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What Is This Collimation Tool Called?

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#26 telesonic

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Posted 25 July 2021 - 04:17 PM

From the discussion It sounds like a Cheshire (like mine) may not be the best tool for collimating a secondary although it can be used. Since this is the only tool I have this is what I used and it seemed to work pretty well. (with help from Vic, Don, Asbytec & others!)

 

Wondering out loud here:

   - If a star test (real star) shows a good airy disk pattern, the primary must be well collimated, yes?

   - Does the good star test automatically mean the the secondary is also well collimated, yes/no/maybe?

   - Is this question going to take me down an optical rabbit hole, yes?

 

As always, many thanks to all for sharing the knowledge.

Well, it might not be the best tool for it - others may have differing opinions here, on this subject.... but it can work. I've used it to good sucess in my F/8 scope, but I also have a few other tools at hand to confirm alignment, a 1.25" AstroSystems Lightpipe, a 1.25" AstroSystems autocollimator (not really needed on my F/8 scope) Celestron Combo tool, and lastly an Orion laser that was way the heck out of alignment, which I've since heavily modified. (collimated, removed the 45 degree window, and added an aperture stop.)

 

I mostly use the Cheshire/Lightpipe - then check with the laser. When they agree, I collimate the primary with the Lightpipe. Keep in mind that at F/8, collimation is rather easy, and forgivable to an extent, but I also have a F/3.6 Schmidt-Newtonian, which is a real bear to collimate.... hence the extra tools like the autocollimator.

 

And I agree, I've definitely learned a ton about collimation from the gurus here, (Don, Vic, Sixela, JasonD... etc.) without them.... I'd probably have no hair left. It get's easier the more you do it, then it eventually just clicks, like an ah-ha! moment.


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#27 sixela

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Posted 25 July 2021 - 05:14 PM

Interesting, sixela's opinion about Hotech collimators.  I like mine.  Granted, I'm no Vic Menard (who of us is?), but it's good enough to quickly get me collimated well enough to enjoy the view.  My two cents.

It can work, but I've seen too many where that centring 'feature' with two rubber rings even made registration of the 1.25" part into the 2" adapter that comes with the HoTech unreliable -- a few days ago I had to use one and it became a lot better when used in the Paracorr tunable top adapter instead, and it was actually well collimated and worked perfectly when used in a Parallizer. Ditto for the same feature on the 2" part -- if you use it then results are usually poor.
 
HoTech's marketing literature extolls the virtue of this arrangement but it really is a misfeature: either you don't need that rubber and then you'd be just as well without it, or you do need it because the focuser is wider than the tool, but then it's more likely to introduce tilt because both rings 'squish' slightly differently than fix anything. Sure, the laser will be centred but you don't care about tiny shifts. What you do care about is that the beam stays parallel to the focuser axis, and in my experience you need to fiddle with that collimator far too much to make it work well.
 
Mind you, apart from that centring feature which I wish simply wasn't there, the mechanical tolerances are good and the holographic patterns it's sold with are really nice (especially the cross). It's not always well collimated (in my experience) but it can actually be collimated and the collimation will hold fairly well (better than on the el cheapo Aliexpress miscollimators).
 
But by and large I prefer the much cheaper  no-fuss (and easy to collimate) Farpoints (for anyone who doesn't want to spring for a Howie Glatter, which is still the best one out there even now Starlight Instrument makes them). Although to be honest I've seen some of these that had fairly narrow barrels and really needed a Parallizer to work well too. Yes, you can't see the return beam but frankly using that is not precise so I'd rather use other tools (e.g. adding a 1.25" TuBLUG or use a good Cheshire).

Edited by sixela, 25 July 2021 - 05:21 PM.

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#28 sixela

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Posted 25 July 2021 - 05:21 PM

From the discussion It sounds like a Cheshire (like mine) may not be the best tool for collimating a secondary although it can be used. Since this is the only tool I have this is what I used and it seemed to work pretty well. (with help from Vic, Don, Asbytec & others!)
 
Wondering out loud here:
   - If a star test (real star) shows a good airy disk pattern, the primary must be well collimated, yes?

Yes.

 - Does the good star test automatically mean the the secondary is also well collimated, yes/no/maybe?

It tells you that the primary was well tilted for the current tilt of the secondary, but you might still have uneven field illumination and a tilted focal plane, so no.

 - Is this question going to take me down an optical rabbit hole, yes?

