26 replies to this topic

[Catchlight]

In an effort to verify the correct position of the secondary mirror in my ONTC 800/200 I tried to calculate the position based on the center position of the focuser tube.

The focuser center is at 176mm from the front of the tube,

the secondary minor axis is 72mm, 72/2= 36, so I calculated that the near edge of the secondary would be 140mm. But when I used the catseye Teletube and the TS Optics Concenter tool, the near edge of the secondary is 147mm. Is this due to the offset? Or have I missed something?
 

 

[Asbytec]

Use you Cat's Eye Teletube to center the focuser visually under the focuser. Focus downward onto the diagonal until it is nearly the same diameter as the bottom edge of the site tube.  A white paper background inside the tube and opposite the focuser helps see the edge of the diagonal mirror when looking down the focuser.

 

Move the diagonal mirror along the optical tube's longitudinal axis, toward or away from the primary mirror, by working the central bolt in the spider hub holding the diagonal mirror. Tighten and loosen the three tilt screws as needed the allow the diagonal to move to the center of the site tube and to hold it in place. 

 

Then, use the three tilt screws, only, to center the diagonal under the site tube in the direction perpendicular to the tube's longitudinal axis. Actually, you really need only two of the diagonal tilt screws NOT in line with the focuser to move the diagonal across the optical tube to center it under the site tube in that direction. 

 

Unless you are trying to cut holes to install your spider, there is no real need to measure the diagonal and focuser so precisely to center the diagonal or "square" the focuser to the tube. The important thing is to center the diagonal under the focuser/site tube. Then it is "square." You can do that visually without measuring it.

Edited by Asbytec, 24 March 2025 - 02:26 AM.

[Catchlight]

Thanks but as stated I have the secondary centered using the Teletube, so that was not really the question.
I am just trying to understand if the 7mm difference from the focuser center is due to the offset or if I have missed something else in my calculation.
I don’t understand why I should not verify the measurement, it only takes a couple of minutes and is an interesting exercise to understand how a reflector works.

Edited by Catchlight, 24 March 2025 - 02:30 AM.

[Asbytec]

Thanks but as stated I have the secondary centered using the Teletube, so that was not really the question.
I am just trying to understand if the 7mm difference from the focuser center is due to due the offset or if I have missed something else in my calculation.
I don’t understand why I should not verify the measurement, it only takes a couple of minutes and is an interesting exercise to understand how a reflectoe works.

Man, I have writer's cramp.

 

Yes, the difference should the offset toward the primary and a similar offset away from the focuser. The light cone is thicker as it hits the diagonal mirror opposite the focuser, so the diagonal needs to sit lower and closer to the primary to catch it.

 

Edit: I don't know, looking at that 7mm difference doesn't make sense. Not sure why, yet. Your illustration, though, doesn't look to have the classical offset. Your offset should be about half of that at around 3.5mm. See here...https://www.bbastrod...ner.html#visual

 

Edited by Asbytec, 24 March 2025 - 02:50 AM.

[Catchlight]

So I guess the calculation for the near edge of the secondary would be (focuser center - 1/2 minor axis of secondary)+calculated offset.

[Asbytec]

So I guess the calculation for the near edge of the secondary would be (focuser center - 1/2 minor axis of secondary)+calculated offset.

Minor axis or major axis? That may be an issue with measurement if you are using the diagonal's minor axis in your calculation. 

 

Offset is usually along the major axis, but it's not halfway along the mirror major axis. The optical axis reflecting straight up the center of the tube is "offset" on the diagonal's surface above the center of the diagonal toward the focuser as shown in the illustration. There is more glass below the spider centerline than above it.

 

It looks like you centered the diagonal directly under the focuser and the optical axis assuming that is a classic offset. It's not. There is a bit of an illusion when looking down on a 45 degree diagonal mirror. It looks circular and centered, but it's actually not. That "actually not" part is the offset. 

Edited by Asbytec, 24 March 2025 - 03:09 AM.

