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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5767952 - 03/31/13 12:33 AM

Good news I am allowed to give the 6" blank away to the winner or person that gets near the weight of the telescope.
My rules: Items to be included in weight of telescope!
1: Focuser
2: secondairy, wire spider and secondary cell.
3: 20 inch quartz mirror 1" thick
4: 18pt mirror cell with side support
5: main rotation bearing.

Not included in weight is drive system and telescope platform/stand.

6:deadline to get your guess in is April 30
7: winner of the free 6" quartz mirror blank will be contacted when the telescope is completed I hope before Oregon Star Party. Good luck


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5768244 - 03/31/13 08:44 AM

Chris,

Good Luck, your goals are enviable, but if you are going to use any thing even vaguely approaching a widefield eyepiece (31 Nagler, etc). You wont come in under 70 lbs for the scope that goes to the field with the tailweights needed to balance, shroud, finder, telrad baffles... This is still great accomplishment as a commercial 20F4.5 would be over 100.

I understand the goals of fast cooling and thin mirrors, but that doesnt get away from the need for tailweights and crazy complex mirror supports to get a good image, as thin as your primary is.

Just looking at your sketch, a couple of other things jump out, I dont think your scope will swing through to Zenith (mirror box may be too short). You will need a significant light baffle opposite side the focuser, as you will have stray light issues (this also will add to the need for tail weights (You are around 5:1 distance to focuser versus distance to mirror), so every pound up top will be 5 pounds in the tail. Telrad, finder etc, to make the scope useable all add counterbalance (also the shroud)..... I am on my 5th or 6th large dob (currently a 25 F5 commercially built), but have built several of my own.... I applaud your ingenuity, and drive, and just want to give you a cuople of look outs....

Edited by gatorengineer (03/31/13 08:47 AM)


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kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5768247 - 03/31/13 08:48 AM

How about Mark C be the judge as to if it is stiff enough .If Mark says it is stiff and you are under 40 lbs I send you a mirror . The X prize The first big telescope who weight is under twice the diameter . Now just get rid of the obstruction. All good fun

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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: kfrederick]
      #5768262 - 03/31/13 08:58 AM

My 14.5 inch scope is at 32LBS on the scale (no bearing stand just scope) photos can be seen at 2012 OSP. This one is going to be better in every way. I have given up some weight with the wood lam but it looks so nice. Still looking good on weight but we will see. Put your guess in.

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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5768267 - 03/31/13 09:10 AM

This is a photo of my 14.5, new telescope is not the same but a combo of it and others I have seen.


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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: kfrederick]
      #5769044 - 03/31/13 03:20 PM

Quote:

How about Mark C be the judge as to if it is stiff enough.




But I'm in the running for the contest, so I won't be impartial.

Chris does some amazing designs that work quite well - if you study the materials he's using and picture how large this will be you'll see where the balance point is going to be... OTOH I don't think it will actually end up looking exactly the design and still balance without weights. But I'm more than willing to be wrong.

Best,
Mark


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: mark cowan]
      #5769127 - 03/31/13 04:26 PM

its clear that part of this interesting thread has been cut off the front when it was transferred. I think there is a segment about mirror support, which I would like to read. 18 pts is no way near enough in common thinking for a 20" F4.5...

Also I though the weight guess was for a finished functional scope? Is it just the ""OTA"" with no shroud baffles, eyepieces, counterweights etc


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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: gatorengineer]
      #5769364 - 03/31/13 05:53 PM

Page 1? It's all here...

Beg to differ, 18-points should be fine. 2.2nm RMS from PLOP 2D for fused silica, 1.8nm RMS from PLOP 3D. I'd go 27-point, but that's just me.

Yes, what are we guessing on? Mine was for push-to configuration including the base and paracorr and 1x finder.

It'll be interesting to see how my wire spider design works with a 3.5" secondary.

Best,
Mark


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: mark cowan]
      #5769695 - 03/31/13 08:55 PM

mark, you are of course right, she calcs out just fine, using Plop with 18 points.... I keep remembering Krieg and Berry..... What the Cruxis program also shows is the criticality of the sling for this thin of a mirror.....

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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: gatorengineer]
      #5769905 - 03/31/13 11:36 PM

I have some more information to help with guesses.

Photo of 8foot by 4foot composit I am using.



and on the scale at 10LBS only a small % of a board is getting used.


I do have a couple of questions:
1 on the primary mirror support triangles can they have some flex and if so how much?

2: my secondary cage is 15" by 22"id with 25"od and will be lined with a wood varniear (yes my spelling sucks sorry) see photo below. how far from the top should the secondary mirror be for best light blocking. Was looking at 6" but not sure? Anyway thank you for the support.


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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5770192 - 04/01/13 06:21 AM

Ok this is about as good as I am going to get. .25 inch = 1" and I think this is very close to what the telescope without base is going to look like. Strut length and secondary placement may change slightly but this is the best information I can give at this time to help you guess the weight. Please comment good or bad, I think I have a very strong bearing / center core like a bike frame most cross members will be hollow light but strong. Anyway we will see how it turns out.



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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5771745 - 04/01/13 09:37 PM

Epoxy coat done just need Urethane top coat on light shrouds. Little bit at a time but going to be a long project.



Weight with shroud. will add another .5 pounds in hardware, 1lbs for secondary, .9lbs for fether touch, .5lbs for finder. and ?lbs for the rest.



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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5772050 - 04/02/13 12:50 AM

Your UC baffling isn't going to be adequate with the secondary that far forward, I suspect...

I'm sticking with my original guess as to the weight, but I'm not going out to buy lottery tickets.

Best,
Mark


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jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5772175 - 04/02/13 05:25 AM

Quote:


I do have a couple of questions:
1 on the primary mirror support triangles can they have some flex and if so how much?





As far as effectively "floating" your primary goes, some flexure in the triangles does not distort the mirror. It can however cause collimation shift. However, symmetry in the flotation system might allow this to cancel out.

I have never seen well quantified tolerances for this (although they are calculable) and I think the tendency therefore is to somewhat err on the side of overbuilding to avoid these issues, which is what I did with my cell.

Still, I have always suspected differential flexure/yielding etc. (i.e. asymmetric) to be an under reported source of collimation shift.
in mirror cells.


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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: jpcannavo]
      #5772187 - 04/02/13 05:54 AM

Thank You JP this is good info, going to use a thin alloy for the triangle and because it is floting with = preshure on all points I would hope they would flex the same amout at the same time. I may beff them up some.

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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5772202 - 04/02/13 06:24 AM

Some info on the Bubinga Wood I used.
Native Regions
There are nearly 20 different species of bubinga trees that grow throughout the world. At least a dozen of those species are found in the West Equatorial Africa Region, where it is warm and humid. They are primarily native to the Cameroon, Gabon, Ivory Coast, and Congo nations, although a few species are found in South America. They typically grow in tropical climates, along jungle waterways such as swamps, streams and lake shores. Bubinga is commercially known as African Rosewood.

Characteristics
Bubinga is a beautiful and interesting wood with colors ranging from pinkish-red to deep red with irregular purple and brown streaks throughout. Its grain pattern primarily varies from straight to interlocking, but can have a variety of other figures such as waterfall, pommel, and quilted. It has a fine texture with small pores. Also, if rotary cut, the patterns can show swirling and veining throughout. It is a very hard, dense, and heavy wood, weighting an average of 58 lbs/cu. ft. Also, it possesses an exceptional strength-to-weight ratio, bending strength, and shock resistance. Bubinga is a very durable wood due to its oily nature, which provides protection from insects, decay, and wet surroundings.


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droid
rocketman
*****

Reged: 08/29/04

Loc: Conneaut, Ohio
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5772242 - 04/02/13 07:13 AM

Id be interested in knowing how the carbon fiber over balsa works. As an amateur rocketeer, I've glassed balsa in the past, and the balsa gets extremely stiff. But no experience with carbon fiber...yet.

My guess would be somewhere in the 40 lb range.


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ctcables
professor emeritus


Reged: 11/14/06

Loc: Oregon
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: droid]
      #5772272 - 04/02/13 07:53 AM

Droid it is just outstanding at how strong and light it is. the 1" by 1/2" by 14"runners in my upper cage have almost no flex when 40lbs is put on them, have not found the breaking point yet. If I have extra I might test it, the only thing I have seen that is better is a nomex (nomax not sure) which is hexgonal core between carbon fiber sheets. It is used on the new Bowing plane and very very expensive.

some one is selling some at this link if you want to take a look.
http://www.ebay.com/itm/Carbon-Fiber-Balsa-Core-Sheet-6-x-18-1-2-Thick-glossy...


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tezster
scholastic sledgehammer
*****

Reged: 07/14/09

Loc: Missisauga, Canada
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: ctcables]
      #5772370 - 04/02/13 09:02 AM

I'm going to throw in my guess at 48.5 lbs

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nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: The Making of my 20" telescope for a Carl Z mirror new [Re: tezster]
      #5774251 - 04/02/13 09:56 PM

It will weigh 51.25 lb

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