Yes. Before you know it you'll be writing a book about it/making wiki articles about it/illustrating it with lots of diagrams/inventing your own collimation tools.
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#29 Orion68

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Posted 25 July 2021 - 05:22 PM

Before short focal ratio sight tubes were available (actually, before most collimating tools of any kind were available), I used my old Tectron Cheshire for assessing the secondary mirror placement (rotation and offset) and I used my Tectron sight tube for secondary mirror tilt. (I also used my Tectron Cheshire for primary mirror alignment--and the square center marker on my primary mirror was calibrated to the Cheshire perforation for optimal precision.)

 

A good star alignment (like Mike Lockwood's http://www.loptics.c.../starshape.html ) is an indicator of optimal primary mirror tilt alignment, it does not indicate optimal secondary mirror alignment (placement or tilt). 

 

Primary mirror tilt adjustment aligns the coma free field to the eyepiece axis.

Secondary mirror tilt adjustment makes the focal plane perpendicular to the eyepiece axis.

Secondary mirror placement centers and balances the field illumination on the eyepiece axis at the focal plane. 

 

Do you see a rabbit hole?

That answers the question - a good star test does not guarantee that the secondary is collimated.

 

Yikes! That is a rabbit hole for me. If the secondary is not properly placed under the focuser or the secondary mirror tilt is off, It seems like none of the light from the primary would be delivered correctly to the eyepiece and therefore a bad star test would result.

 

I think I need to buy your book...

Thanks Vic


Edited by Orion68, 25 July 2021 - 05:22 PM.


#30 Orion68

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Posted 25 July 2021 - 05:29 PM

Yes.
It tells you that the primary was well tilted for the current tilt of the secondary, but you might still have uneven field illumination and a tilted focal plane, so no.

Yes. Before you know it you'll be writing a book about it/making wiki articles about it/illustrating it with lots of diagrams/inventing your own collimation tools.

This stuff is fascinating but also frustrating. Kudos to you and the other true experts on CN for so willingly sharing your extensive knowledgebow.gif  



#31 sixela

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Posted 25 July 2021 - 05:33 PM

That answers the question - a good star test does not guarantee that the secondary is collimated.
 
Yikes! That is a rabbit hole for me. If the secondary is not properly placed under the focuser or the secondary mirror tilt is off, It seems like none of the light from the primary would be delivered correctly to the eyepiece and therefore a bad star test would result.


Put your eye in a collimation cap behind a well collimated scope. The image of the primary appears centred and the secondary catches all of the image of the primary.

Now imagine that you shift both the secondary and the image of the primary off-centre, and that you then used a collimation cap to tilt the primary correctly again. The object without coma will still be centred in the field (after all, the peephole of the Cheshire is in the centre and that's where you made the optical axis cross the focuser axis) but the bundles come from one side and the focal plane is tilted. That means that at low power the edge of the field will not be in focus everywhere, but at higher power you won't notice a small tilt -- your eye will accommodate slightly for very slight focus changes when you redirect your gaze. 

Suppose that you now tilt the secondary again to centre the primary's centre spot. Now the focal plane tilt is gone, but the secondary might not catch the whole of the primary if your eye is in the centre -- you may have to move your eye laterally to see the entire primary again. That means the fully illuminated field is not centred.

Yes, if the secondary placement is bad enough so that it vignettes even the bundle for the on-axis object then that'll be visible in the star test (although the effects close to focus are subtle) but that'd usually only happen with a huge error.


Edited by sixela, 25 July 2021 - 05:35 PM.

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#32 Orion68

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Posted 25 July 2021 - 05:49 PM

Put your eye in a collimation cap behind a well collimated scope. The image of the primary appears centred and the secondary catches all of the image of the primary.

Now imagine that you shift both the secondary and the image of the primary off-centre, and that you then used a collimation cap to tilt the primary correctly again. The object without coma will still be centred in the field (after all, the peephole of the Cheshire is in the centre and that's where you made the optical axis cross the focuser axis) but the bundles come from one side and the focal plane is tilted. That means that at low power the edge of the field will not be in focus everywhere, but at higher power you won't notice a small tilt -- your eye will accommodate slightly for very slight focus changes when you redirect your gaze. 

Suppose that you now tilt the secondary again to centre the primary's centre spot. Now the focal plane tilt is gone, but the secondary might not catch the whole of the primary if your eye is in the centre -- you may have to move your eye laterally to see the entire primary again. That means the fully illuminated field is not centred.

