82 replies to this topic

[Gil V]

Here’s the target I used last time I made one.

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[DAVIDG]

 When I have rebuild Schmdt Cass, I use a target like Gil has shown  in place of the corrector and then a laser collimator in the focuser. The rear cell is adjusted so the laser hits the center of the target.  Now the mechanical axis of the mirror mount is concentric with the mechanical axis of the tube, but that doesn't mean the primary is correctly aligned.

   The target is removed and I  tape two  pieces of string across the front of the scope at  90 degrees to each other and they cross exact in the middle of the opening for the corrector. Many time you can use the screw holes for the retainer ring as reference points to place the string at exactly 90 degrees to each other. 

   Now you stand back a couple  of  feet and look at  the primary. You'll see the reflections of the strings in the primary. Move you head so the reflection are directly behind the strings. If the mirror is adjusted correctly the reflection should be exactly centered behind the strings if not then the mirror is tilted and needs to be adjusted as well so the optical axis of the mirror and the mechanical axis of the tube and mirror cell are the same. If not then you can try to collimate the scope until the cows come home and you'll never get it right. 

  Here is a  picture that shows what I'm describing.

 

 

 

  Now once you have the primary and mirror cell correctly aligned you can move on to the secondary and corrector. Check that the secondary is centered on the mounting plate. The figure on the secondary is NOT spherical so it is critical that the mechanical and optical axis are the same and that happens when it it centered on the mounting plate.  Install the secondary cell in the corrector but don't fully tighten down it so you can move it.

 Center the the corrector in it cell the best you can because they are not perfectly round. Now measure from the ID of the corrector cell to the center of the secondary cell.   Now you snug up the secondary cell so it can not rotate it in the corrector. Rotate the whole assembly so one of the alignment screws is at 12:00 o'clock. This makes aligning the secondary much easier when viewing a star since the movement of the secondary matches the position of the alignment screw ie if you need to move the secondary up and down you only need to adjust one screw not a combination of screws. 

   Now install the retainer ring for the corrector. Tighten the screws evenly  in  a star pattern to tighten them and only a little past finger tight so not to warp the corrector. 

   With the optical axis and mechanical axis all concentric it will  be easy to align the secondary to get round star images.

 

                         - Dave 

             

 

          

[Gil V]

David, those are outstanding instructions, with my favorite part being “you can try to collimate the scope until the cows come home and you’ll never get it right”.

God, I love this forum.

[RDFox]

 When I have rebuild Schmdt Cass, I use a target like Gil has shown  in place of the corrector and then a laser collimator in the focuser. The rear cell is adjusted so the laser hits the center of the target.  Now the mechanical axis of the mirror mount is concentric with the mechanical axis of the tube, but that doesn't mean the primary is correctly aligned.

   The target is removed and I  tape two  pieces of string across the front of the scope at  90 degrees to each other and they cross exact in the middle of the opening for the corrector. Many time you can use the screw holes for the retainer ring as reference points to place the string at exactly 90 degrees to each other. 

   Now you stand back a couple  of  feet and look at  the primary. You'll see the reflections of the strings in the primary. Move you head so the reflection are directly behind the strings. If the mirror is adjusted correctly the reflection should be exactly centered behind the strings if not then the mirror is tilted and needs to be adjusted as well so the optical axis of the mirror and the mechanical axis of the tube and mirror cell are the same. If not then you can try to collimate the scope until the cows come home and you'll never get it right. 

  Here is a  picture that shows what I'm describing.

 

 

mirror alignment string test.jpg

 

  Now once you have the primary and mirror cell correctly aligned you can move on to the secondary and corrector. Check that the secondary is centered on the mounting plate. The figure on the secondary is NOT spherical so it is critical that the mechanical and optical axis are the same and that happens when it it centered on the mounting plate.  Install the secondary cell in the corrector but don't fully tighten down it so you can move it.

 Center the the corrector in it cell the best you can because they are not perfectly round. Now measure from the ID of the corrector cell to the center of the secondary cell.   Now you snug up the secondary cell so it can not rotate it in the corrector. Rotate the whole assembly so one of the alignment screws is at 12:00 o'clock. This makes aligning the secondary much easier when viewing a star since the movement of the secondary matches the position of the alignment screw ie if you need to move the secondary up and down you only need to adjust one screw not a combination of screws. 

   Now install the retainer ring for the corrector. Tighten the screws evenly  in  a star pattern to tighten them and only a little past finger tight so not to warp the corrector. 

   With the optical axis and mechanical axis all concentric it will  be easy to align the secondary to get round star images.

