Barlow Lazer Method

[sbooder]

Hi All,

           I have just tried the barlow laser method of collimation for the first time...Outstanding!! It was so nice to stand at the back of the scope and get a really good view of the donut reflection on the laser target, cool.  And it agreed with my trusty Cheshire sight tube.

EDIT, just realised I spelt laser wrong in title!

[ScottS]

This is how I set up my primary also. Works great.

[John]

Yes, it's a really good collimation method for the primary mirror. 

[earth titan]

When I had a reflector (considering buying another right at this moment) this was the method I preferred.

Also takes out play in the focuser and laser mis-collimation.

[Bart]

Yes, great method. Glad I discovered it early

Barry

[sbooder]

Oh well, I have not had a chance to test the collimation yet as the clouds rolled in as soon as I polar aligned. I left the scope out for two more hours but it just complete cloud now, so I have just packed away.

[Jason D]

Also takes out play in the focuser and laser mis-collimation.

Barlowed laser can only be used for primary mirror adjustment. You still need to use unbarlowed laser to adjust the tilt of the secondary mirror.

Neither barlowed laser or unbarlowed laser can be used to optimally position the secondary mirror under the focuser.

Jason

[sbooder]

Barlowed laser can only be used for primary mirror adjustment. You still need to use unbarlowed laser to adjust the tilt of the secondary mirror.

Neither barlowed laser or unbarlowed laser can be used to optimally position the secondary mirror under the focuser.

Jason

Personally I don't use a laser for the secondary at all, a laser with the secondary can produce false alignment.  So I stick to my Cheshire/sight tube for the secondary.

[Jason D]

Personally I don't use a laser for the secondary at all, a laser with the secondary can produce false alignment.  So I stick to my Cheshire/sight tube for the secondary.

Quality lasers do not produce false alignment when adjusting the secondary mirror. There are two distinct adjustments for the secondary mirror. The first has to do with adjusting the secondary mirror to reflect the laser beam at the primary center. This adjustment is meant to eliminate the tilt between the primary mirror and the eyepiece focal planes. The second adjustment has to do with centering the secondary mirror under the focuser. This adjustment is meant to optimize the illumination field at the eyepiece. A typical laser collimator can't perform the latter. 

The point I am trying to make is that quality laser collimators perform superbly but only when they are used correctly and for their intended adjustments.

Jason

[sbooder]

I can get a laser to centre on the primary with both a rotation error and tilt errors and because of that I do not use them.

[earth titan]

Personally I don't use a laser for the secondary at all, a laser with the secondary can produce false alignment. So I stick to my Cheshire/sight tube for the secondary.

Same here!

[sbooder]

I thought I would explain how I know that the laser can give false reading for secondary alignment.
 

After watching one Dion Hello again astronomy shed's videos on collimating he bought to our attention that the laser collimator can give false reading when used for secondary alignment.

So being a good science buff I decided to re-run the experiment for myself.
 

Here is what I did.
 

I collimated my 10" Newt as accurately as I could get it using my Cheshire/sight tube, I then used my laser collimator to test for agreement and it did.
 

Then holding a metal rule against the side to the scope wall resting of the upper edge of my secondary I introduced a 1mm rotation error to the secondary.  This rotation was enough to move the laser from centre of the primary to the donut wall.
 

I then used the three adjustment bolts (mine are Bobs Knobs) to bring the laser back to centre.
 

So the secondary now has a rotation error and tilt errors in both left right and up down directions but it is reading centred on the primary.
 

The point here is it is hard to notice just a 1mm rotation from the end of the OTA and cannot be seen down the focuser tube because the laser collimator is using it.  I checked the rotation error before re-centering the laser and using the adjustment bolts with the Cheshire and sure enough the rotation although small was noticeable in the circle of the sight tube and was no longer a circle.
 

I suppose that the combination of both the Cheshire/sight tube and a laser is a good way but I find every time I use the Cheshire to collimate the secondary the laser just agrees and becomes redundant.

[earth titan]

I suppose that the combination of both the Cheshire/sight tube and a laser is a good way but I find every time I use the Cheshire to collimate the secondary the laser just agrees and becomes redundant.

Which is fine for solid tube scopes, but if you arrive in a dark site and assemble your truss tube scope in the dark, then a laser is a real must.

[opticalpath]

I'd like to pass on this tip that I learned recently. If you happen to own a scope that has an elliptical ring marking at the correct optical centre of the *secondary*, as well as a primary mirror centre mark (as a few scopes have such as the MN190), you CAN actually use a barlowed laser to adjust tilt of the secondary. And this method has the same benefit as with primary adjustment of being insensitive to laser alignment accuracy.

You still have to adjust centring of the secondary outline in the drawtube by other methods, but if you have a centre marking on the secondary, it works for tilt adjustment of the SM as well as primary.

