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Need clarification on tools for collimation

collimation equipment
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#1 will0wtr33

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Posted 14 January 2020 - 12:56 AM

So I've heard mention of Cheshire eyepieces, sight tubes, and collimation caps for collimating the mirrors in a reflector telescope. During a bit of research I've seen that what some mean by Cheshire is actually the longer sight tube/Cheshire combo tool. When I see JUST a Cheshire it seems to be a shorter eyepiece with a hole in the center and what I'm assuming is a reflective area on the inside. Is a Cheshire just a collimation cap with a mirror in it, or are they the same thing? What is the difference between a Cheshire and the Cheshire/sight tube combo tool?
Could someone please explain the differences between these tools and any other common collimation tools?

In the near future I plan on buying a reflector for my first scope, so I would like to be as well versed in collimation as possible!

Sorry for the noob question, thanks in advance!



#2 gene 4181

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Posted 14 January 2020 - 02:44 AM

 Sight tubes and combination tools have crosshairs  , chesires don't .   At the top of the reflector forum there are pinned  articles on collimation.   Catseye sells  tools  , I suggest the booklet  New Perspectives on Newtonian Collimation  by  Vic Menard  , all things Newtonian .    There are also collimation  explanations on the Catseye site , left side / bottom


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#3 Cames

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Posted 14 January 2020 - 02:57 AM

Welcome to CloudyNights Forums will0wtr33.

 

Three components of the combination tool can be used like this:

 

Sight Tube - Adjust position of secondary mirror. For those reflector telescopes whose secondary mirror position can be adjusted longitudinally up-and-down, the long tube structure aids in achieving the correct amount of 'offset'.  Offset is necessary to insure that the cone of reflected light from the primary mirror is centered on the focuser tube so that none of the precious reflected light is wasted.  It's a matter of optical efficiency so-to-speak.  Collimation can theoretically be achieved without offsetting the secondary mirror.  The shorty cheshires lack the secondary mirror positioning utility.

 

Reticle or crosshairs - For accurately adjusting the tilt of the secondary mirror.  The reticle of the combo tool has to be combined with a center spot on the primary mirror.  Most reflector primaries are center-spotted.  The secondary mirror tilt is adjusted so that the intersection of the reticle appears coincident with the primary mirror center spot.

 

Cheshire-like reflection - For accurately adjusting the tilt of the primary mirror.  I've seen  different configurations of the Cheshire utility. Some use an angled, brushed-metal finish combined with a peep-hole, some use translucent plastic with a peep-hole, etc. The reason for the angle opening or translucence is so that one can make the surface light up in the dark for nighttime collimating. The angle of the reflective surface has to be 45° so that it will appear round when you see it's reflection on the primary mirror. The original closed-dome Cheshires work better in bright, ambient light that enters the front aperture of the telescope.

 

The collimating cap works just about as well as a Cheshire for adjusting tilt of the primary mirror.  Like the Cheshire, the cap has a reflective surface but it's not mirror-like.  It's not as convenient or as accurate as the reticle of the combo tool for adjusting tilt of the secondary.  Nor much effect in position of the secondary.  

 

If you see a Cheshire-looking device with a mirror dome instead one that is diffusely reflective, that's called an 'autocollimator'.  Autocollimator can improve accuracy of secondary mirror tilt and primary after the reticle and Cheshire adjusting procedure is done. Not of much use for positioning of secondary mirror.

 

If you haven't already done so you may like to visit AstroBaby's Guide to Collimating Pages .  There you can see some of these features in action.

------------

C


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#4 Beeham

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Posted 14 January 2020 - 06:47 AM

For what it's worth, one thing I wish I had purchased earlier (since you mentioned this will be your first reflector scope) was the laser collimator - while not super-reliable by itself, when combined with a barlow eyepiece (which you'll probably end up buying anyway) you can use the "barlowed laser collimation" method, which I find the easiest and most reliable way to tweak collimation quickly and easily.  

 

Good luck with your new telescope!


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#5 macdonjh

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Posted 14 January 2020 - 07:38 AM

 

Reticle or crosshairs - For accurately adjusting the tilt of the secondary mirror.  The reticle of the combo tool has to be combined with a center spot on the primary mirror.  Most reflector primaries are center-spotted.  The secondary mirror tilt is adjusted so that the intersection of the reticle appears coincident with the primary mirror center spot.

