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Do I really have to get the secondary alignment exactly right?

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16 replies to this topic

#1 bokemon

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Posted 26 July 2021 - 12:16 AM

Hello folks, 

I have three collimation tools- a Farpoint Cheshire, laser, and Catseye Autocollimator.

The scope in question is a BIG 12" F4 imaging newtonian, which means that it is an on equatorial mount and the focuser points downwards to minimize center of gravity.

The primary mirror cell is not that great, so I need to check collimation before every session.

 

The problem is that I can not get consistent alignment with the laser due to the focuser and its thumbscrews.  The only consistent way is zero thumbscrews while pressing the laser against the lip of the drawtube.  None of the permutations of one thumbscrew, two thumbscrews, tighten one first, barely tighten them, tighten while pressing against the drawtube, etc give a consistent result.

 

On the other hand, the tilting problem doesn't affect the Cheshire since that only has to do with lateral displacement, which is small.  I can leave it clamped with the thumbscrews and adjust the primary mirror back and forth.

 

It's very hard to use the AC because I have to look from underneath the scope and can't really adjust the secondary at the same time.

 

Finally, my coma corrector / camera combo does seem to have some tolerance to tilt if it is not pointed exactly along the optical axis, as I use this with good results on one of my other scopes, also with thumbscrews on the drawtube.

 

With that long intro out of the way, the question is this:  Assume I can get the cheshire (primary) collimation spot on.  And then I put the laser or AC in the draw tube, and wriggle it around, and find that for SOME orientation, it is spot on.  Does that mean that this telescope now makes a perfect image plane, even if it may not be exactly perpendicular with the drawtube, and probably won't be exactly the same as the camera's optical axis?  From the perspective of the camera sensor, it might be a little offset from the ideal image plane (not that big of a deal) and barely tilted (maybe the coma corrector's design is tolerant of this?).



#2 mayhem13

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Posted 26 July 2021 - 05:24 AM

I use this

 

 https://www.highpoin...ZRoCtf8QAvD_BwE

 

for my laser in 12” Dob......I had the thumbscrew problem too.

 

my method is secondary alignment with the laser and then primary with a 3x Barlow and laser........now you just center the scattered laser reflection of the primary center dot on the laser face sight......takes about 5 minutes all together.


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#3 Asbytec

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Posted 26 July 2021 - 05:58 AM

...(maybe the coma corrector's design is tolerant of this?).

Probably not tolerant. With a coma corrector, the tighter focuser axial tolerance in millimeters is 25.4 * 0.005 * Din = 1.5mm. I doubt fiddling with your laser is going to be that accurate. You may need to use your autocollimator registered in the focuser in a set way, then register your camera adapter the same way. Have you considered a Parallizer? (Maybe the self centering adapter above will work, too). 


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#4 bokemon

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Posted 26 July 2021 - 07:20 AM

I can't use my 2" coma corrector in either device, so what's the point?



#5 sixela

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Posted 26 July 2021 - 08:10 AM

The point is to make sure that once you _do_ insert it and put eyepieces in there, the optical axis is indeed centred in the Paracorr and also not tilted.

 

BTW, I collimate with a Howie Glatter laser collimator with the SIPS installed. The diffraction pattern gets widened a bit but is still usable. 


Edited by sixela, 26 July 2021 - 08:10 AM.


#6 SteveG

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Posted 26 July 2021 - 06:56 PM

 

 

With that long intro out of the way, the question is this:  Assume I can get the cheshire (primary) collimation spot on.  And then I put the laser or AC in the draw tube, and wriggle it around, and find that for SOME orientation, it is spot on.  Does that mean that this telescope now makes a perfect image plane, even if it may not be exactly perpendicular with the drawtube, and probably won't be exactly the same as the camera's optical axis?  From the perspective of the camera sensor, it might be a little offset from the ideal image plane (not that big of a deal) and barely tilted (maybe the coma corrector's design is tolerant of this?).

Your AC is for correcting residual errors from the initial secondary tilt adjustments. I would simply start with the laser, tighten with (1) set-screw the same you do with your eyepieces, and adjust secondary tilt.

Next use the Cheshire to get your primary tilt, then use the Auto Collimator to confirm secondary tilt. Be sure to go back to your Cheshire and double confirm the primary tilt as your last step.



#7 Bean614

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Posted 26 July 2021 - 08:18 PM

The OP's Title for the thread is:

 

"Do I really have to get the secondary alignment exactly right"

 

Truthfully,  you don't HAVE to do anything.   You don't even have to collimate your scope, if you so choose.  No Federal,  State, or Local ordinance requires you to collimate a telescope. 

 

However, if you seriously want to experience the BEST views your scope can give, I would think you would WANT to achieve the most perfect collimation possible.   Most in this forum, I suspect,  feel that way.

 

But, if you don't,  that's OK too.  Just remember,  you then can never, ever, complain about the quality of your optics, because you will never have discovered how really good they can be!


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#8 MitchAlsup

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Posted 26 July 2021 - 08:28 PM

Given this::

"All my questions have to do with imaging, not visual, unless stated explicitly..."

