so the double thickness bits are the rocker box base and sides. The feet are a decent size so this big dob doesn't sink into the ground - mind you the place I call home these days is igneous ... lumps of quartz all over the property (unfortunately no gold!) so I doubt we're going to disappear into the folds of the Earth. I put a hole in one of the feet to take the XLR plug that will power the Stellarcat system before I glued it all up. I think it was a 22mm spade blade that I used on the drop saw ... a little bigger than what I wanted but the plug will cover the hole just fine.

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#27 Aperturefever

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Posted 13 August 2019 - 06:25 AM

Production line. I glued all but the front board of the rocker box, then glued in the first set of braces for the mirror box. I will glue in the next set when I can be sure of exactly where all the hardware sits. This doesn't worry me too much as only half the mirror cell and no mirror means low structural load at this point. This shows the attachment of the mirror cell brace supports.

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  • Second_Glue.jpg


#28 Aperturefever

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Posted 13 August 2019 - 06:34 AM

The mirror cell rotates out for mirror access, so I needed to shave some wood off the side braces to facilitate this. Basically, the process was sit the mirror box upside-down, place some 1/16th shims (for clearance), clamp the side braces, then sit the cell in and mark the positions of the four t-nuts. Then I undid everything and drilled the t-nuts on the drop-drill. They are a tricky business. You'll notice in this shot I shamelessly over-glued all the internals. Structural integrity is everything and I finished off all joins by running a line of glue along the internal joint. Let's face it ... they will be all coated in flat black paint ... it will be dark ... it's our little secret ...

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  • Third_Glue.jpg


#29 Kunama

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Posted 13 August 2019 - 03:49 PM

Following this build keenly cool.gif waytogo.gif

Takes me back to my build in 2015:  http://www.iceinspac...uinevere&page=4


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#30 Aperturefever

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Posted 14 August 2019 - 05:28 AM

I remember seeing that thread ages ago Matt, but that was ages ago and it's good to recap for moral support. I imagine you get to the end and think of how much more fun you could have had, and enjoyed the process, if you knew at the start what you learned through the process. Dangerous knowledge ... you could almost go back for another turn ... but of course it's too late to warn you of that!

 

The beauty of this hobby is it has so many facets you could get lost in. Instrument building is a hobby in itself, like mirror making. And then when you get done there's the observing. Or astrophotography, or EAA ... deep sky, planets, solar ... how could you ever get bored with astronomy?

 

I will say that I will pass on the good karma I have enjoyed whenever I happen across someone building a scope - I have been blessed with so much good advice from patient mentors. Astronomy people are a special breed.



#31 Aperturefever

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Posted 14 August 2019 - 05:45 AM

With great trepidation the clamps are removed and I dummy fit the bottom pieces together to check for fit and measure. Truly nerve-wracking stuff. This thing is kinda big ... of course I knew that, but setting the mirror box in the rocker, I am glad to live under dark skies and not needing to travel. With great relief I find my 90% of thinking has brought success. I have a dummy mirror in the cell and everything sits where it's supposed to. The nice thing about the double-thickness rocker side walls is the side bearing insets just a little. It's a nice touch and I once again mentally thank Pete Read at SDM Telescopes for his advice.

 

 

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  • First_fit1.jpg
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#32 Aperturefever

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Posted 14 August 2019 - 06:06 AM

Following this build keenly cool.gif waytogo.gif

Takes me back to my build in 2015:  http://www.iceinspac...uinevere&page=4

Actually Matt just going slowly back through this now and some great ideas. You must have shares in a clamp company lol.gif. Love that stain ... but I love to be different and being a guitar player ... there are some crazy stains out there ... I am thinking green. The Green Machine?



#33 Aperturefever

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Posted 15 August 2019 - 07:29 AM

You would probably describe me as well-read when it comes to astronomy, but with work and a young family I am what you would describe as a bit of a lone wolf astronomer. The online age makes it possible to get all the information and contact you need. Don't get me wrong, I would love to get to a few star parties and club nights. Sometimes I even try. But at the end of the day, if I get a few spare moments I'd just as soon drag the scope out of the barn and get under the stars.

