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Cheshire and Collimation

Accessories Beginner Collimation Reflector
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#1 kas20amc02

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Posted 20 July 2021 - 09:31 AM

I need some advice regarding collimating my 10 inch f/5 Dob.  I read through an article (https://www.astro-ba...nian-reflector/) and realized I may have been doing this incorrectly.  I usually just use a cheap laser and align the secondary with the primary and the primary with the bullseye on the laser.  After reading the article, I realized I skipped several of the first steps.  I also realized what I thought was simply coma artifact and difficulty focusing may have been collimation error (thanks to maroubra_boy).  

 

My question now is what should I buy to rectify the problem?  I found several item below from High Point but am open to suggestions.

 

Many thanks,

Karl

 

 

 

https://www.highpoin...-focusers-94182

 

https://www.highpoin...site-tube-combo


Edited by kas20amc02, 20 July 2021 - 10:05 AM.


#2 Speedy1985

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Posted 20 July 2021 - 09:46 AM

I use this one, along with their Cheshire. Oddly, I paid the same price for the 1.25/2” laser and 2” Cheshire combo kit, but I see now that they are sold separately. The kit is still available as a 1.25” though. Seems odd that they would do this. https://www.highpoin...5-2-combo-fp220



#3 Starman1

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Posted 20 July 2021 - 01:10 PM

Collimation has 3 steps, and I'll identify the tools to do it:

1) centering the secondary under the focuser

Tool needed: Sight tube or Combination sight tube/Cheshire

2) aligning the focuser axis with the primary by adjusting the secondary tilt and rotation

Tool needed: Sight tube, or combination sight tube/Cheshire, or collimated laser, or autocollimator

3) aligning the optical axis with the focal plane by adjusting primary mirror tilt

Tool needed: Cheshire, or barlowed laser

 

I'll mention tools I have experience with and can recommend for each process:

1) Astrosystems Light Pipe

2) Astrosystems Light Pipe, Farpoint laser, Howie Glatter Laser, Astrosystems laser, Farpoint Autocollimator, Catseye autocollimator

3) Farpoint Cheshire, Catseye Cheshire, Astrosystems Light Pipe, Howie Glatter barlow attachment, Howie Glatter Blug, Howie Glatter Tublug, Astrosystems laser with barlow attachment, 

Note: as long as it is barlowed and the screen is before the barlow lens on the return of the beam, any laser, no matter how bad, will work for this process.

 

The Astrosystems Light Pipe does it all, but, like all passive tools (including Cheshires and autocollimators), requires light to work.

Lasers provide their own light, so work in the dark.

Personally, I always collimate in the light of the late afternoon, so I prefer the sight tube + Cheshire + autocollimator processes.

If I check collimation in the dark, I use the laser + barlowed laser processes, but that is not very often.


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#4 Bonco2

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Posted 20 July 2021 - 03:33 PM

I have had excellent results just using a Cheshire. Not much success with a laser.

Bill


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#5 Starman1

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Posted 20 July 2021 - 03:56 PM

I have had excellent results just using a Cheshire. Not much success with a laser.

Bill

I presume you are referring to a combination sight tube + Cheshire as a "Cheshire".

A Cheshire has no crosshairs.



#6 Paul R.

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Posted 20 July 2021 - 06:59 PM

I ONLY use an old Tectron Cheshire and position the secondary and primary everytime I assemble my scope. Once you know what your looking for it's really quite simple. It just takes a small bit of practice.
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#7 daquad

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Posted 20 July 2021 - 07:02 PM

I presume you are referring to a combination sight tube + Cheshire as a "Cheshire".

A Cheshire has no crosshairs.

 

Right.  I removed  the crosshairs on my Orion Cheshire, because they made it more difficult to see the reflections in a refractor and did nothing for centering with a Newt.

 

Dom Q.



#8 Paul R.

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Posted 20 July 2021 - 07:12 PM

I have had excellent results just using a Cheshire. Not much success with a laser.
Bill

I sometimes get torched for this, but I've always thought laser collimators, although fun and cool, for my purposes anyway, are superfluous. A good Cheshire imo are just as accurate if not more. Their just not as easy to use as learning to read them takes some practice.

Edited by Paul R., 20 July 2021 - 07:13 PM.

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#9 Vic Menard

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Posted 20 July 2021 - 08:22 PM

I sometimes get torched for this, but I've always thought laser collimators, although fun and cool, for my purposes anyway, are superfluous. A good Cheshire imo are just as accurate if not more. Their just not as easy to use as learning to read them takes some practice.

It's unfortunate that so many "economy" lasers are 1.25-inch and more often than not used in 2-inch focusers with 2- to 1.25-inch adapters that rarely deliver consistent registration. This is a recipe for poor collimation.

 

That said, not all lasers are the "economy" variety and there are a couple that have a solid reputation for delivering good axial alignment. The 1.25-inch "good" lasers, when used in a 2-inch focuser with a Glatter Parallizer deliver consistent and precise alignment. 

 

A "good" Cheshire (there are several available) is an excellent primary mirror alignment tool, and a "good" combination Cheshire/sight tube (there are a few available--but way too many that are poorly designed) can be used as an excellent "all around" collimating tool.

 

For a quick review of Newtonian alignment signatures and the tool(s) that make these alignment reads precise and easy, read here:  https://www.cloudyni...obs/?p=4651500 



#10 kas20amc02

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Posted 20 July 2021 - 08:41 PM

Thanks to everyone who responded.  I am still trying to soak it all in.  

   ~Karl



#11 Paul R.

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Posted 20 July 2021 - 08:42 PM

It's unfortunate that so many "economy" lasers are 1.25-inch and more often than not used in 2-inch focusers with 2- to 1.25-inch adapters that rarely deliver consistent registration. This is a recipe for poor collimation.

That said, not all lasers are the "economy" variety and there are a couple that have a solid reputation for delivering good axial alignment. The 1.25-inch "good" lasers, when used in a 2-inch focuser with a Glatter Parallizer deliver consistent and precise alignment.

A "good" Cheshire (there are several available) is an excellent primary mirror alignment tool, and a "good" combination Cheshire/sight tube (there are a few available--but way too many that are poorly designed) can be used as an excellent "all around" collimating tool.

For a quick review of Newtonian alignment signatures and the tool(s) that make these alignment reads precise and easy, read here: https://www.cloudyni...obs/?p=4651500


Wow, I read your book WAY back in like 1994..in fact, I first learned 'your perspectives' on collimation by reading that book. I still use my old Tectron Cheshire..that I had purchased along with my Tectron 15" and your book! I have though long lost the sight tube and autocollimator. Good to see you're still around Vic!

#12 Gschnettler

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Posted 20 July 2021 - 09:02 PM

For this step:  1) centering the secondary under the focuser

 

I do have this tool:  1) Astrosystems Light Pipe

 

but, I’m not sure how to do it.  What adjustment screws do you turn in order to center the secondary?  Is it the main one in the middle of the secondary holder?

 

also, when I look in the sight tube, the primary isn’t perfectly centered.  How should I fix that?



#13 Vic Menard

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Posted 21 July 2021 - 07:29 AM

For this step:  1) centering the secondary under the focuser

I do have this tool:  1) Astrosystems Light Pipe

but, I’m not sure how to do it.  What adjustment screws do you turn in order to center the secondary?  Is it the main one in the middle of the secondary holder?

The center mounting screw in the middle of the secondary holder (spider hub) attaches the secondary holder to the spider, so you need to be careful not to loosen it too much as the secondary can fall off of the spider (and on to the primary mirror). Usually, this screw is preset from the manufacturer/resaler and it shouldn't require further adjustment. When it does require adjustment, it's to move the secondary mirror closer to, or farther from the primary mirror, which is typically referred to as "offset". The other secondary mirror adjustment that usually needs adjustment when centering the secondary mirror under the focuser is rotation. See here  https://www.cloudyni...ment/?p=2742900

 

...also, when I look in the sight tube, the primary isn’t perfectly centered.  How should I fix that?

If you've adjusted the secondary mirror tilt to align the sight tube cross hairs to the primary mirror center marker/donut, the reflection of the primary mirror will be centered in the focuser/sight tube. If it "looks" off center, it's most likely the secondary mirror isn't "perfectly centered". The good news is that the secondary mirror doesn't need to be perfectly centered, as a small error in the secondary mirror placement only affects the balanced/centered illumination in the eyepiece, which isn't usually noticeable visually. But the reflection of the primary mirror should be centered in the focuser/sight tube as an error here tilts the focal plane and impacts the focus in the eyepiece.

 

The important point here is this: the visibility of the primary mirror clips in the secondary mirror is NOT a valid alignment signature IF the reflection of the primary mirror isn't centered in the focuser/sight tube!

 

(Edit: In the link I included above, if you click on the images in posts 3, 4 and 5, the animations will play. If you watch the animations carefully, you'll notice that the primary mirror reflection is always centered. Also note that of the three errors shown in the animations, the tilt/rotation error is the most common.)


Edited by Vic Menard, 21 July 2021 - 09:31 AM.



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