danm
sage
Reged: 04/27/08
Posts: 231
Loc: Northern California
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Hi,
I've got a 12.5" f/4 mirror from my Dad I'd like to turn into a scope. I was aiming to put it into an Obsession, but it's f/4 not f/5, and after all the spinning around I'm thinking of just doing a DIY.
I have access to a machine shop and a brother in law's wood shop. What would you do with the above mirror?
Dan
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TV Ethos 13mm
Oberwerk Ultra 15x70
Celestron C6 f/10 & AT Voyager Alt/Az mount
8" f/6.3 Newtonian (Dad's ATM) & AT66 red tube
12.5" f/4.1 Telekit (Dad's ATM Mirror)
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Adventurer
professor emeritus
Reged: 11/22/07
Posts: 713
Loc: Amman, Jordan
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I will do reflector telescope with it  
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"man is not made for defeat....A man can be destroyed but not defeated"
Regards
Rasim Ramadan
Home made 10 inches reflector.
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mconnelley
super member
Reged: 03/14/06
Posts: 198
Loc: Fremont, CA
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Hello:
First, I'd ask Obsession if they are willing to do a special job for you. To accomodate an f/4 mirror, they just need to use a larger secondary (which they have for the larger scopes), adjust the angle of the pole seats on the mirror box, and use shorter poles. Another dob maker may also be willing to do this for you as well.
Since the mirror is so short, you may want to consider just using a solid tube. I have a 12.5" Cassegrain in a tube made from rolled sheet aluminum. It has an f/3.8 mirror and is 46" long. A tube this long should fit pretty easily in most cars. I suspect that a conventional truss tube wouldn't weight much less than a sheet metal tube for a 12.5" scope when all is said and done. I chose to go with a sheet metal tube since it was much easier to make than a truss tube. Setting up the scope is much faster than with a truss, and I don't have to worry about forgetting the truss poles at home. A round solid tube also makes mounting the OTA on an equatorial mount a bit easier, especially if you're going to use tube rotation. The down side is that it's more vulnerable to wind shake and tube currents, and I get to lift the 50 lbs of the scope all at once rather than in several pieces. Think about making an old-fashioned solid tube scope.
Cheers
Mike Connelley
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Vic Menard
Carpal Tunnel
Reged: 07/21/04
Posts: 2682
Loc: Bradenton, FL
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Have you looked here:
http://www.astrosystems.biz/telekits.htm
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RRavneberg
professor emeritus
Reged: 12/02/05
Posts: 632
Loc: Columbus, OH
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Quote:
Have you looked here:
http://www.astrosystems.biz/telekits.htm
I would second Vic's comment. Randy Cunningham at AstroSystems will prepare a custom TeleKit based upon the parameters of your specific mirror. It's tough to go wrong with his stuff.
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Ron Ravneberg
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gatorengineer
professor emeritus
Reged: 02/28/05
Posts: 725
Loc: Hellertown, PA
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Do not dob that mirror without some serious consideration of oberving height. This scope will have ep at 4 feet at zenith in a traditional dob. At 45 degrees around 3 feet. This would be very uncomfortable for me at least. I have a solid tube on my f3.5. And believe its the way to go. I am going to be the guinea pig for a new ultra heavy alt av design from a very reputable manufacturer and should be posting a review shortly. My fall back will be putting it on a cge which is in the mail. Think 14" sonotube parallax rings, a moonlight focuser. Do not skimp on mirror cell or spider. Collimation will need to be very solid. Of course a paracorr or better would be required.
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16" & 20" Dobs
4" Triplet Refractor
10" LX200R OTA
8"F20 Dall Kirkham
12.5" F3.5 Newt
CGE
Lots of binos---
Nikon 8x30, 10x35, Prostars,10W/18 x 70 astrolux; 10x50 Fujis. Collectors items 12x60 BLC, 10x80 Flaks, Gas Masks, US Mark 28, 30, 41, 43's
Telescopes Past - 8" Stf Mak, C8, Meade LX10-10", SN8, TAL150K, Orion 150MC, Jason 60mm refractor, ATM 6" F8, WO 110FLT, 92mm Off Axis Newt, Nikon 20x120 bino's, 15x110 Boarderguards, Kuhne Flaks
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danm
sage
Reged: 04/27/08
Posts: 231
Loc: Northern California
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Mike,
I was thinking of solid tube. Where did you find a ~14" tube?
