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Barlowed laser primary collimation ... I wish I'd done it before

Equipment Barlow Collimation
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Andy Wray avatar
Andy Wray avatar
Just for those people who have been using a laser on its own to align the primary:

A few people mentioned barlowed laser collimation and I hadn't really clicked how useful and simple it would be.

Stick a 2X barlow in the focusser and then stick the laser into that.  Then just align the reflection of the primary centre spot around the centre spot in the laser target.

It's much easier to then adjust the primary mirror adjusting screws to get alignment rather than guessing where the laser disappeared into the hole.  I also understand that it is less susceptible to slight secondary mirror alignment errors.

I know this is second-nature and old-hat to many on here, but thought I'd post for people who haven't tried it.

And the additional cost:  a 2X 1.25" barlow is around $25.

I tried to capture what it looks like, but struggled a bit.  It's not as bright as this in reality, but you can see the donut reflection of the primary center spot that you need to align.

What was a pleasant surprise for me was that the enlarged donut (after the laser had been through the barlow twice) aligned almost perfectly to the rings on the target

@andrea tasselli Thank you for pointing me down this path.


and in real-life it looks more like this:
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Arun H avatar
Arun H avatar
Andy - the great benefit of the barlowed laser method is that it is completely insensitive to proper collimation of a laser. The focuser axis can still be tilted slightly with respect to axis of the secondary mirror if the barlowed laser shows collimation, which is why it is necessary to use a second method to align those two and repeat until both methods agree.
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Andy Wray avatar
Andy Wray avatar
Andy - the great benefit of the barlowed laser method is that it is completely insensitive to proper collimation of a laser. The focuser axis can still be tilted slightly with respect to axis of the secondary mirror if the barlowed laser shows collimation, which is why it is necessary to use a second method to align those two and repeat until both methods agree.

So, right now I am using:

* A TS Concenter eyepiece to get the secondary aligned under the focus tube (about $75 - expensive for what it is, but I struggled to fix my secondary alignment to the focusser tube without it)
* The laser to do initial alignment of secondary to primary (about $30)
* The barlowed laser to align the primary (about $25 for the barlow)
* A Cheshire to do some double-checks and tweaks (about $30)

Maybe go round the loop to check they all agree.

Anything else I have missed?  An autocollimator maybe, although that could be overkill for my gear.
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Mike Dobres avatar
Mike Dobres avatar
Nice photo Andy!  I'd still suggest a star test with an artificial star as the ultimate acid test. - see Chapter 5 of Harold Suiters Star Testing Astronomical Telescopes - A manual for optical evaluation and adjustment. If your backyard is to small, you can always beam across to neighbors house, or set-up in the street

https://www.amazon.com/Star-Testing-Astronomical-Telescopes-Evaluation/dp/0943396905

Mike
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Andy Wray avatar
Andy Wray avatar
Mike Dobres:
Nice photo Andy!  I'd still suggest a star test with an artificial star as the ultimate acid test. - see Chapter 5 of Harold Suiters Star Testing Astronomical Telescopes - A manual for optical evaluation and adjustment. If your backyard is to small, you can always beam across to neighbors house, or set-up in the street

https://www.amazon.com/Star-Testing-Astronomical-Telescopes-Evaluation/dp/0943396905

Mike

Ooops ... I forgot the star test as the ultimate check.
Jason Patterson avatar
Jason Patterson avatar
The version of this I found had the barlow masked down to a relatively small hole (like the size of the hole in notebook paper), and that's what I've been doing.  The resulting ring from the center mark on my mirror winds up projecting onto the mask instead of the laser target.  The method works and provides good collimation.

Are you doing it with no mask at all?

Anyone know if it matters one way or the other? 

Honestly, it seems kind of pointless to mask a laser, since it's going right through the center of the barlow anyway.  It also requires you to look into the telescope to see where the center is, so you wind up losing the convenience of being able to see the laser's target while adjusting the primary (just have to walk back and forth a bit, so it's not the end of the world).
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andrea tasselli avatar
andrea tasselli avatar
Jason Patterson:
The version of this I found had the barlow masked down to a relatively small hole (like the size of the hole in notebook paper), and that's what I've been doing.  The resulting ring from the center mark on my mirror winds up projecting onto the mask instead of the laser target.  The method works and provides good collimation.

Are you doing it with no mask at all?

Anyone know if it matters one way or the other? 