[CrazyPanda]

Just to add clarity about the offset, I modeled this optical schematic based on information I could collect about the scope and focuser.

 

This assumes you have placed the mirror at the 90mm backfocus position, which calculates a diagonal to focal plane distance of 255mm (1/2 of the 230mm tube diameter, 50mm min height of focuser, 90mm backfocus above the focuser).

 

 

It gives an offset towards the primary of 4.286mm, and away from the focuser also by 4.286mm. The offset would change depending on how you've got your primary mirror cell position configured.

 

At the 35mm backfocus position (200mm to the focal plane), then the offset changes to 3.886mm. Since this is a negligible difference, I'm guessing they've just picked an offset somewhere in the middle (4mm) for the secondary mirror.

 

Note that this schematic *does* account for the sagittal depth of the mirror (which you can see modeled in the image)

Edited by CrazyPanda, 25 March 2025 - 08:02 AM.

[Asbytec]

Considering this thread to be an academic exercise, if we center the diagonal on the focuser draw tube/site tube, offset will be what it will be. Either classical or the new model. Position and rotate the diagonal under the focuser/Site tube and know the offset is correct. 

[Catchlight]

Considering this thread to be an academic exercise, if we center the diagonal on the focuser draw tube/site tube, offset will be what it will be. Either classical or the new model. Position and rotate the diagonal under the focuser/Site tube and know the offset is correct. 

I struggled for a couple of months when I got my first Newtonian since I wanted to buy and use the Catseye tools, but they were not readily available in Europe. So I spent a lot time trying to get the secondary correctly centered under the focuser with a homemade cheshire, and it was always a little off. Even with the Catseye tools I always found it hard to see the edge of the secondary nearest the tube opening. If only I had had a formula to get me close it would have saved me a lot of time and made collimation so much easier.

[Vic Menard]

...If only I had had a formula to get me close it would have saved me a lot of time and made collimation so much easier.

As Norme explained in post #8, a "formula" isn't important.

 

Just make these three circles concentric: the bottom edge of the TeleTube or TeleCat, the actual edge of the secondary mirror, and the reflected edge of the primary mirror. If these three circles are concentric, your secondary mirror is offset*.

 

Generally speaking, the spider is "centered" in the tube assembly (each spider vane is equal in length, inside edge of the tube assembly to the spider hub). If the secondary mirror is offset relative to the spider hub, your secondary mirror placement will be (more-or-less) "classic" offset. If your secondary mirror is centered relative to the spider hub, your secondary mirror placement will be (more-or-less) "the New Model" offset. In BOTH cases, the offset will center the illuminated field at the native focal plane. If you're using a coma corrector, you may need to make a small adjustment to the offset to accommodate the illumination profile at the coma corrector field lens (or you can correct any small error with software).

 

*To remove all offset, you'll need to adjust the secondary mirror to center the reflection of the secondary mirror relative to the primary mirror. With a no-offset secondary mirror placement, the bottom edge of the TeleTube or TeleCat will be concentric with the reflected edge of the primary mirror, but the actual edge of the secondary mirror will NOT be concentric, but will instead appear offset (in your case, ~4mm) toward the spider hub! 

[Asbytec]

I always found it hard to see the edge of the secondary nearest the tube opening. 

Oh, I see. Yes, that edge of the diagonal can be difficult to see because we cannot see the white paper background due the spider hub holding the diagonal. Remember, though, we are centering only the reflective surface of the diagonal. We place a white paper background inside the tube behind the diagonal opposite the focuser.

 

Then we focus the site tube downward, so the diagonal appears a little smaller than the bottom of the site tube. We will see a thin white "crescent" between the edge of the diagonal and the edge of the site tube. Center the diagonal using the thin white space. I normally extrapolate that crescent around the spider hub to estimate diagonal centering under the focuser.

 

Another trick you can use is based on the fact when the focuser axis is aligned to the primary center, then the reflection of the primary is necessarily centered under the focuser. Because the primary mirror is centered under the focuser, we can use the reflection centered under the focuser as a reference to center our diagonal, too.

