Collimation - again

[Richard N]

Forums like this always have many threads on collimation of Newtonian reflectors. Endless discussion. All very interesting. What I have never seen is any objective way to assess the effects of various degrees of miscollimation. I suspect that the answers are different for visual and photography? I also suspect (with no evidence at all) that we can obsess over very tiny errors that make no discernable difference. 

[bosun21]

Depends on your definition of  “very tiny“.

[Richard N]

1 minute ago, bosun21 said:

Depends on your definition of  “very tiny“.

That's the point really. I can find no objective data that links the magnitude of errors to detectable deterioration of the image. Everything that I have seen is anecdotal.

[bosun21]

Just now, Richard N said:

That's the point really. I can find no objective data that links the magnitude of errors to detectable deterioration of the image. Everything that I have seen is anecdotal.

I have indeed read several articles from learned folk that small errors in alignment will make such small indiscernible differences in the image quality (at least with your eye). But once again they didn’t define small.

[dweller25]

My philosophy is to get it spot on, then there is no doubt the scope is performing at it’s best.

HOWEVER , my scopes collimation was slightly off but performing well so I got it spot and the views……… were the same 🙂

Maybe its more important for imaging or short f ratios ?

Edited August 19, 2023 by dweller25

[John]

This is worth a look:

Thierry Legault - The collimation (astrophoto.fr)

I think accurate collimation does matter but the differences are seen when pushing the optics towards their limits. Many folks don't push that far so maybe don't notice any improvements 🤔

[dweller25]

7 minutes ago, John said:

This is worth a look:

Thierry Legault - The collimation (astrophoto.fr)

I think accurate collimation does matter but the differences are seen when pushing the optics towards their limits. Many folks don't push that far so maybe don't notice any improvements 🤔

That’s a good read 👍

[PeterStudz]

2 hours ago, John said:

This is worth a look:

Thierry Legault - The collimation (astrophoto.fr)

I think accurate collimation does matter but the differences are seen when pushing the optics towards their limits. Many folks don't push that far so maybe don't notice any improvements 🤔

Yes, that’s a good article - thanks for that!

[inFINNity Deck]

Most important line from that article is "Anyway, if no Airy pattern can be discerned, no high resolution result can be expected (except in big telescopes for whom the Airy pattern is rarely or even never visible)."

Here in the Netherlands seeing the Airy disc is a rare treat, most of the time the seeing too poor to see it. That seeing has a disastrous effect on your imaging. As I explain in my second white paper on the optimal focal ratio, the oversampling rapidly increases with seeing and thus any miscollimation will hardly be noticeable under such conditions:

In certain cases a little bit of miscollimation can even be beneficial to planetary imaging. In his article "The Mars Edge-Rind Artefact", Martin R Lewis shows that a slight miscollimation can reduce the edge-rind.

Nicolàs

 

 

[wimvb]

Small collimation errors are barely noticeable. Small focus errors are barely noticeable. Small guiding errors (if you're imaging) are barely noticeable. But if you have all three, you have a fuzzy image.

Edited August 19, 2023 by wimvb

[Mr Spock]

I'll have to disagree. Near enough isn't good enough. So many people are convinced you can't get above x200 as you can't see anymore detail due to seeing. However we do get good seeing and most of that misconception is people trying to use mis-collimated scopes. A 7mm eyepiece gives x217 in my 12"; that is the lowest magnification I've used, and that is in poor seeing.

You are free to convince yourself of whatever. Meanwhile I'll be cruising the moon's surface, razor sharp, at x461, in good seeing conditions, and wondering what all the fuss is about 

[John]

It's worth noting that the collimation "sweet spot" gets progressively smaller as the focal ratio reduces. This is from Nils Olaf Carlin writing for Sky & Telescope magazine:

"Surprisingly, the size of the "sweet spot" depends only on the main mirror's focal ratio (the mirror's focal length divided by its diameter) and not its size. For instance, even a perfect f/4.5 mirror, small or large, can provide "diffraction limited" performance only within a 2-millimeter (0.08-inch) circle at the focal plane. An f/10 paraboloid's sweet spot, by contrast, spans 22 mm (0.87 inch). (For the mathematically inclined, the sweet spot's diameter is proportional to the cube of the f/ratio.)"

