Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Collimating GSO Classical Cassegrain

cassegrain catadioptric collimation
  • Please log in to reply
10 replies to this topic

#1 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 194
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 16 January 2019 - 11:41 AM

Collimating GSO Classical Cassegrain

 

First thing is to remember is that the mechanical axis and optical axis in a budget scope will 99% of the time not match. This is true even in high end Classical Cassegrain with tight tolerances.

 

I just wanted to let people know that it is not necessary to run out and purchase a Cheshire or Laser collimator to line up your new Classical Cassegrain. The telescope was set up at the factory and may be out of collimation due to shipping but can’t be off all that much. If your focuser is like mine with a built in tilt you’re going to throw the whole system out of whack collimating with it. Under the stars when testing you might have to totally undo everything that you did previously with those tools.
Due to a slight tilt of the GSO focuser base to the focuser draw tube its best to forget using normal collimating tools like a Cheshire or collimation laser. Unless you machine the focuser base to match the focuser tube, replace the focuser completely or use the GSO tilt plate to correct the slight tilt of the focuser. If using the latter, once lined up you must leave the focuser in the position all the time or you will compound this issue by turning the focuser on the scope. It also has to be redone every time you adjust the primary. I chose to machine the existing focuser which worked out quite well.

 

NOTE: A fixed primary does not mean it doesn’t need to be collimated! All it means is that it doesn’t move to achieve focus!

 

Collimating Steps:

Do not use a star diagonal while performing this procedure; use a 4” extension tube in focuser. Diagonals can introduce their own set of problems.

 

*Primary Mirror    
Center star (focused) in eyepiece and examine for any on axis coma, tighten the silver screw and loosen the lock screw at the same time on the bright side opposite of the flared side to eliminate most if not all the on axis coma. You can also loosen the silver screw and tighten the lock screw on the flared side if the screw matches orientation of flare. This is looking from the back of scope. Re-center again and repeat till gone. If you examined the star and you don’t have any on axis coma just go to the next step.

 

1st Step.jpg
                    

*Secondary Mirror 
Next defocus outward (Extra Focal) the focuser to show the secondary in the light path (Large Doughnut)
This is when you adjust the secondary Allen wrench screws. I like to move the mount to make the star de-center in the eyepiece in the direction that makes the donut look symmetrical. (Use a low power eyepiece.) Now turn the adjustment Allen screws by tighten one and loosen one or two screws to bring the doughnut back to the center of the eyepiece. Re-center and repeat if needed. If you loosen one screw and your star image is gone out of the field of view, just do the reverse and tighten the same screw to bring it back. No need to panic. Only minor adjustments should be required to achieve collimation

 

2nd Step.jpg

 

                    

* Primary Mirror

Insert medium power eyepiece and slightly defocus outward on the centered star (not as much as secondary adjustment) you may see only a brightening of the outer ring, this is the side you need to tighten the silver screw and loosen the lock screw of primary mirror at the same time. You can also loosen the silver screw and tighten the lock screw on the duller side if the screw matches orientation of duller side of the outer ring.

 

Tip: Look thru the eyepiece as you turn the silver and lock collimation screws and watch how the two interact with each other.

 

3rd Step.jpg

 

                   

* Secondary Mirror
Insert high power eyepiece and defocus outward. At this point you can probably start seeing the Poisson Spot in the doughnut; this will allow you to achieve a higher degree of centering using the adjustment Allen screws. (Make the spot land in the center of the secondary shadow/large doughnut) You should be using an eyepiece equal to the focal length of the scope so a 12mm or so.

 

4th Step.jpg

 

                    

* Primary Mirror
Now increase the power again by changing the eyepiece and observe a star at focus. You need to see the ring around the airy disc for this next step. (A night of good seeing is necessary). Look for any brighten of the ring and once again you need to tighten the silver screw a tad more on the bright side but don’t touch the lock down screw. Loosening the lock down screw will make too much of a movement.
This should still be within the factory tolerance for spacing between the primary and secondary mirrors. This can be checked by inserting a Ronchi Eyepiece and if the lines run straight you are good. If they bow out the mirrors are too close together (Under corrected wavefront) or if they bow inward they are too far apart. (Over corrected wavefront)

 

5th Step.jpg

 

                     

* Secondary Mirror
Double check secondary but all should be good as the last turn on the primary screw should have been like 1/64th of a turn. If not adjust and you are collimated!
 

 

 

Always remember that moving one mirror affects the other mirror. This method I have been using for years and it works for me as always use at your own risk smile.gif

 

Hope this helps.

 

 


  • George9, Starman27, GlenM and 12 others like this

#2 waso29

waso29

    Surveyor 1

  • *****
  • Posts: 1,737
  • Joined: 12 May 2010
  • Loc: Chi-Town, mother earth

Posted 16 January 2019 - 07:39 PM

thank you.

this sure beats the instructions provided by orion telescopes.