 

In this case, focus outward until the reflection of the primary is nearly the same diameter as the diagonal mirror. Any error in the diagonal position will be evident if it is not centered on the primary mirror reflection, too. As Vic explains, all three collimation circles should be concentric with each other under the site tube, including the edge of the diagonal. 

 

I believe this visual method is more accurate than measuring all of those parameters to place the diagonal or square the focuser, and it's so much easier. 

Edited by Asbytec, 25 March 2025 - 05:44 PM.

[Catchlight]

Oh, I see. Yes, that edge of the diagonal can be difficult to see because we cannot see the white paper background due the spider hub holding the diagonal. Remember, though, we are centering only the reflective surface of the diagonal. We place a white paper background inside the tube behind the diagonal opposite the focuser.

 

Included in the Catseye tools is a red led pointer to illuminate the center spot during collimation. Yesterday I figured out that if I clip that led to the top edge of the tube and then point at the near edge of the secondary, it will illuminate the secondary profile in red, allowing me to finally see the edge clearly. So I think I have finally got it right...

[Asbytec]

Included in the Catseye tools is a red led pointer to illuminate the center spot during collimation. Yesterday I figured out that if I clip that led to the top edge of the tube and then point at the near edge of the secondary, it will illuminate the secondary profile in red, allowing me to finally see the edge clearly. So I think I have finally got it right...

Excellent. I've used a small piece of tape to highlight that dark edge. Now, I just use the crescent, then focus outward to see if the diagonal edge is concentric with the primary reflection. Whatever works for you. God job. Glad you got it.

 

I think it looks pretty good. Maybe Vic will put his concentric circles on it to see how you did. Or you can focus outward and see for yourself. Now, if you want to learn about offset, you can read up on classic offset and the new model. The proper offset happens (almost) automatically. 

Edited by Asbytec, 26 March 2025 - 03:15 AM.

[Vic Menard]

The focuser axis isn't fully corrected. 

 

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Edited by Vic Menard, 26 March 2025 - 07:44 AM.

[Vic Menard]

The Cheshire view confirms the TeleCat view:

 

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[Vic Menard]

...Another trick you can use is based on the fact when the focuser axis is aligned to the primary center, then the reflection of the primary is necessarily centered under the focuser. Because the primary mirror is centered under the focuser, we can use the reflection centered under the focuser as a reference to center our diagonal, too.

To be clear, it's not a trick. Making all three circles concentric (ideally, with the apparent size of the actual edge of the secondary mirror only slightly larger than the apparent size of the reflection of the primary mirror, which places the pupil near the apex) is the standard procedure for optimal secondary mirror placement. In fact, it's more difficult to assess the actual secondary mirror placement when the focuser axis is not correct.

 

There are far too many internet collimation sources that use only two circles (the bottom edge of the focuser drawtube or bottom edge of the sight tube and the actual edge of the secondary mirror) for secondary mirror "alignment". Several of these references actually suggest placing a paper between the secondary and primary mirrors to completely remove the "confusing" reflection of the primary mirror. And too many consider this two circle alignment to be the more critical focuser alignment, or go on to suggest "centering the reflection of the primary in the secondary mirror" as the focuser alignment. 

 

1.) Concentricity of the three circles indicates optimal secondary mirror placement (rotation and offset).

2.) Alignment of the sight tube cross hairs or the outgoing thin beam laser relative to the primary mirror center marker (usually corrected by adjusting the secondary mirror tilt) is focuser axial alignment.

3.) Alignment of the return thin beam laser relative to the laser emitter is primary mirror axial alignment. Or, using a Cheshire derivative (including a collimation cap or Barlowed laser), aligning the reflection of the Cheshire ring to the primary mirror center marker or the silhouette of the primary mirror center marker relative to the Barlowed laser target is also primary mirror axial alignment.

