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Laser collimation accuracy question.

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#1 Atlantic Devil

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Posted 21 November 2020 - 03:09 PM

When i try to collimate my orion StarBlast II 4.5 with my laser collimator, i notice that depending on how i tighten the screws to hold the collimator in place, the laser point will be a little off in the circle at the bottom of the scope. Does that little bit of play between the collimator and the inside of the socket where the eyepiece goes make a big difference? Sometimes the laser point is perfect. Sometimes it's off to the top left.



#2 spereira

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Posted 21 November 2020 - 03:22 PM

Moving to Reflectors.

 

smp



#3 wrvond

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Posted 21 November 2020 - 03:49 PM

The ideal thing is to get the collimator centered in the focuser every time. Failing that, you should tighten the screw(s) the same way you do when you insert an eyepiece.

The same thing applies if you use a barlow to collimate. Insert the same way you insert your eyepieces.


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#4 SteveG

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Posted 21 November 2020 - 04:04 PM

When i try to collimate my orion StarBlast II 4.5 with my laser collimator, i notice that depending on how i tighten the screws to hold the collimator in place, the laser point will be a little off in the circle at the bottom of the scope. Does that little bit of play between the collimator and the inside of the socket where the eyepiece goes make a big difference? Sometimes the laser point is perfect. Sometimes it's off to the top left.

That is the main problem with the Lasermate and it's rebranded cousins. Yes, it makes a difference. Also, do not use the return beam to the bullseye on the laser. It is not accurate enough, especially with the poor fit in your focuser. 

 

The recommendations of only tightening the set screw the same as you would or an eyepiece are you best options. Note that if you use a barlow for your primary axis collimation (recommended), the play won't matter. For your primary adjustment, I recommend a simple collimating cap.


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#5 philinbris

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Posted 21 November 2020 - 05:06 PM

The Barlow method works very well, I can wiggle the laser from side to side and the primary doughnut silhouette on the Barlow device face does not move despite the laser dot on the primary moving.

Its made my collimating a lot quicker and easier. No need for a super collimated laser - close is good enough. I did find a laser collimater that had a good round dot as opposed to the rice grain shape better.

Now all I need is for my Howie Glatter TuBlug to show up - will alleviate my makeshift Barlow arrangement and one less party in the optic axis..

Cheers


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#6 MitchAlsup

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Posted 21 November 2020 - 08:58 PM

When i try to collimate my orion StarBlast II 4.5 with my laser collimator, i notice that depending on how i tighten the screws to hold the collimator in place, the laser point will be a little off in the circle at the bottom of the scope. Does that little bit of play between the collimator and the inside of the socket where the eyepiece goes make a big difference? Sometimes the laser point is perfect. Sometimes it's off to the top left.

a) it depends if the laser is actually at the center of the collimator body and whether the laser is axially concentric with the collimator body. I use a set of V-block and slowly rotate the collimator until it does not move on a wall 30' away to get the laser axially concentric.

 

b) once the laser is axially concentric, then it does not mater AS MUCH if the focuser moves the collimator a bit off axis (and the slower your scope is the less it maters.)

 

c) a couple of wraps with Teflon tape might make the collimator a more-perfect fit to the focuser.


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#7 MOwen

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Posted 22 November 2020 - 02:12 PM

To quote Mitch...

c) a couple of wraps with Teflon tape might make the collimator a more-perfect fit to the focuser.

 

That's the ticket.  I just used plain old cellophane tape and it worked like a champ.


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#8 Atlantic Devil

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Posted 22 November 2020 - 03:08 PM

What's this about using a barlow? I just tried using my 2x barlow with my collimator and it was completely off from when i collimated it without the barlow. How do i know which one is the accurate one?

Btw, i'm using the this Astromania collimator.

 

https://www.amazon.c...06075649&sr=8-3



#9 wrvond

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Posted 22 November 2020 - 04:24 PM

Checking that type of collimator is extremely easy. Here is how I check mine:

 

https://www.cloudyni...tor/?p=10668399


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#10 Jon Isaacs

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Posted 22 November 2020 - 04:57 PM

What's this about using a barlow? I just tried using my 2x barlow with my collimator and it was completely off from when i collimated it without the barlow. How do i know which one is the accurate one?