 

haven’t seen any yet from agena/ astronomics.


  • NZRoger likes this

#3 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 194
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 17 January 2019 - 11:46 AM

No problem waso29.

I think this fall under, Do you want to talk to the guy in the office running the place or the guy actually do it. LOL


  • 100MPG likes this

#4 quilty

quilty

    Mariner 2

  • -----
  • Posts: 246
  • Joined: 07 Oct 2019

Posted 13 November 2019 - 01:25 PM

this sounds to be a good one. Better than any available from any provider.

Thanks


  • Spacedude4040 likes this

#5 macdonjh

macdonjh

    Fly Me to the Moon

  • *****
  • Posts: 6,861
  • Joined: 06 Mar 2006

Posted 13 November 2019 - 02:16 PM

       

* Secondary Mirror
Insert high power eyepiece and defocus outward. At this point you can probably start seeing the Poisson Spot in the doughnut; this will allow you to achieve a higher degree of centering using the adjustment Allen screws. (Make the spot land in the center of the secondary shadow/large doughnut) You should be using an eyepiece equal to the focal length of the scope so a 12mm or so.

 

Minor quibble: "focal ratio"...

 

Thanks for this, Spacedude4040.  One other suggestion: when I was collimating my classical Cassegrain the first time, a friend made sure I verified my focuser was properly aligned before I started moving the mirrors.  He was right, it wasn't properly aligned.  Talk about chasing your tail.

 

An observation: as you note, the farther you get in your procedure, the better the seeing has to be.  So I've taken to zeroing in on proper collimation over several attempts rather than trying to get it just right on a night of exceptional seeing.  That way I spend a few minutes on some random night getting as close as I can.  On subsequent nights, if I think the seeing is better than the last time I collimated, I'll spend a few minutes trying to improve things then get on with observing.  After three or four iterations, I'm done.


  • eros312 and Spacedude4040 like this

#6 coinboy1

coinboy1

    Surveyor 1

  • *****
  • Posts: 1,696
  • Joined: 03 May 2011
  • Loc: Tulsa, OK

Posted 15 November 2019 - 08:45 PM

Great guide! Thanks for your contribution!
  • Spacedude4040 likes this

#7 nfotis

nfotis

    Mariner 2

  • -----
  • Posts: 284
  • Joined: 22 Jun 2020
  • Loc: Athens, Greece

Posted 16 September 2020 - 12:42 PM

Nice guide, I would like to read impressions from users if it was helpful.

N.F.

#8 MikeECha

MikeECha

    Messenger

  • *****
  • Posts: 410
  • Joined: 15 Sep 2018
  • Loc: Charlotte, NC

Posted 16 September 2020 - 05:34 PM

Collimating GSO Classical Cassegrain

 

First thing is to remember is that the mechanical axis and optical axis in a budget scope will 99% of the time not match. This is true even in high end Classical Cassegrain with tight tolerances.

 

I just wanted to let people know that it is not necessary to run out and purchase a Cheshire or Laser collimator to line up your new Classical Cassegrain. The telescope was set up at the factory and may be out of collimation due to shipping but can’t be off all that much. If your focuser is like mine with a built in tilt you’re going to throw the whole system out of whack collimating with it. Under the stars when testing you might have to totally undo everything that you did previously with those tools.
Due to a slight tilt of the GSO focuser base to the focuser draw tube its best to forget using normal collimating tools like a Cheshire or collimation laser. Unless you machine the focuser base to match the focuser tube, replace the focuser completely or use the GSO tilt plate to correct the slight tilt of the focuser. If using the latter, once lined up you must leave the focuser in the position all the time or you will compound this issue by turning the focuser on the scope. It also has to be redone every time you adjust the primary. I chose to machine the existing focuser which worked out quite well.

 

NOTE: A fixed primary does not mean it doesn’t need to be collimated! All it means is that it doesn’t move to achieve focus!

 

Collimating Steps:

Do not use a star diagonal while performing this procedure; use a 4” extension tube in focuser. Diagonals can introduce their own set of problems.

 

*Primary Mirror    
Center star (focused) in eyepiece and examine for any on axis coma, tighten the silver screw and loosen the lock screw at the same time on the bright side opposite of the flared side to eliminate most if not all the on axis coma. You can also loosen the silver screw and tighten the lock screw on the flared side if the screw matches orientation of flare. This is looking from the back of scope. Re-center again and repeat till gone. If you examined the star and you don’t have any on axis coma just go to the next step.