 

Axial alignment correction delivers image performance, including optimal field focus (focuser axis) and optimally centered coma free images (primary mirror axis). Optimal secondary mirror placement delivers centered field illumination--which has little (or no) impact on the image in an eyepiece, and minimal (but easily corrected) impact on the image in a camera.

 

...if you want to learn about offset, you can read up on classic offset and the new model. The proper offset happens (almost) automatically. 

I would argue that, since we're talking potential errors around ~1mm (or less), each error impacting the various geometries (squared focuser, spider, centered primary, spider, secondary mirror offset (holder/mirror) and possible skew)... pretty much all "proper" secondary mirror placements are possible because, in one way--or many ways, the vast majority of Newtonian collimation enthusiasts all utilize the New Model paradigm (and some sort of offset) to some "workable" solution.  

[Asbytec]

...(ideally, with the apparent size of the actual edge of the secondary mirror only slightly larger than the apparent size of the reflection of the primary mirror, which places the pupil near the apex) is the standard procedure for optimal secondary mirror placement.

Several of these references actually suggest placing a paper between the secondary and primary mirrors to completely remove the "confusing" reflection of the primary mirror.

 

I shouldn't have called it a trick. Focusing out to just inside the apex a good thing to know and use. It's a great standard procedure to assess the diagonal position once we realize the primary reflection is centered on the focuser when the focuser axis is aligned to the primary center.

 

Yea, never hide the reflections in diagonal. We can learn to use those confusing reflections for better diagonal placement and rotation and easier collimation. Otherwise, we have to tilt and rotate the diagonal so much we tend to get confused pushing the diagonal away from center trying to align the focuser axis. 

[Catchlight]

1.) Concentricity of the three circles indicates optimal secondary mirror placement (rotation and offset).

2.) Alignment of the sight tube cross hairs or the outgoing thin beam laser relative to the primary mirror center marker (usually corrected by adjusting the secondary mirror tilt) is focuser axial alignment.

3.) Alignment of the return thin beam laser relative to the laser emitter is primary mirror axial alignment. Or, using a Cheshire derivative (including a collimation cap or Barlowed laser), aligning the reflection of the Cheshire ring to the primary mirror center marker or the silhouette of the primary mirror center marker relative to the Barlowed laser target is also primary mirror axial alignment.

 

Axial alignment correction delivers image performance, including optimal field focus (focuser axis) and optimally centered coma free images (primary mirror axis). Optimal secondary mirror placement delivers centered field illumination--which has little (or no) impact on the image in an eyepiece, and minimal (but easily corrected) impact on the image in a camera.

 

I would argue that, since we're talking potential errors around ~1mm (or less), each error impacting the various geometries (squared focuser, spider, centered primary, spider, secondary mirror offset (holder/mirror) and possible skew)... pretty much all "proper" secondary mirror placements are possible because, in one way--or many ways, the vast majority of Newtonian collimation enthusiasts all utilize the New Model paradigm (and some sort of offset) to some "workable" solution.  

I just discovered that the concentricity of the annulus gap varies depending on the placement of the focuser and sight tube. If I raise the sight tube to the level where the primary and secondary are nearly the same size the annulus gap is almost perfectly even, but as I lower the sight tube or switch to the cheshire they are increasingly less concentric. Is this normal? And is the highest position the one I should be concerned about or is there another issue? I measured the squareness of the focuser and it was square...

 

[sixela]

Is this normal?

 

Yes. The apparent offset between the geometrical centre of the face of the secondary and the apparent centre of the outline as seen from the focuser depends on the distance.

 

Why almost everyone advises to tune it so the primary and secondary appear equally large is because that centres the fully illuminated field (which is pretty much the smallest thing it makes sense to centre).

 

If you'd want to centre the 70% illuminated field then the required offset would be different.

[Catchlight]

 

 

Why almost everyone advises to tune it so the primary and secondary appear equally large is because that centres the fully illuminated field (which is pretty much the smallest thing it makes sense to centre).

 

Aha! Thanks for clarifying!
 

