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Collimation with Catseye - Stacking reflections?

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#1 Corpze

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Posted 21 September 2020 - 02:50 AM

Hi, i have been collimating my newtonian astrograph f4 with the Concenter eyepiece from Teleskop Service, Infinity eyepiece and Blackcat but I can't get all the reflections to stack up according to the image below.

hotspot_xlk_offset.jpg

It is either the offset pupil reflections OR the center pupil reflections that stack up, not both at the same time


The primary mirror is aligned according to the image below using the Blackcat

hotspot_cheshire2.jpg

Is there something I have been missing?

I believe the secondary mirror is center very well when looking through the Concenter eyepiece.

/Daniel
 



#2 ButterFly

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Posted 21 September 2020 - 05:04 AM

The offset pupil is offset.  You should see two sets of stacks when it's collimated because you are looking at an angle.  The central pupil is not offset.  There should only be one stack (with inverted reflections gone) when it's collimated because you are looking dead center.

 

Watch the Catseye video.  Learn CDP and move the primary around when the secondary is aligned to the focuser using the correct inverted reflection.  It is very instructive as to what the autocollimator is actually doing.  In practice, I find the iterative approach easier because the proper inverted reflection is brighter and less fuzzy.

 

You don't mention which pupil you have taken the image through, but it's obvious by looking at the reflection of the pupil itself.  This scope is not collimated.  Here, you did CDP with the wrong inverted reflection.

 

Very nice images though.  You even captured the inverted pupil reflection in the autocollimator!  Watch what happens to those pupil reflections when the correct inverted reflection is used for CDP.



#3 Corpze

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Posted 21 September 2020 - 06:29 AM

The offset pupil is offset.  You should see two sets of stacks when it's collimated because you are looking at an angle.  The central pupil is not offset.  There should only be one stack (with inverted reflections gone) when it's collimated because you are looking dead center.

 

Watch the Catseye video.  Learn CDP and move the primary around when the secondary is aligned to the focuser using the correct inverted reflection.  It is very instructive as to what the autocollimator is actually doing.  In practice, I find the iterative approach easier because the proper inverted reflection is brighter and less fuzzy.

 

You don't mention which pupil you have taken the image through, but it's obvious by looking at the reflection of the pupil itself.  This scope is not collimated.  Here, you did CDP with the wrong inverted reflection.

 

Very nice images though.  You even captured the inverted pupil reflection in the autocollimator!  Watch what happens to those pupil reflections when the correct inverted reflection is used for CDP.

I might have been bad at explaining enough...
I will try again :)

The images is from Catseye website.

When i look through the centrel pupil, the reflections interlock (and fades away just like the video) exactly like the image i have attached
When i change to the Blackcat, the hotspot is dead center, just like the attached image of the blackcat.

When i then changes back to the infinity, the central pupil stays just like before, they interlock and fades away, when i look through the offset pupil, they do not interlock.
If i adjust so that the reflections interlock watching through the offset pupil, the reflections looking through the center pupil dos not interlock.

Every iteration is double checked with the Blackcat.

I simply put does not manage to get the reflections to stack up via the central pupil and the offset pupil at the same time, without adjusting the secondary mirror :/

I have read something about the infinity eyepiece not beeing at the right focal plane might have something to do with it?

/Daniel



#4 Vic Menard

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Posted 21 September 2020 - 03:33 PM

I have read something about the infinity eyepiece not being at the right focal plane might have something to do with it?

Bingo!

 

The autocollimator is sensitive to the pupil placement relative to the focal plane--this is more of an issue for imaging scopes that have the focal plane high above the focuser. If you're too far from the focal plane, you will also notice that the reflections of the primary mirror center marker are different sizes. 

 

Also remember that stacked reflections in the center pupil indicate the two axes are crossing at the center of curvature, and stacked pairs in the offset pupil indicate the two axes are parallel. CDP indicates the axes are crossing at the primary mirror center marker, and Cheshire alignment indicate the axes are crossing at the focal plane.



#5 ButterFly

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Posted 21 September 2020 - 03:46 PM

So those are not the pupils, but the flashlight?



#6 sixela

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Posted 21 September 2020 - 04:45 PM

Have you rotated the tool to see if the stacks change?

#7 Corpze

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Posted 24 September 2020 - 02:45 AM

Bingo!

 

The autocollimator is sensitive to the pupil placement relative to the focal plane--this is more of an issue for imaging scopes that have the focal plane high above the focuser. If you're too far from the focal plane, you will also notice that the reflections of the primary mirror center marker are different sizes. 

 

Also remember that stacked reflections in the center pupil indicate the two axes are crossing at the center of curvature, and stacked pairs in the offset pupil indicate the two axes are parallel. CDP indicates the axes are crossing at the primary mirror center marker, and Cheshire alignment indicate the axes are crossing at the focal plane.

Thank you for the confirmation, i will try to collimate at the focal point to see if the reflections is stacked up properly in either pupil at the same time (that will indicate that i have the right focal plane i presume? together with the same size of the hotspot reflection mark?)

 

Have you rotated the tool to see if the stacks change?

No i have not, i can try that to just to see if there is anything wierd going on.

/Daniel



#8 Blueox4

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Posted 26 September 2020 - 07:55 AM

This may help.

 

https://youtu.be/Vj12cx3tnsM


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#9 Corpze

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Posted 29 September 2020 - 02:26 AM

Thanks for the input Blueox! That video is very good, I have watched it several times.

I tried to rotate the infinity eyepiece and the Blackcat in the focuser and the reflections stays put so i believe the eyepieces are good!

I also tried to hold the infinity 1" above the maximum extended drawtube and reached the focalpoint, so i ordered a 2" extension tube, i hope this might sort things out not beeing able to stack the reflections properly.

/Daniel



#10 Corpze

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Posted 02 October 2020 - 04:10 AM

Update:

i just recieved my 2” extension tube and now the reflections are stacking up just perfectly!

 

thanks for all input guys, i missed the importance of collimating at the focal point!




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