Honestly, it seems kind of pointless to mask a laser, since it's going right through the center of the barlow anyway.  It also requires you to look into the telescope to see where the center is, so you wind up losing the convenience of being able to see the laser's target while adjusting the primary (just have to walk back and forth a bit, so it's not the end of the world).

The way Andy does it makes it sensitive to the focuser parallax (how well centred is) while the one with the proper mask on the barlow lens isn't but you loose the convenience of seeing everything from the back of the telescope which is a plus if you use an autocollimator to refine on the primary axial error anyway. Plus, you don't need to buy or modify anything if you already have a barlow.
Arun H avatar
Arun H avatar
Andy Wray:
Anything else I have missed?  An autocollimator maybe, although that could be overkill for my gear.


Both the Chehsire and barlowed laser are sensitive to primary axial  error (PAE or whether the coma free field is centered with respect to the focuser).  What this means is that the line defining the intersection of the focal plane of the mirror and the plane of the focuser passes through the center of the focuser. The focal plane could be tilted with respect to the plane of the focuser (focuser axial error)  and neither method would detect it unless it were large. It is quite easy to convince yourself of this with a thought experiment.

To eliminate FAE requires that you use an autocollimator. So the sequence recommeded is to collimate the primary with the barlowed laser, then collimate using the autocollimator by only changing the tilt of the secondary. Then go back and redo the primary collimation if needed - iteratively until both agree. Which generally takes only a couple of iterations. 

PAE is the more critical of the two errors, so the barlowed laser is probably the more important tool. Besides, you can fully eliminate FAE using an autocollimator, but if you have sensor tilt, it will have been for nought.

Geometrically - Newtonian collimation is assuring that the normal from the plane of the focuser is parallel to the optical axis of the primary. Two lines are parallel if the distance between them is the same measured at two points. That's the logic behind the two tool strategy put in simple terms.
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Andy Wray avatar
Andy Wray avatar
and here a better photo of the barlowed laser.  It's at least twice as sensitive as just using a laser on its own in terms of aligning the primary and also eliminates any minor laser collimation issues, so this is my goto for initial alignment now.  I'm now just waiting for my autocollimator to arrive so that I can get the whole imaging train sorted
Rob Johnson avatar
Rob Johnson avatar
Andy Wray:
Andy - the great benefit of the barlowed laser method is that it is completely insensitive to proper collimation of a laser. The focuser axis can still be tilted slightly with respect to axis of the secondary mirror if the barlowed laser shows collimation, which is why it is necessary to use a second method to align those two and repeat until both methods agree.

So, right now I am using:

* A TS Concenter eyepiece to get the secondary aligned under the focus tube (about $75 - expensive for what it is, but I struggled to fix my secondary alignment to the focusser tube without it)
* The laser to do initial alignment of secondary to primary (about $30)
* The barlowed laser to align the primary (about $25 for the barlow)
* A Cheshire to do some double-checks and tweaks (about $30)

Maybe go round the loop to check they all agree.

Anything else I have missed?  An autocollimator maybe, although that could be overkill for my gear.

Hi Andy, you said a laser collimator for $30, are these the ones on eBay? Are they any good?
Rob
Andy Wray avatar
Andy Wray avatar
Rob Johnson:
Hi Andy, you said a laser collimator for $30, are these the ones on eBay? Are they any good?
Rob


I use the SVBony Laser Collimator which is available in the US for less than $30 on Amazon.com.  In the UK it is nearer £38.  I'd say it is pretty good for the money.  I checked mine and it was pretty well collimated from the factory.

For primary alignment using a Barlow it should be fine.
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Rob Johnson avatar
Rob Johnson avatar
Thanks Andy I’ll have a look 👍
Andy Wray avatar
Andy Wray avatar
Rob Johnson:
Thanks Andy I’ll have a look 👍

One nice thing about it is that it has 7 levels of brightness which is useful:  at level 7 it is great for the barlow stuff as you need some brightness to really show the reflection of the centre spot.  When aligning the secondary to the primary, I turn it down so that I can see exactly where in the centre spot it is pointing.  It also works off one standard CR2032 button cell which I have loads of for other things around the house.  I'm sure there are many better options, however you can at least practise with this for so little spend and can always upgrade (if you need to) without having spent too much.

I would say that we should all follow the laser up with an autocollimator to get really accurate allignment, however I am struggling to buy one as they don't seem to be available anywhere right now
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