16 replies to this topic

[68Kustom]

Hi there,

 

So, while I was assiduously collimating my $100 factory-misaligned M---e collimator, I discovered the laser 'dot' was not a dot at all.  

 

I made a cradle which bolts solidly to a door frame and projects the laser to a wall about 24 feet (7.3 m) distant or about 3 times my scope's total focal length.  The laser dot cast on the wall is actually a highly-flattened ellipse aligned parallel with two of the adjustment screws.  It's not even aligned with the viewing port but at a weird slant to it.  Makes no design sense that I can see.

 

I had to get each ellipse position to cross the other two positions precisely.  This made it reeeeeally difficult to align the collimator.  What's the true center of the ellipse?  Where it's brightest?  Or one end?

 

I ended up with a near-perfect asterisk on the wall as I had to draw each ellipse associated with each adjustment screw.  Took three days of futzing!

 

I thought collimators projected a small laser light disc, though.  Anyone else's show an oblong or ellipse?

Edited by 68Kustom, 22 May 2017 - 04:59 PM.

[xiando]

Most solid state lasers don't produce a true circular beam, since they're generally emitted from a rectangular opening. Even on glatter collimators, the beam is only circular when the pinhole mask is applied.

 

Use the brightest point towards the center of the ellipse as the reference. It's about the best you can do.

[havasman]

Glatter lasers and Farpoint lasers address that matter by orificing the laser projection so that their lasers produce a MUCH more round point. Additionally, the Glatter dimmer attachment has the effect of shrinking the point as it is dimmed so that the eventual dot size is very small. Tinier, rounder point = better gauge resolution.

[Starman1]

Add an aperture stop to an inexpensive laser:

https://www.cloudyni...-a-cheap-laser/

[68Kustom]

Use the brightest point towards the center of the ellipse as the reference. It's about the best you can do.

Thanks! 

 

I ended up with this (or as near to it tweaking M4-0.7 setscrews from 7.315 m away):

Edited by 68Kustom, 22 May 2017 - 04:58 PM.

[Jon Isaacs]

Just a point of reference reference. 

 

Jon

[Redbetter]

I actually find the elongated shape a benefit.  I removed the aperture stop on my Glatter as it caused other problems.  Instead, I can use its dash to get good alignment on my mirror on the 20".  [I am told the alignment of the secondary is not critical anyway...so why use the aperture stop that causes trouble in the next step?]  With my Zhumell laser the dash shape is almost perfectly matched to the inner portion of the etched ring around center on the Z10.  This allows a more precise positioning than I could easily do with a narrow dot.  Indeed, switching back and forth with the 1,25" Zhumell laser fully collimated and the 2" Glatter w/Tublug is not showing any need for adjustment. 

 

The Glatter Barlow Tublug works well in the 20" particularly when checking for collimation shift from high to low tube angles and eliminating registration issues as a concern, but it isn't as sensitive as I anticipated.  While the nature of the Barlow makes it more trustworthy, that doesn't seem to guarantee better precision in my experience.  That was not what I expected, but is what I observed.     

 

The main negatives with any Barlow technique involve wasted time using the collimator.  With the Glatter one has to fiddle with screws in the Tublug to insert the Barlow (because it doesn't fit in the case with the screws extended) and take it in and out to check the secondary vs. checking the primary.   A home made Barlow/aperture arrangement is no better of course, and possibly worse.   [A Tublug with a flip in/flip out Barlow would be really nice as it would be truly all-in-one without unscrewing multiple things.]  Not having to Barlow at all, simply using the return beam on a Zhumell style collimator is a much easier/quicker and can provide the same result...but only if the method has been worked out to address registration errors/collimating the collimator, etc.  If one hasn't done a Barlow check to work out the method then the results could be poor. 