Yes, if the secondary placement is bad enough so that it vignettes even the bundle for the on-axis object then that'll be visible in the star test (although the effects close to focus are subtle) but that'd usually only happen with a huge error.

Thanks for that detailed explanation!

 

I can't say that I fully understand it yet but it's making more sense now. I'll try to visually recreate your description while viewing the secondary and the focuser from the open end of the scope. Like you said, at some point it will come together for that Aha! moment.

 

Many thanks sixela. 

CS



#33 sixela

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Posted 25 July 2021 - 05:59 PM

Here's a small diagram of a scope with a secondary that's way off to the side (the green visualises what you see in the centre of a collimation cap, when looking along the focuser axis, and you can see that the green line does not travel to the centre of the primary) but that will deliver a good star test:
Assencollimatie_geen_pae.png

 

Here's a diagram with correct axial collimation but with the focuser axis pointing to the real physical centre of the face of the secondary (assume that it was e.g. centre spotted without taking into account the proper offset):

Assencollimatie_ok.png

 

Here axial collimation is correct (and the focal plane is not tilted) but the secondary is not optimally placed. You can see it just about catches the entire bundle for the on-axis image, but if you stray just a bit on one side it will immediately start to clip the bundles (of course on the other side it's OK).


Edited by sixela, 25 July 2021 - 06:00 PM.

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#34 Vic Menard

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Posted 25 July 2021 - 06:01 PM

...If the secondary is not properly placed under the focuser or the secondary mirror tilt is off, It seems like none of the light from the primary would be delivered correctly to the eyepiece and therefore a bad star test would result.

OK, first, if your secondary mirror placement is good enough to show these three circles:

the bottom edge of the Cheshire collimator,

the actual edge of the secondary mirror, and

the reflected edge of the primary mirror... 

are concentric with each other, your secondary mirror placement (and subsequent field illumination) will be "good enough" for your 6-inch f/5 scope.

 

Secondary mirror tilt is another problem, but maybe not too big of a problem depending on how you use your 6-inch scope. The high magnification (50X per inch of aperture) error tolerance for your scope (without a coma corrector) is around 4.5- to 6mm. Without a cross hair reticle or a simple thin beam laser, this error can be difficult to read. If you use mostly moderate magnifications (less than 25X to 30X per inch of aperture), you might be able to get away with twice the error. So, how can you tell if you're "close enough"? 

 

If the view in the eyepiece seems to all come to focus at the same time (snap!), you're well within the tolerance window for the magnification you're using. If, on the other hand, the focus seems "mushy" and almost seems to slide across the field of view, you still have some work to do. (In either case, you should still be able to find a focus position that brings the center of the field of view into sharp focus.


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#35 Orion68

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Posted 25 July 2021 - 06:37 PM

Here's a small diagram of a scope with a secondary that's way off to the side (the green visualises what you see in the centre of a collimation cap, when looking along the focuser axis, and you can see that the green line does not travel to the centre of the primary) but that will deliver a good star test:
Assencollimatie_geen_pae.png

 

Here's a diagram with correct axial collimation but with the focuser axis pointing to the real physical centre of the face of the secondary (assume that it was e.g. centre spotted without taking into account the proper offset):

Assencollimatie_ok.png

 

Here axial collimation is correct (and the focal plane is not tilted) but the secondary is not optimally placed. You can see it just about catches the entire bundle for the on-axis image, but if you stray just a bit on one side it will immediately start to clip the bundles (of course on the other side it's OK).

 

The diagrams are really helpful. Thank you!

 

In the first diagram, would the incorrect secondary tilt be fixed by simply adjusting the secondary adjustment screws so that the outline of the actual secondary mirror is concentric with the circular bottom of the sight tube/Cheshire?

 

In the second diagram, is the secondary too far away from the primary? Would this error be fixed by adjusting the large center screw on the secondary?

 

oops, just saw Vic's post above and he answered this question:

 

Would it be accurate to say this?

 

 - If the view in the Cheshire/sight tube shows concentric circles for:

    1. the bottom of the Cheshire/sight tube,

    2. the outline of the physical secondary mirror,

    3. and the reflection of the primary mirror in the secondary

 

  - Then

     1.The secondary is reasonably well collimated

     2. A better collimation would require better tools

 

Thanks again sixela for sharing your knowledge.

