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Looking at Moonshane's diagram above reminded me that you could have the secondary slightly rotated. While the secondary adjusts may be able to correct this the error may be too large.

On my home made Newts I needed to rotate the secondary, about it's central support axis, to align the center spot with the horizontal cross hair as shown on the diagram.  I.e. it needs to be at right angles to the direction the center spot moves when you rotate it. Then adjust the secondary screws to fully center the secondary. 

Regards Andrew

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19 minutes ago, Moonshane said:

.....I ignore the clips (not all mirrors have them)

Point taken. 

Though do ensure if you  have to re-apply a donut ( after cleaning maybe? )  that its is accurately replaced! ( that's another topic)

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Looking at Moonshane's diagram above reminded me that you could have the secondary slightly rotated. While the secondary adjusts may be able to correct this the error may be too large.

On my home made Newts I needed to rotate the secondary, about it's central support axis, to align the center spot with the horizontal cross hair as shown on the diagram.  I.e. it needs to be at right angles to the direction the center spot moves when you rotate it. Then adjust the secondary screws to fully center the secondary. 

Regards Andrew

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On 12/8/2016 at 06:34, Charic said:

Moonshanes image is  quite detailed.
From the image provided, the secondary alignment (Pink) is achieved by adjusting the secondary mirror center bolt +/- and if required, with  a small rotation (by hand - don't touch the mirror surface, the edge is fine!)  to get the secondary perfectly circular and equidistant   with the  Cheshire (Blue).

Then the secondary adjusters ( three of them) are turned (one at a time - no specific order )  until you can see the three primary clips on the main mirror. Gently (GENTLY) tighten these adjusters until their just snug, recheck the mirror is still centred and the primary clips are  still visible.

Then the primary mirror is adjusted, loosen the locking nuts/grub-screws/Bobs knobs? and adjust the primary adjusters until the centre spot coincides with the centre axis/cross hair. Nip up the locking nuts, re-check everything, probably good to go at this stage.

Keep at it, your getting there.
 

I believe what you meant to say is to adjust the primary mirror until the cheshire pupil reflection is centered with respect to the chesire's cross hairs. 

If you center the secondary mirror under the focuser and able to see all the primary mirror clips then the primary center spot will most likely be centered with respect to the cheshire cross hairs.

Jason

 

 

 

 

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Mathematically speaking,  adjusting the three secondary mirror set screws will introduce both a rotational and linear displacements -- not just the latter. That is, you do not need to be very precise in getting the correct rotation of the secondary mirror by hand. Get as close as you can then fine adjust the rotational placement using only the set screws.

To illustrate the above point, start off with a perfectly aligned scope. Place a laser collimator in the focuser and the beam will reflect off the secondary mirror striking the primary center. Now introduce a "small" rotation to the secondary mirror. By adjusting ONLY the secondary mirror set screws  you should be able to get the laser beam to strike the primary mirror center again.

Jason

 

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5 hours ago, Jason D said:

I believe what you meant to say is..........

NO! I said it the way I interpreted it from memory at the time of writing?

If you study the Astro Baby guide, there is an image that shows a view through the Cheshire, the 'Pattern'. This shows  the Cheshire cross hairs dissecting the primary mirrors centre spot if all is well and good.
It goes on to enlarge that image to show that there is infact,  a centre spot misalignment.
The solution is to adjust the primary adjusters to align the pattern, not to dissimilar to my statement above!

Whatever the interpretation, following the Astro Baby guide will see you right. 

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If anyone is trying to get a copy of Astro_Baby's 'Guide to Collimation,' her site is currently down on this date. But I would be happy to send a Pdf. of it to you. Send me a PM, and you should have it soon. Just put 'Collimate' in the thread-title.

Dave - Keeper of the Oracle of Delphi :p

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4 hours ago, Charic said:

NO! I said it the way I interpreted it from memory at the time of writing?

You stated "adjust the primary adjusters until the centre spot coincides with the centre axis/cross hair"

Adjusting the primary mirror knobs will not move the center spot reflection. The center spot reflection will virtually remain the same with respect to the cheshire's cross hairs no matter what you do with the primary mirror knobs. What will move is the reflection of the cheshire's pupil. The goal of adjusting the primary mirror knobs is to move the cheshire's pupil reflection to the center of the primary mirror spot reflection.

Jason

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We could continue via PM save hijacking this thread!

30 minutes ago, Jason D said:

Adjusting the primary mirror knobs will not move the center spot reflection

I know were talking about  Cheshire Collimation in this thread,  but  your statement  has  just blown the  Barlowed Laser technique out of the water if the Primary mirrors centre spot reflection does not move!

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4 hours ago, Charic said:

We could continue via PM save hijacking this thread!

I know were talking about  Cheshire Collimation in this thread,  but  your statement  has  just blown the  Barlowed Laser technique out of the water if the Primary mirrors centre spot reflection does not move!

You are mixing things. Assume you are standing 10 feet away from me holding a flash light. Moving/twisting only the hand holding the flash light will not change the flash light apparent position as observed by me; however, the spot light projected by the flash light on the wall behind me will move all over the wall. 

Use a collimation cap or a cheshire. Have someone fiddle with the primary mirror knobs while you keep your eye on the primary center spot reflection through the collimation cap. The apparent position of the center spot reflection will not move. Try it.

Jason

 

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On 2016-12-03 at 14:33, tehmac said:

and have watched and read just about every collimation thing online.

As you have done that, I'm sure you know that Jason is among the most knowledgeable person about collimation. :smiley: If I were you,  I would without any hesitation follow his advices if there's different opinions about collimation.

Good luck with your collimation.

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.......I`ll also be updating my knowledge, just in-case ? 
There's always two sides?  till then,  communication continues?  though  I'm more than satisfied with my present results, and of the advice I have given in the past, but what if?..............always  open minded!

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YSKE, Gerry, thank you for your kind words.

Charic, let me explain my point with the attachment. Refer to the left diagram. The solid red arrow represents the reflection of the center spot as observed from the focuser. The primary mirror is tilted in some direction. If we fiddle with the primary mirror knobs to tilt the primary mirror in the opposite direction (right diagram), the center spot position in 3D space will "virtually" remain the same. Therefore, no matter what we do with the primary mirror knobs, the apparent location of reflection of the primary mirror center spot as observed from the foucser will remain the same.

However, if we insert a laser, the reflection of the laser beam (represented by the dotted red arrows) will change as the primary mirror tilt changes.

You stated "adjust the primary adjusters until the centre spot coincides with the centre axis/cross hair" . The center spot reflection will not move no matter what you do with the primary mirror adjusters.

Jason

 

 

reflection.png

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10 hours ago, Jason D said:

Charic, let me explain..............

I understand your point with regards to the fixed position of the centre spot.

Quote

......I believe what you meant to say is to adjust the primary mirror until the cheshire pupil reflection is centered with respect to the chesire's cross hairs.  [sic]

In principle, that's what I meant.
The method I described above, in my mind does  the same thing, I've described this  method before but never had it questioned?  ( there's always a first)  but I now understand the  reason for  your  concern over the specific wording I used.

So, yes,  unless the mirror is  physically moved laterally, the centre  spot cannot move! 
 

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  • 2 weeks later...

Sorry to piggy back on this one but I'm also having trouble! I can't seem to get the pupil reflection in the doughnut no matter what I do! I can get it to about a 50%overlap but that is about as good as it gets?

Any thoughts?

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I'm using a Cheshire collimator. Yeah I can see the the clips without a problem and they all seem pretty even, I'm just struggling to get it in that last little bit. I did a star test last night in a gap in the clouds and it looked ok but it's still not perfect.

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