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# Newtonian spider collimation mechanics

7 replies to this topic

### #1 Dodde

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Posted 07 April 2021 - 03:15 PM

How does the mirror holder actually gimbal on the center bolt? Does the collimation screws just force or bend the material in the center bolt and/or holder or is there an actual mechanical joint in there somewhere.

What is the typical maximum angle interval the holder can be or needs to be moved through by means of turning the collimation screws?

### #2 CosmoSat

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Posted 07 April 2021 - 03:29 PM

The center bolt is loose in the holder without threads, the secondary stays put because of the three collimating screws pushing at it. The small play is enough to achieve the degree of angle required

### #3 MitchAlsup

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Posted 07 April 2021 - 08:57 PM

What is the typical maximum angle interval the holder can be or needs to be moved through by means of turning the collimation screws?

And F/3 mirror has a light cone that converges at 19º, so about 9.5º moves the laser from the edge of the mirror to the center. In practice, ±1º is sufficient for any build using reasonable tolerances.

A much harder requirement is that the position is repeatable/stable to small fractions of a millimeter on the primary at any pointing angle, than how much adjustment can be provided. I build my secondaries with just over 1º of adjustment and I build the rest of the scope with enough stiffness than the laser point on the primary does not move any measurable/visible amount.

### #4 Dodde

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Posted 08 April 2021 - 04:03 AM

And F/3 mirror has a light cone that converges at 19º, so about 9.5º moves the laser from the edge of the mirror to the center. In practice, ±1º is sufficient for any build using reasonable tolerances.

A much harder requirement is that the position is repeatable/stable to small fractions of a millimeter on the primary at any pointing angle, than how much adjustment can be provided. I build my secondaries with just over 1º of adjustment and I build the rest of the scope with enough stiffness than the laser point on the primary does not move any measurable/visible amount.

Does the gimbal point between the center bolt and the holder need to be where it is (seems fiddly and inaccurate) and to also be semi-spherically movable as it is with the traditional three-bolt design or can this movement be mechanically restrained to three fixed planes or axies, i e

1) tilting the secondary around it s minor axis, locating the actual axis center point of rotation in height with the optical surface

2) rotating it around the main mirror optical axis

3) moving it "up" and "down" along the main mirror optical axis

If it seems sound, what would a sensible intervall be for each of the respective movements.

### #5 CosmoSat

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Posted 08 April 2021 - 11:04 AM

Does the gimbal point between the center bolt and the holder need to be where it is (seems fiddly and inaccurate) and to also be semi-spherically movable as it is with the traditional three-bolt design or can this movement be mechanically restrained to three fixed planes or axies, i e

1) tilting the secondary around it s minor axis, locating the actual axis center point of rotation in height with the optical surface

2) rotating it around the main mirror optical axis

3) moving it "up" and "down" along the main mirror optical axis

If it seems sound, what would a sensible intervall be for each of the respective movements.

It's the combination of these movements you mention of the secondary mirror that will help one to coincide the primary axis with the eyepiece/focuser axis.

### #6 MitchAlsup

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Posted 08 April 2021 - 11:28 AM

Does the gimbal point between the center bolt and the holder need to be where it is (seems fiddly and inaccurate)

The top figure is the way I design secondary holders, the one at the bottom is the "typical" way to hold secondaries.

In my design, the center of least movement is almost coincident with the optical axis, so the tilt and tip are not moving the secondary laterally (or longitudinally). In the typical design the center of least movement is almost at the top of the secondary mirror, so, tilt and tip adjustments move the secondary off the optical axis. This is more important as optical speed goes up.

Notice, also, that the point where forces are applied to the secondary mirror have:: a) almost no distance for bending moments to transpire, b) impinge directly on the silicone glue holding the secondary to the holder. Compare this to the typical secondary design and we see fairly large distances where bending moments can show up, and large cantilevering distance--leading to being vibration prone.

1) tilting the secondary around it s minor axis, locating the actual axis center point of rotation in height with the optical surface

You want the secondary tilt/tip to articulate the secondary as close to the optical axis as possible.

2) rotating it around the main mirror optical axis

3) moving it "up" and "down" along the main mirror optical axis

These are what my design avoids.

### #7 Vic Menard

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Posted 08 April 2021 - 11:31 AM

How does the mirror holder actually gimbal on the center bolt? Does the collimation screws just force or bend the material in the center bolt and/or holder or is there an actual mechanical joint in there somewhere.

What is the typical maximum angle interval the holder can be or needs to be moved through by means of turning the collimation screws?

It depends on the design. The common, 3-screw tilt adjustment secondary mirror holders utilize three push screws that are restrained (loosen one, tighten one, or loosen two, tighten one) by the center mounting screw. The Novak/AstroSystems 4-screw adjustment secondary mirror holders utilize four push screws restrained (loosen one, tighten opposite) by a center mounting stud. Rotation and/or offset on either is accomplished by loosening (or tightening) the center mounting hardware.

The tilt adjustment range is typically a degree or two.

Secondary mirror adjustment is usually two parts:

Offset adjustment-- adjustment closer to or farther from the primary mirror (center mounting hardware) with connected tilt, and

"Skew" -- rotation adjustment (center mounting hardware) with connected tilt.

Edited by Vic Menard, 08 April 2021 - 11:40 AM.

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### #8 Vic Menard

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Posted 08 April 2021 - 11:37 AM

You want the secondary tilt/tip to articulate the secondary as close to the optical axis as possible.

This is possible with four tilt screws (the two associated with offset) or one tilt screw with a hinge attachment (which eliminates skew adjustment).

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