 

attachicon.gif1st Step.jpg
                    

*Secondary Mirror 
Next defocus outward (Extra Focal) the focuser to show the secondary in the light path (Large Doughnut)
This is when you adjust the secondary Allen wrench screws. I like to move the mount to make the star de-center in the eyepiece in the direction that makes the donut look symmetrical. (Use a low power eyepiece.) Now turn the adjustment Allen screws by tighten one and loosen one or two screws to bring the doughnut back to the center of the eyepiece. Re-center and repeat if needed. If you loosen one screw and your star image is gone out of the field of view, just do the reverse and tighten the same screw to bring it back. No need to panic. Only minor adjustments should be required to achieve collimation

 

attachicon.gif2nd Step.jpg

 

                    

* Primary Mirror

Insert medium power eyepiece and slightly defocus outward on the centered star (not as much as secondary adjustment) you may see only a brightening of the outer ring, this is the side you need to tighten the silver screw and loosen the lock screw of primary mirror at the same time. You can also loosen the silver screw and tighten the lock screw on the duller side if the screw matches orientation of duller side of the outer ring.

 

Tip: Look thru the eyepiece as you turn the silver and lock collimation screws and watch how the two interact with each other.

 

attachicon.gif3rd Step.jpg

 

                   

* Secondary Mirror
Insert high power eyepiece and defocus outward. At this point you can probably start seeing the Poisson Spot in the doughnut; this will allow you to achieve a higher degree of centering using the adjustment Allen screws. (Make the spot land in the center of the secondary shadow/large doughnut) You should be using an eyepiece equal to the focal length of the scope so a 12mm or so.

 

attachicon.gif4th Step.jpg

 

                    

* Primary Mirror
Now increase the power again by changing the eyepiece and observe a star at focus. You need to see the ring around the airy disc for this next step. (A night of good seeing is necessary). Look for any brighten of the ring and once again you need to tighten the silver screw a tad more on the bright side but don’t touch the lock down screw. Loosening the lock down screw will make too much of a movement.
This should still be within the factory tolerance for spacing between the primary and secondary mirrors. This can be checked by inserting a Ronchi Eyepiece and if the lines run straight you are good. If they bow out the mirrors are too close together (Under corrected wavefront) or if they bow inward they are too far apart. (Over corrected wavefront)

 

attachicon.gif5th Step.jpg

 

                     

* Secondary Mirror
Double check secondary but all should be good as the last turn on the primary screw should have been like 1/64th of a turn. If not adjust and you are collimated!
 

 

 

Always remember that moving one mirror affects the other mirror. This method I have been using for years and it works for me as always use at your own risk smile.gif

 

Hope this helps.

Very good guide. I have been learning how to collimate my AT6RC and I use a mixture of step from several techniques from post on this forum including some of yours.

 

I find that the GSO focuser on my scope does not have a tilt. What creates the tilt is the compression fitting. I removed the secundary assembly and with a laser on the focuser compression ring, the beam would hit close to the center hole but not right thru. When I loosened the compression ring I was able to put the laser beam thru the center hole by very slightly tillting the nosepiecs while still fully inserted into the focuser. That was just using the slop between the focusef and the nosepiece. I now have a ZWO Tilter II on order to permanently fit it to the nosepiece.

 

Another thing I learned with my disaasembly and experimentation is that mirror distance/focal length adjustment is not done with the center screw but with the big, fine tread on the secundary assembly. That allowse you to make very fine adjustment without affecting the tilt. If you are carefull you can change the mirror distance without affecting collimation.



#9 Nicole Sharp

Nicole Sharp

    Mercury-Atlas

  • -----
  • Posts: 2,613
  • Joined: 12 Jun 2018
  • Loc: Cumberland, Maryland, USA

Posted 09 September 2021 - 11:04 PM

Will a laser collimator work in a Classical Cassegrain?



#10 Gen 1:16

Gen 1:16

    Explorer 1

  • -----
  • Posts: 60
  • Joined: 03 Jan 2021

Posted 13 October 2021 - 09:33 AM

Will a laser collimator work in a Classical Cassegrain?

I'd like to know that myself?



#11 macdonjh

macdonjh

    Fly Me to the Moon

  • *****
  • Posts: 6,861
  • Joined: 06 Mar 2006

Posted 13 October 2021 - 01:23 PM

Will a laser collimator work in a Classical Cassegrain?

 

 

I'd like to know that myself?

A simple laser is of limited use.  The problem is when the laser is reflected off the convex secondary mirror is diverges, turning into a large blob by the time it returns to the laser tool.  It's not a precise task to judge when the center of the laser blob is centered over the center of the target.  It can help get rough alignment, but you still have to finish collimation of both mirrors by traditional methods.

 

I don't have any experience using the "holographic" laser collimators.


  • eros312 likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: cassegrain, catadioptric, collimation



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics