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My Howie Glatter Laser Collimator Experience

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#51 Jason D

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Posted 07 April 2011 - 09:56 AM

One thing I find is the aperture stop works better with the Tublug.

I believe I tried this combination but I do not recall an improvement. I will try it again and pay closer attention.


I tried several attachments in combination with the TuBlug.
The following photo shows the results:

Posted Image

Top left: No attachments
Top right: In combination with the 1mm aperture stop attachment
Bottom left: In combination with the square grid attachment
Bottom right: In combination with the self-barlow attachment

I guess the results will vary between scopes.

Jason

#52 94bamf

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Posted 07 April 2011 - 10:58 AM

Great pictures Jason. Your picture illustrates what I was talking about with the laser brigting up the center hole in the tublug when the aperture stop isn't used. You can see the glare in the center hole in all the pictures except the one with the aperture stop. It almost looks like you can get a read on the center of the hotspot when the aperture stop is used, it looks like I can see a bit of red all the way around the center hole in that picture. With the glare in center hole of the tublug in the other pictures, it is harder to see..

Ken

#53 Jason D

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Posted 07 April 2011 - 12:11 PM

Indeed, the 1mm aperture stop did reduce the central glare; however, it also reduced the spread of the laser and its intensity.
But in my case the Hotspot perforation shadow still looks little smaller then the TuBlug opening. I have to rely more on the outer edge alignment.

#54 Jason D

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Posted 07 April 2011 - 12:14 PM

I went back and made a new template but this time I laminated it with package transparent tape on both sides of the template for protection and stiffness. Now I have a durable and reusable template without the need to tape it against the OTA.

Posted Image

I found a rotating test at the end of the procedure will help reaffirm correctness -- (Click here to view)

End result looks great

Posted Image

I could have used the procedure where a sheet is placed opposite to the focuser. The goal is to balance the three protrusions of the grid. It is a good an easy procedure; however, it is not as accurate for scopes with oversized secondaries. In the following photo, you will see the bottom protrusion is greater than the two balanced side ones. Still, it is a great and easy procedure that produces good results. Few comments about this procedure:
1- Ensure the central beam of the grid strikes the dead center of the primary mirror before evaluating the secondary position. You can ignore adjusting the primary mirror for this procedure.
2- I recommended diagonal grid as opposed to parallel grid for better readability.

Posted Image

#55 Jason D

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Posted 08 April 2011 - 02:24 PM

I love this photo. Stare at it for few seconds and you should get that 3D effect. Rock your head side to side by a little amount.
Unlike the typical dark secondary mirror silhouette, this one is more vibrant and interesting :)

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  • 4505267-3D.jpg


#56 Starman1

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Posted 09 April 2011 - 09:19 AM

Jason,

I notice you checked the alignment of the laser without the aperture stop. Since it is so difficult to detect the center of the laser "spot" that way, because it is a small "slot" shape instead of a point, I would have recommended using a piece of graph paper on the wall, and having the aperture stop in place. The movement of the small pointed spot on the graph paper would have been easier to see, and the diffraction pattern easier to see on the primary mirror when aligning the secondary tilt.
Thinking further about it, I see to reason not to use the aperture stop even in the TuBlug. The diverged beam will be rounder and more uniformly lit.


A side note (sorry for the segue) on DSC accuracy: it is very easy with Tangent-manufactured DSCs to simply add a Star Fix alignment on a couple stars near a part of the sky you will be observing in if you wish better accuracy in that part of the sky. I typically add alignment stars all night as the sky rotates or my target area changes. It only takes a few seconds.

#57 Jason D

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Posted 09 April 2011 - 09:50 AM

The 1mm aperture stop will actually enlarge the central beam. At 25 feet, it will be few mm in diameter. That is why I conducted by testing with the plain laser beam.

The combination of 1mm aperture stop and TuBlug was covered few posts back. I noticed it eliminated the glare around TuBlug laser opening.

I still need to collect my thoughts about the DSC error.

Jason

#58 Vic Menard

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Posted 09 April 2011 - 10:23 AM

...on DSC accuracy: it is very easy with Tangent-manufactured DSCs to simply add a Star Fix alignment on a couple stars near a part of the sky you will be observing in if you wish better accuracy in that part of the sky. I typically add alignment stars all night as the sky rotates or my target area changes. It only takes a few seconds.

I'm not fully "up to speed" with "Tangent" DSCs, but that sounds similar to the newer SkyCommander DSCs (additional "realigns" are added to the original "setup" star alignment). I know there are other DSCs that discard, first in, first out, as a new realign is added. ArgoNavis offers both "realign" and "local sync" options, which makes me think the realign function discards previous alignment points (local sync has no affect on setup alignment and can be disengaged and reengaged at will).

