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collimation of refractor

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#1 Doug Doonan

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Posted 14 November 2019 - 09:49 PM

I am at  the stage of collimating my DIY 85mm f5.7  scope. I can't seem to get it , somehow.

I Collimate straight through by adjusting the optic axis to the center of the FA of the objective, by adjusting the focuser. I then use the collimating EP to center the circles. I go back and check with the laser and then back with the collimating EP . Everything is OK. I now have an optic axis in the scope.

I now put the diagonal in and adjust the mirror screws until I get the circles in line. I then put the laser in and check if the beam is in the center of the objective. It is off center. The laser beam is centered as it exits the diagonal which means that there is a tilt to the optic axis. If I try to get the beam on the optic axis, the collimation circles are not concentric when checking with the collimation EP. 

I have gone back and forth doing this procedure, checked and double checked, to be sure that I haven't done some thing  wrong or be sloppy. 

I just can't seem to get it together. Any ideas??

Doug Doonan



#2 ButterFly

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Posted 14 November 2019 - 10:05 PM

Putting the diagonal in consistently can be difficult to learn.  Both the scope and the diagonal have their own quirks.

 

Collimate the diagonal by itself, without the scope.  Practice putting it on consistently, checking with the cheshire to make sure it's on well and centered.  Take it off, then do it again, and again ... .  Only once you are consistently putting it on, tweak the diagonal collimation when it is on the scope.

 

Step 0 is to check whether the laser is collimated - must be said.


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#3 JamesMStephens

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Posted 14 November 2019 - 10:23 PM

Can you post some pictures of your telescope?  Reftractor?

 

Jim



#4 Doug Doonan

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Posted 15 November 2019 - 08:56 PM

Hi,

Here is a crude shot of the DIY project. It is ES 2.5 Hex focuser with ES 2 " diagonal. Tube is 4" plastic soil pipe with a 4" to 3" adapter holding the focuser. Had to saw slots and spread out the results to fit the 3" to  the 80 mm connection on the focuser. There are set screws under the hose clamp to hold that connection secure. The other two hose clamps hold the vixen plate to the tube. The objective can be manipulated from the front of the tube in order to collimate the scope. 

Attached Thumbnails

  • IMG_2943.JPG


#5 ButterFly

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Posted 17 November 2019 - 03:23 AM

Very nice.  Could be flexure with the weight of the diagonal.  Test that by pushing the focuser down with a finger while viewing with a cheshire without diagonal (ES diagonal plus Ethos is three pounds).  A gasket between the slits and the clamp may help stiffen it by widening the contact area.  A rectangle of rubber or painted sheet metal, maybe, held by two clamps near the ends.  If that's not enough, you may need to brace it to the tube.



#6 Doug Doonan

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Posted 17 November 2019 - 05:11 PM

Hi Butterfly,

Thank you for your response.

I did what you suggested and also lubricated the screws to get them to hold better. On and off  a lot of times to get used to it.

There was no flexure  when I looked straight through. That joint where the focuser is fastened to the tube is really solid. The hose clamp is tightened to the point where I am afraid I might break something.

The laser is collimated.

The problem is in the diagonal. I have a not-so-fancy optical bench made from a piece of angle aluminum four feet long. I clamp the diagonal to the bench and square it up in two dimensions, put in the laser, and the beam is not in the center of the exit aperture of the diagonal. When I adjust the mirror to get the beam in the center, the beam is then not on the geometric axis of the diagonal. I put a target at the exit aperture and note where the laser is hitting and then move the target down the bench a meter and see where the laser is hitting. It is quite a ways off. So I guess I have a problem that I will have to address, somehow. 



#7 hamishbarker

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Posted 18 November 2019 - 04:22 PM

I built a refractor recently. I collimated by star testing. Put high power eyepiece in, check focus then rack in and out, adjust the objective cell a bit and work out which way to tilt it to improve the symmetry of the out of focus views. At first I had pointy egg shapes, then got it to a nice airy ring view. At very high power I can see very slight triangularity of the airy disc, indicating that the cell is pinching Tue objective. This is because the screw heads are too big. So I need to change them.

My focuser is a simple friction tube slider affair so the only collimation adjustment I have is at the objective.

#8 dogbiscuit

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Posted 19 November 2019 - 12:24 AM


The laser is collimated.