What happens is that the barlowed laser reflects a shadow of the secondary centre mark on to the primary. You can look into the OTA and adjust secondary tilt until the shadow aligns with the primary centre marking.

So if you are fortunate enough to have a correctly centre-marked secondary, as well as primary, barlowed laser is a good method for adjusting both mirrors.

This is a view looking down on to the primary of my MN190:

 

 

 

Against the diffuse patch of barlowed laser light, you can clearly see the PM centre mark and the shadow of the SM centre mark.  I deliberately tilted the SM slightly here to misalign them for clarity.  You can also see a shadow of the small black dot that I marked on the secondary (because the factory centre 'circle' mark was not in the correct offset position).

 

 

Adrian

[Jason D]

I thought I would explain how I know that the laser can give false reading for secondary alignment.

After watching one Dion Hello again astronomy shed's videos on collimating he bought to our attention that the laser collimator can give false reading when used for secondary alignment.

So being a good science buff I decided to re-run the experiment for myself.

Here is what I did.

I collimated my 10" Newt as accurately as I could get it using my Cheshire/sight tube, I then used my laser collimator to test for agreement and it did.

Then holding a metal rule against the side to the scope wall resting of the upper edge of my secondary I introduced a 1mm rotation error to the secondary.  This rotation was enough to move the laser from centre of the primary to the donut wall.

I then used the three adjustment bolts (mine are Bobs Knobs) to bring the laser back to centre.

So the secondary now has a rotation error and tilt errors in both left right and up down directions but it is reading centred on the primary.

The point here is it is hard to notice just a 1mm rotation from the end of the OTA and cannot be seen down the focuser tube because the laser collimator is using it.  I checked the rotation error before re-centering the laser and using the adjustment bolts with the Cheshire and sure enough the rotation although small was noticeable in the circle of the sight tube and was no longer a circle.

I suppose that the combination of both the Cheshire/sight tube and a laser is a good way but I find every time I use the Cheshire to collimate the secondary the laser just agrees and becomes redundant.

I am aware of Dion's video and claim. The problem is with the interpretation of the experiment you have described.

But first let me include a quote from my last post in this thread: "There are two distinct adjustments for the secondary mirror. The first has to do with adjusting the secondary mirror to reflect the laser beam at the primary center. This adjustment is meant to eliminate the tilt between the primary mirror and the eyepiece focal planes. The second adjustment has to do with centering the secondary mirror under the focuser. This adjustment is meant to optimize the illumination field at the eyepiece. A typical laser collimator can't perform the latter. "

Laser collimators should only be evaluated against what they are intented to be used for. They are not meant to center/round the secondary mirror under the focuser. They are only meant to achieve what is known as axial alignment which is defined as ensuring the eyepiece and primary mirror focal planes and focal points coincide. Axial collimation impacts the sharpness of the view. Centering/rounding the secondary mirror under the focuser impacts the illumination of the field-of-view but as I mentioned this is not the job of the laser collimator.

When a laser collimator beam hits the primary center then retraces its path, axial alignment is achieved regardless of any introduced rotation or/and tilts.

When you rotated the secondary mirror, you did introduce an error; however, by tilting the secondary mirror to redirect the beam back to the primary center the adjusting the primary mirror to have the laser retraces its path, you have eliminated the rotational error and axial alignment is re-achieved.

As far as the shape of the secondary mirror, there is nothing sacred about it appearing as a perfect circle. The manufacturer could have installed square shaped secondary mirrors but that does not mean such mirrors can’t be collimated. The fact you ended up with a slightly oval shaped secondary is OK as long as the secondary mirror is properly sized and reasonably centered.

Jason

[Jason D]

 because the factory centre 'circle' mark was not in the correct offset position).

 Adrian

if the factory marked the secondary mirror above its geometric center then they probably did it correctly to account for the offset.

As far as the method you have described, it will not help you achieve good collimation. Unlike the primary mirror, you can't align the secondary mirror with a diffused laser beam.

Jason

[sbooder]

As far as the shape of the secondary mirror, there is nothing sacred about it appearing as a perfect circle. The manufacturer could have installed square shaped secondary mirrors but that does not mean such mirrors can’t be collimated. The fact you ended up with a slightly oval shaped secondary is OK as long as the secondary mirror is properly sized and reasonably centered.

Jason

I'm sorry but i must disagree, it has to appear as a perfect circle when looking through the focuser, if it does not you are either not square to the focuser or you are not in the correct position in the up down direction in the OTA.  The whole reason secondary's are elliptical (in Newtonian reflectors) is so the offset shows as a circle.  There has to be a three way relationship between focuser, secondary and primary and that relationship should be square on.

[opticalpath]

if the factory marked the secondary mirror above its geometric center then they probably did it correctly to account for the offset.

 

 

 

As far as the method you have described, it will not help you achieve good collimation. Unlike the primary mirror, you can't align the secondary mirror with a diffused laser beam.