CS


Edited by Orion68, 25 July 2021 - 06:38 PM.


#36 Orion68

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Posted 25 July 2021 - 07:01 PM

OK, first, if your secondary mirror placement is good enough to show these three circles:

the bottom edge of the Cheshire collimator,

the actual edge of the secondary mirror, and

the reflected edge of the primary mirror... 

are concentric with each other, your secondary mirror placement (and subsequent field illumination) will be "good enough" for your 6-inch f/5 scope.

 

Secondary mirror tilt is another problem, but maybe not too big of a problem depending on how you use your 6-inch scope. The high magnification (50X per inch of aperture) error tolerance for your scope (without a coma corrector) is around 4.5- to 6mm. Without a cross hair reticle or a simple thin beam laser, this error can be difficult to read. If you use mostly moderate magnifications (less than 25X to 30X per inch of aperture), you might be able to get away with twice the error. So, how can you tell if you're "close enough"? 

 

If the view in the eyepiece seems to all come to focus at the same time (snap!), you're well within the tolerance window for the magnification you're using. If, on the other hand, the focus seems "mushy" and almost seems to slide across the field of view, you still have some work to do. (In either case, you should still be able to find a focus position that brings the center of the field of view into sharp focus.

Most of my viewing is well within the 25x to 30x per inch of aperture range because I use the scope mainly for wide field views of star clusters. But, I do occasionally use higher powers for double stars and for final collimation of the primary using a focused star. 

 

The three circles are concentric. Tonight I'll check for a snap-to focus and note at what power it becomes mushy. Based on your feedback, I think the secondary is just fine for my purposes. What a pleasure it is to use a well collimated scope!

 

Thanks for the feedback Vic and for all of the previous help. I have learned a lot here on CN but much, much more to go.

CS


Edited by Orion68, 25 July 2021 - 07:02 PM.


#37 Vic Menard

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Posted 25 July 2021 - 08:43 PM

Would it be accurate to say this?

 

 - If the view in the Cheshire/sight tube shows concentric circles for:

    1. the bottom of the Cheshire/sight tube,

    2. the outline of the physical secondary mirror,

    3. and the reflection of the primary mirror in the secondary

 

  - Then

     1.The secondary is reasonably well collimated

     2. A better collimation would require better tools

If the first 1 through 3 circles are precisely concentric, then the secondary mirror tilt is also precisely correct. The unanswered question is just how close to "precisely concentric" are the three circles? The critical concentricity is the primary mirror reflection relative to the bottom edge of the Cheshire--NOT the primary mirror reflection relative to the actual edge of the secondary mirror. Too many beginners fixate on the visibility of the primary mirror clips in the secondary mirror, which is NOT a valid alignment if the primary mirror reflection isn't concentric to the bottom edge of the collimator.

 

As noted at the beginning of this discussion, a Cheshire collimator is an excellent primary mirror tilt alignment tool. It's not an ideal secondary mirror placement tool, and it's typically inadequate for assessing and correcting the secondary mirror tilt. There are better tools for assessing and correcting those alignments.


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#38 Vic Menard

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Posted 25 July 2021 - 08:47 PM

Most of my viewing is well within the 25x to 30x per inch of aperture range because I use the scope mainly for wide field views of star clusters. But, I do occasionally use higher powers for double stars and for final collimation of the primary using a focused star. 

 

The three circles are concentric. Tonight I'll check for a snap-to focus and note at what power it becomes mushy. Based on your feedback, I think the secondary is just fine for my purposes.

When you have only one tool to use, you should give it your best shot. If it works, and you get good image performance, I'd say it was worth the effort. 


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#39 Orion68

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Posted 25 July 2021 - 11:41 PM

"The critical concentricity is the primary mirror reflection relative to the bottom edge of the Cheshire--NOT the primary mirror reflection relative to the actual edge of the secondary mirror. Too many beginners fixate on the visibility of the primary mirror clips in the secondary mirror, which is NOT a valid alignment if the primary mirror reflection isn't concentric to the bottom edge of the collimator."

 

This is extremely useful to know. I'm going to re-check my secondary with this in mind. I remember reading this in one of your other posts so I may have ignored the clips, best go back and make sure.

 

Thanks Vic.

.



#40 Asbytec

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Posted 26 July 2021 - 06:19 AM

"The critical concentricity is the primary mirror reflection relative to the bottom edge of the Cheshire--NOT the primary mirror reflection relative to the actual edge of the secondary mirror. Too many beginners fixate on the visibility of the primary mirror clips in the secondary mirror, which is NOT a valid alignment if the primary mirror reflection isn't concentric to the bottom edge of the collimator."

This is extremely useful to know. I'm going to re-check my secondary with this in mind. I remember reading this in one of your other posts so I may have ignored the clips, best go back and make sure.

Thanks Vic..

The instructive thing about the "critical concentricity" is when the focuser axis is aimed directly at the primary center, then the edge of the Cheshire and the edge of the primary reflection must be concentric. When the centers (focuser axis and primary center) of two circles coincide, their respective edges (Cheshire and primary reflection) must be concentric. As Sixela shows in his upper illustration, this can happen regardless of the 'position' of the secondary.

In fact, you can use the fact the Cheshire and primary reflection are concentric to better position the secondary mirror. The third circle (collimation signature) is the edge of the properly placed secondary being concentric with the Cheshire/Focuser. When the focuser axis is aligned to the primary center with their edges concentric, the properly placed secondary edge will also concentric with the already concentric Cheshire and primary reflection.

Now, all three signatures are concentric with each other. You can see all three clips and your field will be fully illuminated and centered in the eyepiece.

Edited by Asbytec, 26 July 2021 - 07:05 AM.

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#41 Vic Menard

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Posted 26 July 2021 - 08:13 AM

The instructive thing about the "critical concentricity" is when the focuser axis is aimed directly at the primary center, then the edge of the Cheshire and the edge of the primary reflection must be concentric.

The "unfortunate" part is that the actual edge of the secondary mirror falls between the two circles, and as you know, getting the secondary mirror placement sorted can be a convoluted task. The reason I say unfortunate is because many beginners give up when the secondary mirror placement is "close enough", and then proceed with centering the primary mirror reflection in the secondary mirror, compromising the concentricity of the primary mirror reflection relative to the bottom edge of the Cheshire! It's also unfortunate that the "error" is easy to see and the correction is "obvious"--although the result is NOT collimation.


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#42 sixela

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Posted 26 July 2021 - 08:15 AM

The diagrams are really helpful. Thank you!

 

In the first diagram, would the incorrect secondary tilt be fixed by simply adjusting the secondary adjustment screws so that the outline of the actual secondary mirror is concentric with the circular bottom of the sight tube/Cheshire?

 

 

The secondary tilt adjustments screws do not move the secondary, at least on their own. Usually you'd either have to tune the push/pull system (in this case, loosen the central bolt and then tighten the tilt screws) or to use the collimation screws on the focuser to tilt its axis to where the secondary is currently located instead (if the mountain does not want to move to Muhammad,...).

 

With only the tilt screws if you fixed axial collimation you'd get into a situation similar to the second diagram (possibly worse).


Edited by sixela, 26 July 2021 - 08:15 AM.

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#43 Orion68

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Posted 26 July 2021 - 08:14 PM

The "unfortunate" part is that the actual edge of the secondary mirror falls between the two circles, and as you know, getting the secondary mirror placement sorted can be a convoluted task. The reason I say unfortunate is because many beginners give up when the secondary mirror placement is "close enough", and then proceed with centering the primary mirror reflection in the secondary mirror, compromising the concentricity of the primary mirror reflection relative to the bottom edge of the Cheshire! It's also unfortunate that the "error" is easy to see and the correction is "obvious"--although the result is NOT collimation.

So, is the solution to go through an iterative process of:

 

  1. Making the primary mirror reflection concentric with the bottom edge of the Cheshire

  2. Then adjusting the secondary to make the edge of the secondary concentric with the Cheshire

 

  Then do #1 again

  Then do #2 again

 

  And on and on until step #2 is no longer needed?



#44 sixela

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Posted 26 July 2021 - 09:06 PM

Watch out — when he mentions the edge of the secondary it is not its silhouette in the primary, but the edge of the secondary in direct view, around the image of the primary.

If you have a sight tube then no, the procedure is to first centre the secondary under the focuser and make it as circular as possible. Then you do axial collimation (tilt the two mirrors), which might have slightly changed the centring of the secondary, but usually not that much, so if needed you start again.

If you don’t have a sight tube but e.g. just a laser collimator and a Cheshire, you start by doing the axial collimation, which makes the image of the primary a correctly centred reference, and then you tweak the secondary position/rotation in smaller steps, redoing the axial collimation every time.