 

So what's my point? Well, I sat out this telescope's components and admit I scratched my head a bit and caught my breath with a few moments of worry. Sure, I'd seen big, fast dobs on the net; just never in the flesh. I have a 16' f4.5 so figured this 24 would look a bit shorter ... but geez. This thing is stumpy. And they used to call f4.5s light buckets. I even went back to the calculations with some trepidation to ensure I shouldn't have taken a left at Albuquerque. But no; this is indeed a sawn-off dob, and one can step on up without fear of heights, apparently.

 

I grin to my vertically challenged self from the depths of the shed and ponder more maths and the considerations of pole clamp placement ...

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#34 macdonjh

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Posted 15 August 2019 - 07:43 AM

If you "take a left at Albuquerque" you will end up "in Coachela, and the carrot festival 'dere'in". No good star gazing at carrot festivals.

Ask me how I know...

#35 Ian Robinson

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Posted 15 August 2019 - 07:43 AM

Looking good …

 

looking forward to seeing what your final result looks like .



#36 Aperturefever

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Posted 17 August 2019 - 06:22 AM

If you "take a left at Albuquerque" you will end up "in Coachela, and the carrot festival 'dere'in". No good star gazing at carrot festivals.

Ask me how I know...

I'm no Bugs Bunny ... but don't carrots make you see better in the dark? You're not telling me my parents were just tricking me into eating vegetables ... and not grooming a super astronomer??!



#37 Aperturefever

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Posted 17 August 2019 - 06:24 AM

Looking good …

 

looking forward to seeing what your final result looks like .

Thanks Ian. Slowly and surely does it. Building a big fast dob is like a Rubik's Cube sometimes ... you're always trying to think a few steps ahead. Cross your fingers for me mate lol.gif



#38 Aperturefever

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Posted 21 August 2019 - 06:57 PM

Time for more deep thinking as I consider my layout with struts roughly in place. I am using recycled Obsession side bearings, which were made in the times of longer focal lengths, and I realise that because of the F3.42 focal length, I will be running a steeper angle on the struts than what these bearings were probably made to accommodate for. It is going to be touch and go; I am reliably informed others have shaved a smidgen off their bearing cut-outs to get the strut past before with no worries. I would just prefer not to do it if possible, but I don't want the pole even resting on the side bearing.

 

I will probably leave any surgery as a last resort. Chances are I will get away with it.

 

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  • TrussCalculation.jpg


#39 Aperturefever

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Posted 21 August 2019 - 07:02 PM

I almost overlooked the first step, but thankfully thought it through in time. I was so preoccupied with the side bearing clearance that I almost forgot to start on the bottom face. I needed to do this to make allowance for the clamp to sit inboard of the side bearing at a slight angle.

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  • Truss_Clacs3.jpg


#40 Aperturefever

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Posted 21 August 2019 - 07:19 PM

So I transferred that inboard measurement to the bottom side clamp area. Now it is a question of how far back I go from the face of the box. The further back I go, the more risk having to caress (bludgeon/hack) the bearing cut-out. The thing is I need that bolt back far enough to retain the structural integrity of the ply. I will have a bracing block inside the mirror box so about 35mm of timber to hold the bolt in position, but I am still trying to decide.

 

Sorry of some of these posts are pretty basic stuff, but I wanted to record the process so others might get some ideas. I reckon there should be more telescope building in this day and age ...

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  • TrussCalcs2.jpg


#41 sparksinspace

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Posted 22 August 2019 - 06:54 AM

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.

 

Should of read this line earlier. Yes threads of mirror making/testing do tend to wander and become animated. I guess it tend to happen when pure science and mathematics collide with interpretation and a "feel" for things


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#42 Aperturefever

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Posted 24 August 2019 - 08:29 AM

It sure does. But I just want to focus on the build here. I've certainly had a great conversation getting the mirror made, that's for sure, but that's another story in itself. I will say mirror making is one of the most satisfying and relaxing things I have ever done. I reckon the key is to not be in a rush. As with everything in this world, if you are fascinated with the process the end result takes care of itself. In all honesty the mirror was far easier than building a box to balance and swing within another box ... I know it's not rocket science but geez ...
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#43 Aperturefever

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Posted 27 August 2019 - 01:11 AM

I got the LTA clamps mounted - I haven't reinforced them yet, but it allowed me to check the lie of the struts. Things were just too tight on the side bearing cut-out, so I elected to wield the angle grinder and shave a little metal out of the way. The second shot is a bit out of focus but that hides my awful metalwork well lol.gif . I will have to chase up some emery paper and neaten things up, but I don't plan on doing that until star tests are over and I know exactly how long (short?!) the poles are going to be.