Dan
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TV Ethos 13mm
Oberwerk Ultra 15x70
Celestron C6 f/10 & AT Voyager Alt/Az mount
8" f/6.3 Newtonian (Dad's ATM) & AT66 red tube
12.5" f/4.1 Telekit (Dad's ATM Mirror)
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
You need to find a metal shop with a rolling machine. They can roll a thin sheet of Aluminium into a tube and a couple of end rings and a stop ring for the tube mount. The problem is finding someone who can weld a seam or the cheapest method is to put a piece of aluminium flat bar under the seam and tap countersunk bolts into it and glue as well which I have done for a friend when we downsized an existing tube slightly to match his Fullerscope rings.
Cheers. Andrew.
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Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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SteveV
member
Reged: 11/26/06
Posts: 68
Loc: Southern Ore.
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Truss scope and an observing chair, DIY.
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Sky90
10" Dob
16" Dob
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danm
sage
Reged: 04/27/08
Posts: 231
Loc: Northern California
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I ended up going with a TeleKit. My Optician (Cary at OWL) will try and push it to f/4.5 or more during polishing, which is a reasonable compromise on the focal length.
--------------------
TV Ethos 13mm
Oberwerk Ultra 15x70
Celestron C6 f/10 & AT Voyager Alt/Az mount
8" f/6.3 Newtonian (Dad's ATM) & AT66 red tube
12.5" f/4.1 Telekit (Dad's ATM Mirror)
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Strgazr27
Scope Junkie
Reged: 10/04/04
Posts: 6025
Loc: StonyHill Observatory
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A wicked astrograph with oversized secondary
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Bobby
StonyHill Observatory
Atlas w EQMOD
Vixen R200SS
Orion ED-80
AT-111
AT66-ED
QHY-8
SSAG
YAHOO TMB 130SS Group
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Starkler
professor emeritus
Reged: 11/04/05
Posts: 692
Loc: Australia, Melbourne
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Quote:
A wicked astrograph with oversized secondary
That was my thought upon seeing the thread title and I'm not even into imaging.
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Geoff
15" SDM truss dob | Vixen r130sf | GSO 10" dob
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danm
sage
Reged: 04/27/08
Posts: 231
Loc: Northern California
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Sorry I don't follow. Why would this mirror be a good astrograph? Seems like it would be a baddie because of the short f ratio & coma.
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TV Ethos 13mm
Oberwerk Ultra 15x70
Celestron C6 f/10 & AT Voyager Alt/Az mount
8" f/6.3 Newtonian (Dad's ATM) & AT66 red tube
12.5" f/4.1 Telekit (Dad's ATM Mirror)
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Joe Cipriano
Entropy Personified
Reged: 09/03/05
Posts: 3830
Loc: Uh... anyone have a GPS?
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A Paracorr would take care of most of the coma; the short f ratio allows shorter subexposure times.
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In the Land of Eternal Light Pollution & Great Pizza (Chicago)
SN-6, ED80, WO 66SD
Meade 208xt, SBIG ST-4
D70 (modified)
CGE (way modified)
A Wife who understands (unmodified)
Some other stuff...
I may not have gone where I intended to go, but I think I have ended up where I needed to be.
Douglas Adams
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Strgazr27
Scope Junkie
Reged: 10/04/04
Posts: 6025
Loc: StonyHill Observatory
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Joe is correct except I'd substitute the Parracor with an MPCC.
Dan, If you change your mind and want to sell the mirror drop me a PM.
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Bobby
StonyHill Observatory
Atlas w EQMOD
Vixen R200SS
Orion ED-80
AT-111
AT66-ED
QHY-8
SSAG
YAHOO TMB 130SS Group
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danm
sage
Reged: 04/27/08
Posts: 231
Loc: Northern California
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Eh, I'm not ready to get into imaging yet, and no my Dad's mirror isn't for sale! 
