Collimation Query + Photo

[cybernautic]

Im not seeing your image just a red circle with a hand in it.

Sorry about that. Not sure why it's not appearing, but it just showed pretty much what I can see through the collimating EP, a white circle, with the spider vanes and a black hole in the centre. I'm not seeing the concentric rings. I've read a little about star testing since and I think I should be using a much higher magnification to see the concentric rings. I'll try again tonight (if it's clear!)

[Moonshane]

the concentric rings would only show with an eyepiece in and at higher powers (magnifications). you need a star that does not move like Polaris and defocus just a midge either side of sharp.

[bambuko]

...Not sure why it's not appearing...

Have you tried to hotlink to someone's picture on their webpage?

[dcwales2012]

I'm a great believer in; collimation tells lies, and I've had a discussion about this before, which Moonshane contributed too.

Your secondary may look centred in the colli cap, or Cheshire eyepiece, but you need to check to see if its square in the spider. Take a small mirror and see if its seated at an equal distance all around in the holder. If its not, then collimation will not be achieved correctly.

You can't see any clips in my 10" flextube and the secondary was way out even when the Cheshire appeared to indicate it was spot on. I tried my 6" 150p where I can see the clips in the Cheshire, yet when checking secondary mirror its was tilted in the holder. So I would suggest getting secondary square first, then align with cap or Cheshire.

[Jason D]

Your secondary may look centred in the colli cap, or Cheshire eyepiece, but you need to check to see if its square in the spider. Take a small mirror and see if its seated at an equal distance all around in the holder. If its not, then collimation will not be achieved correctly.

What do you mean by "check to see if it is square in the spider?"

Jason

[Merlin66]

Jason et al,

Just a question....is it normal nowadays to put a "centre spot" (off set as needs be) on the secondary mirror???????

[Jason D]

Jason et al,

Just a question....is it normal nowadays to put a "centre spot" (off set as needs be) on the secondary mirror???????

Spotting the secondary mirror was never the norm and it should not be the norm. Following the collimation steps using a cheshire eyepiece will get the job done without the need to spot the secondary mirror.

Jason

[dcwales2012]

Not sure how much you know, so I'm sorry if I'm teaching you how to suck eggs. Basically, the secondary mirror fixes to the spider hub by the means of the large centre screw, which one turns left or right to move the secondary up and down the tube to centre it in the focuser, and 3 small Allen screws. Sometimes when the scope comes from supplier the secondary is badly fitted and can be tilted or rotated in the spider holder. Yet, it can still appear centred in the Cheshire/Cap because the 3 allen screws have been adjusted to compensated for this. To see if the secondary is square, i.e. equal distant; all around, away from the spider hub, one uses a mirror held inside spider to be able to see where secondary joins spider hub to check this.

If your secondary is not set at equal distance all the way around, your probably end up adding more rotation and tilt. So I would suggest secondary alignment to spider holder first, then move secondary up/down tube as required to centre in focuser tube, then line up with Cheshire using 3 Allen screws.

Hope this helps.

[wookie1965]

What dcwales2012 is trying to make sure is that your mirror looks like the picture (tilt2) not (tilt1) these pictures are from astrobaby`s guide.

[dcwales2012]

Cheers Wookie.

[Scooot]

You can place a piece of card inside the tube to block out the reflection of the primary, and opposite the focusser to make the secondary more prominent, and then look down the focusser to judge how central and round the mirror looks. This is a photo of mine I took through the collimation cap with my phone.

[Jason D]

I'm a great believer in; collimation tells lies, and I've had a discussion about this before, which Moonshane contributed too.

Uh, you are talking about what is known as secondary tilt/rotate error. "Squaring the secondary" is unfamiliar term in the collimation world.

Bear in mind that the error can be introduced even when the spider vanes are centered in the tube.

Why do you think little rotate/tilt error is a problem? Why did you use the strong word "lies"?

There is nothing wrong with little rotate/tilt error especially for mass produced scopes where the secondary mirror size is little oversized.

If the scope owner does not have the proper skills to eliminate this error then it is absolutely OK to leave it.

Attached illustration shows an exaggerated form of this error.

Jason

[cybernautic]

Thanks for the replies. I'm going to try another star test tomorrow.

[dcwales2012]

Collimation World, aaah so the place does exsist.

Collimation tells lies - why do I believe that. A secondary can look centred in the Cheshire, plus one can see the clips, and the crosshairs line up. Adjust primary so donut lines up with secondary and collimation is spot on. Yet on the right hand side when viewing down Cheshire one can see the focuser guide wheel. Secondary too far down tube, yet collimation indicated everything was correct and no error.

Another error I have found, is put tilt onto secondary, and then twist the mirror until it appears centred in Cheshire, adjust allen screws to align secondary mirror in Cheshire. Align with primary, and your scope is in theory collimated. What I found out was secondary had a rotation error, and a tilt error which placed a strain on one allen screw. But again, collimation appeared to be correct.

I have changed the way I collimate from the norm, to get better results because of the errors.

I agree a small amount of tilt and rotation is not critical.

Happy collimating. :-)

[Jason D]

A secondary can look centred in the Cheshire, plus one can see the clips, and the crosshairs line up. Adjust primary so donut lines up with secondary and collimation is spot on.

You just described a well-collimated scope

Yet on the right hand side when viewing down Cheshire one can see the focuser guide wheel. Secondary too far down tube, yet collimation indicated everything was correct and no error.

How can the secondary be too far down the tube when you stated it was centered in the cheshire eyepiece?

Secondary mirror tilt/rotate error is not considered a "lie". It is the least important collimation error and can be ignored unless it is severe.

Scopes with some tilt/rotate error can still achieve perfect axial collimation.

Jason

[Moonshane]

Collimation World, aaah so the place does exsist.

Collimation tells lies - why do I believe that. A secondary can look centred in the Cheshire, plus one can see the clips, and the crosshairs line up. Adjust primary so donut lines up with secondary and collimation is spot on. Yet on the right hand side when viewing down Cheshire one can see the focuser guide wheel. Secondary too far down tube, yet collimation indicated everything was correct and no error.

seeing the clips is incidental to collimation. what would you do if your mirror had no clips?

the critical issue for this stage is that the whole of the primary can be 'seen' by the secondary and ideally that its reflection is placed in the centre of the secondary mirror (which has already been centred in the drawtube in the previous step).

when you say the cross hairs line up, what are you lining up with? if it's the spider vanes then this will not be correctly collimated (although as above this stage makes little difference in the real world) as generally you should ignore the relationship between the spider vanes and the cross hairs in the Cheshire/sight tube. if it's the reflected donut (which the cross hairs should be centred upon) then you might just find that the first time you collimate you need to iterate through the process a few times as large adjustments on certain aspects can affect others. after that it will be usually only the primary that needs any adjustment.

the physical position of the secondary in relation to the drawtube cannot change unless things are loose or you make really big adjustments with the tilt portion (which could make it not round). adjusting the tilt should be very minor if your first stage of centering the secondary in the drawtube is accurately done.

then again if you have found your own way to collimate that gives you regular and reliable sharp images with lots of contrast keep on doing it your way