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  #21  
Old 13-09-2018, 12:52 PM
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Jason D (Jason)
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Quote:
Originally Posted by bojan View Post
Yes and yes

It is possible (but not easy) to see the designed offset on attached images.
However, on design drawing, the offset from optical axis of the secondary mirror centre is evident
I see the offset clearly in the photo.

Check my first attachment.

Refer to the left-hand diagram. It represents a reflector with its secondary mirror mounted with the proper offset away from the focuser. This is the typical diagram most have in mind when they think of Newtonians.

Refer to the middle diagram. It represents a reflector with its secondary mirror mounted without the proper offset away from the focuser. Many would claim this is a problem because it will miss part of the light cone. I presume this is the issue you had in mind for a secondary mirror mounted without an offset, correct? This is a common misconception. There is really no optical issues because after the proper collimation steps are completed the primary mirror will end up tilted forward towards the focuser automatically -- as shown in the right hand diagram. Now the central star in the FOV is no longer coincident with the OTA axis which has no impact on the optical alignment.

The second attachment illustrates better how both secondary mirror mounts are equivalent from an optical perspective.

One thing to watch out for is to ensure the now tilted light path does not get eclipsed by the edge of the front opening of the scope. Unless the OTA opening is within few millimetters from the primary mirror size, this is not a concern.

I have my secondary mirror mounted without an away-from-focuser offset and I do not have any optical issues with this setup.

Jason
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  #22  
Old 13-09-2018, 02:05 PM
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Yep.. all clear.
As demonstrated on your drawings, I also wanted to reduce misalignment of light beam relative to tube (truss frame) axis... because this scope will be mounted on EQ6 and used for AP, and misalignment such as this potentially reduces pointing accuracy somewhat (it is possible to compensate that with Bartels software, but it needs to be done every time collimation was changed… not very often).


Last edited by bojan; 13-09-2018 at 02:20 PM.
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  #23  
Old 13-09-2018, 03:29 PM
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Quote:
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It may also be a bit of wrong secondary offset...
Because the light beam reflected from primary is conical, for it to pass the secondary unaffected by vignetting, it needs to be positioned a bit away from focuser (2~4mm, depending on f/ ratio and other factors).
Do you still believe that an away-from-focuser secondary mirror offset is needed to avoid vignetting? That is what I understood from your above statement.

Jason
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  #24  
Old 13-09-2018, 05:37 PM
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Well, it's still up the creek! Took it out for a test and I can't get a star to a pinpoint. The secondary shows as a perfect circle, in the center of the focus tube. The donut is aligned with the spider vanes in the center. The laser says it's collimated. See the attached pic for what I see when I rack the focus in and out.
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  #25  
Old 13-09-2018, 06:05 PM
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Quote:
Originally Posted by Jason D View Post
Do you still believe that an away-from-focuser secondary mirror offset is needed to avoid vignetting? That is what I understood from your above statement.

Jason
Yes, in specific circumstances (narrow tube for example), as illustrated on your drawings above.
And to have mechanically correctly designed telescope system.
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  #26  
Old 13-09-2018, 10:44 PM
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Not necessarily...
Then put the the secondary back in place (make sure it is offset away from focuser to avoid vignetting) and adjust the laser beam to point to the centre of the mirror.. then adjust the returning laser beam such it is hitting the secondary at the same spot by tilting primary.
Above is a quote from the first reply to the OP which happened to be yours. You asked the OP to offset the secondary mirror away from the focuser to avoid vignetting which later you explained to be light cone vignetting. You did not question the OP to find out if such an offset is needed. I can tell
from the OP's photo that the secondary mirror is mounted centrally on its stalk without an offset. Asking the OP to move the secondary mirror away from the focuser unnecessarily will cause opposite spider vanes to be at an angle with respect to each other which will worsen the diffraction spikes. In addition, suggesting that the away-from-focuser secondary mirror offset is needed to avoid light cone vignetting is incorrect.

Jaosn
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  #27  
Old 14-09-2018, 06:17 AM
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Quote:
Originally Posted by Jason D View Post
Above is a quote from the first reply to the OP which happened to be yours. You asked the OP to offset the secondary mirror away from the focuser to avoid vignetting which later you explained to be light cone vignetting. You did not question the OP to find out if such an offset is needed. I can tell
from the OP's photo that the secondary mirror is mounted centrally on its stalk without an offset. Asking the OP to move the secondary mirror away from the focuser unnecessarily will cause opposite spider vanes to be at an angle with respect to each other which will worsen the diffraction spikes. In addition, suggesting that the away-from-focuser secondary mirror offset is needed to avoid light cone vignetting is incorrect.

Jaosn
I am not sure why you insist I am in wrong here.

Yes, of course this statement was mine, the post is here to stay for all to see for all times, and I do not see any problem with it.

All I did in this thread was I gave an (free!) advice in attempt to help clarifying the cause of the problem (based on my own experience with Newtonian scope design), in an evolving discussion which at the end produced some results (and I never intended to claim at any time that my contribution is crucial for resolution).

Quote:
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.... In addition, suggesting that the away-from-focuser secondary mirror offset is needed to avoid light cone vignetting is incorrect.

Jaosn

I disagree with this - offset may be needed to avoid the vignetting in case the scope tube is too narrow, situation clearly illustrated with your third drawing, and this is what I wanted to avoid in my design. Plus I wanted the optical axis of my scope to be paralel with mechanical axis of my tube (frame), also illustrated with your own drawing.


This offset is of course meaninless with open designs (and oversized tubes, used in Dobsonians).

Last edited by bojan; 14-09-2018 at 06:32 AM.
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  #28  
Old 14-09-2018, 08:31 AM
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If I can get a word in...... turns out I have a pinched primary.
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  #29  
Old 14-09-2018, 08:49 AM
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Elimination process is obviously finished now
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  #30  
Old 14-09-2018, 04:44 PM
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Quote:
Originally Posted by bojan View Post
Yep...
It may also be a bit of wrong secondary offset...
Because the light beam reflected from primary is conical, for it to pass the secondary unaffected by vignetting, it needs to be positioned a bit away from focuser.
You were clearly not thinking of the front UTA/OTA opening when you referenced vignetting. You were clearly referencing vignetting introduced by any secondary mirror mounted without an offset. I decided to contribute to this thread to explain this is incorrect. There are reasons when a secondary mirror offset is needed but what you have stated in the above quote is not one of these reasons.

OK, I will stop now. This exchange is becoming extremely uncomfortable.

Jason
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