[Vic Menard]

I use a Glatter laser with the newer screw-on 1mm aperture stop to quickly assess and correct the axial alignments on my 22-inch f/4. The 1mm aperture stop creates a diffraction pattern that surrounds the laser "dot". I can easily see the reflected silhouette of my triangular center spot embedded in the diffraction pattern on the white face on the 1mm aperture stop. This means I can see both the outgoing beam alignment (laser dot centered in the triangular silhouette) and the primary mirror alignment (triangular silhouette centered on the 1mm hole in the aperture stop). This gives me all of the advantages of the thin beam laser and the Barlowed laser (since I can see the silhouette). Another advantage is that the alignment read takes place directly under the focuser, so I can use my reading glasses for both alignments!

[Vic Menard]

Here's what it looks like under the focuser...

 

 

Attached Thumbnails

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[Jon Isaacs]

I can easily see the reflected silhouette of my triangular center spot embedded in the diffraction pattern on the white face on the 1mm aperture stop. This means I can see both the outgoing beam alignment (laser dot centered in the triangular silhouette) and the primary mirror alignment (triangular silhouette centered on the 1mm hole in the aperture stop). This gives me all of the advantages of the thin beam laser and the Barlowed laser (since I can see the silhouette).

 

 

What happens if the secondary is within tolerance but not perfectly centered?  

 

As I understand it, the tolerance on the secondary (or more properly the focuser axial alignment) in a 22 inch without a coma corrector is about 17 mm.  With a type 1 Paracorr the tolerance is still about 3 mm.

 

So if the secondary is aligned at 2 mm offset, how does this effect your collimation based on the return spot, assuming you can see it?

 

Jon

[Vic Menard]

...if the secondary is aligned at 2 mm offset, how does this effect your collimation based on the return spot, assuming you can see it?

If the outgoing beam is 2mm offset, it will appear 2mm out of alignment relative to the silhouette of the triangular center spot. This has no impact at all when aligning the silhouette of the triangular center spot to the hole in the 1mm aperture stop.

 

The image I added shows both axes corrected. I usually decollimate the primary mirror first so I can clearly see the laser dot (surrounded by the diffraction pattern) and the silhouette of the triangular center spot away from the hole in the 1mm aperture stop. Then it's easy to center the dot inside the perforation of the silhouette--there's nothing quite like fine adjusting the secondary mirror tilt while watching the alignment only inches away! After the secondary tilt is corrected, collimating the primary mirror tilt is just like using a Barlowed laser (including the 2X error magnification)--just center the silhouette of the primary mirror center spot (and the laser dot and the diffraction pattern since they have just been aligned to the silhouette in the first step) on the hole in the 1mm aperture stop.  

[Starman1]

 I removed the aperture stop on my Glatter as it caused other problems.  

 

Aperture stop caused problems?  The diffraction rings around the dot help you put the dot dead center on the primary, and the rings also help see the return center mark shadow, as Vic illustrated. 

 

The Glatter Barlow Tublug works well in the 20" particularly when checking for collimation shift from high to low tube angles and eliminating registration issues as a concern, but it isn't as sensitive as I anticipated.  While the nature of the Barlow makes it more trustworthy, that doesn't seem to guarantee better precision in my experience.  That was not what I expected, but is what I observed. 

 

Perhaps you could gain better precision by using an autocollimator.  If you use a coma corrector, or have a scope of f/5 or faster (likely), you should be using one anyway.   It will reveal errors you cannot see in the other tools. 

 

The main negatives with any Barlow technique involve wasted time using the collimator.  With the Glatter one has to fiddle with screws in the Tublug to insert the Barlow (because it doesn't fit in the case with the screws extended) and take it in and out to check the secondary vs. checking the primary.   A home made Barlow/aperture arrangement is no better of course, and possibly worse.   Not having to Barlow at all, simply using the return beam on a Zhumell style collimator is a much easier/quicker and can provide the same result...but only if the method has been worked out to address registration errors/collimating the collimator, etc.  If one hasn't done a Barlow check to work out the method then the results could be poor. 