The problem is in the diagonal. I have a not-so-fancy optical bench made from a piece of angle aluminum four feet long. I clamp the diagonal to the bench and square it up in two dimensions, put in the laser, and the beam is not in the center of the exit aperture of the diagonal. When I adjust the mirror to get the beam in the center, the beam is then not on the geometric axis of the diagonal. I put a target at the exit aperture and note where the laser is hitting and then move the target down the bench a meter and see where the laser is hitting. It is quite a ways off. So I guess I have a problem that I will have to address, somehow. 

Mirror is misplaced fore or aft in diagonal body relative to nosepiece and eyepiece tube.

with focuser axis aligned on optical axis and mirror at proper 45° angle but misplaced fore or aft the axis of the eyepiece is parallel to but not on the optical axis.

StarDiag1.jpg

 

 

 

Without placing mirror in proper fore/aft position, adjusting tilt of mirror to bring eyepiece axis to intersect optical axis at front of nosepiece results in eyepiece axis and optical axis tilt relative to each other.

StarDiag2.jpg

 

 

 

So, in addition to adjusting tilt of the mirror you must also adjust position fore and aft along the optical axis.
With mirror at proper position and angle you have this:

StarDiag3.jpg


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#9 dan_h

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Posted 19 November 2019 - 01:35 PM

Mirror is misplaced fore or aft in diagonal body relative to nosepiece and eyepiece tube.

with focuser axis aligned on optical axis and mirror at proper 45° angle but misplaced fore or aft the axis of the eyepiece is parallel to but not on the optical axis.

attachicon.gif StarDiag1.jpg

 

 

 

Without placing mirror in proper fore/aft position, adjusting tilt of mirror to bring eyepiece axis to intersect optical axis at front of nosepiece results in eyepiece axis and optical axis tilt relative to each other.

attachicon.gif StarDiag2.jpg

 

 

 

So, in addition to adjusting tilt of the mirror you must also adjust position fore and aft along the optical axis.
With mirror at proper position and angle you have this:

attachicon.gif StarDiag3.jpg

While I agree in principal with your description of the fore/aft alignment of the mirror, in all fairness, one has to acknowledge just how little such errors actually affect the views. Likewise with the mirror angle error.

 

Your mirror angle diagram shows a gross error that could never occur in reality. There would need to be a mirror error of tens of degrees to deflect the axis to the side of the eyepiece holder. Such errors are physically impossible in a diagonal assembly.  If the mirror is tilted 1 degree off the 45 degrees, the deflection error at the focal plane is less than 1mm. It is of course dependent on the mirror to focal plane distance. In terms of the image, the shift off axis amounts to a few arcminutes of view.  No one can see errors in the image resulting from such a small distance. There is also a resulting tilt in the image plane as a result of mirror angle error but such tilt is minimal compared to the usual field curvature of a refractor and like field curvature, tilt does not affect the centre of the field. .  

 

When the mirror angle is correct but there is a fore/aft displacement of the mirror, the resulting shift in the optical axis is roughly equal to the displacement error of the mirror surface. This too is a small amount, maybe a millimetre, maybe less, and certainly not enough to see with a casual visual inspection. Given that there are literally thousands and thousands of diagonals with the mirror glued in place without an accurate reference for the surface location, it can't be a major problem. Nor can it be corrected with the resources most observers have access to. Determining the correct position for the mirror surface requires a means of verifying the location and accuracy of the eyepiece holder and diagonal nosepiece. 

 

IMHO, the diagonal collimation is blamed for far more problems than can actually be observed.   Just because we can determine that an error exists, doesn't mean that it affects the views. As your diagrams so clearly illustrate, the location and alignment of the eyepiece holder and nosepiece are interconnected with the mirror surface. When we use a laser or other tool to correct an error we are simply compensating one error by introducing another and claim incredible improvements that lie beyond the realm of reality. .

 

dan    



#10 dogbiscuit

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Posted 19 November 2019 - 02:17 PM

I intentionally made no statement about what degree of error would cause unacceptable degradation of the views.  I don't really know that.

The drawings are exaggerated so the principle can easily be seen, and this was what caused the effects described by the OP about his collimation attempts.  I don't know the range of adjustment of the mirror position in that particular star diagonal, but it may be that perfect adjustment is within the range.  In any case one should adjust as near perfect as is reasonably possible.