If you only have a Cheshire then you really don’t _have_ a tool that can precisely centre the image of the primary; you just ballpark that aspect and live with the focal plane tilt that remains. It’s less critical than making sure the coma Frée object is centred in the field (which the Cheshire always does if the focal plane is mid-way between the pupil and the physical Cheshire ring location).

You still have an indirect queue about whether the focuser axis points to the middle of the primary, though: if it’s significantly off the Cheshire reading changes when you rack the focuser in and out (which is why you want to place it close to the focal plane for the final tweak). It’s not impossible to use the shift you’d see to tweak secondary tilt but usually if you’re smart enough to do that you’re smart enough to also have a sight tube or (good) laser collimator in the first place ;-).

Edited by sixela, 26 July 2021 - 09:13 PM.

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#45 Asbytec

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Posted 27 July 2021 - 12:34 AM

So, is the solution to go through an iterative process of:

1. Making the primary mirror reflection concentric with the bottom edge of the Cheshire
2. Then adjusting the secondary to make the edge of the secondary concentric with the Cheshire

Then do #1 again
Then do #2 again

And on and on until step #2 is no longer needed?

For your initial collimation, the steps above are reversed. First step is to center the secondary under the Cheshire (focuser draw tube). During secondary placement in step 1, the idea is to center and rotate the secondary under the focuser or Cheshire (your step 2 above). Then step 2 is to align the focuser axis with the primary center. Once that is done, then the reflection of the primary mirror will be centered under the focuser. That's your step two.

But yes, once you have done your initial secondary placement and focuser axial alignment, the primary reflection will be centered under the focuser. You can refine your secondary position relative to the centered primary reflection. This is best done with the focuser extended out to the apex where the primary reflection and the actual edge of the secondary are the same apparent size.

When the focuser axis is aligned to the primary center, the primary reflection will be centered under the Cheshire. If the actual secondary edge is not (also) centered under the focuser, the primary reflection will not appear to be centered in the secondary. You can see this best focused out to the apex. However, since the primary reflection is centered under the Cheshire because the focuser axis is aligned to the primary, you can use the primary reflection as a reference for centering the secondary under the Cheshire, too.

Edited by Asbytec, 27 July 2021 - 02:08 AM.


#46 sixela

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Posted 27 July 2021 - 03:22 AM

For your initial collimation, the steps above are reversed. First step is to center the secondary under the Cheshire (focuser draw tube).

If the secondary placement is so way off that you can see it's not well centred even though you could drive a truck through your two references (the far end of the focuser drawtube and the edge of the secondary) then you have a point, might as well get it roughly OK first.

 

But if the placement is not that far off, it simply does not work well in practice in my experience. There is usually so much room between the far end of the focuser draw tube and the secondary that it is illusory to think you can evaluate correct centring precisely with that tool.

 

But as you quite rightly say, you can re-evaluate that after axial collimation, when the edge of the primary becomes a more convenient reference.

 

[Disclaimer: I have a TeleTube, so I rarely end up in those circumstances if I plan to help someone collimate his scope.]


Edited by sixela, 27 July 2021 - 03:24 AM.


#47 Asbytec

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Posted 27 July 2021 - 04:44 AM

Sixela, yes, I agree. It's nice to get the aperture of the tool close to the apparent size of the secondary. I have a 2" light pipe and a lot of leeway with focuser travel. We're discussing a Cheshire here, which really means the focuser draw tube for secondary placement. I was working within the discusion in that context and hopefully with understanding of your point. You're correct. Thanks, thanks for making it clear. (Plus, I wrote this reply during a pool match waiting my turn on the table. <yawn> :) )

Edited by Asbytec, 27 July 2021 - 04:47 AM.


#48 sixela

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Posted 27 July 2021 - 09:16 AM

I wasn't disagreeing, just pointing out that to see if two circles of very different diameters are concentric is not that easy, so you usually have to ballpark it and then find references that you can compare precisely more easily.

 

If you take digital pictures through a Cheshire (with a lot of depth of field) then it's different, as Vic has shown in numerous threads. Then you can use sketching and annotation tools to verify if what should be concentric is indeed concentric.


Edited by sixela, 27 July 2021 - 09:18 AM.

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#49 Asbytec

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Posted 27 July 2021 - 09:51 AM

Sixela, absolutely. :)


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