4 replies to this topic

[bob.locascio]

Hello All,

 

I recently acquired a 10' Meade Starfinder (Dob).  I am having trouble with the collimation, specifically adjusting the secondary mirror.

 

The manual states:

"the 3 hex screws on the secondary mirror assembly must be unthreaded slightly to refine the tilt-angle of the secondary mirror until the primary mirror can be seen centered within the secondary mirror assembly."

 

I think I understand the goal of this step, and can see that the primary mirror is not quite centered.  I can loosen the screws and the secondary mirror become wobbly, which I assume is not the end state,  Loosening very slightly does not seem to do much, that I can correlate with the loosening. 

 

And so, I'm not sure what the step is asking me to do:

• Try various combinations of slight unloosening until I achieve the alignment
• Loosen the screws and tilt the mirror to the desired angle "manually", then retighten to lock in place
• Something else.

I have attached a pdf of the manual pages I found online (for the Equatorial Mount version.  The collimation is the same) so you can see the figures, if that is any help.

 

Thanks in advance for your assistance.  

 

Bob

 

Attached Files

•  Meade Starfinder Manual Pages.pdf   337.96KB   12 downloads

[CloudyMatt]

Dine and dash

Edited by CloudyMatt, 05 September 2024 - 12:13 AM.

[SoCalPaul]

In my experience, there is usually a center bump of some kind, upon which the secondary holder rests. The three screws each pull the holder in their respective directions around this bum.

 

Therefore, you can't just start tightening one screw, as it might not result in any movement due to the other two screws holding fast. Also, it could overly increase the pressure against that central bump.

 

So you have to loosen one or both of the other two screws, then tighten the target screw.

 

This may sound complicated and difficult, but in practice, I find it relatively easy. Give this a try, see if it works for you:

 

1) Slightly loosen all 3 screws. This should not make the secondary holder loose or floppy. You are just relieving a bit of the pressure.

2) Figure out which single screw to tighten, to move the primary mirror image as centered as possible. (If this screw starts to feel overly tight, slightly loosen the other two a bit more.)

3) Once you are done with the first screw, continue to iterate on all three screws until the primary mirror image is well centered.

 

This will maintain a reasonable amount of pressure on the center bump.

 

What is reasonable? Again, in my experience, that would be enough to hold the secondary holder firmly in place so that the usual bumps from transport don't throw it off, while at the same time, not damaging anything such as stripping the threads, warping the secondary holder, etc. (That would take quite a bit of torque, eh? LOL.)

 

Note that you might also have to rotate the secondary holder around its stalk so that it squarely faces the focuser. That adjustment is relatively easy and should be done before the steps above. (And in some cases, the secondary could be too low or high in the tube, so that it is not squarely under the focuser. That usually can also be adjusted with the center bolt and a couple nuts.)

 

Lots of good resources on the internet about collimation. I bet there are even Youtube videos showing the process. Good luck!

 

Clear skies,

Paul

[apfever]

Is the 10" the same as the 8" Starfinder secondary assemblies, only bigger?

[Vic Menard]

...I'm not sure what the step is asking me to do...

That's because the manual doesn't know there are (at least) two other adjustments!

 

Correcting the secondary mirror placement involves three distinct adjustments:

1.) The offset adjustment (the secondary mirror position closer to or farther from the primary mirror end of the tube assembly)

2.) The rotation adjustment (relative to the spider) and

3.) The tilt adjustment (tightening and loosening those three screws the manual talks about).

 

This assumes the spider is more-or-less centered in the tube assembly and more-or-less perpendicular to the tube axis, and the focuser (axis) is more-or-less perpendicular to the tube axis. It also assumes that the secondary mirror is mounted correctly relative to the spider so that correcting offset, rotation, and tilt will bring the focuser axis into alignment with the center of the primary mirror.

 

So, correcting the secondary mirror placement is a process that ultimately makes these three circles more-or-less concentric:

1.) The bottom edge of the focuser (outermost light blue circle),

2.) The actual edge of the secondary mirror (green circle), and 

3.) The reflected edge of the primary mirror (red circle)

(Note that the reflection of the secondary mirror (violet circle) will NOT be concentric, but will appear offset toward the primary mirror end of the tube assembly (see illustration below).

 

Here's the deal--sorting out the secondary mirror placement can be difficult, especially for beginners, especially without the right tools. And small(ish) secondary mirror placement errors have little or no impact on image performance. So here's what I suggest:  

 

1.) Using a simple thin beam laser or a combination Cheshire/sight tube collimating tool, adjust the secondary mirror tilt to center the outgoing laser beam (or the sight tube cross hairs) relative to the primary mirror center marker/donut. 

2.) Using a simple collimation cap, adjust the primary mirror tilt to center the primary mirror center marker/donut relative to the collimation cap pupil.

 

If, after completing the above two alignments, you can still see most of the primary mirror reflected in the secondary mirror, your collimation should be "good enough" to deliver good image performance. Do NOT try to adjust the secondary mirror tilt to center the reflection of the primary mirror! If you want to resolve the secondary mirror placement error, read here:  https://www.cloudyni...ment/?p=5260727

Attached Thumbnails

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Edited by Vic Menard, 05 September 2024 - 01:35 PM.