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Refractor collimation

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#1 psuaero

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Posted 14 February 2020 - 12:03 PM

How difficult is this task? Is it complete black magic only left to the pro's? Does it require a special dilithium powered phaser and tri-corder? Are there reputable places that do it for you? Is it expensive?

 

I see a 80mm triplet scope that the seller is claiming is only slightly out of collimation and selling for 1/2 off. It might be worth the price of admission if it isn't too difficult or expensive to have collimated.

 

Thanks,

John



#2 junomike

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Posted 14 February 2020 - 12:12 PM

It's possible and actually fairly simple to collimate a Doublet,  but a Triplet is in another league and best left for the Pro's IMO.


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#3 StarBurger

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Posted 14 February 2020 - 12:16 PM

I would expect to have a look through it first.. It might be worse than just mis-collimation.

I recall a post recently where someone bought /acquired a refractor that seemed to be missing a lens element!

Caveat emptor...



#4 scopewizard

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Posted 14 February 2020 - 12:21 PM

Actually it is not voodoo magic. I use a Cheshire eyepiece with a light. Shine the light into the 45 degree angle while you look into the Cheshire. For a doublet or triplet the reflection on the back of the lens will have a bright circle with a dark interior and a fainter one, easily visible if out of collimation, they will look like a de-focus star in a Newtonian. First, the bight circle should be in the center of the lens, if is not the focuser needs to be adjusted. Second, adjust the front lens until the circles are on top of each others. This will achieve perfect collimation.


Edited by scopewizard, 14 February 2020 - 12:24 PM.

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#5 Jeff Struve

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Posted 14 February 2020 - 12:35 PM

If it is an Explore Scientific, they may do it for you for the cost of shipping.



#6 ButterFly

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Posted 14 February 2020 - 01:47 PM

Not all cells are collimatable with push/pull screws - that's something you need to find out first.  If there are no screws, it is a much more involved job if you want the optical axis and the physical axis to align.  Everything is done at the focuser and the cell mounting at that point.  Leave that to those with a proper setup for that.

 

A triplet and doublet with collimation screws are no different to collimate in cell tilt.  Centering a triplet (getting the individual elements aligned with respect to each other, not just the tilt of the combination) is much harder than centering a doublet and you should NOT attempt that unless you know what you are doing (it's microns of precision).

 

With push/pull screws, it's a fairly easy task.  You first align the focuser to the physical axis.  Place a laser in the focuser and have it hit the center of the objective (as noted with a template with crosshairs on the cell).  Make sure it still hits that target as you rack the focuser in and out.  The laser itself needs to be colimated for this to work.  Next, you use a chesire eyepiece to get the reflections off the back of the various elements to all line up concentrically.

 

The reflections are different sizes, so blocking half the chesire window with a strip of paper and rotating it around the winidow helps to see whether the differently sized reflections are concentric.  You get semi-circles instead of full circle reflections that way.   Rotating the strip helps determine which screw to push/pull.  It also helps to have a pair of close focusing binoculars to make the reflections look bigger.  2.5x monoculars come in handy for this.

 

All you need are a collimated laser, a template for the objective, a chesire eyepiece (preferably without crosshairs becuase they get in the way), a strip of paper, and prefereably some kind of magnification device to make the reflections look bigger.  This gets you on the pitcher's mound of the ballpark when you finish by tweaking the collimation on a star in good seeing.  If the cell does have push/pull screws, it is worth knowing how to do it.  As with all fairly easy tasks, it's easy once you have done it a few times.


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#7 Jeff B

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Posted 14 February 2020 - 04:06 PM

John:

 

Beware.

 

The traditional meaning for collimation for refractors concerns lining up the center axis of the focuser draw tube with the center of the objective lens (using a laser), then lining up the optical center of the objective with the center axis of the focuser draw tube (using a Cheshire eyepiece).  

 

Now some people are, as well,  using the term collimation to describe the radial centering and tilt of each individual element in the objective relative to each other.  Adjusting the centering/tilting of the elements is a completely different, and much more complex, procedure than traditional collimation and best left to the OEM (who has the right optical tools), especially for triplets.  

 

So be very careful.

 

Now what is the owner describing to you as inaccurate "collimation"?  Astigmatism, coma, weird looking stuff...?

 

Jeff


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#8 LDW47

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Posted 15 February 2020 - 09:53 AM

Based on it being a refractor how would it all of a sudden need collimation ? Did the owner, one of the owners drop it or throw it around ? Aren’t the cells normally pretty solid in their alignment ? Is the seller the original owner ? My take is that very, very rarely does a refractor need collimation !



#9 psuaero

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Posted 15 February 2020 - 12:58 PM

Thanks. I think I'm still way to much of a newb to attempt this. Maybe in a few years.

 

For those looking for a project, it was a scope from Astronomics (CN sponsor/host). The Astro-Tech AT80EDT was $399 and the description said it was just slightly out of collimation. I can't find it now so maybe someone else picked it up for a great price.

 

Thanks everyone.




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