 

 

Can't you also achieve this by lining up the refection of the spider vanes in the primary mirror with the cross hairs of the Cheshire/ sight tube or the reticle?  That way you're not dependent on a center spot on the primary (hard to center spot a Cassegrain mirror, but these tools are helpful for that optical design as well).

 

Of course, doing this assumes the scope has good mechanical construction and the mechanical bits are actually aligned with the optical axes.  I have found this useful for getting Cassegrains close enough to collimated to make star testing useful for final collimation.


Edited by macdonjh, 14 January 2020 - 07:47 AM.


#6 Cames

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Posted 14 January 2020 - 11:13 AM

Can't you also achieve this by lining up the refection of the spider vanes in the primary mirror with the cross hairs of the Cheshire/ sight tube or the reticle?  That way you're not dependent on a center spot on the primary (hard to center spot a Cassegrain mirror, but these tools are helpful for that optical design as well).

 

Of course, doing this assumes the scope has good mechanical construction and the mechanical bits are actually aligned with the optical axes.  I have found this useful for getting Cassegrains close enough to collimated to make star testing useful for final collimation.

I wasn't taught to use the spider vanes as you suggested. The primary mirror in the dobsonian telescope that I'm using is not actually centered in its mirror cell. The mirror rests on two support pads and is held down simply by gravity. I can visually see that it isn't held in the exact center of the circular mirror cell. 

 

In my early years of collimating this telescope I did an experiment.  I would get the collimation as close as I could with collimation tool. Then, I would stand back away from the front of the telescope and look in at the actual spider vanes and the reflected vanes on the primary.  They never would coincide at the level of center spot. If I readjusted the tilt of the mirrors so that the spider vanes coincided with their reflection on the primary mirror, and then checked with the collimation tool, the collimation was definitely off.  Ever since, I have tended to ignore the spider vanes. I haven't done an engineering analysis of the problem. I've simply adopted the path of least resistance.

 

From what I understand about adjusting the tilt of the secondary mirror, it is done to insure that the imaginary radial centerline of the eyepiece (the light ray that passes through the exact center of the upper and lower lenses) will impact the center of the primary mirror.  The line of sight between the peep hole in the collimator to the intersection of its reticle is analogous to that special central light ray from the eyepiece.  It is difficult and time consuming to collimate a telescope using an eyepiece. But with the collimating tool, it usually becomes relatively painless after a several practice sessions.

 

I should have specified dobsonian in my answer. For reflector telescopes where there is no center spot possible, you are probably right.  But it is beyond the level of my training. Sorry for the confusion.

----------

C


Edited by Cames, 15 January 2020 - 01:11 AM.


#7 Asbytec

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Posted 14 January 2020 - 11:28 AM

"Can't you also achieve this by lining up the refection of the spider vanes in the primary mirror with the cross hairs of the Cheshire/ sight tube or the reticle?"

Not for a Newt. The cross hairs define the focuser axis, which is what you are collimating. The spider vanes do not define the focuser axis, and they are not necessarily on the optical axis. Secondary offset usually requires the two be different. Bottom line, the cross hairs (and a thin beam laser) define the focuser axis, the spider vanes do not. The vanes cannot be used that way.

A Cassegrain might be different. The vanes may be on the primary axis. Need to put my thinking cap on.

Edited by Asbytec, 14 January 2020 - 11:44 AM.


#8 macdonjh

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Posted 14 January 2020 - 11:30 AM

Cames,

 

Interesting.  I followed your link to Astro-Baby's website and can see what you mean about the spider vanes in a Newtonian.  I've used Astro-Baby's information for rebuilding Synta mounts, but never seen the collimation tutorial.  I've only had two Newtonians.  For one I taught my ten-year-old son to use a laser collimator to get close enough so he could enjoy observing.  If I was using the scope and thought it needed more I used a defocused star.  The other Newtonian is actually a Parks hybrid Newtonian/ classical Cassegrain.  It's tough to collimate because of the perforation in the primary mirror.  I need to practice the technique you outlined above.



#9 Starman1

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Posted 14 January 2020 - 01:49 PM

So I've heard mention of Cheshire eyepieces, sight tubes, and collimation caps for collimating the mirrors in a reflector telescope. During a bit of research I've seen that what some mean by Cheshire is actually the longer sight tube/Cheshire combo tool. When I see JUST a Cheshire it seems to be a shorter eyepiece with a hole in the center and what I'm assuming is a reflective area on the inside. Is a Cheshire just a collimation cap with a mirror in it, or are they the same thing? What is the difference between a Cheshire and the Cheshire/sight tube combo tool?
Could someone please explain the differences between these tools and any other common collimation tools?