Btw, i'm using the this Astromania collimator.

 

https://www.amazon.c...06075649&sr=8-3

 

The Barlowed laser is a specific technique and properly done, the most accurate way to align the primary with a laser.

 

You need a circular white paper target that fits the lens of your Barlow on the bottom of the Barlow. 

 

It needs to have a hole in the very center maybe 1/8"-1/4" in diameter.

 

The Barlow is inserted in the focuser, the laser in the Barlow. This produces a diffuse beam that covers a small area around the center marker on the primary mirror.

 

This is reflected back with a circular donut shaped shadow to the paper target. You center the donut around the bright outgoing beam to align the primary mirror.

 

Jon


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#11 SteveG

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Posted 23 November 2020 - 01:14 PM

What's this about using a barlow? I just tried using my 2x barlow with my collimator and it was completely off from when i collimated it without the barlow. How do i know which one is the accurate one?

Btw, i'm using the this Astromania collimator.

 

https://www.amazon.c...06075649&sr=8-3

That is the same as the LaserMate. When you place it in the focuser, what does the dot do when you spin the laser while in the focuser? If it stays in the exact same place, then your laser is likely collimated. Next question, does the dot move when you tighten the set-screw on your focuser? Does it move exactly the same way every time you insert it?

 

See Jon's description of the barlowed laser technique.


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#12 Asbytec

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Posted 25 November 2020 - 12:33 PM

"Does that little bit of play between the collimator and the inside of the socket where the eyepiece goes make a big difference? Sometimes the laser point is perfect. Sometimes it's off to the top left."

The focuser axial tolerance is 0.03D, or a little more than 3mm for a 114mm aperture. If it moves more than that, say outside of the central perforation, then it becomes a problem. The real problem, though, is using the return beam to align the primary mirror axis. At f/4, the tolerance 0.02f^3 = 1.3mm, or less, is much tighter. Use at least the Barlowed method, a Cheshire, or collimation cap.

Edited by Asbytec, 25 November 2020 - 12:34 PM.

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#13 philinbris

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Posted 26 November 2020 - 03:23 AM

That is the same as the LaserMate. When you place it in the focuser, what does the dot do when you spin the laser while in the focuser? If it stays in the exact same place, then your laser is likely collimated. Next question, does the dot move when you tighten the set-screw on your focuser? Does it move exactly the same way every time you insert it?

 

See Jon's description of the barlowed laser technique.

I think the most important concept to understand is the laser passes through the Barlow to make a diffuse pattern, reflects back from the primary to the face of the target before re-entering the Barlow.

Using a laser with a standard Barlow does not work because the return beam comes back through the Barlow - i.e. passes through twice. Using the Barlow technique like Howie Glatter does only relies on the laser passing trough the Barlow lens once.

 

Cheers


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#14 Jon Isaacs

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Posted 26 November 2020 - 12:43 PM

"Does that little bit of play between the collimator and the inside of the socket where the eyepiece goes make a big difference? Sometimes the laser point is perfect. Sometimes it's off to the top left."

The focuser axial tolerance is 0.03D, or a little more than 3mm for a 114mm aperture. If it moves more than that, say outside of the central perforation, then it becomes a problem. The real problem, though, is using the return beam to align the primary mirror axis. At f/4, the tolerance 0.02f^3 = 1.3mm, or less, is much tighter. Use at least the Barlowed method, a Cheshire, or collimation cap.

 

Norme:

 

The coma free diameter is 0.022 x F3 = 1.4 mm 

 

The tolerance is half the coma free radius which is 0.005 x F3 = 0.35 mm and closer is better.

 

Jon



#15 Pinbout

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Posted 26 November 2020 - 02:38 PM

What's this about using a barlow? I just tried using my 2x barlow with my collimator and it was completely off from when i collimated it without the barlow. How do i know which one is the accurate one?

Btw, i'm using the this Astromania collimator.

 

https://www.amazon.c...06075649&sr=8-3

Use a sight tube / autocollimator to slight the focuser to secondary to primary 

 

use a barlowed laser to collimate the primary to secondary/focuser.

