11 replies to this topic

[Okcman]

So I drug a 13.1 inch Coulter Odysey red tube home from an elderly gentleman who decided he couldn’t get it outside much less move it around - I found the collimation screws stripped out . So I replaced the screws and added locking nuts - although I have discovered that my mirror cell - in this case a monolithic block of mdf -  I have to go and remove the monolith and try to restore it to its former state- it may not be the best idea but it was pointed out in my pictures it was missing the duct tape and iron strap system - that is on me and if I decide to put a proper mirror cell - that is a another discussion - I have inherited all of the decisions and modifications in the past - so now my question - I collimated the best I could - I could see the moon quite vividly at 107x but as I focused my sights on Saturn and used the helical focuser - again it’s what I inherited - it seems that Saturn was like the wheels on the bus go round and round - 

 

so my instinct is that I have a collimation error - i have looked through a scope or two and it seems that some questions come up - could my focuser be misaligned ? Could the secondary be slightly askew ? How do I test the alignment of the focuser ? Is a 45 degree secondary critical to the performance or should I say optimal performance ? 

[Bob4BVM]

A picture taken thru the focuser tube would help us help you here !

[SaturnRacer32]

You could buy a cheap combination system here from cloudy nights classified and analyze your collimation.  All of the things you mentioned could be out of whack but the focuser will likely cause the least amount of problems. Get your secondary precise and then call me the main mirror.  Plenty of articles describing how to do that.

[Okcman]

Well today I found that I was off by that much when I mounted the mirror cell back in the tube ! I included a 

before and after through the focuser ! 

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[Vic Menard]

Well today I found that I was off by that much when I mounted the mirror cell back in the tube ! I included a 

before and after through the focuser ! 

Assuming you held your eye carefully centered above the empty focuser, I've annotated your "after" image. For good image performance, the bottom edge of the focuser drawtube (the outermost blue circle/cross hairs) should be aligned to the center of the primary mirror reflection (red circle/cross hairs). The allowable error (without coma correction) for a 13.1-inch primary is about 1/2-inch. Your error is about 1/4 of the primary mirror diameter, about 3.5-inches. But this is strictly dependent on your eye being carefully centered above the empty focuser, and I doubt that's the case. And that lack of precision effectively casts doubt on your primary mirror alignment (the underside of the focuser/yellow circle centered relative to the primary mirror reflection/red circle).

 

A simple collimation cap (like the 2-inch Rigel Aline) will improve your overall collimation assessment and your primary mirror tilt alignment correction. A good combination tool (like the Catseye TeleCat XLS) will work even better with your short focal ratio scope, assessing and correcting your secondary mirror offset and rotation, your secondary mirror tilt alignment, and your primary mirror tilt alignment. 

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[Okcman]

lol I will have to digest all that - I did get to up to 136x using a 11mm 84 degree  explore scientific eyepiece  and the views of Saturn rings was much better - although the rings are on end !!! Although between my bad eyes , light polluted skies , etc . It’s getting a lot better views - The skies in Oklahoma City was turbulent to say the least ! But thank you so much for your help ! 

[NinePlanets]

Okcman:

 

A quick and dirty collimation tool (to center your eye in the focuser tube) is just an old plastic 35mm film can with the bottom cut off and a little peep hole poked into the center of the lid. It fits right in the focuser and works great.

 

Well, that is if you happen to have an old 35mm film can laying around. I use them for dust plugs in focusers and star diagonal prisms and such. They are handy to keep spare set screws and stuff in.

[Bob4BVM]

Here is a Q&D method which will get you very close.No credit to me, I copied from someone here on CN a while back, may have been Vic !

 

Q&D Collimation !

 

1. Mask over the secondary mirror with a piece of white paper that just fits over the reflective part. (Laid flat, the paper will be an ellipse- i like to mark the center of it by folding it along the hor & vert axis and marking cross-point with a dot.)

 

2. Looking through the focuser fully extended, adjust the spider mounting and the secondary rotation so the white area is exactly centered in the focuser view and appears round.

3. Remove the paper mask and insert a "Cheshire" type eyepiece (or a plug that fits into the focuser with a .5 to .8mm hole in the exact center.  An old 35mm film can works perfect for this)).

 

4. Adjust the secondary mirror alignment screws so that the main mirror appears centered in the view seen through the Cheshire eyepiece. Ignore the reflection of the secondary in it.

 

5. Now adjust the main mirror (while looking thru the Cheshire eyepiece) until the reflection of the Cheshire eyepiece hole appears centered exactly in the main mirror.  If the main mirror has a dot in the center, that will help.  This should get you in pretty good shape.

[Vic Menard]

Here is a Q&D method which will get you very close.No credit to me, I copied from someone here on CN a while back, may have been Vic !

 

Q&D Collimation !

 

1. Mask over the secondary mirror with a piece of white paper that just fits over the reflective part. (Laid flat, the paper will be an ellipse- i like to mark the center of it by folding it along the hor & vert axis and marking cross-point with a dot.)

