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Anthony RS

Advice on Secondary Mirror Collimation

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Hello!

I've been having some offset coma issues with my f5 Newtonian so I've been trying to eliminate some elements that might be causing the issue so for now, just to remove collimation out of the equation, I need advice on whether or not my secondary appears to be well in place (not forgetting offset).

Below are some images I took, my question is should the black outer ring(pointed by the red lines) be concentric with the rest of the rings? if yes, how do I go about fixing that? is it by adjusting the tilt or moving the secondary inward or outwards (towards primary) or moving secondary away or towards focuser?

I also have another image with a crosshair which might help letting me know if the secondary is accurately positioned.

Edit: It's not apparent in the image but the crosshair in the image is pointing right on the primary center marker.

Thanks!

Secondary no mire.jpg

Secondary with mire.jpg

Edited by Anthony RS

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That patterns looks very good, are you sure you need an offset? My f/5 300mm dob has no offset and doesn't need it, the secondary is 70mm across, large enough to grasp the whole light cone. 

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Just now, Ben the Ignorant said:

That patterns looks very good, are you sure you need an offset? My f/5 300mm dob has no offset and doesn't need it, the secondary is 70mm across, large enough to grasp the whole light cone. 

I don't really know much about offset. All that I did is center the secondary mirror in the focuser and collimated but I've been reading that a fast newtonian will automatically have the secondary mirror offset if collimated properly. So now I have 2 choices, either leave it as is with the reflection offset, or move the secondary mirror a bit towards the primary which would offset the actual secondary mirror, but the reflection should appear centered and not offset. This is too complicated for me and I don't know which method to follow. Apparently centering both the secondary mirror and the reflection is impossible with my newtonian?!

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Many commercially produced newtonians have the offset designed into the secondary holder so you don't need to create it yourself. What brand / model of newtonian is yours ?

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32 minutes ago, Anthony RS said:

I don't really know much about offset. All that I did is center the secondary mirror in the focuser and collimated but I've been reading that a fast newtonian will automatically have the secondary mirror offset if collimated properly.

No. Only the very fast, f/4 or so might require an offset, and only if the secondary is smaller to retain contrast. Sacrificing a liitle contrast with a larger secondary allows it to grasp the complete light cone without a need for offset.

And the high-power star test is the only true proof of collimation. Defocus your star a tiny, tiny bit so you see only two or three rings around the center dot. When the dot is centered, collimation is perfect regardless of anything else.

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1 minute ago, John said:

Many commercially produced newtonians have the offset designed into the secondary holder so you don't need to create it yourself. What brand / model of newtonian is yours ?

I have a celestron 8 inch C8N (the one with the AVX mount). I know the offset is created automatically once I center the seconday in the focuser but my questions are:

1- Could that offset result in an offset coma while using a coma corrector?

2- Is it better if I move the secondary mirror inwards towards the primary so that the secondary mirror is offset but the reflection is actually centered? would that produce better images?

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Just now, Ben the Ignorant said:

No. Only the very fast, f/4 or so might require an offset, and only if the secondary is smaller to retain contrast. Sacrificing a liitle contrast with a larger secondary allows it to grasp the complete light cone without a need for offset.

And the high-power star test is the only true proof of collimation. Defocus your star a tiny, tiny bit so you see only two or three rings around the center dot. When the dot is centered, collimation is perfect regardless of anything else.

This is where I'm having problems, star test. After collimating and doing a star test, I tried to center the black dot however the results were terrible which leads me to my other question! :D If the seconday is offset so the relfection isn't actually at the center, doesn't this mean that when I do a star test, the black dot should also be offset and centered?

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Nope, when you star test you don't look at the large black disk you see when you defocus a lot, that is way too inaccurate. Instead, you focus on a star (artificial is better for it doesn't move), and then defocus it an extrememy small amount, so you see a couple of rings around a luminous dot that is created by diffraction. That is the proper and sensitive way to make an accurate collimation.

By the way, I'm not sure we say the same thing when we talk about the offset. I seem to understand your idea of the offset is moving the secondary along the optical axis only, but the offset is actually also pushing the secondary away from the focuser. Sorry if I misunderstood but here is a picture to clarify that.