 

a laser never sees if the secondary is centered in the focuser, nice a round, and the autocollimator makes sure the center of the primary is in the sweet spot, which isn’t the center of the secondary on the long axis cause the offset.

 

or get a howie laser cause they are made well collimated. Hit it with a hammer and it’s still good.


Edited by Pinbout, 26 November 2020 - 09:08 PM.

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#16 Ihtegla Sar

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Posted 26 November 2020 - 03:31 PM

or get a howie laser cause they are made well collimated. Hit it with a hammer and it’s still good.


A word of caution about caring for the Glatter laser. The laser itself is very robust but the threads holding the aperture stop are delicate and there have been reports of the threads wearing out over time or being damaged by cross threading. If one is removing or attaching the aperture stop or any of the other attachments that can be purchased for the Glatter laser, one should take great care not to damage the threads.
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#17 Asbytec

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Posted 26 November 2020 - 05:06 PM

Norme:

The coma free diameter is 0.022 x F3 = 1.4 mm

The tolerance is half the coma free radius which is 0.005 x F3 = 0.35 mm and closer is better.

Jon

Jon, agreed the closer the better. Shooting for D/4 = 0.0055F^3 = 0.35mm is preferred. Especially at F/4 for high power, smaller true field viewing. As the true field gets smaller, the coma free field needs to move closer to the center FOV. The closer the better. That's the tolerance on the focal plane half way to the center of curvature. The "read error" at the primary mirror is twice that being 0.7mm.

Edited by Asbytec, 26 November 2020 - 06:50 PM.

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#18 Jon Isaacs

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Posted 26 November 2020 - 08:44 PM

A word of caution about caring for the Glatter laser. The laser itself is very robust but the threads holding the aperture stop are delicate and there have been reports of the threads wearing out over time or being damaged by cross threading. If one is removing or attaching the aperture stop or any of the other attachments that can be purchased for the Glatter laser, one should take great care not to damage the threads.

 

That happened to mine. I bought my 2 inch in 2007. I used it with the self Barlow attachment and eventually the threads became worn. I sent it to Howie and he did what he could but said the next step would be a new body.

 

Since then, I've been using the TuBlug and only remove the aperture stop when I need use the holographic tool or occasionally when measuring the aperture of a fast refractor.

 

But, it's likely that my collimator got more use than many, I observe quite a bit, both from my backyard and from dark skies and most of its with Dobs.

 

Jon


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#19 Starman1

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Posted 29 November 2020 - 01:34 PM

I discovered my Cheshire and the barlowed laser agreed to the limit of my ability to see a difference.

So, like Jon, I leave the aperture stop on the Glatter laser, but I use a Cheshire to align the primary because I can more easily see small fractions of a mm miscollimation with the Cheshire than with the barlowed laser.

[Note that my Cheshire is not a combination tool and is designed for a large primary center marker, like the Hotspot, which I use.]

And when all the other tools agree with my 2-pupil autocollimator, I know I'm there.

 

Then there is the ability to HOLD that level of collimation, but that is a different discussion.



#20 SteveG

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Posted 30 November 2020 - 01:22 AM

Use a sight tube / autocollimator to slight the focuser to secondary to primary 

 

use a barlowed laser to collimate the primary to secondary/focuser.

 

a laser never sees if the secondary is centered in the focuser, nice a round, and the autocollimator makes sure the center of the primary is in the sweet spot, which isn’t the center of the secondary on the long axis cause the offset.

 

or get a howie laser cause they are made well collimated. Hit it with a hammer and it’s still good.

The Farpoint laser is equally good, but won't work with a Tublug. It does work with a Blug, and would work with a self-made barlow with a white paper target.



#21 SteveG

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Posted 30 November 2020 - 01:27 AM

I think the most important concept to understand is the laser passes through the Barlow to make a diffuse pattern, reflects back from the primary to the face of the target before re-entering the Barlow.

Using a laser with a standard Barlow does not work because the return beam comes back through the Barlow - i.e. passes through twice. Using the Barlow technique like Howie Glatter does only relies on the laser passing trough the Barlow lens once.

 

Cheers

As noted in post #10, you can use a standard barlow with a paper target.

 

The OP has a non-barlowed laser with the return beam target, which is insufficient IMO.


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