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#44 Aperturefever

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Posted 30 August 2019 - 11:03 PM

With my bottom end clamps sorted, it was time to turn my attention to the top clamps. The first step was get the four clamp positions accurately marked - I did this by using the assembly plans John had sent me for the spider, as he is no doubt a lot more accurate than me, then took my lead from there.

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  • UTA_calcs.jpg


#45 Aperturefever

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Posted 30 August 2019 - 11:07 PM

The second step was ensuring the UTA clamps sit at the right diameter so everything sits square and the struts run straight and true from the mirror box to the UTA. I did the maths, but if I can I prefer to see things in the flesh, so to speak. So I created this jig with the mirror box, clamps and set squares, just to be sure. I think I am good. I can see a dry initial fit of everything coming very soon, and am encouraged to press ahead.

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#46 Aperturefever

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Posted 02 September 2019 - 02:05 AM

The project reached a nice milestone as I screwed the infrastructure in place to check it all went together. It did. As nice as it was to see this thing take shape, there is still much to do ... so I will take it all apart now. At least it allowed me to eyeball the cut-outs I will need in the front rocker box board to accommodate the bottom poles. So I will get that done along with loads of other more fiddly stuff, like reinforcing all the clamping points, screwing in the focuser and finder boards, etc ... I think I will skin the UTA after I have star tested.

Then there is the front board on the mirror box ... a ventilation system ... wheels and handles ... the Stellarcat. OK. So I don't need to order the stain just yet!

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#47 GUS.K

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Posted 02 September 2019 - 02:31 AM

Nice work, scope looks great.Thanks for sharing your build progress.


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#48 Aperturefever

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Posted 06 September 2019 - 11:00 PM

Nice work, scope looks great.Thanks for sharing your build progress.

Thanks for the encouragement mate!



#49 Aperturefever

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Posted 06 September 2019 - 11:22 PM

After enjoying the sight of an almost-there telescope, I have pulled it down and get back to serious work. I need to get the focuser mounted to run star tests, so this is next on the list. For the uninitiated, it is of course not as easy as it would seem. A 2 inch hole for a 2 inch focuser plonked squarely under the secondary, right?

Wrong!

I matched the hole with the Feathertouch base plate hole, which is 70mm, so well over the 2 inch mark. Then I needed to set the focuser in the right place: to find the optical centre on the diagonal I firstly found the mechanical (actual) centre on the diagonal, then marked a spot 1.414 times the offset amount above the centre dot.

So that's 1.414 x .35"= 0.4949"

Converted to metric = 12.57046 mm

So after the hole is cut I will use 1/2" long 6G panhead screws to mount the focuser plate. They fit perfectly and hold better than the standard screws.

I will tack the focuser board into the UTA with smaller screws than I will eventually use to preserve the strength of the wood. But I need the focuser board in place to get the scope tested and poles cut to exact length. I won't mount the Telrad and finder boards until final assembly as I don't need these to run the tests.

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  • Focuser_board.jpg


#50 Kunama

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Posted 07 September 2019 - 12:17 AM

After enjoying the sight of an almost-there telescope, I have pulled it down and get back to serious work. I need to get the focuser mounted to run star tests, so this is next on the list. For the uninitiated, it is of course not as easy as it would seem. A 2 inch hole for a 2 inch focuser plonked squarely under the secondary, right?

Wrong!

I matched the hole with the Feathertouch base plate hole, which is 70mm, so well over the 2 inch mark. Then I needed to set the focuser in the right place: to find the optical centre on the diagonal I firstly found the mechanical (actual) centre on the diagonal, then marked a spot 1.414 times the offset amount above the centre dot.

So that's 1.414 x .35"= 0.4949"

Converted to metric = 12.57046 mm

So after the hole is cut I will use 1/2" long 6G panhead screws to mount the focuser plate. They fit perfectly and hold better than the standard screws.

I will tack the focuser board into the UTA with smaller screws than I will eventually use to preserve the strength of the wood. But I need the focuser board in place to get the scope tested and poles cut to exact length. I won't mount the Telrad and finder boards until final assembly as I don't need these to run the tests.

Been there, done that, and had a lot of fun doing it cool.gif

Still following this keenly and maybe one day I get to have look at it in person flowerred.gif




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