13 replies to this topic

[sky_lounge]

I feel like I should be seeing the 3 mirror clips here instead of just 2 out of the 3 when I look through the cheshire. 

 

Is all of the primary mirror being presented to the secondary, or is it somehow being cut off from the secondary not being in correct position?

 

I'm not sure what's expected.  If there's an issue, I'm not sure what else I should do since it currently looks like the cheshire collimator is in center alignment with the primary (see picture). 

 

There's also the center screw for the secondary but I'm not sure I should be messing with that.  (fwiw the laser collimator also looks fine)

 

Or, is this simply the expected view from the cheshire and I'm thinking too much about it?

 

(view through cheshire in 8" dob f/5.9)

[ngc7319_20]

Interesting.   What happens if you run the focuser inward more and/or push the Cheshire farther into the focuser?

 

I suppose in a situation were the secondary mirror is undersized in a fast F-ratio scope, it may not be possible to see the mirror clip opposite the focuser.  But I would not say F/6 is especially fast.

 

Probably the focuser is tilted by a tiny amount -- it is not aimed exactly at the center of the secondary mirror.

Edited by ngc7319_20, 20 May 2025 - 08:38 PM.

[Vic Menard]

1.) I feel like I should be seeing the 3 mirror clips here instead of just 2 out of the 3 when I look through the cheshire. 

 

2.) Is all of the primary mirror being presented to the secondary, or is it somehow being cut off from the secondary not being in correct position?

 

3.) I'm not sure what's expected.  If there's an issue, I'm not sure what else I should do since it currently looks like the cheshire collimator is in center alignment with the primary (see picture). 

 

4.) There's also the center screw for the secondary but I'm not sure I should be messing with that.  (fwiw the laser collimator also looks fine)

 

5.) Or, is this simply the expected view from the cheshire and I'm thinking too much about it?

1.) You should see all 3 mirror clips, but it's not critical.

2.) I'm not sure. It depends on the location of the combination tool pupil relative to the focal plane. What combination tool are you using and how far above the focuser is the tool pupil?

3.) I can't see the primary mirror center marker, but I'm assuming it's inside the bright Cheshire ring. I've annotated the center marker in red and the Cheshire ring in yellow.

4.) The center screw behind the secondary mirror sets the offset of the secondary mirror. Your secondary mirror looks like it has a bit too much offset. To reduce the offset, the three secondary mirror tilt adjustment screws need to be loosened equally and the center screw needs to be tightened. The solid line green circle shows the secondary mirror placement error and the dashed green circle shows the optimal placement. When the offset is correct, the reflection of the secondary mirror (violet circle is optimal) will also show less offset.

5.) It is not the expected "optimal" view, but as I noted in the response to 1.), the error is unlikely to impact the scope's image performance. You can use this collimation "as is"--and save the secondary mirror placement error for a rainy day (if you can adjust the combination tool pupil enough to see all three of the outer circles:

the bottom edge of the combination tool (light blue circle),

the actual edge of the secondary mirror (green circle), and

the reflected edge of the primary mirror (red circle).

You can also make the actual edge of the secondary mirror more visible by placing a sheet of white copy paper behind the secondary mirror against the inside of the tube wall opposite the focuser.

Attached Thumbnails

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[sky_lounge]

1.) You should see all 3 mirror clips, but it's not critical.

2.) I'm not sure. It depends on the location of the combination tool pupil relative to the focal plane. What combination tool are you using and how far above the focuser is the tool pupil?

3.) I can't see the primary mirror center marker, but I'm assuming it's inside the bright Cheshire ring. I've annotated the center marker in red and the Cheshire ring in yellow.

4.) The center screw behind the secondary mirror sets the offset of the secondary mirror. Your secondary mirror looks like it has a bit too much offset. To reduce the offset, the three secondary mirror tilt adjustment screws need to be loosened equally and the center screw needs to be tightened. The solid line green circle shows the secondary mirror placement error and the dashed green circle shows the optimal placement. When the offset is correct, the reflection of the secondary mirror (violet circle is optimal) will also show less offset.