 

The return of a simple beam going into the hole the laser came out of allows for some movement of collimation screws before you can see the beam come out of the hole.
On the other hand, moving a collimation screw a tiny amount produces an immediately visible change in the shadow position in the Barlowed laser technique.
So I disagree that the result is the same, because the barlowed laser technique is more sensitive to revealing errors.

Also, the Barlowed technique doesn't depend on the laser being collimated and it is totally independent of the accuracy of the outbound beam in determining primary collimation.
Hence, the return beam of the simple laser beam may or may not equal the accuracy of the Barlowed technique, with a tilt, percentage-wise, toward the "NOT equal the accuracy of the barlowed technique.

If accuracy is required, the barlowed technique is superior to the use of an out-and-back simple laser beam.

Edited by Starman1, 25 May 2017 - 02:40 PM.

[Redbetter]

Don,

 

You can disagree about the precision of the return vs. the Barlow, but when I check with the Glatter Tublug Barlow after getting the return centered there is typically nothing to adjust.   (In other words, the Glatter Tublug is confirming my assessment.)   There are some problems with your thesis about the return beam.  The first is that the beam has some size/shape, rather than being a point that simply disappears, this helps in finding the minimum inside the hole.  The second is that the hole on the Zhumell collimator is roughly half the diameter of that of the Glatter target; aim small, miss small.  The third is that I center the beam approximately in that hole--by going back and forth slightly to find the middle.  As a result the remaining error is not the radius of the hole, but some fraction of that...and I am starting with a much smaller target to begin with. 

[Starman1]

Well, all that is well and good, but it does not describe the majority of real-world uses in the manner you describe.

But you cannot get around the problem that even a perfect centering of the laser in the hole upon return is not going to be worth much if the outbound laser beam

does not hit the center of the mirror with precision.

The advantage to the barlowed technique is that it is a single pass--from primary to focuser--and not a double pass.

[Jon Isaacs]

Well, all that is well and good, but it does not describe the majority of real-world uses in the manner you describe.

But you cannot get around the problem that even a perfect centering of the laser in the hole upon return is not going to be worth much if the outbound laser beam

does not hit the center of the mirror with precision.

The advantage to the barlowed technique is that it is a single pass--from primary to focuser--and not a double pass.

 

If one is using the return beam you are dependent on the precision of the alignment of the secondary, the error in the alignment of the secondary is included in the return beam, it's the sum of the errors.  This is not the case with the Barlowed laser...  A second advantage of the Barlowed laser is that it magnifies the error in primary tilt making it easier to see. 

 

Jon

[Redbetter]

...and yet a free collimator is able to duplicate the performance of the Barlow on a regular basis.    Funny how that works.

[Vic Menard]

...and yet a free collimator is able to duplicate the performance of the Barlow on a regular basis.    Funny how that works.

By "free" I assume you mean the laser was included in the scope purchase. That said, I'm not that surprised that you've been able to achieve good axial alignment with your simple thin beam laser. If the laser itself is correctly aligned internally, and the registration in the focuser drawtube (and the 2- to 1.25-inch adapter if you're using one) is consistent, all that's left is "reading" the alignments (outgoing and return) to the precision necessary to meet your performance expectation.

 

As I noted above, I use a simple thin beam laser with my f/4 Newtonian (I also use the diffraction pattern and the embedded center spot silhouette to improve my read precision, but that's what's quick and easy for me). But for someone else who may be using a 1.25-inch economy laser with less than perfect internal alignment and mounting it in an equally less than perfect 2- to 1.25-inch adapter--a Barlowed laser still manages to deliver remarkably good primary mirror collimation (maybe not perfect, but considering the mechanical shortfalls and "read" issues...remarkably good!) 

 

If you really want to push the primary mirror alignment precision, a calibrated Cheshire is probably the best choice. But then you start wondering about the centering of the primary mirror center spot, and whether or not mechanical centering actually matches optical centering... It's those considerations which make me smile when I can routinely (and easily) do better than the high performance error tolerance for my f/4 mirror.