 

Jason

Sorry, Jason, I have to disagree. What makes you say that the method I described for adjusting tilt of the secondary doesn't work? Have you tried it?

I agree that barlowed laser is a well-known and recommended method for collimating the primary, and that you don't see it described as a method for SM adjustment. I'm just saying that it DOES work for SM tilt adjustment if the SM is centre-marked correctly as you described. The photo I attached to my post illustrates that the diffused laser does indeed cast a sharp shadow of the SM mark on to the primary; I got excellent collimation of my secondary by aligning the shadow with the PM centre mark. And I'm not the only one; I got this idea from another SGL member who has used the method successfully ( see page 5 of this thread: http://stargazerslounge.com/topic/141012-skywatcher-190mn-fitting-of-a-moonlite-focuser/page-5).

I know the method is not well-known, but it is very effective, I can assure you. I wanted to draw it to the attention of others who may be able to use it. Please don't dismiss it without trying it.

Adrian

[Jason D]

Sorry, Jason, I have to disagree. What makes you say that the method I described for adjusting tilt of the secondary doesn't work? Have you tried it?

Adrian

Hello Adrian,

Refer to the attached illustration.

Figure A - A well-collimated scope with a quality laser collimator. This figure is meant as a reference

Figure B - We remove the quality laser collimator and replace it with a poor quality laser collimator. Now the laser strikes the primary mirror off-center.

Figure C - We adjust the secondary mirror to center the laser beam. Now the scope is miscollimated.

Figure D - To prove the scope is miscollimated, we re-insert the quality laser collimator and sure enough it is off-center.

I hope you agree with the above straightforward description

Figure E - Starting with figure "A" again (the well-collimated scope), we will use the barlowed laser method you have suggested. Your marker is at the center of the secondary mirror. The shadow is off-center. 

Figure F - We adjust the secondary mirror to move the shadow to the center. Now the scope is miscollimated.

Figure G - Starting with figure "A" gain, you might suggest to account for the secondary offset and move the marker above the secondary mirror center. Now the scope is collimated and the shadow is centered.

Figure H - However, if we move the secondary mirror little higher then adjust  the secondary mirror to center the shadow we will end up with a miscollimated scope again.

Jason

[Scooot]

I tried the laser Barlow method but my Barlow wasn't long enough, couldn't see the bottom at the end of the focuser, which I think you need to, unless I misunderstood how to do it.

I agree you can use the laser for the secondary once it's centred, and rotated correctly in the focuser. If it isn't the laser won't help fix it.

[Jason D]

I tried the laser Barlow method but my Barlow wasn't long enough, couldn't see the bottom at the end of the focuser, which I think you need to, unless I misunderstood how to do it.

You can use a mirror as shown in the attached photo

[sbooder]

I notice in all of those diagrams the primary has not been altered, if in C you tilted the primary because you think the secondary is aligned because it is hitting the primary in the centre you can actually tilt the primary to make the return laser beam hit the secondary in the correct place to follow its original path back to the laser target but you will not be collimated.  I tried it today and it was so off when I checked with my Cheshire.

[Jason D]

I'm sorry but i must disagree, it has to appear as a perfect circle when looking through the focuser, if it does not you are either not square to the focuser or you are not in the correct position in the up down direction in the OTA.  The whole reason secondary's are elliptical (in Newtonian reflectors) is so the offset shows as a circle.  There has to be a three way relationship between focuser, secondary and primary and that relationship should be square on.

Refer to attachment. Left figure is for a well-collimate scope with the secondary mirror perfectly rounded and centered. Middle figure is for the same scope but with a slightly larger secondary mirror. A quality laser collimator hits the primary center then retraces its path; however, as you can tell the secondary mirror is little off-center and little elongated. Would you consider this scope to be collimated? Bear in mind that the secondary mirror in the middle figure overlaps the secondary mirror in the first figure.

I did not suggest that the secondary mirror centering/rounding is unimportant. What I said is that there is nothing "sacred" about having a perfectly rounded/centered secondary mirror. A "reasonably" rounded/centered secondary mirror is acceptable especially for mass produced scopes that come with slightly oversized secondary mirrors.

Jason

[sbooder]

No it is not acceptable, well not for me anyway.  Why would I bother doing a half job when I can do a good job on the secondary with a site tube.

[Jason D]

I notice in all of those diagrams the primary has not been altered, if in C you tilted the primary because you think the secondary is aligned because it is hitting the primary in the centre you can actually tilt the primary to make the return laser beam hit the secondary in the correct place to follow its original path back to the laser target but you will not be collimated.  I tried it today and it was so off when I checked with my Cheshire.

You are correct. I did not factor the primary mirror into the figures because it is irrelevant to our discussion. We are discussing the alignment of the secondary mirror which pertains only to the path from the laser to the primary mirror. I did not want to clutter the figures with the laser return paths. 

Jason