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Collimation Issues

Celestron Collimation
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#1 Metanoia

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Posted 18 October 2021 - 10:54 PM

(Celestron C8-N telescope 8 inch Reflector.)

 

Either I'm really bad at this (highly possible), I have a bad secondary lens and need a new one, or with some help maybe there's something I'm missing. Over the last few years I've collimated this telescope, I've always had what seems like two issues. One, the secondary mirror doesn't line up perfectly inside of the focus tube, it's always SLIGHTLY off a tinnny bit. And two, the "true" center when viewing through the eyepiece, is not the center when viewing through the focus tube. Essentially, whenever I focus I feel like I sacrifice in one direction. When doing the star test to check for collimation, I'm always a tiny bit off. Last week it seems like it resulted in somewhat of a double vision of the planet. Typically collimation for me involves an extremely tedious combination and dance between the primary mirror and secondary mirror and sometimes it drives me absolutely crazy. And I feel like I'm never able to get this thing collimated perfectly. 

Does anyone have experience with this specific telescope, and does anyone have suggestions on how to fix this problem? I was thinking potentially a different secondary mirror replacement. Or perhaps I'm missing something. 

 

 



#2 m11

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Posted 19 October 2021 - 12:24 AM

Hi Metonoia,

 

I am no expert when it comes to collimation.

When I do my collimation I use the barlowed laser collimation method and used other laser collimators with the bullseye - found them easier.

 

 

Using a laser collimator with a bullseye:

  1. I usually start by putting in the laser first into the focuser.
  2. Check the laser dot as to where it is to the centre donut on the primary mirror (looking from the front of the scope in) - adjust the secondary mirror collimation screws - there are usually 3 of them.
  3. Then once its near or on the centre donut I go to the back of the scope (where the primary mirror is) 
  4. There should be 3 knobs for collimation and 3 screws for locking the mirror in place - loosen the screws locking the mirror ( you can tell the collimation knobs as they have springs between the knob and mirror.
  5. Look at the laser collimator bullseye and adjust until the laser dot is in the donut.  ( You might find if its completely off you will need to locate the dot by where the red laser reflections move on the bullseye.
  6. Go back to the front of the scope and repeat step 2
  7. Go back to the back and check if the laser dot with the bullseye has moved
  8. Once both are ok- test the collimation ( planets really do show if the collimation are off)

This is banking on the fact your laser is in alignment as they can be off - I have a number of them.

 

I generally use the barlowed laser method for my sdm scope which is different as you are looking for the reflected donut on the laser collimator.

 

Hope it helps,



#3 Metanoia

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Posted 19 October 2021 - 10:48 AM

Hi Metonoia,

 

I am no expert when it comes to collimation.

When I do my collimation I use the barlowed laser collimation method and used other laser collimators with the bullseye - found them easier.

 

 

Using a laser collimator with a bullseye:

  1. I usually start by putting in the laser first into the focuser.
  2. Check the laser dot as to where it is to the centre donut on the primary mirror (looking from the front of the scope in) - adjust the secondary mirror collimation screws - there are usually 3 of them.
  3. Then once its near or on the centre donut I go to the back of the scope (where the primary mirror is) 
  4. There should be 3 knobs for collimation and 3 screws for locking the mirror in place - loosen the screws locking the mirror ( you can tell the collimation knobs as they have springs between the knob and mirror.
  5. Look at the laser collimator bullseye and adjust until the laser dot is in the donut.  ( You might find if its completely off you will need to locate the dot by where the red laser reflections move on the bullseye.
  6. Go back to the front of the scope and repeat step 2
  7. Go back to the back and check if the laser dot with the bullseye has moved
  8. Once both are ok- test the collimation ( planets really do show if the collimation are off)

This is banking on the fact your laser is in alignment as they can be off - I have a number of them.

 

I generally use the barlowed laser method for my sdm scope which is different as you are looking for the reflected donut on the laser collimator.