Because of their proportionally larger secondary obstruction and very "fast" primary mirror (usually F/2 or F/2.5) I believe SCT's are even more sensitive to collimation errors with contrast and sharpness declining markedly when collimation is "off". On the plus side, SCT's tend to hold their collimation better than newtonians once it is correctly adjusted. 

 

 

[inFINNity Deck]

4 minutes ago, John said:

It's worth noting that the collimation "sweet spot" gets progressively smaller as the focal ratio reduces. This is from Nils Olaf Carlin writing for Sky & Telescope magazine:

"Surprisingly, the size of the "sweet spot" depends only on the main mirror's focal ratio (the mirror's focal length divided by its diameter) and not its size. For instance, even a perfect f/4.5 mirror, small or large, can provide "diffraction limited" performance only within a 2-millimeter (0.08-inch) circle at the focal plane. An f/10 paraboloid's sweet spot, by contrast, spans 22 mm (0.87 inch). (For the mathematically inclined, the sweet spot's diameter is proportional to the cube of the f/ratio.)"

Because of their proportionally larger secondary obstruction and very "fast" primary mirror (usually F/2 or F/2.5)

Interesting quote from Carlin that is,  thanks.

4 minutes ago, John said:

 I believe SCT's are even more sensitive to collimation errors with contrast and sharpness declining markedly when collimation is "off". On the plus side, SCT's tend to hold their collimation better than newtonians once it is correctly adjusted.

That is only when the SCT is properly constructed. I use a Celestron C11 EdgeHD and had a C11 XLT Carbon, both of which suffer from mirror-flop to an extend that it affects collimation. The current one has been back to Celestron a few years ago and was 'repaired', but upon arrival back here it showed exactly the same issue as it had before. Inquiries prior to its return learned that Celestron does specify mirror-shift (the tilting of the main mirror due to focusing), but not mirror-flop (the tilting due to change in gravity). I have an optical bench with a collimator and can measure it, so I asked for these figures so that I could check if the return journey had affected their repair. They simply refused to specify mirror-flop ("...we would not be able to divulge that information."), only telling me that "We will to make sure your telescope is returned to factory specifications." Upon arrival back here the test on the optical bench showed it was still affected by mirror-flop (the animation shows the collimation in two 180 degrees rotated positions of the scope before and after repair):

Needless to say that I was not amused (the shipment alone was more than a grand). Ever since I collimate the C11 each time when I want to image the planets or the Sun in detail, even if that is on two consecutive nights and especially after a meridian flip.

Nicolàs

[Richard N]

Nice to see some science emerging behind the usual anecdotal noise. Thanks all. 

[bosun21]

30 minutes ago, inFINNity Deck said:

Interesting quote from Carlin that is,  thanks.

That is only when the SCT is properly constructed. I use a Celestron C11 EdgeHD and had a C11 XLT Carbon, both of which suffer from mirror-flop to an extend that it affects collimation. The current one has been back to Celestron a few years ago and was 'repaired', but upon arrival back here it showed exactly the same issue as it had before. Inquiries prior to its return learned that Celestron does specify mirror-shift (the tilting of the main mirror due to focusing), but not mirror-flop (the tilting due to change in gravity). I have an optical bench with a collimator and can measure it, so I asked for these figures so that I could check if the return journey had affected their repair. They simply refused to specify mirror-flop ("...we would not be able to divulge that information."), only telling me that "We will to make sure your telescope is returned to factory specifications." Upon arrival back here the test on the optical bench showed it was still affected by mirror-flop (the animation shows the collimation in two 180 degrees rotated positions of the scope before and after repair):

Needless to say that I was not amused (the shipment alone was more than a grand). Ever since I collimate the C11 each time when I want to image the planets or the Sun in detail, even if that is on two consecutive nights and especially after a meridian flip.

Nicolàs

Maybe this is the reason why Celestron felt the need to incorporate the mirror locks into their Edge HD design.

[Richard N]

55 minutes ago, John said:

It's worth noting that the collimation "sweet spot" gets progressively smaller as the focal ratio reduces. This is from Nils Olaf Carlin writing for Sky & Telescope magazine:

"Surprisingly, the size of the "sweet spot" depends only on the main mirror's focal ratio (the mirror's focal length divided by its diameter) and not its size. For instance, even a perfect f/4.5 mirror, small or large, can provide "diffraction limited" performance only within a 2-millimeter (0.08-inch) circle at the focal plane. An f/10 paraboloid's sweet spot, by contrast, spans 22 mm (0.87 inch). (For the mathematically inclined, the sweet spot's diameter is proportional to the cube of the f/ratio.)"

Because of their proportionally larger secondary obstruction and very "fast" primary mirror (usually F/2 or F/2.5) I believe SCT's are even more sensitive to collimation errors with contrast and sharpness declining markedly when collimation is "off". On the plus side, SCT's tend to hold their collimation better than newtonians once it is correctly adjusted. 

 

 

Is the term “sweet spot” defined in any objective manner?

[bosun21]

2 minutes ago, Richard N said:

Is the term “sweet spot” defined in any objective manner?

I would say that this sweet spot gets smaller as you increase to high magnification (300x plus) regardless of f ratio. However if you have a fast newtonian and are using it at high magnification then the sweet spot will be really small if at all.

[Captain Scarlet]

2 hours ago, Richard N said:

Is the term “sweet spot” defined in any objective manner?

It’s described here, if you have the patience for it:

https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

and

https://www.telescope-optics.net/newtonian_collimation.htm

Edited August 20, 2023 by Captain Scarlet

[John]

I've seen the "sweet spot" also called the "coma-free field" and the "diffraction limited field".

I found these references on another forum which I think are relevant:

Again from Nils Olof Carlin:  

"The allowable tilt of the focuser axis as measured on the primary (e.g. with a laser spot) is some 1/25 of the primary's diameter (actually, the focal ratio is eliminated in the expression) - here, the defocusing from the tilt of the focal plane is negligible compared to the coma everywhere within the FOV.

For an F/5.5, the "diffraction limited field", or more accurately the field where the Strehl ratio is not lowered more than 0.2 from its maximum, is some 3.5 mm diameter...."

and from another poster:

"As far the size of that "coma free" field, watch out, because there are two common criteria; one for the "photographically acceptable" field for prime focus photography (where the comatic spot size in the Newtonian focus reaches typical film resolution, 0.025mm), and one for the "diffraction limited" coma-free field (where the comatic spot size reaches the size of an Airy disc).

The latter dimension is much smaller."

 

[inFINNity Deck]

2 hours ago, bosun21 said:

Maybe this is the reason why Celestron felt the need to incorporate the mirror locks into their Edge HD design.

That is what I thought and the reason why I swapped the C11 Carbon for the C11 EdgeHD. The optical bench measurements of C11 EdgeHD in the animation were done directly upon arrival and with the mirror-locks engaged, so the locks do not have the expected result (in my C11 at least). The issue appears to be that the main mirror is not properly attached to the outer focusing-tube. The Italian company Blue Atelier, a technical division of the astronomical park La Torre del Sole near Milan can correct this, but they insist that the telescope is collected in person as they fear that transport may undo the repair. During the pandemic I could not travel there myself, so they explained all the steps required to do the repair, which I should now be able to do so. The repair is, however, quite an undertaking and requires a lot of additional (and at times costly) tools currently not present in my workshop. Apart from the collection costs, it is cheaper to have it repaired in Italy, than to do it myself, so maybe I will send it off somewhere in near future and then have a nice few days in Italy... 😉

Nicolàs

[John]

Many years ago I had a C8 SCT which suffered from mirror flop. It was an older one though. Annoying even as a visual astronomer. When the collimation was accurate the scope performed excellently and showed me some of the best views of Saturn that I've ever had. But when the scope moved past a certain angle you could actually see the collimation slip out of alignment and the sharpness and contrast just vanished 😬

[lunator]

4 hours ago, Mr Spock said:

I'll have to disagree. Near enough isn't good enough. So many people are convinced you can't get above x200 as you can't see anymore detail due to seeing. However we do get good seeing and most of that misconception is people trying to use mis-collimated scopes. A 7mm eyepiece gives x217 in my 12"; that is the lowest magnification I've used, and that is in poor seeing.

You are free to convince yourself of whatever. Meanwhile I'll be cruising the moon's surface, razor sharp, at x461, in good seeing conditions, and wondering what all the fuss is about 

I have to agree. I have an F6 200mm Newt. It used to be on a GEM. If you got the collimation spot on and the seeing was good then the performance of the scope was amazing. It is worth the effort to aim for the best collimation you can.

[bosun21]

8 hours ago, inFINNity Deck said:

That is what I thought and the reason why I swapped the C11 Carbon for the C11 EdgeHD. The optical bench measurements of C11 EdgeHD in the animation were done directly upon arrival and with the mirror-locks engaged, so the locks do not have the expected result (in my C11 at least). The issue appears to be that the main mirror is not properly attached to the outer focusing-tube. The Italian company Blue Atelier, a technical division of the astronomical park La Torre del Sole near Milan can correct this, but they insist that the telescope is collected in person as they fear that transport may undo the repair. During the pandemic I could not travel there myself, so they explained all the steps required to do the repair, which I should now be able to do so. The repair is, however, quite an undertaking and requires a lot of additional (and at times costly) tools currently not present in my workshop. Apart from the collection costs, it is cheaper to have it repaired in Italy, than to do it myself, so maybe I will send it off somewhere in near future and then have a nice few days in Italy... 😉

Nicolàs

It’s a poor showing from Celestron when their flagship models of SCT have this inherent fault. From what you have said it’s neither simple or cheap to address this fault. As I said they must be aware of this problem hence the addition of the mirror locks to try and rectify the issue. Alas from what you said this has not had the desired outcome.

[inFINNity Deck]

18 hours ago, bosun21 said:

It’s a poor showing from Celestron when their flagship models of SCT have this inherent fault. From what you have said it’s neither simple or cheap to address this fault. As I said they must be aware of this problem hence the addition of the mirror locks to try and rectify the issue. Alas from what you said this has not had the desired outcome.

Even better: the RASA initially had the same mirror suspension and mirror-locks, but that has been completely redesigned (by a user! (but I cannot find the thread in which that was discussed)) a few years ago, so, yes, they are very well aware that the mirror-mounting is not adequate. The new RASA are equipped with "NEW Ultra-Stable Focus System - six precision sealed ball bearings virtually eliminate focus shift". And that was only to mitigate mirror-shift. The mirror-flop is caused by the silicon filler between tube and mirror not properly holding the mirror.

Still, once properly collimated these are very nice scopes (but please do use it with a third-party focuser)!

In below image (taken on 9 November 2022 around 20:53UTC with a C11 EdgeHD @f/20 and ASI174MM) we can even see correct detail on Ganymede (far left):

Nicolàs

 

[lawsio]

I need collimation advice (just had my first atyempt!) so will jump on this thread to avoid starting a new one!

Basically, am I collimated yet? After a good hour of farting about everything seems to more or less line up apart from the crosshairs on the cheshire, which I guess wouldn't be there if I used a standard cap for it.

Am I close enough? Need to go back and try again?