 

                         - Dave 

 

 

David, those are outstanding instructions, with my favorite part being “you can try to collimate the scope until the cows come home and you’ll never get it right”.

God, I love this forum.

Hey, thanks for the information, I will follow the instructions and let you know how it turns out. We’ll see if I can screw this up! LOL

[ccwemyss]

So Dave, would that procedure also work for getting the tube and front cell assembly aligned to the optical axis?

 

I have a C14 that came back from service at Celestron with the screws holding the tube to the rear cell loose. They had done a collimation on an optical bench, and I assume that the screws loosened in shipping. Their advice was to set it on its face, make sure all of the screws were loose so the rear cell would sit flat on the end of the tube, and then tighten them. But despite collimating it at over 600X, it still isn't as sharp as it was before the service. So I suspect the tube is out of alignment. 

 

Chip W. 

[DAVIDG]

So Dave, would that procedure also work for getting the tube and front cell assembly aligned to the optical axis?

 

I have a C14 that came back from service at Celestron with the screws holding the tube to the rear cell loose. They had done a collimation on an optical bench, and I assume that the screws loosened in shipping. Their advice was to set it on its face, make sure all of the screws were loose so the rear cell would sit flat on the end of the tube, and then tighten them. But despite collimating it at over 600X, it still isn't as sharp as it was before the service. So I suspect the tube is out of alignment. 

 

Chip W. 

 Yes, it  will work on any of the different flavors of Cassegrain system. 

 

               - Dave 

[deSitter]

 When I have rebuild Schmdt Cass, I use a target like Gil has shown  in place of the corrector and then a laser collimator in the focuser. The rear cell is adjusted so the laser hits the center of the target.  Now the mechanical axis of the mirror mount is concentric with the mechanical axis of the tube, but that doesn't mean the primary is correctly aligned.

   The target is removed and I  tape two  pieces of string across the front of the scope at  90 degrees to each other and they cross exact in the middle of the opening for the corrector. Many time you can use the screw holes for the retainer ring as reference points to place the string at exactly 90 degrees to each other. 

   Now you stand back a couple  of  feet and look at  the primary. You'll see the reflections of the strings in the primary. Move you head so the reflection are directly behind the strings. If the mirror is adjusted correctly the reflection should be exactly centered behind the strings if not then the mirror is tilted and needs to be adjusted as well so the optical axis of the mirror and the mechanical axis of the tube and mirror cell are the same. If not then you can try to collimate the scope until the cows come home and you'll never get it right. 

  Here is a  picture that shows what I'm describing.

 

 

mirror alignment string test.jpg

 

  Now once you have the primary and mirror cell correctly aligned you can move on to the secondary and corrector. Check that the secondary is centered on the mounting plate. The figure on the secondary is NOT spherical so it is critical that the mechanical and optical axis are the same and that happens when it it centered on the mounting plate.  Install the secondary cell in the corrector but don't fully tighten down it so you can move it.

 Center the the corrector in it cell the best you can because they are not perfectly round. Now measure from the ID of the corrector cell to the center of the secondary cell.   Now you snug up the secondary cell so it can not rotate it in the corrector. Rotate the whole assembly so one of the alignment screws is at 12:00 o'clock. This makes aligning the secondary much easier when viewing a star since the movement of the secondary matches the position of the alignment screw ie if you need to move the secondary up and down you only need to adjust one screw not a combination of screws. 

   Now install the retainer ring for the corrector. Tighten the screws evenly  in  a star pattern to tighten them and only a little past finger tight so not to warp the corrector. 

   With the optical axis and mechanical axis all concentric it will  be easy to align the secondary to get round star images.

 

                         - Dave 

This is basically the same way I collimate a fast Newtonian, only I don't need strings because I have the spider hub for that, and I use a dressing mirror to gaze down the tube from behind where I can get to the adjustment screws. Get the primary right first, then the secondary - gives better accuracy than a laser, whose spot is actually larger than the diffraction limited field at f/4.5.

 

-drl

[DAVIDG]

  Using the spider to replace the strings works well. I do it all the time. The issue with a cassegrain is that the focuser and primary baffle need to be aligned with the mechanical axis to the tube. If not you can have the primary and secondary correctly aligned but if the focuses is tilted again you'll never get the scope collimated correctly. One of the symptoms that the focuser or in  the case of a  commercial SCT  that moves the mirror  to focus is  tilted is you'll get the scope to look collimated on one side of  focus with the shadow of the secondary centered in the defocused star and then when you go the other side of focus the shadow will be off center.

 So you need to start with the focuser to make sure it is  mounted so it  is correctly aligned and then go from there.

 

                            - Dave 

Edited by DAVIDG, 26 January 2022 - 09:14 AM.