[Vic Menard]

I just discovered that the concentricity of the annulus gap varies depending on the placement of the focuser and sight tube. If I raise the sight tube to the level where the primary and secondary are nearly the same size the annulus gap is almost perfectly even, but as I lower the sight tube or switch to the cheshire they are increasingly less concentric. Is this normal? And is the highest position the one I should be concerned about or is there another issue? I measured the squareness of the focuser and it was square...

As Sixela pointed out, for optimal secondary minor placement (at the native focal plane) the ideal placement for the pupil is near the apex (where the apparent diameters of the reflection of the primary mirror and the actual edge of the secondary mirror are more-or-less equal (as noted in post #16).

 

That said, you have another problem. As you rack the focuser in and out, you appear to be changing the focuser axial alignment. I recommend avoiding focusing all the way in or all the way out as the drawtube may be torqued out of alignment at these extremes. I also recommend using only one locking screw when securing the collimating tool in the focuser drawtube. Finally, you can verify the tool registration to the focuser drawtube by rotating it. Do NOT "free" rotate the tool as this can cause the alignment to precess. Register the tool flush to the top edge of the drawtube, gently secure the lock screw, and note the alignment of the cross hairs relative to the primary mirror center marker. Loosen the lock screw, rotate the tool 60- to 90-degrees, retighten the lock screw, and take the next alignment read. Continue the process until the tool has been rotated 360-degrees. If the alignment changes inside a 1/10-inch circle, the tool cross hair alignment is off 1/20-inch.

 

Your alignment images show a significant axial alignment error. At the apex (first image) the alignment is about 0.02-inch off toward the lower right. The second image (between the first and last), the alignment is about 0.2-inch off toward the upper left. The third (Cheshire) image, the alignment appears to be about 0.3-inch off toward the upper right. I'm not sure what alignment tools you're using--assuming they're 2-inch tools, they're passing the rotation test, and the pupils are small, the focuser could be the problem.

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[Vic Menard]

Compare with your first images (post #12).

 

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[Catchlight]

As Sixela pointed out, for optimal secondary minor placement (at the native focal plane) the ideal placement for the pupil is near the apex (where the apparent diameters of the reflection of the primary mirror and the actual edge of the secondary mirror are more-or-less equal (as noted in post #16).

 

That said, you have another problem. As you rack the focuser in and out, you appear to be changing the focuser axial alignment. I recommend avoiding focusing all the way in or all the way out as the drawtube may be torqued out of alignment at these extremes. I also recommend using only one locking screw when securing the collimating tool in the focuser drawtube. Finally, you can verify the tool registration to the focuser drawtube by rotating it. Do NOT "free" rotate the tool as this can cause the alignment to precess. Register the tool flush to the top edge of the drawtube, gently secure the lock screw, and note the alignment of the cross hairs relative to the primary mirror center marker. Loosen the lock screw, rotate the tool 60- to 90-degrees, retighten the lock screw, and take the next alignment read. Continue the process until the tool has been rotated 360-degrees. If the alignment changes inside a 1/10-inch circle, the tool cross hair alignment is off 1/20-inch.

 

Your alignment images show a significant axial alignment error. At the apex (first image) the alignment is about 0.02-inch off toward the lower right. The second image (between the first and last), the alignment is about 0.2-inch off toward the upper left. The third (Cheshire) image, the alignment appears to be about 0.3-inch off toward the upper right. I'm not sure what alignment tools you're using--assuming they're 2-inch tools, they're passing the rotation test, and the pupils are small, the focuser could be the problem.

The focuser is a TS-Optics UNCN2-G2, which only has 22mm of adjustment, so in these images it was in the middle of its range and I didn't change the focuser position, since it just doesn't do that much. Instead I moved the Astro-systems light pipe (which has much less play in the 2" focuser tube than the Catseye XLS sight tube) further down into the tube, but probably used more than a single screw to tighten.
I did a simple check of the focuser by putting a translucent ruler under the crosshairs and then with a laser collimator checking that the beam falls at the same distance on the far side of the tube, and it was identical. There is no tilt adjustment on the focuser, so I was hoping to get everything else as correct as possible to evaluate if I need to do something about the focuser...
Considering that I am using my telescope for astrophotography, so with a backfocus of 55mm above the CC, where should I set the sight pipe to evaluate concentricty of the mirrors?