5.) It is not the expected "optimal" view, but as I noted in the response to 1.), the error is unlikely to impact the scope's image performance. You can use this collimation "as is"--and save the secondary mirror placement error for a rainy day (if you can adjust the combination tool pupil enough to see all three of the outer circles:

the bottom edge of the combination tool (light blue circle),

the actual edge of the secondary mirror (green circle), and

the reflected edge of the primary mirror (red circle).

You can also make the actual edge of the secondary mirror more visible by placing a sheet of white copy paper behind the secondary mirror against the inside of the tube wall opposite the focuser.

> What combination tool are you using and how far above the focuser is the tool pupil?

 

This tool https://www.amazon.com/dp/B073QP7W93 extends 112mm above the focuser.

 

However, the focuser itself is racked in as far as it can go.

 

Thanks for the diagram and comments; definitely helpful.

Edited by sky_lounge, 21 May 2025 - 12:16 AM.

[Vic Menard]

> What combination tool are you using and how far above the focuser is the tool pupil?

This tool https://www.amazon.com/dp/B073QP7W93 extends 112mm above the focuser.

I was afraid it might be one of those--it's a bad design. The tool should insert all the way into the focuser until all you can see is the top part with the side illumination window. Then you will also be able to see the tube protruding into the light path when you look into the tool.

 

For 1.25-inch focusers, I recommend the AstroSystems LightPipe. For 2-inch focusers, at f/5.9, I recommend the Catseye TeleCat XL (and if you're considering a bigger Dob with a shorter focal ratio, I recommend the Catseye TeleCat XLS).

 

And welcome to Cloudy Nights!

[sky_lounge]

I was afraid it might be one of those--it's a bad design. The tool should insert all the way into the focuser until all you can see is the top part with the side illumination window. Then you will also be able to see the tube protruding into the light path when you look into the tool.

 

For 1.25-inch focusers, I recommend the AstroSystems LightPipe. For 2-inch focusers, at f/5.9, I recommend the Catseye TeleCat XL (and if you're considering a bigger Dob with a shorter focal ratio, I recommend the Catseye TeleCat XLS).

 

And welcome to Cloudy Nights!

 

Ok, thanks - will make the switch!

[star acres]

Don't force any screws. That's the most important thing.

[Oldfracguy]

> What combination tool are you using and how far above the focuser is the tool pupil?

 

This tool https://www.amazon.com/dp/B073QP7W93 extends 112mm above the focuser.

 

However, the focuser itself is racked in as far as it can go.

 

Thanks for the diagram and comments; definitely helpful.

I use a similar long Cheshire with crosshairs to collimate my f/6 Newtonian:

 

https://agenaastro.c...reflectors.html

 

It works very well.

 

Be sure to verify that those crosshairs do indeed intersect in the exact center.  If you rotate the Cheshire in the focuser's 1.25" adaptor and see the intersection of the crosshairs remain fixed over a certain point, that is good.  If the intersection of the crosshairs traces out a little circle as you rotate the Cheshire, then the crosshairs need to be repositioned.  It's the same idea as checking a laser collimator to make sure the collimator itself is collimated.

Edited by Oldfracguy, 05 June 2025 - 10:26 PM.

[Vic Menard]

I use a similar long Cheshire with crosshairs to collimate my f/6 Newtonian:

https://agenaastro.c...reflectors.html

It works very well.

It might be better suited for f/6 than f/5.9?

 

That said, if you look at the OP's annotated image (with the focuser fully racked in), you can just see all three of the alignment circles (light blue, green and red). The dashed green circle clearly shows that there is an optimal secondary mirror placement that will have all three circles concentric and all three mirror clips visible. 

 

Since this is the Beginners forum, I suggested a different sight tube/Cheshire combination tool to make the three circle alignment read easier. Moving the tool pupil closer to the secondary mirror will make the secondary mirror appear larger than the primary mirror reflection, and using a shorter sight tube/Cheshire combination tool will make the bottom edge of the sight tube appear larger than the secondary mirror. This makes small secondary mirror placement errors (like the OP's) less likely to obscure the primary mirror clips.

