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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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#51 bridgman

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Posted 07 September 2019 - 12:40 AM

24 inch f/3.4... is that even legal ?

 

Enjoying watching your progress. Best of luck with the build. 


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#52 John Miele

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Posted 07 September 2019 - 02:37 PM

funnypost.gif lol.gif lol.gif lol.gif



#53 Aperturefever

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Posted 08 September 2019 - 05:52 AM

Thanks Matt, glad to show you the scope but hope you're a patient man!
F3.4 is legal ... but take a squiz at Ed's f2.75 24" "Elvira". Now that's against the law. Which makes Ed a wanted man ... and his scope even more wanted.
Ok ... that's the best big dob gag I got ...
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#54 Mike Lockwood

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Posted 08 September 2019 - 12:32 PM

24 inch f/3.4... is that even legal ?

 

Enjoying watching your progress. Best of luck with the build. 

I think you need a permit for that in Canada.  grin.gif



#55 stargazer193857

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Posted 08 September 2019 - 10:57 PM

Fantastic avatar. It suits you.

#56 Aperturefever

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Posted 09 September 2019 - 06:14 AM

Haha thanks mate - I wondered if anyone would pick the Green Man mythology ...

#57 Aperturefever

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Posted 12 September 2019 - 06:21 AM

As much as I like the Webster clamping system, I wasn't fussed when I put things together and saw first-hand my 19mm of ply holding the clamps in place. Maybe it works, but you need to question everything you see, and if you aren't satisfied, then change it, regardless of what has gone before you. Reinforcing the clamp areas has added a little weight, but in the grand scheme of things, this scope isn't travelling far, so I am happy to take on a few extra grams.

On the mirror box I bonded two layers of 6.5mm py together to get exactly the depth I wanted, then used a mitre saw to cut 45 degree angles and come up with the hole reinforcements seen in the first image at the mirror box corner. This gives the bolts a bit more to hang on to. I will field test this, and if I am not satisfied, I will bore out the the holes and add aluminium sleeves. No biggie. In the meantime I will sand things off, then glue on the 6.5mm board that sits on the face of the mirror box. The reinforcements just give the front board something extra to stick on to.

The second image shows an extra layer of 19mm ply I added to the upper tube assembly point. It's stronger now, and the bolts are longer. Again, no biggie with the few extra grams gained. But although I am looking at stars with this thing, and not the telescope, I reckon these blocks currently look tripe. I will put my thinking hat on, get the Dremel tool out, and fashion these reinforcements into something a bit less blocky. I can't help but feel a little pressure as an instrument maker to take a little pride in appearance ...

Attached Thumbnails

  • LTA_clamp_reinforce.jpg
  • UTA_reinforced.jpg


#58 Aperturefever

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Posted 20 September 2019 - 08:41 PM

So I've hit a couple of snags. The first is I don't like the design of my UTA braces. I think I will make them more streamlined to sit entirely under the bracket. Shame I glued them on ... so I reckon I will cut a new bottom hoop and try again. The second issue is I drilled and attached my new flat focuser base, but it the round adapter ring on the bottom of the Lightweight Feathertouch doesn't fit it. Wayne has offered to machine something new, but in the meantime I can't do much about it. I did think of screwing on the curved base just to run some scope tests, but the screw holes don't match. Sigh. Now I know how the James Webb Telescope team feel ...

I will move on to other fiddlier bits. Rome wasn't built in a day.

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  • Focuser_base.jpg


#59 Aperturefever

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Posted 28 September 2019 - 08:09 AM

So no pretty pictures today, just a bit of mathematics and having my thinking cap placed squarely on my non-thinking head!

Bottom line is I am building towards a bit of scope testing. I need a new round adapter to get my lightweight Feathertouch focuser to attach to my new flat base plate. OK, so with a young family in mind, this is pushing the astrobudget slightly. So as I wait for Santa to come sliding down the chimney with that required bit of kit, I realise I have plenty of scrap plywood. So I have knocked out a stunt focuser board so I can screw on my Meade 16" focuser baseplate and do some testing that way. Not ideal, but this temporary measure should confirm that everything is in order and I can get other - cheaper - stuff done as I await the Festive Season.

The second component to scope testing is some double-sided tape and the side bearing teflon pads. So I refer to the Kriege and Berry encylopedia for the formula and come up with this:

My UTA is 12.75kg

Poles are 5.12kg

Primary mirror is 24.4kg

LTA is 44.5kg

So the scope wieight is 86.77 kg or 216.925 pounds.

This is resting on four azimuth bearing pads. That means that each pad bears a quarter of the weight; or 216.925/4 = 54.23 pounds per square inch.

I have Ebony Star Laminate on the side bearing, which typically operates happily at 15 pounds per square inch. So if I have 54.23 pounds for each pad, and 15 pounds per square inch, I end up with 3.62 square inches of teflon per pad, cut in a nice rectangle. I have 4.5mm deep teflon - nice and deep, so I can countersink some screw in to hold the pads in place. But as mentioned, I will stick them in with double sided tape as I run the tests and make everything is OK.

Tomorrow's lesson is mounting a secondary mirror - not as easy as you would think - and based entirely on the teachings of Mike Lockwood.

 


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#60 Aperturefever

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Posted 30 September 2019 - 04:29 PM

I've got the scope transported up to the back astro-barn as I turn my attention towards some scope tests to see what I might have gotten wrong (surely not!). So with everything in place and the primary covered up by a lump of ply in case anything drops down (!!) I clear off the study desk to mount my secondary mirror. It is an Antares Optics Supremax job ... about 3.1 inches from memory.

The picture you see is a carbon copy of the image shown on A Modern Guide to Mirror Support, an online article by Mike Lockwood that appears on the Lockwood Custom Optics website. If you have a reflector, take the time to read it - even if you never build a scope. A deep understanding of this topic is crucial to getting the best out of your reflector.

I won't reiterate Mike's carefully chosen words - he knows far better than me and I'm just doing as instructed. Suffice to say that, left to right, this is my make-up. The noteworthy point is the majority of the filling behind the mirror is the foam plug. It was surprising how little wadding was needed. It took some time to get the mirror to sit just right as prescribed: slight movement up and down; slight movement side to side. Basically, just like the primary, you are giving the optics just enough support to stay in place, but allowing the glass to do its thing. Funny to think of chunks of glass as living, breathing things, but that's pretty much what you've got to do.

Down the back end of the telescope, the primary slips right into the JP Astrocraft cell as though its lived there forever. John Pratte is a genius.

Attached Thumbnails

  • SecondaryOK.jpg

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#61 Aperturefever

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Posted 30 September 2019 - 10:33 PM

With some trepidation I checked everything was screwed up tight as night fell, and then used my Glatter laser collimator to get my secondary right. Having temporarily marked the centre point of my uncoated mirror, I went ahead and adjusted the primary with the Tublug. The bellevue washers on the mirror cell are quite tight, but this was first use and tight is good. Five minutes got the optical train set up.

So on with an eyepiece and what did I see?

I saw that I can't add up!

Or I can, I suppose, I just stupidly took a measurement from the back of the mirror box and not the face of the mirror when I was sorting out a rough pole length. Can't believe I had a brain snap like that. Where did that leave me? About half an inch short of focus. I could have squeezed out maybe that by playing round with the clamps, but I did not want to risk any structural failures. What I could see was that by backing out the eyepiece hand-held, I could come into focus and get my star points, so at least that was good news. But no star testing for the moment.

Anyone know a good metal recycler?!

So there I am, tripped up just short of the finish line, snapped a Webster T-nut (reckon I might look for a stronger version online), and a decent blood blister to boot on the tip of my index finger (don't ask).

The first coat of stain remains some time away ... the life of a (very) ATMer can be tough!

Attached Thumbnails

  • InTheBarn.jpg
  • Barn2.jpg

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#62 Bob S.

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Posted 01 October 2019 - 01:53 PM

At least you are fully "in the game". Others can talk the talk but you have been walking the walk which is realistically much tougher! If we all had a few dollars for every brain fart that we have had, we would all be rich. Keep up the good work. The light at the end of the tunnel is NOT a train! 


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#63 Aperturefever

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Posted 07 October 2019 - 12:01 AM

At least you are fully "in the game". Others can talk the talk but you have been walking the walk which is realistically much tougher! If we all had a few dollars for every brain fart that we have had, we would all be rich. Keep up the good work. The light at the end of the tunnel is NOT a train! 

Thanks Bob for the encouragement. Rich? A billionaire! And yes I would prefer starlight but just at this early point a train would be just fine if I could get the **** thing in focus (before it clobbered me)!