 

Star diagonals I am familiar with have mirrors glued into a machined cell and would not be easily adjustable.  Well machined mechanical parts and accurate thickness mirrors could make a very accurate assembly.  I'm surprised the OP's is adjustable. It may be that although adjustments are available they would not be advised or needed with most star diagonals, but the OP had already made some changes so maybe the degree of error had become significant and alignment is now needed.

 

My post did assume the nosepiece and eyepiece tubes axes intersect at 90°.  If they intersect at other than 90° the mirror angle would be different.

Of course another problem could be that the axes of the two tubes do not precisely intersect. :-)

As near as possible the mirror should be placed so the axis of the eyepiece tube and axis of the nosepiece tube intersect on the mirror surface and the angle of the mirror adjusted so the reflected optical axis is congruent with the eyepiece tube.


Edited by dogbiscuit, 19 November 2019 - 03:50 PM.


#11 dan_h

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Posted 20 November 2019 - 10:45 AM

The problem is in the diagonal. I have a not-so-fancy optical bench made from a piece of angle aluminum four feet long. I clamp the diagonal to the bench and square it up in two dimensions, put in the laser, and the beam is not in the center of the exit aperture of the diagonal. When I adjust the mirror to get the beam in the center, the beam is then not on the geometric axis of the diagonal. I put a target at the exit aperture and note where the laser is hitting and then move the target down the bench a meter and see where the laser is hitting. It is quite a ways off. So I guess I have a problem that I will have to address, somehow. 

Here is a link to a simple diagonal test/adjustment procedure.  If your diagonal passes these tests it is as good as it needs to be and probably better than most. 

 

https://www.youtube....h?v=8wFDkl6FZxc

 

hope this helps,

 

dan


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#12 ButterFly

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Posted 21 November 2019 - 03:49 AM

I like dogbiscuit's logic.  My ES dagonal has screw holes for collimating, but no screws.  I use layers of tinfoil at the edges so not much shift there.  Backing off the screws a few turns each may be a very simple fix.  You can file them down later if it works and they stick out too far.   It's well worth the effort even if there is no real noticable gain - other than knowing you tried your best.

 

I suggest changing the title to collimation of diagonal so others can find this more easily.



#13 Starman1

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Posted 21 November 2019 - 06:57 PM

Well, the error in the diagonal will be reduced to inconsequential by collimating the diagonal with the same laser you used to collimate the focuser.

 

The laser goes to a point in the center of the objective lens when collimating the focuser.  I use a translucent template over the lens cell with a center indicator

and adjust the focuser to put the beam dead center.  Then I collimate the objective cell in the scope.

To adjust a star diagonal that is off, insert the laser where the eyepiece goes and adjust the diagonal mirror to put the laser dead center on the objective lens, just like you did the focuser.

Yes, it may be off a tad in angle, but it won't matter much.  And the laser might not exit the exact center of the diagonal's forward tube.

Yes, this doesn't guarantee the diagonal mirror is centered under the eyepiece, but that won't matter much, either.  It's unlikely to be way off unless the mirror is a replacement

for the original mirror used in the diagonal.

 

One caveat: If you collimate the diagonal like that, there is no guarantee that rotating it will keep the same alignment.  You should plan on keeping the star diagonal in one orientation only.



#14 Doug Doonan

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Posted 21 November 2019 - 09:04 PM

Hi everybody,

Thank you for all of your comments.

I had to do major surgery on the diagonal to get it near where it should be. The mirror was stuck with double sided sticky tape  in a saddle which in turn was screwed to the diagonal 45 degree surface. As received the laser beam on the output side was 4 mm (!) high. The saddle and mirror combination was clearly too thick.The mirror was too far forward or upward.

After a lot of iterations of my surgery, I finally ended up with mirror glued to the 45 degree face with hot glue. The hot glue is flexible enough to allow me to tweak the aiming of the mirror with the jack screws in the back, to get the laser beam on the geometric axis (and optic axis) of the diagonal. I checked the alignment on the my optical bench and the beam is on  axis a meter away, and centered in the out put aperture.

I put the diagonal back in the scope and the circles are very good when using the collimating EP and the laser beam is in the center of the objective.

I checked the alignment with my artificial star arrangement and there was very little ,if any, tailing or cometting that I could see.

I will be using the scope with the diagonal, so next time out in the stars, I will teak the objective for best focus.

I am optimistic about my surgery.

Thanks again,

Doug


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