Why should this make any difference? Depending on the computation model, I suspect that many alignment points will actually reduce accuracy over parts of the sky. To better understand how DSCs interpret setup alignment stars, and how to choose the optimal alignment stars, I suggest reviewing the documentation at Best Pair. Note that this is not the same as TPAS or TPoint.

#59 Vic Menard

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Posted 09 April 2011 - 10:33 AM

...I still need to collect my thoughts about the DSC error.

Try here, specifically, the bottom of the page, "Such a pointing error is referred to on Alt/Az mounts as Collimation Error in Azimuth (CA) or on equatorial mounts as Collimation Error in Hour Angle (CH)."

For Alt/Az DSCs, a collimation error in azimuth will cause the pointing axis to swing in a wide cone as altitude changes, and when pointed (as close as possible) to the zenith, will swing in a narrow cone when azimuth is rotated. A collimation error in altitude still keeps the pointing axis within the plane of rotation (altitude), and the zenith is still accessible, which is why these errors can be disregarded when using DSCs.

#60 Jason D

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Posted 09 April 2011 - 03:08 PM

Don, attached is a comparison between 1 mm aperture stop attachment versus plain laser at 12 feet. I had to adjust the camera settings for the 2nd photo to reduce glare.

The 1mm aperture stop diffracts the laser beam to concentric rings hence reducing the intensity of the central beam but it does not reduce its size.

Vic, thanks for the DSC link.

Jason

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  • 4507371-stop.jpg


#61 Jason D

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Posted 12 April 2011 - 03:17 PM

I have been reading as much material I can find in CN and the web on the use of the holographic attachment and I noticed a common misconception about what to look for and how to interpret what is seen.

Seeing the whole grid as shown below does not mean much especially when the attachment is located below the focal plane.
Posted Image

Some believe that as long as the grid is symmetrical with respect to the primary mirror perimeter then the secondary mirror is well positioned under the focuser. That is incorrect. When the central beam of the grid is aligned with the primary mirror center spot, then the grid will always be seen symmetrical. There is no need to check for symmetry – just aligned the central beam with the primary center spot. You will never run into the scenario shown below:
Posted Image

What you need to look for is the truncation or clipping of the grid by the secondary mirror edge. As you rack the laser collimator out, the grid will get larger and will eventually get clipped. Depending on how oversized the secondary mirror and the F-ratio of the scope, The grid clipping might not be seen concentric with the primary mirror for a well-positioned secondary – shifted slightly towards the focuser as shown below:
Posted Image

If you rack the attachment above the focal plane, then you you will get to a point where the clipped grid covers only the primary mirror. This can only occur for a well-positioned secondary mirror as shown below:
Posted Image

I had to use a 2” drawtube extension to take the following photo.
Posted Image

If the holographic attachment is located at or below the focal plane and the grid is clipped but does not cover the whole primary mirror then the secondary mirror is too small.
Posted Image

On the other hand, if the the whole (unclipped) grid is visible but smaller than the primary mirror then it does not mean much. Always remeber that a "clipped" grid will tell us something about the position of the secondary mirror with respect to the focuser because the "clipping" is a direct result of the interaction of the secondary mirror edge with the grid.
Posted Image

In the following photo, the whole grid is visible within the primary mirror which does not tell me much about the secondary mirror placement.
Posted Image

By the way, you could use the first circle of the concentric circles holographic attachment against the outer edge of the HotSpot to align the focuser axis.
Posted Image


Jason
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#62 dvb

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Posted 12 April 2011 - 03:38 PM

This is excellent information, Jason.

I hope Howie will find a way to put such excellent information on his website (with appropriate attribution etc. to yourself.)

#63 Jason D

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Posted 12 April 2011 - 07:23 PM

In an earlier post, I measure the distance between the optical and mechanical axes to be around 8mm at the OTA opening which did not make sense – Vic pointed that out. I applied the same procedure again but this time I updated the template to give me the ability to place it with more precision. The new nd more accurate measured distance is a tad below 4mm which now makes sense.
Posted Image
Posted Image
Posted Image
Posted Image

Incidentally, note the shadows of the primary mirror clips. I know that the front of my OTA is not vignetting.
Jason

#64 Jason D

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Posted 12 April 2011 - 07:41 PM

In the procedure I outlined earlier for positioning the secondary mirror using the special template, I mentioned that the primary mirror needs to be aligned each time the secondary mirror is adjusted. I did not like the idea of running back-and-forth. As it turns out, adjusting the primary mirror is not really required as long as it is roughly collimated. See below:

Referencing a new laminated template with better fit – still no adhesive tape.
Posted Image

Below is a well positioned secondary mirror and a collimated primary mirror
Posted Image

Now by decollimating the primary mirror by several millimeters, the returned grid projection did not move.
Posted Image

It is only when the primary is decolliamted by more than 1 inch, the grid projection moves by few millimeters
Posted Image

Bottom line, as I move the secondary mirror up/down/left/right to align the returned grid projection against the template, I no longer have to run behind the scope to adjust the primary mirror – assuming the primary alignment was reasonably close. However, it is still a requirement to ensure the central beam of the grid is centered with the primary spot.