#64 Aperturefever

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Posted 07 October 2019 - 12:19 AM

Time to get some lessons learnt from Barn Test #1. The first thing to take a look at is the Webster clamps on the mirror box. Two of the T-knobs have failed. The first sort of came apart straight out of the box with no pressure applied, so I am wondering if I got a faulty batch. The second T-knob snapped on one side of the "T" during the first time I put the scope together for fit, the other side snapped off in the barn as I set the scope up for testing (see pic).

I now know exactly what these Webster clamps really are, and could replace the whole clamp cheaply, but that would not solve the problem of potential future failures of the T-nuts. To solve this I went to the local musical shop, which has a dedicated drum shop, and purchased the pictured cymbal wingnut with 8mm thread. It's a lot stronger than the original clamp nuts; the only issue is I will need to add an unthreaded collar to make the neck a little longer. It actually tightens up OK as is but the collar is an easy fix, and as you never completely undo the nut to take off the poles I will not lose the collars in the dark. As fate would have it they only had a couple in stock, so I have enough to cover the two broken nuts and will trial them. If all goes well in further tests I will replace them all. At two bucks a pop you can't go wrong!

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  • Busted_Clamp.jpg
  • New_Knobbed.jpg

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#65 Aperturefever

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Posted 28 October 2019 - 03:50 AM

After attending an intensive session at the Midvale School for the Gifted (see Larson's Far Side famous cartoon to see why a Stellarcat is a far better option for me than a push-pull dob) I return to the barn with a new-found ability to add up, and with a bunch of new, slightly longer poles in tow. I can't quite get my primary mirror aligned - that will take a trade of a Bellevue washer down the back - but it is close.

Because of this, I only use a 21mm eyepiece, but the stars focus right where they are supposed to. I am so relieved I wonder if something is wrong! With a coma corrector, the stars sit tight to the edge of the field. I am greatly relieved, and today I swap around some washers to allow for a better fit with my Glatter gear and a high-power eyepiece star test for a more accurate idea of what's going on.

Pity about the high clouds ...

I have found out in the past few days Australia's only mirror coater is retiring by year's end, so the pressure is on. The only other option is the Angel Gilding silvering, and I await their correspondence to see if I can even get it into the country. I guess there's nothing like a deadline to get stuff done.



#66 Oberon

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Posted 28 October 2019 - 04:30 AM

I have found out in the past few days Australia's only mirror coater is retiring by year's end, so the pressure is on. The only other option is the Angel Gilding silvering, and I await their correspondence to see if I can even get it into the country. I guess there's nothing like a deadline to get stuff done.

Whut!!

 

This is serious?

Who? Why? What happens to the equipment?



#67 Aperturefever

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Posted 28 October 2019 - 04:47 AM

I spoke to Wayne from Saintech and alas it is true. I have until end of November to make things happen. But after this, who knows what will happen. A sad state of affairs to be sure.

#68 clivemilne

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Posted 28 October 2019 - 06:29 AM

I think you need a permit for that in Canada.  grin.gif

You need a prescription for that in Australia.



#69 Aperturefever

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Posted 08 November 2019 - 07:10 AM

I imagine my far-flung neighbours discussing my ATMing efforts in the distance:

 

"Bob ... look ... it's that strange man ... there he goes again. Happens all the time. I swear. I see him striding out to that barn of his - disappears a while - then comes stomping out, looking irritated."

"Whaddya mean, irritated, Gladys?"

"He's muttering to himself. Glancing up at the sky and shrugging his shoulders. Sometimes I think it's like he's forgotten something and trudges back down to the other end of the property - then he's back a few minutes later with something in his hand. Like, with a screwdriver. Or one of those right-angle thingoes."

"Don't worry about it, Gladys. He seems harmless."

"Harmless? Harmless, you say. Well I dunno what he's up to in that barn of his, but I reckon he might have a few kangaroos loose in the top paddock, that's all I can say."

"Well he does get about in the middle of the night with that red 44-gallon drum thing of his. I got no idea what that is all about. I did try and turn the spotlight on so he can see what he's doing in the dark ... but like you say ... he seems to get awful irritated."

"I'm keeping my eye out, I tell you, Bob. Who knows what somebody like that could be cobbling up in the depths of his shed? It could be anything. Anything, I tells ya."

