Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Reflectors

Pages: 1 | 2 | 3 | (show all)
alonzo
super member


Reged: 06/06/12

different views thru collimation cap and cheshire
      #6211171 - 11/23/13 11:38 AM

...When I go to center my secondary with a cheshire things look as they should but when I replace the cheshire with a collimation cap the secondary appears too far forward. Which one should I believe.

Thanks
Alonzo


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6211265 - 11/23/13 12:18 PM

Did you mean cheshire/sight-tube combo? I would believe it over the collimation cap. However, when both are used properly with a well-collimated scope they should agree -- assuming the tools are machined accurately.
Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6211442 - 11/23/13 01:48 PM

Maybe he needs to sq up the focuser.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: Pinbout]
      #6211479 - 11/23/13 02:07 PM

Quote:

Maybe he needs to sq up the focuser.



Squaring the focuser has nothing to do with the discrepancy the OP described. If the secondary looks centered using a cheshire/sight-tube then it should also look centered with the collimation cap.
With the collimation cap, the eye axis need to be coincident with the drawtube axis which could be difficult. If the eye is placed at an angle then the secondary mirror might appear off. This is why the collimation cap is not a good tool to center/round the secondary mirror.
Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6211580 - 11/23/13 02:51 PM

Quote:

...When I go to center my secondary with a cheshire things look as they should...



Did you place a sheet of white paper behind the secondary mirror? You need to be able to see the edge of the secondary mirror centered relative to the bottom edge of the Cheshire combo tool.


Post Extras: Print Post   Remind Me!   Notify Moderator  
alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6211957 - 11/23/13 06:33 PM

...Thanks for the comeback and to let you know my cheshire somehow got cocked in the focuser tube and at the time I didn't notice it. All is well now and thanks. Now another problem has emerged. With the secondary centered I can get the three mirror clips in view but when I start to tweak my secondary I start to loose one of the mirror clips. I am going nuts over this and about ready to call it quits.

Vic yes I had a white piece of paper inserted in the tube at that time but as I said I totally wasn't aware at the time the cheshire was in a serious tilt in the focuser.

Alonzo

Edited by alonzo (11/23/13 06:37 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6211982 - 11/23/13 06:48 PM

Try the following steps:

1- Get the cross-hairs to line up with the primary mirror center spot. Now the primary mirror reflection is centered under the focuser. Note I said "the primary mirror reflection is centered" -- not the secondary mirror.

2- Now imagine the secondary mirror is a window overlooking the primary mirror. You need to move the window "secondary" to get the whole primary mirror into view. Should you lower the window down the tube? Should you raise it up the tube?

3- Make the movement without looking down the sight-tube.

4 Repeat above steps until you are done. Refer to the animations below to help you visualize my description:





Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6212198 - 11/23/13 09:16 PM

....Jason I am puzzled over step three.

Alonzo


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212217 - 11/23/13 09:27 PM Attachment (26 downloads)

Assuming you end up with this view

Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212220 - 11/23/13 09:28 PM

Quote:

...With the secondary centered I can get the three mirror clips in view but when I start to tweak my secondary I start to loose one of the mirror clips...



If the secondary is already centered (concentric with bottom edge of the sight tube and cross hairs centered on primary mirror center spot), I don't understand why you need to "tweak" your secondary?

If, OTOH, you mean the primary mirror reflection is centered in the secondary mirror, but the cross hairs in the sight tube are not centered on the primary mirror center spot, then I suggest these two steps:

1.) Use the secondary mirror tilt adjustment screws to align the cross hairs with the primary mirror center spot.

2.) Now, to center the primary mirror reflection on the face of the secondary mirror, use either the secondary mirror rotation adjustment and/or the position of the secondary mirror closer to or farther from the primary mirror to make the alignment. Do NOT use the secondary mirror tilt adjustment.

Repeat steps 1 and 2 until the two alignments are corrected.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6212224 - 11/23/13 09:31 PM Attachment (21 downloads)

It means the secondary mirror is too low in the tube.
I would tighten the central bolt of the secondary holder clockwise to raise the secondary mirror. I would do this without worrying about the sight-tube. After completing this step, I would start all over again to adjust the rotation/tilt of the secondary mirror to align the cross-hairs again on the top of the primary mirror center spot.
Hopefully you will end up with the following. If not, repeat the steps.