In the near future I plan on buying a reflector for my first scope, so I would like to be as well versed in collimation as possible!

Sorry for the noob question, thanks in advance!

 

THE TOOLS OF COLLIMATION
The most commonly used (there are other exotic, do-it-yourself tools that also work, but they are uncommon) Newtonian collimation tools are:

1. Collimation cap (A simple peep-hole, though not good enough for scopes below f/10.  I don’t recommend these except, perhaps, as a quick check to see if the optical elements are at least in gross alignment at the start).  It is used solely for primary mirror tilt adjustment.

2. Laser collimator (not useful unless perfectly collimated itself, possessed of a small beam diameter, and not accurate enough for primary mirror collimation unless used with a Barlow lens, but quite useful in the dark. Mfrs.: Glatter, FarPoint, Astrosystems, etc.  Used for focuser axis alignment (usually referred to as secondary mirror tilt adjustment).

3. Sight Tube (Cylinder with peep hole at one end and crosshairs at the other. Tectron (1.25”), Catseye (2”). Tectron is no longer in business, but the tools are common in the used market.  Combination tools contain this tool).

4. Cheshire (Cylinder with interior annular reflective white surface and blackened center area, producing a reflection from the primary mirror that appears as a bright ring with a dark center. Tectron, Catseye. Combination tools contain this tool).

5. Autocollimator (Cylinder with a reflective mirror inside the peep-hole cap. The internal mirror is perpendicular to the optical axis of the focuser. It produces multiple reflections of the primary mirror’s center marking. Catseye, Tectron, Farpoint, and others).

6. Combination Sight Tube/Cheshire (least expensive, but a little harder to use. It combines the crosshairs of the sight tube and the reflective interior surface of the Cheshire. There are many inexpensive models in the market, and a couple better ones: 1.25” and 2" from AstroSystems called the Light Pipe/Sight Tube, and a 2” from Catseye called the TeleCat).

 

Collimation is 3 to 4 steps:

--centering the secondary under the focuser--use a sight tube or holographic display laser

--aligning the focuser axis by secondary mirror adjustment--use a sight tube, laser, or combination tool

--aligning the primary mirror axis--use a collimation cap, Cheshire, or barlowed laser, or Krupa collimator.

--(optional) eliminating residual errors--use an autocollimator.

 

Send me your email address in a private message, and I'll send you a couple collimation tutorials--one simple and one more detailed.  Both are illustrated.


Edited by Starman1, 14 January 2020 - 01:49 PM.

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#10 will0wtr33

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Posted 15 January 2020 - 12:56 AM

Wow, first let me say thanks everyone for the detailed and informative responses.

I appreciate the time everyone took to help explain this to me. After reading these comments and the collimation guide linked here I'm feeling much more confident about being able to collimate my first scope when the time comes.

I would prefer to avoid laser collimators for now, due to the fact that the tool itself may require collimation.

What I've gathered from the responses and the guide is that a nice Cheshire/sight tube combo will pretty much do the trick. With that I can adjust the secondary mirror placement with the sight tube (provided the entirety of the second mirror can be seen through the tool, the AstroBaby guide mentioned this being an issue for the scope they were using), the tilt of the secondary with the crosshairs, and the tilt of the primary with the Cheshire.

If I want to get extremely accurate past that I can get an autocollimator to fine tune the tilt of my primary and secondary mirrors, but the autocollimator is no good for adjusting the position of the secondary mirror.

Please correct me if any of this is wrong.

Any recommendations on a Cheshire/sight tube combo tool? I've seen people speak highly of the AstroSystems Lite Pipe.

The scope I plan on getting has a 1.25" focuser, so the TeleCat isn't an option. 


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#11 Starman1

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Posted 15 January 2020 - 01:00 AM

In 1.25" tools, the LightPipe is the best combination tool, though I have heard the First Light Optics Premium Cheshire Collimating eyepiece is also quite good.

The one thing I like about the Light Pipe is that the illumination comes from a 360° angle.

I use a 2" version on my own scope.


Edited by Starman1, 15 January 2020 - 10:36 AM.

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#12 Kipper-Feet

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Posted 15 January 2020 - 01:53 AM

Will0wtr33, welcome to the CN forum, and kindly check your Personal Messenger. You've got mail.




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