Flux
member
Reged: 11/02/06
Posts: 26
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I did a picture to show the problem that I think I am having. I also did another post on this but I thought it would be better to a picture this time. My question is should I be seeing the spider vain screws when I collimate? The screws do not show as much in reality as they do in the picture, I just wanted to emphasize the issue.
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Vic Menard
Carpal Tunnel
Reged: 07/21/04
Posts: 2681
Loc: Bradenton, FL
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Quote:
...should I be seeing the spider vain screws when I collimate? The screws do not show as much in reality as they do in the picture, I just wanted to emphasize the issue.
It's not uncommon, although I'm curious why only two are visible.
It could mean that your front aperture is a bit undersized, but it's more likely that your pupil (collimation cap) is some distance inside the focal plane which makes the front aperture (and spider mounting screw attachments) more visible.
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sixela
Postmaster
Reged: 12/23/04
Posts: 9499
Loc: Boechout, Belgium
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The third, of course, is hidden behind the focuser. The lack of offset away from the focuser will somewhat hide the one away from the focuser (as the optical axis frequently ends up tilted slightly towards the focuser).
Mind you, that's not indicative of real a problem, as long as the front of the tube does not clip the bright primary's reflection.
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400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1960
Loc: California
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Quote:
The third, of course, is hidden behind the focuser. The lack of offset away from the focuser will somewhat hide the one away from the focuser (as the optical axis frequently ends up tilted slightly towards the focuser).
That was my initial thought but the fact the vertical vanes reflection are centered suggests the mirror was mounted with full-offset and the right spider vanes screw should be visible. Had the vertical vanes reflection been shown center aligned with respect to the secondary silhouette, then the right spider vanes screw might not be visible.
But then again, it is only an illustration and we are being aggressively picky 
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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Flux
member
Reged: 11/02/06
Posts: 26
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So, over all does the collimation look good?
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1960
Loc: California
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Quote:
So, over all does the collimation look good?
Yes it does 
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
Loc: Los Angeles
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This looks like near-perfect collimation to me.
The outline of the secondary shadow is always offset as you show (though it's exaggerated slightly).
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Flux
member
Reged: 11/02/06
Posts: 26
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Thank you every one for your comments and help.
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deSitter
scholastic sledgehammer
Reged: 12/09/04
Posts: 779
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This is off by a substantial amount!
It appears the secondary is pre-mounted with the proper offset - meaning the secondary post itself should be centered in the tube, and if you can see any of the spider joints, you should be able to see all of them, equally.
With a secondary that is mounted with the proper offset, the final result should show everything perfectly concentric, other than the outline of the diagonal reflected in the main mirror.
-drl
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sixela
Postmaster
Reged: 12/23/04
Posts: 9499
Loc: Boechout, Belgium
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Quote:
This is off by a substantial amount!
No, it just shows the optical axis is probably not along the tube axis. But this is a Newt, so this is not a collimation error ("off") in se. Many scopes are shipped without any offset of the secondary away from the focuser.
Also, this is a diagram and nothing is really on scale, so anything that shows as "substantial" on the drawing isn't sure to be substantial in real life.
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400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1960
Loc: California
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Quote:
This is off by a substantial amount!
 -- I do not see the "substantial" error.
Quote:
With a secondary that is mounted with the proper offset, the final result should show everything perfectly concentric, other than the outline of the diagonal reflected in the main mirror.
The above statement implies an optical axis coinciding with the OTA axis.
A scope with a fully offsetted secondary implies its focuser and optical axes are perpendicular to each other when axial alignment is achieved. The optical axis is not guaranteed to coincide with the OTA axis.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
|
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