Edit: Spelling correction

Edited by Jason D (11/24/13 12:20 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6212233 - 11/23/13 09:37 PM

...Vic you guessed it correctly. When all looks well and I attempt to center the cross hairs on the center mark things go askew.

Alonzo

Edited by alonzo (11/23/13 09:47 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6212239 - 11/23/13 09:40 PM

Quote:

Assuming you end up with this view



Jason, I wonder if you could create a graphic like this one showing the secondary mirror offset downward (toward the six o'clock position) to demonstrate when the user needs to consider realigning the focuser (assuming the spider/secondary mirror are properly centered). A second graphic, showing the secondary skewed toward the center of the fov (an ellipse with the centered primary mirror reflection) would demonstrate the other possible "best" alignment with a misaligned focuser.

This would help clear up what looks like rotation/offset/tilt combined errors when in fact, the focuser needs to be shimmed/leveled...


Post Extras: Print Post   Remind Me!   Notify Moderator  
alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212245 - 11/23/13 09:45 PM

..Jason are you refering to a fore and aft adjustment when you use the word tie and ifso I am to understand the secondary mirror is a little to close to the primary and needs to go forward towards the front of the tube.

Alonzo


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212247 - 11/23/13 09:47 PM

Quote:

...you guessed it correctly. When all looks well and I attempt to center the cross hairs on the center mark things go askew.



Try repeating the two steps I suggested.
If your best alignment leaves the secondary mirror looking like a skewed ellipse (tilted toward two o'clock or four o'clock in Jason's graphic) with the reflection of the primary mirror centered in the secondary mirror, your focuser will need to be realigned...

If your best alignment leaves the secondary mirror looking like a skewed ellipse (tilted toward two o'clock or four o'clock as in Jason's first animation) with the reflection of the primary mirror OFF-center in the secondary mirror (like in Jason's animation), you have an unresolved tilt/rotation error.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212250 - 11/23/13 09:50 PM

Quote:

..Jason are you refering to a fore and aft adjustment when you use the word tie and ifso I am to understand the secondary mirror is a little to close to the primary and needs to go forward towards the front of the tube.



I believe he meant "tighten" (spell checker?)
And yes, he meant fore and aft.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6212256 - 11/23/13 09:59 PM

FTR, I call the "fore and aft" adjustment the "offset" adjustment--when it's set correctly (centered under the focuser with the primary mirror reflection centered in the secondary mirror) the alignment is "offset". If you continue to move the secondary mirror away from the primary mirror in small increments, you will eventually achieve a "centered" alignment (the secondary mirror is NOT centered under the focuser but the primary mirror reflection IS centered under the focuser and the silhouette reflection of the secondary mirror that normally looks offset toward the primary mirror will instead be centered in the primary mirror reflection).

Offset alignment is the optimal alignment for visual work.
Centered alignment can be useful (when the secondary mirror is large enough to accommodate the unbalanced illumination caused by the lack of offset) to improve DSC accuracy (assuming the spider/secondary mirror is centered).


Post Extras: Print Post   Remind Me!   Notify Moderator  
alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6212262 - 11/23/13 10:08 PM

....Vic,Jason I am going retire for the night for its been a hectic day. In the morning I am going to neutralize the secondarys tilt screws and the primarys adjusting screws and start this mess all over again. Please don't give up on me for I really need some help. Just keep an eye out on this post for I will be getting back to you tommorrow.

Thanks
Alonzo


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212276 - 11/23/13 10:20 PM



Post Extras: Print Post   Remind Me!   Notify Moderator  
alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6212999 - 11/24/13 11:05 AM

....Today I am going to neutralize my primary adjusters and tilt screws on my secondary. I will try the best I can to center the secondary with the cheshire. This is done by extending the cheshire very carefully up the focuser with an extension tube. I don't like doing it this way but if I can just get halfway close to collimation I can do a star test later and bring it in. After the procedure I just described I will use a collimation cap to view the results. The collimation cap offers a much wider view of my efforts. Once this is done I will attempt to put the cheshires cross hair on the primarys center spot. This is were I think things will go wrong and I will report back on my post with the results.If you guys wish to comment on the procedure I just outlined or if I need to do something different please comment. As always I appreciate the time and help you guys are giving me.

Alonzo

Edited by alonzo (11/24/13 11:08 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | (show all)


Extra information
12 registered and 21 anonymous users are browsing this forum.

Moderator:  ausastronomer, Knuklhdastrnmr, Phillip Creed, JayinUT 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1334

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics