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rchrusciel
member
Reged: 07/26/09
Posts: 12
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I have been out the last few nights trying to finalize my 13" homemade truss tube dob. The problem I am seeing is that when I collimate, my laser is showing that the focuser places the focus point approx. 1/2 to the left of center on the secondary. Obviously I mounted the focuser off a little. Will this greatly affect the overall view. The scope is a 13.1" f4.5 with a 2.5" secondary. This is my first real atm project/scope, so any help would be great.
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Remember when you were a kid, and all you had to do was look up at night to see the milky way. Imagine what our kids will be missing...
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2270
Loc: Kent, UK
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Yes it will affect collimation, better to sort it now or it will plague you forever. Assuming the spider is centred first, pack one side of the focuser to square it up before you continue with collimation. A washer or two is a quick way, or drill and tap grub screws for adjustment for a better job.
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
PST
DSI 1
and a curious mind
Edited by John Carruthers (11/07/09 03:00 AM)
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sixela
Postmaster
Reged: 12/23/04
Posts: 10850
Loc: Boechout, Belgium
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Watch out: because of perspective effects, the focuser axis *will not* intersect the secondary in its geometrical centre; this effect is called diagonal offset:
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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GaryS
super member
Reged: 10/30/06
Posts: 191
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Quote:
I have been out the last few nights trying to finalize my 13" homemade truss tube dob. The problem I am seeing is that when I collimate, my laser is showing that the focuser places the focus point approx. 1/2 to the left of center on the secondary. Obviously I mounted the focuser off a little. Will this greatly affect the overall view. The scope is a 13.1" f4.5 with a 2.5" secondary. This is my first real atm project/scope, so any help would be great.
I'm curious. Why do you say it's your focuser that's off and not your secondary out of position? Rare is the scope that has a focuser tilted so much that you can't compensate with the secondary.
A piece of advice: If you're new to collimation, I'd put the laser away and use a simple collimation cap or Cheshire eyepiece instead. The laser can easily lead one astray.
Regards,
Gary
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Sky & Telescope Contributing Editor
SkyNews Columnist & Blogger
www.GarySeronik.com
A place for stargazing enthusiasts.
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RossSackett
professor emeritus
Reged: 08/17/07
Posts: 691
Loc: Memphis, TN
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...all good advice. I'd add: get yourself a book on collimation like Vic Menard's New Perspectives on Collimation and check out websites like Nils Olof Carlin's
http://web.telia.com/~u41105032/kolli/kolli.html
These will explain where you are headed, and what you do and don't need to worry about.
Ross
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"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
Anagrams: Amateur astronomer = A mature moon-starer; Dobsonian maker = Debonair as monk
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backwoody
Pooh-Bah
Reged: 01/08/07
Posts: 1114
Loc: Idaho USA
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Gary's quote: "I'm curious. Why do you say it's your focuser that's off and not your secondary out of position? Rare is the scope that has a focuser tilted so much that you can't compensate with the secondary."
I think Gary is probably correct, and suspect as he does that you have a collimation problem (with secondary optical alignment). Using a sight tube and cheshire in the standard manner, you ought to be able to align the secondary and primary mirrors optically for the focuser, even if the focuser is slightly tilted.
Hope you can give it a try...
c/s,
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woody
a parsec farther out...
12.5" f/4.8 custom truss dob, EQ platform
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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You need to shim the focuser so the laser shoots directly to the opposite side of the tub/ring/upper cage assembly. This can be easily measures with a tape ruler.
Then position the secondary under the focuser so that the latteral center is directly under the focuser. The center of the secondary mirroed surface will be longitudinally displaced towards the primary and radially away from the focuser.
I placed a little yellow dot on the stray light shield of my dob just so I could square up the focuser to the optical axis.
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