 

(Edited to correct apparent sizes of the three circles with a closer pupil and a shorter sight tube.)

Edited by Vic Menard, 06 June 2025 - 04:57 PM.

[Oldfracguy]

It might be better suited for f/6 than f/5.9?

 

That said, if you look at the OP's annotated image (with the focuser fully racked in), you can just see all three of the alignment circles (light blue, green and red). The dashed green circle clearly shows that there is an optimal secondary mirror placement that will have all three circles concentric and all three mirror clips visible. 

 

Since this is the Beginners forum, I suggested a different sight tube/Cheshire combination tool to make the three circle alignment read easier. Moving the tool pupil closer to the secondary mirror will make the outermost circle (the bottom edge of the sight tube) appear larger than the secondary mirror, and using a shorter sight tube/Cheshire combination tool will make the secondary mirror appear larger than the reflection of the primary mirror. This makes small secondary mirror placement errors (like the OP's) less likely to obscure the primary mirror clips.  

I've been following your "three circle" approach for over three years now, ever since I got my first Newtonian.  It never fails to get a reflector collimated.  I often do not even need to make a final adjustment of the primary mirror screws using a real star at 25x per inch.

 

The first thing I do is to make sure the secondary mirror is centered underneath the focuser by adjusting that large central screw on the front of the secondary mirror housing if required.  The picture is Post #1 looks to me that the secondary might need to be moved to the left a little to get it centered, but the OP does mention that the laser collimation looks fine.  If the laser itself has been proven to be accurate and collimated, then perhaps that Cheshire Collimation Tool does not fit snugly inside the 1.25" adaptor and is tilted off to one side.

Edited by Oldfracguy, 06 June 2025 - 04:41 PM.

[Vic Menard]

...The picture is Post #1 looks to me that the secondary might need to be moved to the left a little to get it centered...

I agree with your assessment--I see the correction from 4:30 to 10:30 (lower right to upper left), which would reduce the offset (you can also see the correction in the reflection of the secondary mirror). But it's a small error and should have no impact on the scope's high magnification image performance.

 

...but the OP does mention that the laser collimation looks fine.

Exactly. The laser collimator is an axial alignment tool, not a secondary mirror placement tool. The fact that the axial alignments are good is why I'm not worried about the small secondary mirror placement error.

 

...If the laser itself has been proven to be accurate and collimated, then perhaps that Cheshire Collimation Tool does not fit snugly inside the 1.25" adaptor and is tilted off to one side.

The laser alignment agrees with the Cheshire/sight tube combination tool axial alignments--the  sight tube cross hairs are aligned to the primary mirror center marker which agrees with the outgoing laser beam which is also aligned to the primary mirror center marker, and the primary mirror center marker is concentric with the Cheshire ring which agrees with the laser return beam which should be aligned to the laser emitter (or if the laser is Barlowed, the silhouette of the primary mirror center marker is aligned to the laser emitter).

 

So both tools seem to agree--with the exception of the secondary mirror placement, which the laser can't see, but with a sight tube, you can see the three circles. 

[Oldfracguy]

I agree with your assessment--I see the correction from 4:30 to 10:30 (lower right to upper left), which would reduce the offset (you can also see the correction in the reflection of the secondary mirror). But it's a small error and should have no impact on the scope's high magnification image performance.

 

Exactly. The laser collimator is an axial alignment tool, not a secondary mirror placement tool. The fact that the axial alignments are good is why I'm not worried about the small secondary mirror placement error.

 

The laser alignment agrees with the Cheshire/sight tube combination tool axial alignments--the  sight tube cross hairs are aligned to the primary mirror center marker which agrees with the outgoing laser beam which is also aligned to the primary mirror center marker, and the primary mirror center marker is concentric with the Cheshire ring which agrees with the laser return beam which should be aligned to the laser emitter (or if the laser is Barlowed, the silhouette of the primary mirror center marker is aligned to the laser emitter).

 

So both tools seem to agree--with the exception of the secondary mirror placement, which the laser can't see, but with a sight tube, you can see the three circles. 

I understand how the laser is not able to provide any useful information regarding the placement of the secondary mirror along the scope's optical axis, but I have read that you can insert the laser (assume here it is properly aligned and collimated itself) into the focuser and look into the front of the scope to see where the laser beam is hitting the primary mirror.  Then you can adjust the three screws on the secondary mirror so that the laser beam hits the center of the donut on the primary mirror.  After that, go back and look at where the laser hits the grid on the 45° angle surface of the laser collimator.  Finally, adjust the primary mirror screws until the laser return beam is centered on the "bull's eye" target on the 45° surface of the laser collimator.  Assuming the secondary mirror is properly placed under the centerline of the focuser, will the laser collimator render a sight tube or Cheshire with crosshairs unnecessary?

 

.  

[Vic Menard]

I've been following your "three circle" approach for over three years now, ever since I got my first Newtonian.  It never fails to get a reflector collimated.  I often do not even need to make a final adjustment of the primary mirror screws using a real star at 25x per inch.

To be clear, aligning the three circles ensures optimal secondary mirror placement. In many cases, with a Cheshire/sight tube combination tool, this circle alignment delivers a usable focuser axial alignment (the sight tube cross hairs  end up "more-or-less" aligned. But users (and many beginners) can also assess and correct the three circle alignment with a collimation cap. Since a collimation cap has no cross hairs, the focuser axial alignment will likely still need to be assessed and corrected with a sight tube or a thin beam laser.

 

Primary mirror axial alignment (using the alignment of the Cheshire ring relative to the primary mirror center marker or the return beam of a laser relative to the laser emitter) is the final step of Newtonian collimation. Assuming you've already verified your primary mirror collimating tool with a star alignment, you should always end your secondary mirror placement and focuser axial alignment procedure with a good primary mirror axial alignment assessment, and correction (as needed).

[Vic Menard]

...I have read that you can insert the laser (assume here it is properly aligned and collimated itself) into the focuser and look into the front of the scope to see where the laser beam is hitting the primary mirror. Then you can adjust the three screws on the secondary mirror so that the laser beam hits the center of the donut on the primary mirror.

Focuser axial alignment assessment, and correction.

 

... After that, go back and look at where the laser hits the grid on the 45° angle surface of the laser collimator.  Finally, adjust the primary mirror screws until the laser return beam is centered on the "bull's eye" target on the 45° surface of the laser collimator.

Primary mirror axial alignment assessment, and correction.

 

...Assuming the secondary mirror is properly placed under the centerline of the focuser, will the laser collimator render a sight tube or Cheshire with crosshairs unnecessary?

No. You'll still need a sight tube (preferably), and one matched to the scope's secondary mirror and focal ratio (ideally), to assess, and correct, the concentricity of the three circles. A laser can't see the secondary mirror placement. This is what got Rick Singmaster in trouble with his StarMaster Dobsonian line in the early 2000's. His scopes decoupled secondary mirror rotation from secondary mirror offset and secondary mirror tilt. Unfortunately, while his scopes were configured with plastic washers that made it easier to correct rotation errors, Rick pretty much assumed rotation problems would not be a big issue. But then he started making bigger Dobsonians, and many users started using thin beam lasers for routine alignment. Bigger Dobs have bigger secondary mirrors, and since his scopes were built to be transportable, they were subjected to a lot of vibration, that gently, and unfailingly, changed the secondary mirror rotation! Lasers can't see rotation, so StarMaster users assumed the error was secondary mirror tilt--every time they collimated. A simple sight tube, or even a collimation cap, would have shown the error--but the outgoing laser beam was aligned to the primary mirror center marker, and the return beam was aligned to the laser emitter--so those StarMaster users assumed their alignment was "good-to-go", until it wasn't "good-to-go". After Rick started fielding (many) phone calls about this "issue", he started calling two of the secondary mirror tilt adjustment screws (StarMaster scopes had a four screw secondary mirror tilt adjustment) the "forbidden" screws! Not too long after that, StarMaster users started including a sight tube (or similar tool) as part of their collimation routine to see what was really going on...