#65 Jason D

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Posted 12 April 2011 - 07:42 PM

This is excellent information, Jason.

Thank you, dvb

#66 Jason D

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Posted 13 April 2011 - 12:58 AM

Updated template

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#67 Jason D

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Posted 13 April 2011 - 12:59 AM

And another

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#68 Jason D

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Posted 13 April 2011 - 01:02 AM

Attached is a word document with the templates. Feel free to download and resize for your scope.

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#69 Jason D

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Posted 13 April 2011 - 01:07 AM

Another word document

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#70 Nils Olof Carlin

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Posted 13 April 2011 - 03:26 AM

But I did not feel comfortable taping a sheet of paper across my OTA between the secondary and primary mirrors.



No meed, really. A cardboard "L" clipped to a spider vane is easy for open and closed tubes - and works great for both truss and closed tubes.

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  • 4514742-cardb.jpg


#71 Nils Olof Carlin

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Posted 13 April 2011 - 03:29 AM

If it doesn't show clearly, here is how to cut the cardboard (no precision needed). Fold 90 deg along the dashed line.

Nils Olof

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#72 Jason D

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Posted 14 April 2011 - 01:19 AM

Good suggestion, Nils Olof.
I assume a hole needs to be cut at the center of the cardboard screen to allow the laser beam goes through to ensure the secondary mirror is aligned with the primary center spot for more accurate results.
Of course, the goal would be to center the grid against the reddish shadow of the secondary mirror. The shadow is the result of residual diffused laser.
We need to go from this
Posted Image

To this
Posted Image

The two left diagrams in the above are meant to explain that the grid and shadow relative sizes could be anything depending on the scope components’ dimensions.

But there is an inherent error when using this method. I contend for a perfectly positioned secondary, the grid will be slightly misaligned with the shadow as shown below:
Posted Image

I will explain in the next post .

#73 Jason D

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Posted 14 April 2011 - 01:20 AM

Given a well aligned secondary mirror under the focuser where the secondary/primary mirror edges coincide as shown below. In this case, the 100% illumination field is well-centered with respect to the optical axis.
Posted Image

If the laser with the grid holographic attachment is placed at the vertex of the cone with perfect fit to both mirrors (typically located above the focal plane), then the grid will be clipped by the secondary mirror and fits perfectly on the primary mirror. In addition, if we place a screen under the secondary mirror then the grid and secondary shadow will be aligned as shown below:
Posted Image

But the above will most likely need a 2” drawtube extension to get the laser that far up. Typically, the laser and attachment will be located below the focal plane. In this case, the grid will overflow the primary mirror but the clipping will be asymmetrical. The far side away from the focuser will be more clipped than the closer side under the focuser. Refer to the following:
Posted Image
Posted Image

If we attempt to center the grid against the secondary shadow, then the only way to do it is by lowering the secondary mirror. Doing so, will minutely rotate the secondary mirror and minutely moves it higher but mostly shifts it sideways with respect to the optical axis –counterintuitive. However, doing so also shifts the 100% illumination field away from the optical axis. The diagram is highly exaggerated. The shift will be in terms of few millimeters.
Posted Image

Referring to the following photo for a well-positioned secondary mirror, note how the background grid and shadow gives the illusion the secondary is little high in the OTA when it is not.
Posted Image

The approach I used makes the laser location with respect to the focal plane mostly insensitive as shown below:
Posted Image

Posted Image


Jason

#74 Howie Glatter

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Posted 14 April 2011 - 05:54 AM

"Referring to the following photo for a well-positioned secondary mirror, note how the background grid and shadow gives the illusion the secondary is little high in the OTA when it is not."

Jason, are you taking into account the fact that the secondary mirror edge protrudes into the light path of the diverging grid beam? You can see the missing portion of the grid projected upon the secondary mirror's edge in your photo.

#75 Jason D

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Posted 14 April 2011 - 09:22 AM

Jason, are you taking into account the fact that the secondary mirror edge protrudes into the light path of the diverging grid beam? You can see the missing portion of the grid projected upon the secondary mirror's edge in your photo.


Howie, I was comparing the lower protrusion against the two side ones. The upper protrusion is blocked by the mirror.

The small error I pointed out in my last post is highly dependent on many factors including the F-ratio, Focal length, relative secondary size to primary, and how far the attachment is located below the focal plane. For larger scopes with minimal sized secondary mirror, the error is negligible especially when the laser is racked out.

Posted Image

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  • 4517438-focuser_view3.jpg



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