 

For astrophoto, yes you need to get the secondary properly offset, properly aligned, and properly collimated; for the whole field of view to be evenly illuminated.

 

For very fast optics (such as my F/3 with minimal sized secondary) here, too, the secondary needs to be properly offset, properly aligned, and properly collimated to avoid all sorts of bad things happening to my views.

 

For slower optics and for secondaries which are larger than minimal, the tolerances increases dramatically.


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#9 Tom Stock

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Posted 26 July 2021 - 08:33 PM

 

my method is secondary alignment with the laser and then primary with a 3x Barlow and laser........now you just center the scattered laser reflection of the primary center dot on the laser face sight......takes about 5 minutes all together.

Right due to laser registration problems due to slop, locking screw pressure, etc, I do the best I can with laser to align secondary, and this could really be done just looking into the focuser,  but when adjusting primary I only use the barlow+laser combo rather than the laser only.  With this method I've it not only easier but more accurate and repeatable.


Edited by Tom Stock, 27 July 2021 - 08:31 AM.

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#10 bokemon

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Posted 26 July 2021 - 10:24 PM

I don't use eyepieces, there is no 1.25" drawtube involved, and my initial complaint is that I can't even get consistent laser alignment with 1 thumbscrew.



#11 bokemon

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Posted 26 July 2021 - 10:26 PM

The OP's Title for the thread is:

 

"Do I really have to get the secondary alignment exactly right"

 

Truthfully,  you don't HAVE to do anything.   You don't even have to collimate your scope, if you so choose.  No Federal,  State, or Local ordinance requires you to collimate a telescope. 

 

However, if you seriously want to experience the BEST views your scope can give, I would think you would WANT to achieve the most perfect collimation possible.   Most in this forum, I suspect,  feel that way.

 

But, if you don't,  that's OK too.  Just remember,  you then can never, ever, complain about the quality of your optics, because you will never have discovered how really good they can be!

I don't use high power eyepieces to observe Airy disks, so I will never know "how good the optics can be" anyway


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#12 Asbytec

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Posted 27 July 2021 - 02:53 AM

I don't use eyepieces, there is no 1.25" drawtube involved, and my initial complaint is that I can't even get consistent laser alignment with 1 thumbscrew.


I presume you checked your thin beam laser is true. Is it the Farpoint laser that is sometimes sold with your Farpoint Cheshire? Either your laser is not true or you have a registration problem. Or both. To ensure good registration you may need a self centering adapter or Parallizer.

Edited by Asbytec, 27 July 2021 - 02:58 AM.


#13 bokemon

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Posted 27 July 2021 - 03:34 AM

yes, the laser is true because I can rotate it.

Not sure how a paralleizer is going to help.  Now I need to use 1.25" collimation tools?  And the parallizer still has to fit in the 2" drawtube via thumb screws.  Then I have to pull it out to use the camera.



#14 sixela

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Posted 27 July 2021 - 04:05 AM

Not sure how a paralleizer is going to help.

It's designed to register in the focuser consistently when using one (properly positioned) thumbscrew (preferably one that sticks out in the slot where there is no compression ring), and to register whatever you put in it correctly also with only one thumbscrew.



#15 Asbytec

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Posted 27 July 2021 - 04:23 AM

yes, the laser is true because I can rotate it.
Not sure how a paralleizer is going to help. Now I need to use 1.25" collimation tools? And the parallizer still has to fit in the 2" drawtube via thumb screws. Then I have to pull it out to use the camera.

Okay, try this. Does your focuser have a compression ring? If so, then remove it and try the laser with one or two screw registration. You should be able to get reasonably consistent registration. One screw should push the laser to one side of the focuser. Maybe 2 screws will, too. If your registration is not consistent, maybe the action of the compression ring is a potential culprit. I removed mine because tightening the screws squeezed my focal extender out of the focuser adapter. It no longer seated evenly. Maybe something like this is happening to your laser.

Edited by Asbytec, 27 July 2021 - 04:27 AM.


#16 Jon Isaacs

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Posted 27 July 2021 - 04:33 AM

My two cents:

 

This scope is an imaging scope used for imaging. 

 

- The Farpoint laser is a 1.25 in/2 inch Collimator.  The Parallizer a 1.25 inch to 2 inch adapter meant to be used with 1.25 inch lasers.  

 

- As far as secondary placement, I think Mitch had it right, this is an imaging scope so one wants the secondary setup to center the illuminated field of view.  Of course if the secondary is large enough that the illuminated field is larger than the camera chip, this provides more wiggle room.

 

- And the secondary tilt/focuser axial alignment want to be right on to avoid tilt of the field.

 

- Spot on primary collimation is probably less important since this is is basically a low power application but if it is off, it shifts the coma correction.

 

Jon 



#17 SteveG

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Posted 27 July 2021 - 03:06 PM

I presume you checked your thin beam laser is true. Is it the Farpoint laser that is sometimes sold with your Farpoint Cheshire? Either your laser is not true or you have a registration problem. Or both. To ensure good registration you may need a self centering adapter or Parallizer.

He also has the Auto Collimater, which will correct for any loose tolerances from the laser, if used properly.


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