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Laser Collimation, & Nils Olof Carlin

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This last weekend was the fourth weekend in a row of researching and experimenting with my Skywatcher Truss 10"

I got caught up in the whole mess with a simple statement to a newbie who was asking "How close does everything have to be for a good view" on the topic of collimation.

I got chastised for replying to him, "Close enough for Horse shoe's and Hand grenade's", and what I meant was that ,,,, well what I meant didn't seem to matter as that I was collimated do near death by a couple of other members of that site.

So I was going to see just how far off center the collimation could be and still provide an acceptable view, 

After researching and purchasing several new collimation tools, Cheshires' Cats Eyes, site tube combo's and two Hotech laser's, one single beam the other is the cross hair model. Working with the process, including removing the secondary mirror to black the edges & back, I was having difficulty with the Hotech laser, seemed to not center up as designed, always changed a bit when it was just removed and re-installed, so wanting something similar to the Glatter Laser that could just set into the eyepiece holder and be freely rotated I set to going through all my 1.25" to 2" adapters until I found one with a true enough bore and snug enough fit in the 2" eyepiece holder that I could rotate the laser w/adapter and the beam stay centered in the primary center doughnut. 

The problem of having a consistent  enough rotation platform for testing all the adapters I decided to set my ES HR-CC into the draw tube and use the turntable top, seemed to work just fine and as I was watching the laser spot on the primary mirror I also noted that the optics of the CC was refracting the laser beam and as that I was using the cross hair laser for this I also noted the refracted beam was almost square, I was setting up the cross hair laser because it was the newest and I hoped best collimated.

Anyway while watching the little square rotate and stay centered I remembered an article I had read during the last few weeks about using a Barlow to collimate the primary mirror and the reflected center mark technique, Nils Olof Carlin was the author, and during the last four weekends I had built a cap to place on the bottom of my Barlow.

Seeing the refracted square I thought about Nils and for the heck of it I placed the cap on the bottom of the CC and there was the Square with a doughnut in the center!

I was like "Wow & cool" at the same time,,,, as I rotated the laser the square would also rotate around the center hole of the cap, as I stood back up from crouching down to peek up the draw tube at the cap I noticed that the pattern was also displayed on the 45 degree target face of the laser! And also centered !

But, the pattern was just a tad too large, the doughnut was there but the outside edge was on the very outer edge of the target face, so I switched to the single dot laser with the adapter and cap on the CC and the pattern was just a bit smaller, I removed the cap then looked at the laser target face, there was the doughnut, dimmer but very distinct.

I then moved to the back of the scope, I have a truss type dob so from the back I can also see the face of the secondary mirror and watch as the laser pattern moves around as I twist the primary mirror adjustment screws, as the two laser patterns merge the doughnut and outgoing refracted dot of the laser to form a a round dot with a bight center.

The image on the screen may not be distinct enough for other's but understanding what's there it can be worked with quite well at night from the back of the scope.

Let me know what you think of this and is it going to hurt to try it before condemning the idea?

This might be a topic for the Astro Lounge rather than the Getting Started forum, but I wasn't quite sure where to post it and figured you'd put it where it would do the most good.

Edited by SloMoe

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After reading the same article by the late Nils, its the only method I use when away from home using may 2nd Gen Laser.
The laser is collimated to the best of my ability, the laser dot is static at 18+ feet, good enough for the scope, but the barlow method is just so sweet and simple, and pretty accurate, as your not chaser laser dots into any holes? the reflected pattern is very distinguishable too, make adjustments (if needed)  apretty simple task.

I've mentioned in the past for folk to keep their cheapo lasers, don't bin them, use them with a barlow for a quick and pretty accurate check.

I still prefer the Cheshire method for initial accuracy, and a Star test finalises my accuracy assessment. Not only that, the last donut I added is larger than usual, rendering a larger image,but the pattern is clear, good enough for my needs.


Edited by Charic

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So i guess with this new idea, how about setting up the laser in the Barlow and center up the out going refracted dot and the center mark on the face of the secondary, well it's truss dob specific as that's the only type you can see the face of the secondary mirror.

If this works then I won't need the Carlin Barlow cap on the bottom of the Barlow, I'll just line up the two images on the secondary.

Going to have to wait until I get home from work to give it a try.

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17 hours ago, SloMoe said:

So I was going to see just how far off center the collimation could be and still provide an acceptable view, 

I only use the bar lowed laser method to collimate my scope, and the result looks good in a star test.

But your original question remains unanswered. How far of can you be in collimation, and still get acceptable quality views/images? I guess the magic word here is "acceptable" and any ones interpretation of it.

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Morning, well got home last night from work, set up the dob again, it's a truss and the reason for working out a quick collimation method with the coma corrector in place.

Tweaked the secondary into position, the refracted beam dot and the etched center doughnut are almost the exact same size so as the laser dot moved over the doughnut i could easily center it, then the fully illuminated doughnut is now a shadow on the face of the laser 45degree target face, and the refracted laser just outlines the doughnuts shadow perfectly.

In a fully darkened room the two images of the reflected beams on the secondary were difficult to distinguish, so no need to use that although if the laser isn't Barlowed or CC'd then yes you can see both tiny dots merge as the primary comes into collimation.


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At f/5 (correct?) I think I remember your primary axis error tolerance (PAE) to be about .85mm or thereabouts. It's charted somewhere on the net, but you can be nearly a mm off axis w/o any notice of image degradation/contrast loss. I don't remember now whether primary aberrations also follow/show up (coma, TDE, etc).  Nils is the boss (RIP), and Don Pensack and Vic Menard also rule for collimation. Another must


And for fun, check out Suiter's Star Testing  for a classic breakdown of collimation errors and understanding. Google PAE  for your focal ratio.

edit: Here's the chart

https://www.catseyecollimation.com/Newtonian Axial Tolerances.pdf

and f/5 has an error tolerance of .63mm 


Edited by laowhoo

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