 

Hope it helps,

Hey Vostok, the thing is, everything is lined up to be what seems perfect, i.e. the dot is in the center of the donut. the only thing that's off is the mirror never sits perfectly. I guess I should take some pictures. 



#4 Vic Menard

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Posted 19 October 2021 - 01:43 PM

From your description, it sounds like you're having secondary mirror placement problems. You can get a better understanding of secondary mirror placement by reading the first 6 posts in this discussion:  

https://www.cloudyni...rror-alignment/

(click on the images in post #3, 4 and 5 to see the animations).

 

It's not clear what tool (or tools) you're using to assess your collimation. To better understand which tool is used for each alignment, and how the alignment is assessed and corrected, read this:

https://www.cloudyni...dobs/?p=4651500

 

Finally, when using a star to fine tune the primary mirror tilt alignment (and that's all you should be trying to do when aligning on a star), it's important that the star should be almost in focus (if you go too far out of focus, you'll see your secondary mirror's offset). I recommend using Mike Lockwood's method, here:

http://www.loptics.c.../starshape.html


  • eyeoftexas and Spile like this

#5 Starman1

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Posted 19 October 2021 - 06:38 PM

(Celestron C8-N telescope 8 inch Reflector.)

 

Either I'm really bad at this (highly possible), I have a bad secondary lens and need a new one, or with some help maybe there's something I'm missing. Over the last few years I've collimated this telescope, I've always had what seems like two issues. One, the secondary mirror doesn't line up perfectly inside of the focus tube, it's always SLIGHTLY off a tinnny bit. And two, the "true" center when viewing through the eyepiece, is not the center when viewing through the focus tube. Essentially, whenever I focus I feel like I sacrifice in one direction. When doing the star test to check for collimation, I'm always a tiny bit off. Last week it seems like it resulted in somewhat of a double vision of the planet. Typically collimation for me involves an extremely tedious combination and dance between the primary mirror and secondary mirror and sometimes it drives me absolutely crazy. And I feel like I'm never able to get this thing collimated perfectly. 

Does anyone have experience with this specific telescope, and does anyone have suggestions on how to fix this problem? I was thinking potentially a different secondary mirror replacement. Or perhaps I'm missing something. 

One thing that always confuses beginners at collimation is this:

These things will be concentric when looking into the focuser in a collimated scope:

--the inside of the focuser

--the outside edge of the secondary's reflective surface

--the reflection of the primary in the secondary

--the reflection of the focuser drawtube's bottom

--the reflection of the tool in the focuser

--the center of the tool reflection in the focuser.

 

What will NOT be concentric, and what will appear offset to one side, is:

--the reflection of the outline of the secondary mirror holder, its silhouette, so to speak.

And it will look a bit like this:

Attached Thumbnails

  • Secondary & Primary Aligned - full view.jpg
  • Secondary & Primary Aligned - closeup.jpg

  • SteveG and eyeoftexas like this

#6 fabianalmarain

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Posted 19 October 2021 - 11:07 PM

after 2 years looking for the solution and having read what these great people teach you! Together with other geniuses who contributed their own, you are going to achieve collimation, the full moon just arrived at full and I cannot test my mn190 with the new collimation, I'll see but the first tests were good. luck! and I followed all his instructions at the bottom



#7 m11

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Posted 19 October 2021 - 11:25 PM

Hey Vostok, the thing is, everything is lined up to be what seems perfect, i.e. the dot is in the center of the donut. the only thing that's off is the mirror never sits perfectly. I guess I should take some pictures. 

 

I guess the question is how does it star test with the scope - thats the ultimate goal. Do you see a wide variation in focus with the planets in particular over many nights? That is what I use as my guide if I need to look at the collimation or tweak it even on the same night. 

 

There will be a bit of an offset with the secondary - what tools are you using?

 

Vic and Starman1 I would listen to when it comes to collimation. 


Edited by m11, 19 October 2021 - 11:26 PM.



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