20181003_214934.jpg.cc71fddce56f00b080beac582acc96c6.jpg

Edited by Ben the Ignorant

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2 minutes ago, Ben the Ignorant said:

Nope, when you star test you don't look at the large black disk you see when you defocus a lot, that is way too inaccurate. Instead, you focus on a star (artificial is better for it doesn't move), and then defocus it an extrememy small amount, so you see a couple of rings around a luminous dot that is created by diffraction. That is the proper and sensitive way to make an accurate collimation.

By the way, I'm not sure we say the same thing when we talk about the offset. I seem to understand your idea of the offset is moving the secondary along the optical axis only, but the offset is actually also pushing the secondary away from the focuser. Sorry if I misunderstood but here is a picture to clarify that.

20181003_214934.jpg.cc71fddce56f00b080beac582acc96c6.jpg

I did not know that about the star test. Thank you! I'll try that. As for the offset, I'm only talking about the offset towards the primary not away from the focuser. Here's what I mean:

I've been reading that there are 2 ways of collimating the secondary. The first way is by actually centering the Secondary mirror in the focuser tube so that it's dead center. This will result in the REFLECTION to be offset (which is the way my scope is currently collimated). The other way is by offsetting the actual mirror towards the primary which should result in the reflection to be centered.

I just want to know if the second method might yield better results for AP and if using the first method might be the reason I'm having the offset coma in my images.

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Sorry, I should have asked from the start, what do you call offset coma? It's a term nobody else uses; please make a drawing because written descriptions are never as clear.

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45 minutes ago, Ben the Ignorant said:

Sorry, I should have asked from the start, what do you call offset coma? It's a term nobody else uses; please make a drawing because written descriptions are never as clear.

I already have a thread regarding the issue 

basically it's offcentered coma, so I' have coma more pronounced in one corner than the others.

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I'd be surprised if a tiny positional error on the secondary of any kind would cause coma. It usually affects light gathering or light transmission rather than image quality does it not? 

The primary is more likely to affect coma but it's usually uniform. More likely to be something not orthogonal in the optical train maybe but I'm no imager.

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1 hour ago, Anthony RS said:

basically it's offcentered coma, so I' have coma more pronounced in one corner than the others.

That's usually the result of tilted optics or a tilted imaging rig.

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12 minutes ago, Ben the Ignorant said:

That's usually the result of tilted optics or a tilted imaging rig.

That's what  I though but I'm trying to eliminate any other possible causes and I thought collimation might be one. I think my main issue is focuser sag. Maybe tilting is caused by the focuser sagging when I change focus with the DSLR attached. Do you think that could be the problem?

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Only you can determine that. To avoid having to wait for clear nights, make an artificial sky with pinholes in a backlit plate, and experiment with one suspicious factor at a time. (I'm not an imager, by the way, I'm just giving general advice)

Edited by Ben the Ignorant

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12 hours ago, Anthony RS said:

Below are some images I took, my question is should the black outer ring(pointed by the red lines) be concentric with the rest of the rings? if yes, how do I go about fixing that? is it by adjusting the tilt or moving the secondary inward or outwards (towards primary) or moving secondary away or towards focuser?

I also have another image with a crosshair which might help letting me know if the secondary is accurately positioned.

Edit: It's not apparent in the image but the crosshair in the image is pointing right on the primary center marker.

Thanks!

The slight misalignment you referenced is negligible and has no impact on your views. I would definitely ignore and not be bothered with it.

Jason

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18 hours ago, Ben the Ignorant said:

That patterns looks very good, are you sure you need an offset? My f/5 300mm dob has no offset and doesn't need it, the secondary is 70mm across, large enough to grasp the whole light cone. 

I presume the middle figure below matches your concern when mounting a secondary mirror without an away-from-focuser offset , am I correct? That is, as shown in the same middle figure, part of the reflected star light cone will miss the secondary mirror unless it is either properly offsetted away-from-the-focuser or the secondary mirror is  large enough to intercept the whole reflected light cone, am I still correct?

Well, this is a common misconception!!!

If the secondary mirror is mounted with the proper away-from-focuser offset then you will get the left-hand side figure which is the "intuitive" picture we have in mind when we think of the reflected cone. However, when the secondary mirror is mounted without any away-from-focuser offset then we can still intercept the whole light cone as shown in the right-hand side figure. As we go through the collimation steps, we will end up tilting the primary mirror towards the focuser without even knowing it. Now the central star in the FOV is not the one intercepted by the OTA axis and our setup will intercept the whole reflected light cone of the FOV central star.

post-5330-0-27502700-1424898584_thumb.png

 

Both methods of secondary mirror mounting will provide perfect collimation if the proper steps are followed as shown below:

post-5330-0-45755500-1364367262_thumb.png

There is one exceptions when it is desired to mount the secondary mirror with the proper offset which is to improve DSC accuracy but the benefit is small. 

There are two exceptions when it is a MUST to mount the secondary mirror with the proper offset  which are:

1- When tilting the primary mirror causes the incoming parallel light to be clipped by the OTA  edge. This only happens with the OTA opening it too tight -- as large as the primary mirror.

2- When the Newtonian has a corrective lens mounted at the OTA opening but most Newtonians do not have it.

One more thing, it is typical for off-center stars to have their reflected light cones clipped by the secondary mirror even for a perfectly collimated scope as shown below:

post-17988-133877736535_thumb.gifpost-5330-0-75128000-1352744931_thumb.gif

Jason

EDIT: Added missing last attachment

 

Edited by Jason D

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9 hours ago, Ben the Ignorant said:

And the high-power star test is the only true proof of collimation. Defocus your star a tiny, tiny bit so you see only two or three rings around the center dot. When the dot is centered, collimation is perfect regardless of anything else.

What you have described only checks for the primary mirror alignment which is the most critical collimation alignment -- the one responsible for coma. However, the method you have described does not evaluate the proper placement of the secondary mirror unless the secondary mirror is grossly (and I mean grossly) misaligned. 

Jason

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9 hours ago, Ben the Ignorant said:

Nope, when you star test you don't look at the large black disk you see when you defocus a lot, that is way too inaccurate. Instead, you focus on a star (artificial is better for it doesn't move), and then defocus it an extrememy small amount, so you see a couple of rings around a luminous dot that is created by diffraction. That is the proper and sensitive way to make an accurate collimation.

By the way, I'm not sure we say the same thing when we talk about the offset. I seem to understand your idea of the offset is moving the secondary along the optical axis only, but the offset is actually also pushing the secondary away from the focuser. Sorry if I misunderstood but here is a picture to clarify that.

20181003_214934.jpg.cc71fddce56f00b080beac582acc96c6.jpg

The following statement in the book "For uniform illumination of the focal plane the diagonal must be displaced away from the eyepiece and toward the primary mirror by equal amounts" is wrong!!!!

Jason

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On 04/10/2018 at 06:15, Jason D said:

The following statement in the book "For uniform illumination of the focal plane the diagonal must be displaced away from the eyepiece and toward the primary mirror by equal amounts" is wrong!!!!

Jason

How old is the book and how old is the F5 scope?

for example, Skywatcher's PDS versions have mirror glued to the holder with the offset already, and no offset has to be introduced during collimation.

Edited by RolandKol

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1 hour ago, RolandKol said:

How old is the book and how old is the F5 scope?

It's Jean Texereau's How to Make a Telescope from 1951. In case you wonder, this a famous book among scope makers, used by many generations of newtonian builders. It also explains how to craft aspherical cassegrainian optics, with optical window and all, even if few people mention it. Okay, he wrote a book, now what real-life credentials does this Texereau guy have, you ask?

Well, this is the optical standard of his workshop:

20190111_173958.thumb.jpg.7b800256fd24deb5f10b5c846f5df046.jpg

His team of amateurs made this 1/50th wave parabolic primary. Ideal shadows meaning perfect shape, and it doesn't get any smoother.

 

But only comparison really allows to judge, so see this other mirror:

20190111_174119.thumb.jpg.4e2d36ab07a080f13d7f290ee3e59a97.jpg

That's the outcome of beginner or incompetent mirror making.

 

Texereau's work was not limited to amateur optics, here he rectifies the 28-inch secondary of the texan MacDonald 82-inch cassegrainian, with his own hands:

20190111_174604.thumb.jpg.c503ffea6e4403ac0a36597fa08976b9.jpg

The Texas scope performed very well after his work. Texereau knows how to make optics and align them. (I got the book to better understand optics, not that I wanted to craft them).

Edited by Ben the Ignorant
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