I wish you guys wouldn't make such a case for an astrograph, makin' me doubt the quick and easy route of a dob
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TV Ethos 13mm
Oberwerk Ultra 15x70
Celestron C6 f/10 & AT Voyager Alt/Az mount
8" f/6.3 Newtonian (Dad's ATM) & AT66 red tube
12.5" f/4.1 Telekit (Dad's ATM Mirror)
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Joe Cipriano
Entropy Personified
Reged: 09/03/05
Posts: 3830
Loc: Uh... anyone have a GPS?
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Quote:
I wish you guys wouldn't make such a case for an astrograph, makin' me doubt the quick and easy route of a dob
You did ask - and there are lots of "yous" here... 
Make whatever scope you think will get the most use. If you're not seriously thinking about imaging, it would be kind of silly to build an imaging scope - you'd probably be happier with a Dob.
Still - that mirror would make one heck of an astrograph...
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In the Land of Eternal Light Pollution & Great Pizza (Chicago)
SN-6, ED80, WO 66SD
Meade 208xt, SBIG ST-4
D70 (modified)
CGE (way modified)
A Wife who understands (unmodified)
Some other stuff...
I may not have gone where I intended to go, but I think I have ended up where I needed to be.
Douglas Adams
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danm
sage
Reged: 04/27/08
Posts: 231
Loc: Northern California
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OK Joe! Now, suppose I were to make this into an imaging scope, how would you do it? Would you just do a classic newtonian, or try and get a hole drilled in it for a classical cassegrain, or ...
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TV Ethos 13mm
Oberwerk Ultra 15x70
Celestron C6 f/10 & AT Voyager Alt/Az mount
8" f/6.3 Newtonian (Dad's ATM) & AT66 red tube
12.5" f/4.1 Telekit (Dad's ATM Mirror)
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Joe Cipriano
Entropy Personified
Reged: 09/03/05
Posts: 3830
Loc: Uh... anyone have a GPS?
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IMHO don't monkey with the primary. Do a newt. You can still use it for visual, even though you'll lose some light from the larger secondary. Drill an extra set of holes in the OTA to allow the primary to be moved (or make sure your FP works for both EPs and your CCD), add a ParaCorr or MPCC, and you'll have a good double-duty instrument. If you're not using a large-chip CCD, you could probably get away with a smaller secondary - but as DSLRs and the QHY8 are very popular (and a good place for a beginner or part-time imager to start out), you want to ensure full chip illumination.
If it were me (95% imaging), I'd be tempted to copy Kerry's astrograph. As his scope is a dedicated astrograph (no visual except via video), you should be pretty serious about imaging before going that route.
Just my $0.02.
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In the Land of Eternal Light Pollution & Great Pizza (Chicago)
SN-6, ED80, WO 66SD
Meade 208xt, SBIG ST-4
D70 (modified)
CGE (way modified)
A Wife who understands (unmodified)
Some other stuff...
I may not have gone where I intended to go, but I think I have ended up where I needed to be.
Douglas Adams
Edited by Joe Cipriano (06/14/08 10:51 PM)
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danm
sage
Reged: 04/27/08
Posts: 231
Loc: Northern California
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Hi Joe,
Thanks for your money! Even if it's only $0.02 ... 
That's a fine idea. I am very interested in imaging, but I was just planning on doing it sometime in the next five years or later. I've got plenty to keep me busy visually until then.
The Kerry scope is cool, but I would want it for DSO imaging, and all that aluminum wouldn't be conducive to that kind of work I think. I'm not interested in video. Besides I'd like a bigger scope for pictures, I'm looking at the AT 16" RC which is supposedly coming out.
Hrmmmm ...
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TV Ethos 13mm
Oberwerk Ultra 15x70
Celestron C6 f/10 & AT Voyager Alt/Az mount
8" f/6.3 Newtonian (Dad's ATM) & AT66 red tube
12.5" f/4.1 Telekit (Dad's ATM Mirror)
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