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Building a 28"

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#1 Mike Harvey

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Posted 22 July 2005 - 06:21 PM

I'm constantly in awe of folks like you who can do such things! I hvae trouble just putting scopes together...let alone BUILDING them! :bow:

You are SO right about Steve Kennedy's 28" mirrors...
I have one of his f/3.66's in my new 28" Starstructure.
My fears that I would sorely miss my 24" Zambuto have proved to be unfounded. I'm totally pleased!

Good luck with the new toy!

#2 Jarad

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Posted 22 July 2005 - 07:14 PM

Wow - great article, and great looking scope! :bow:

I gotta get me one of those one of these days.... That, and move to Florida where the seeing can take advantage of it.

Jarad

#3 Don W

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Posted 22 July 2005 - 07:16 PM

What article?

#4 asaint

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Posted 22 July 2005 - 07:47 PM

The article link is http://www.cloudynig...hp?item_id=1124

Appears the original thread got deleted.

Allister

#5 Rusty

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Posted 22 July 2005 - 07:47 PM

Wow - great article, and great looking scope! :bow:

I gotta get me one of those one of these days.... That, and move to Florida where the seeing can take advantage of it.

Jarad


Florida for seeing? The last time any of my scopes was out was the first week in May, at the Georgia Sky View...

All of our Club sessions from May through tonight have been cancelled. It was the same last year. :p

The next time the scopes come out will be 7/31-8/5 - at the Nebraska Star Party.... :jump:

#6 MrLunch

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Posted 22 July 2005 - 07:50 PM

Bravo on a well written article and beautiful scope. Especially thanks for the detailed lists and completeness of the article. I saw the scope at SSP this year, thought it was a pro job! Really great work.

#7 hbanich

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Posted 22 July 2005 - 07:50 PM

Don - the article "Building a 28 Dob" is posted in "Recent Additions" and in the ATM contest page.

And thanks Mike and Jarad! As much fun as I had building the scope I'm having way more fun using it. I'm frothing at the mouth for the upcoming new moon cycle and Mars...

By the way, I was set up right next to Steve Kennedy's personal 28" f3.66 at Shingletown and we did some comparative star testing one night, and not surprisingly found there's practically no difference between his f3.66 and my f4.

He's an amazing optician, a true perfectionist and a real nice guy to boot. After he finishes bench testing each mirror he star tests them - with the diagonal - until it produces a great result. No wonder his mirrors are so good.

Howard

#8 Mike Harvey

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Posted 22 July 2005 - 09:44 PM

Don - By the way, I was set up right next to Steve Kennedy's personal 28" f3.66 at Shingletown and we did some comparative star testing one night, and not surprisingly found there's practically no difference between his f3.66 and my f4.

He's an amazing optician, a true perfectionist and a real nice guy to boot. After he finishes bench testing each mirror he star tests them - with the diagonal - until it produces a great result. No wonder his mirrors are so good.

Howard


Yeah, I've had my f/3.66 alongside Mike Zammit's (Starstructure builder) 28" f/4.15 (also a Kennedy mirror)
and there simply is no discernable difference in performance. Amazing consistency in these mirrors!

And after owning an Obsession and 4 Starmasters, all wooden scopes, I'm convinced that metal is the way to go.
The Starstructure will fully dampen out from a solid rap against the UTA in less than 2"...even at over 1,000X!
And I can hold the focus knob in my hand and rack it in and out at over 500X with absolutely no visible vibration.
The wooden scopes took at least twice as long to dampen and were very 'jittery' when focusing at high powers.

There is also no 'focuser sag' whatsoever when using the binocular viewer. And, after transporting and reassembling(a job that takes less than 5 minutes!)the collimation is always almost dead on. Normally it only takes a slight tweak to center the laser dot in the center ring (triangle actually). While the Starmaster was usually pretty close too (but not THIS close) the Obsession ALWAYS needed more attention.

I really like the rotating light shield you designed and hope to have something similar made for my scope.

Mike Harvey

#9 Cerberus

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Posted 23 July 2005 - 12:49 AM

What a beauty!!!

#10 Starman1

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Posted 23 July 2005 - 01:17 AM

I believe I saw this scope at RTMC? In telescope alley? Pity the viewing was mediocre. There were a couple 28" scopes there. The other one, on the field, wasn't quite the solid design of yours, though.
I've had the chance to look through 2-28" scopes on the same night at Mt. Pinos, both Kennedy mirrors--one Kennedy's own scope, one belonging to someone else.
Both were impressive. Until these scopes, I didn't think a 28" could have sharp star images, but I was wrong.
And such simple design, yet elegant.
Impressive. Most impressive.

#11 hbanich

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Posted 23 July 2005 - 11:31 AM

Hi Don,

Yes,I had the scope set up on telescope alley at RTMC, but all the old timers were telling me the viewing was pretty good for Camp Oaks! I had a blast regardless as it was the first time I had the scope under a starry sky after painting it.

I love sharp, bright images, which to me is why a big scope is worth all the effort and expense to build.

Howard

#12 StarStructure Telescopes

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Posted 24 July 2005 - 09:48 AM

Howard,
Nice work !! Glad to see more aluminum scopes being built.
Hope to get out there again and see one first hand.
Michael

#13 ericpd

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Posted 24 July 2005 - 08:49 PM

Wow! Just wow.

The only downside I see to that scope is that I don't own it.:)

How do the truss clamps from Aurora Precision work? Are they aluminum as well? I'm in the design phase of my own scope and I'm always looking for good products and options for it. I'd also like to avoid wood for the truss attachment at least.

Eric

#14 hbanich

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Posted 24 July 2005 - 09:07 PM

Hi Eric,

The lower truss clamps are made of Delrin, but they are available in aluminum for a lot more $. The Delrin clamps have worked fine so far although every now and then I think I might upgrade to aluminum.

The upper tube brackets are aluminum, and hold two poles together as a unit. That bracket fits another bracket on the bottom of the cage with both a registration post and a threaded post with a hand knob. This works nicely and is a very solid connection.

Howard

#15 ericpd

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Posted 25 July 2005 - 11:40 AM

Howard,

Thanks for the info. I e-mailed the company for more information. I'm planning an 18" right now and I'm considering all options before I get all froggy.

Also, God bless you for being a stubborn star hopper. There's not enough of us around. :)

Eric

#16 hbanich

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Posted 25 July 2005 - 12:46 PM

Eric, here are a few photos:

Attached Files



#17 ericpd

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Posted 25 July 2005 - 11:26 PM

Thanks for the pics. IS the hand knob on the upper clamps loose?

Eric

#18 thelittleman

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Posted 26 July 2005 - 10:01 AM

I hope you dont mind me asking, but how much did you pay for the mirror?

#19 BluewaterObserva

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Posted 26 July 2005 - 10:31 AM

Man, I would love to have Mr. Kennedy do a re-work on my 30".... I just have a hard time messing with a mirror that works well, even though it is only 1/8th wave RMS. (estimated by star testing)

#20 hbanich

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Posted 26 July 2005 - 02:23 PM

Hi Eric,

The upper clamp knobs are not attached to the bracket, so in that sense they are loose. Although this allows the possibility of dropping the knobs they are a lot easier to screw on.

I should have included a photo of the upper clamp knobs, but they are just a smaller version of the knobs on the lower clamps.

Howard

#21 hbanich

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Posted 26 July 2005 - 02:32 PM

I paid $8200 for the Kennedy 28 inch f4, and the 5.5 inch diagonal was another $600. I expected to wince when writing those checks but I was so excited that I forgot!

Last month I traded the 5.5 inch diagonal for a 5 inch from Steve that he tested for flatness, and it works very well - couldn't see any difference in the star test.

Howard

#22 hbanich

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Posted 26 July 2005 - 02:38 PM

Important correction! I inaccurately stated in "Aligning the alt-az drive" section (about 2/3 of the way into the article)that Dan Gray was going to introduce his own goto box - I didn't realize that he had stopped working on that project and had decided to stick with Argo Navis for goto.

My apologies to Dan, and sorry for any confusion this causes.

Howard

#23 Starman1

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Posted 26 July 2005 - 02:49 PM

I paid $8200 for the Kennedy 28 inch f4, and the 5.5 inch diagonal was another $600. I expected to wince when writing those checks but I was so excited that I forgot!

Last month I traded the 5.5 inch diagonal for a 5 inch from Steve that he tested for flatness, and it works very well - couldn't see any difference in the star test.

Howard


Bartels' calculator indicates the 5" should be fine if my estimate of 19" from center to focal plane is nearly correct.
That's 18% for the secondary--pretty decent.

#24 hbanich

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Posted 26 July 2005 - 03:17 PM

Hi Don,

I'd have to check to be sure but I think the focal plane to diagonal distance is about 18 inches, so that's a little better illumination yet.

Also, I need to add a small counter weight to the cage to rebalance the scope because of the smaller diagonal - a scope really is never finished...

Howard

#25 StarStructure Telescopes

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Posted 26 July 2005 - 04:53 PM

Howard,
I used the 5.5 on the 28 F3.66 also (19.6% central obstruction) I was considering using the 5” (18% central obstruction) as well. My intersect is 19.37” (30.5” clear aperture) (15.25 + 1” for the focuser board + 1.625” for the Feathertouch 1.125” for back focus + .37” for off centered secondary away from focuser) First off I really look at the 5.5” as a 5.3 and the 5” as a 4.8 because of the 1/10” lip on the holder. Putting the 5” in the 3.66 clearly clipped of about .3” of the primary mirrors edge. I know the math, but after comparing the 28 F3.66 with a 5.5” secondary to the 28” F4.1 with a 5” secondary I saw no discernable difference (as did a few others) in the view as a result of the 1.6% difference in obstruction. Just curious to why you changed out to a smaller secondary??
Mike


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