 

And here's today's lesson, folks: collimating a big, fast newtonian - one that has not arrived pre-packaged - is not an easy task. It may even send you around the parabolic bend to the extent your neighbours notice!

 

 

 


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#70 Aperturefever

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Posted 08 November 2019 - 07:27 AM

I will run through the nuances (!) of wrestling out collimation with a quick and big newtonian when I get to the finish line. In the said wrestling ring, however, I am finding the T-nuts on the LTA Webster clamps are failing miserably. I had already broken two of eight; that count went to three of eight when I tried to tighten a strut without thinking and being super-careful. These things are just way too brittle (see first image).

Happily, a trip to the local music shop drummed up even better cymbal nuts than I had found previously. The second image shows how the far right nut has a longer neck, and this works better than the far left. It was discontinued stock, and I got eight plus a spare for thirty bucks.

They are not black, but they look kinda nice. Nothing like a bit of bling on your dob, right?!

Attached Thumbnails

  • TNut_Failure.jpg
  • New_TNuts.jpg
  • New_nut.jpg

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#71 Aperturefever

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Posted 10 November 2019 - 07:06 AM

"And welcome to another episode of ATMers Who Can't Count!!

And who's our special guest today? Aperturefever! from Down!! Under!!!"

(Tinned applause)

So ... Aperture ... tell us your story!

"Well ... I thought I could count. Then I tried to focus my new ultra-fast dob.

"Wow ... and what happened buddy?!"

(Shuffles feet nervously) Turns out I got my math wrong. I drilled a focuser hole ... then found my stars wouldn't focus."

"Hmmm ... that's not good, Aperture. What happened?"

"My Glatter laser was focused a little too far up the secondary. I think I plussed a minus. Or something."

(Tinned laughter)

"OK. So Now what?!"

"So now I drill another hole on my focuser board a little further down. Thankfully it is a back-up focuser board. So now I have a focuser oval  - not a focuser hole."

(More tinned laughter)

"Well never mind, Aperture. And a big shout-out to Mike Lockwood, who might want to add another section to his Why Aren't My Stars Round article: because I'm a boofhead who can't add up!!"

(Tinned laughter and applause)

Thanks for tuning in folks ... and tune in tomorrow for another enthralling episode of ATMers Who Can't Count!!!

(Tinned applause)


Edited by Aperturefever, 10 November 2019 - 07:07 AM.

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#72 Mike Lockwood

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Posted 10 November 2019 - 04:24 PM

It's just an alternative implementation of a sliding focuser.  No big deal.  Nothing to see here.

 

Sometimes it's Almost Telescope Masochism......


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#73 Pinbout

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Posted 10 November 2019 - 06:57 PM

 

I am finding the T-nuts on the LTA Webster clamps are failing miserably

Just to clarify

 

tnut

D6ECFA17-DEBA-48D1-95DA-92065EE6B30B.jpeg

 

threaded knob

 

33663FCC-CD8E-4C41-8E53-57B2BC5E7905.jpeg


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#74 Aperturefever

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Posted 11 November 2019 - 04:39 AM

You got me, Dan. I believe I shoulda called them "cymbal hi-hat wing nuts", to be precise.


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#75 Aperturefever

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Posted 11 November 2019 - 04:53 AM

With my Sideways Sliding Focuser ™ firmly in place (thanks Mike!) things go a lot better as a rising moon wrestled the long twilight for the last blue sky of the day. With my focuser where it was supposed to be, I bolt on the secondary and swivel it until the Glatter dot gets as close as I can get it to the centre of the primary, which happily was bang on. The slightest of adjustments to tighten the mount bolt and the collimation knobs and I was done.

I began the whole process by using a bubble level: I ensured the mirror box face was dead flat. Then I made sure the UTA was bubble levelled dead flat all the way around. This part of the process was yesterday, as I wanted to be confident nothing was slipping or sliding over the course of 24 hours.

Using an a JP Astrocraft mirror, you have to be sure all your ducks line up. John uses Bellevue washers stacked together instead of springs, and there is not a lot of give. You need to be precisely lined up to begin with. A few turns here and there and my primary centre spot sits pride of place in the Tublug.

I think I am good now to see if I can get this thing precisely tuned with star collimation, but with the moon so bright and no real sideways movement (yet to do the base teflon/laminate) I decide to let things go a few nights until the moon is out of the way early evening. it's time to bunker down and sweat out some real dangerous fire conditions.




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