 

2. Looking through the focuser fully extended, adjust the spider mounting and the secondary rotation so the white area is exactly centered in the focuser view and appears round.

3. Remove the paper mask and insert a "Cheshire" type eyepiece (or a plug that fits into the focuser with a .5 to .8mm hole in the exact center.  An old 35mm film can works perfect for this)).

 

4. Adjust the secondary mirror alignment screws so that the main mirror appears centered in the view seen through the Cheshire eyepiece. Ignore the reflection of the secondary in it.

 

5. Now adjust the main mirror (while looking thru the Cheshire eyepiece) until the reflection of the Cheshire eyepiece hole appears centered exactly in the main mirror.  If the main mirror has a dot in the center, that will help.  This should get you in pretty good shape.

Sorry, that's not my method!

 

1.) First--I don't recommend masking the secondary mirror. If the OP wants to minimize reflections, he could mask the primary mirror with a 13.1-inch circle (with a visible center marker). This minimizes the secondary mirror placement correction to three circles--the bottom edge of the focuser, the actual edge of the secondary mirror, and the reflection of the masked primary mirror. The additional center marker on the primary mirror mask is also useful as a centering reference for the focuser axis (when using a cross hair (sight tube) collimator or a simple thin beam laser). 

 

2.) By "adjust the spider mounting..." I assume you mean, ...adjust the secondary mirror tilt and rotation (and the secondary mirror tilt and offset). These adjustments will ultimately make all three circles concentric and, more critical than perfect concentricity, the primary mirror center marker will be centered relative to the bottom edge of the focuser. Note that this means the actual edge of the secondary mirror may mot be perfectly concentric to the other two circles, which is OK since a small secondary mirror centering error will have a minimal, visually undetectable, impact on field illumination. It's more critical that the reflection of the primary mirror is centered relative to the bottom edge of the focuser, which ensures optimal focus across the eyepiece field of view.

 

4.) When adjusting the secondary mirror tilt, the "quick and dirty" adjustment is referenced to the primary mirror center marker OR the bottom edge of the focuser. The actual edge of the secondary mirror is NOT a good reference for this adjustment, and more often than not, when used as the reference for secondary mirror tilt, will actually misalign the focuser axis. 

 

5.) Adjusting the primary mirror tilt is always the final step of "quick and dirty" tool alignment. This adjustment centers the coma "free" field diameter (aka, the sweet spot) in the center of the eyepiece field of view.

 

If the secondary mirror placement is more or-less correct (the three circles are more-or-less concentric), steps 4 and 5 are the typical "Q&D" collimation. 

 

Assessing, and correcting, the secondary mirror placement (usually rotation and/or offset assuming the other mechanicals ("squared" focuser and spider, "centered" spider, primary and secondary, etc.) are more-or-less correct) is not Q&D--even when you have the right tools.

 

The collimation order is always:

1.) Assess and correct the secondary mirror placement (three circles).

2.) Assess and correct the secondary mirror tilt (the primary mirror center marker is centered relative to the bottom edge of the focuser)

3.) Assess and correct the primary mirror tilt (the reflection of the underside of the focuser (inside the reflection of the secondary mirror) is centered relative to the primary mirror center marker).

 

If you change the secondary mirror placement (step 1), you must repeat steps 2 and 3.

If you change the secondary mirror tilt (step 2), you must repeat step 3.

 

Last but note least, there is a 4th step--verify the primary mirror tilt tool alignment using a star alignment. I like Mike Lockwood's method;  http://www.loptics.c.../starshape.html

(Note that this only verifies the primary mirror tilt adjustment! Once verified, you can trust your tool alignment to deliver the correction necessary for optimal performance.) 

Edited by Vic Menard, 15 September 2024 - 12:24 PM.

[Bob4BVM]

OK Vic, thanks for the corrections !

That method has worked well enough for me, but I do like your suggestions for changing and improving it.

Beyond that, i am not about to argue with the expert

CS

Bob

[Vic Menard]

...thanks for the corrections ! That method has worked well enough for me, but I do like your suggestions for changing and improving it.

When it comes to Q&D, my intention is the same as yours, keeping the Newtonian collimation procedure as simple as possible. I posted my "5-step" sight tube collimation procedure in this forum 16 years ago ( https://www.cloudyni...nian/?p=2009767  )

 

The first four steps (yours and mine) invariably lead to assessing and correcting secondary mirror tilt/rotation and tilt/offset errors, which is still the most difficult part of the procedure (and is why I eventually evolved the assessment into the 3-circle paradigm). 

[John Bali]

I just picked up a 13.1" red tubed Coulter. Mine has a focuser made from plumbing parts. It is glued in place.  Just looking at it can see it does not point exactly at the secondary. I also noticed it is mounted about .25" off off in rotational axis. Have you checked to see that your focuser is pointed to the center of the tube?

 

John