I should mention that when photographing through the sight pipe, there is a lot of potential movement of the camera lens, I don't think I can capture the entire field of view in the sight pipe, so some of the outside circle may be skewed due to how the camera is placed over the hole...

Edited by Catchlight, 17 April 2025 - 03:32 PM.

[Asbytec]

I should mention that when photographing through the sight pipe, there is a lot of potential movement of the camera lens, I don't think I can capture the entire field of view in the sight pipe, so some of the outside circle may be skewed due to how the camera is placed over the hole...

Yes, Vic's assumption was the pupil is small so there is less room for parallax error positioning the camera over the pupil. Especially a cell phone camera. I have the 2" light pipe, as well, and I can move my eye just enough so the focuser axis looks to be aligned with the center mark when it's not. You have to keep your eye over the center of the pupil. I use my hand to hold my eye in position centered on the pupil. 

 

I also center the diagonal with the light pipe most of the way down, but not at the end of the focuser travel. In my experience, it's easier to center the diagonal with a thinner crescent between the diagonal edge and the bottom of the site tube than a thicker crescent. Once the focuser axis is aligned to the primary center, I focus out to just inside the apex to evaluate the diagonal position. For visual, it's usually very good or good enough.

 

I have also fought with focuser drift through focus for a long time. Racking from nearly all the way down to nearly all the way out, the cross hair or laser will drift laterally over the diameter of the center mark. Not good. As Vic explains, that is a focuser problem. I managed to fix the Crayford style focuser using a thin plastic shim inside the tube as "guide," but have since sent it in for repair. However, you have an apparently nice focuser, so not sure what to do.

[Vic Menard]

...I should mention that when photographing through the sight pipe, there is a lot of potential movement of the camera lens, I don't think I can capture the entire field of view in the sight pipe, so some of the outside circle may be skewed due to how the camera is placed over the hole...

I expect some parallax with any sight tube, but the variation with the LightPipe is enough to warrant another tool for assessing and correcting secondary mirror tilt (either a good thin beam laser or an autocollimator). Either the LightPipe (or the TeleCat) should be fine for assessing and correcting secondary mirror placement (three circles).

 

...I did a simple check of the focuser by putting a translucent ruler under the crosshairs and then with a laser collimator checking that the beam falls at the same distance on the far side of the tube, and it was identical.

Did you also check the focuser travel using the laser beam on the far side of the tube?

 

...Considering that I am using my telescope for astrophotography, so with a backfocus of 55mm above the CC, where should I set the sight pipe to evaluate concentricty of the mirrors?

Generally speaking, I suggest starting with the pupil near the apex relative to the native focal length (where the apparent primary mirror diameter is a bit smaller than the apparent secondary mirror diameter. From the apex, you want to make these three circles concentric: 

the bottom edge of the sight tube,

the actual edge of the secondary mirror, and

the reflected edge of the primary mirror.

When the three circles are concentric, you'll need to assess and correct the focuser axial alignment by adjusting the secondary mirror tilt to align the sight tube cross hairs (or the outgoing thin beam laser, or P-3 in an autocollimator) to the primary mirror center marker.

Assuming the three circles are still more-or-less concentric and the focuser axial alignment is correct, the final alignment is the primary mirror axis. This is best assessed and corrected with a Cheshire derivative (ideally with a calibrated Cheshire).

 

Once the telescope is correctly collimated relative to the native focal length, you'll need to assess the vignetting profile in your imaging train. Usually, assuming the secondary mirror is large enough to fully illuminate the coma corrector field lens (or nearly so), the vignetting profile is then attributed solely to the coma corrector.