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Impact of less than perfect collimation

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#1 dgcms

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Posted 15 July 2013 - 10:10 AM

Collimation gets quite a bit of attention on the forum. Is there any qualitative way of stating how being out of collimation affects visual quality at the eyepiece? If the scope is slightly out how much is resolution affected? How much is luminance affected? Any pictures on the web depicting out of collimation views to in collimation views? Is the importance of perfect collimation being overly emphasized?

#2 REC

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Posted 15 July 2013 - 10:21 AM

There is.....just don't know where to point you? It shows a picture of a close up on the Moon's craters and also on Saturn and if you can see clearly something like the Cassini division depending how accurate your collimation is. I just don't know where I saw it? Maybe someone else will chime in and in the meantime I'll go back and view my bookmarks as I should have marked that excellent article.

BTW, welcome to Cloudy Nights!

Bob

#3 howard929

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Posted 15 July 2013 - 10:23 AM

A slightly mis-collimated secondary will result in a small amount of light loss. Possibly noticeable under light polluted conditions and not so much under really dark skies.

Primary mirror mis-collimation of small degrees will result in a loss of fine detail at high powers. Usually dependent on seeing conditions which vary widely and viewer experience. The way I see the bottom line is perfection is nice but good enough is good enough for most observers most of the time. Exceptions vary with seeing conditions and experience level. Also, it's much more rewarding IMO to look through then to look at. ;)

EDIT to add:

Is the importance of perfect collimation being overly emphasized?

Yes. Of course it is! It's something we learn to fret over, THEN we learn not to. Personally I always get a kick out of someone who states "...and the collimation was perfect" Of course it was... :tonofbricks:


#4 Pharquart

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Posted 15 July 2013 - 11:35 AM

I think the page that REC is referring to is linked below. It shows simulated images of the moon, Jupiter, and Saturn at varying levels of miscollimation.

http://legault.perso....fr/collim.html

Brian

#5 panhard

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Posted 15 July 2013 - 12:13 PM

dgcms: What scope do you have? With fast scopes collimation is more critical Lets say below f5. Slower scope collimation isn't as critical and there is also larger sweet spot.

#6 Kraus

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Posted 15 July 2013 - 12:25 PM


I bring a star out of focus.

Well Tim, is the black disc pertineer in the center of the large white disk?

Yuperroo! It sure looks it to me.

Then we're go to go.

#7 AngryHandyman

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Posted 15 July 2013 - 12:30 PM

Brian - great link, thanks for posting it I've come across it before but neglected to book mark it.

I have noticed a difference in views when I've messed up collimation and then corrected it. I'm most certainly not perfect but my noob experiences can tell between terrible and what most would probably say is good enough. At that point I stop fiddling and get on with the show!

#8 dgcms

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Posted 15 July 2013 - 01:36 PM

dgcms: What scope do you have? With fast scopes collimation is more critical Lets say below f5. Slower scope collimation isn't as critical and there is also larger sweet spot.


I have a Celestron C8N newtonian at f5 so sounds like collimation may be more critical in my case.

#9 dgcms

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Posted 15 July 2013 - 01:38 PM

Thanks Brian. That's exactly what I was looking for.

#10 REC

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Posted 15 July 2013 - 01:43 PM

Yep, that's the one...thanks!

#11 Feidb

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Posted 15 July 2013 - 02:27 PM

It all depends on how much you want to obsess over specs and what you think the results might be, which they usually aren't. You can obsess over zero perfect collimation, tweak it to death but wouldn't be able to tell the difference on most nights because the sky conditions won't allow you to notice, what with the stars blurring in and out of focus. Maybe between blurs, you might catch a glimpse of clarity and see something almost perfect, but you can get that with a slight misalignment depending where the object is in the field of view and depending on magnification and focal ratio.

Using an f/4.5, I pop in my plastic cap with the reflective inside, get the dots centered "about right" and don't worry about it unless I get up to 390X or so and notice the stars shapes change from one side to the other as I rack in and out of focus. Then I know I'm off a tad. Since I mostly observe between 102X and 229X, I'm not usually bothered by that. If I'm planning on pushing above 390X, I may spend another minute or two tweaking things, but that's about it. I don't spend five minutes, ten or an hour and spend big bucks on tools I don't need to get perfection for that one day a year when I might get good enough sky conditions to take advantage of extreme high magnification.

#12 Vic Menard

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Posted 15 July 2013 - 02:56 PM

...Is the importance of perfect collimation being overly emphasized?

I hope "perfect collimation" isn't being overly emphasized!

FWIW, I'm not sure there's such a thing as "perfect" collimation. I do think it's important that collimation should be "good enough" to realize your (realistic) performance expectation.

If you observe from a location that rarely has good seeing, your optics are "pretty good", and even though your focal ratio is f/6 or higher, you prefer low power views because all your high magnification views are fuzzy--a quick, "ball park" alignment (whatever that is, although I suspect it varies from one observer to the next) may get the job done.

If, OTOH, you are fortunate enough to observe from a location that often has good to excellent seeing, you have high Strehl optics, your focal ratio is shorter than f/6, and you routinely use moderate to high magnification quality eyepieces (and a coma corrector)--you need to know what "good enough" collimation is so you can determine whether or not you have it, and if not, what correction is necessary to get it. (And, FWIW, these routine axial alignment adjustments rarely take more than a few minutes.)

#13 Mxplx2

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Posted 15 July 2013 - 03:10 PM

If you have a refractor, do a comparison test with the reflector. I did on the moon where you have detail to work with, and the reflector's image was no way as crisp as the refractor's. Sooo...

#14 GeneT

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Posted 15 July 2013 - 03:16 PM

If the scope is slightly out how much is resolution affected?


It is all a matter of degree. Out of collimation only slightly, then only a slight impact on images. However, with a little practice, its not that difficult to get collimation dead on. The faster the mirrors, the more important it is to get and hold accurate collimation. There have been telescope reviews reporting that collimation can't be held throughout the viewing arc. If I were to buy a new telescope, I would check out that aspect very thoroughly.

#15 CeleNoptic

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Posted 15 July 2013 - 05:38 PM

I think the page that REC is referring to is linked below. It shows simulated images of the moon, Jupiter, and Saturn at varying levels of miscollimation.

http://legault.perso....fr/collim.html

Brian


Somehow this link doesn't work. Is it only me?

P.S. OK, it works now, they probably had maintenance at the time I tried to access.

#16 Jon Isaacs

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Posted 15 July 2013 - 08:41 PM

Collimation gets quite a bit of attention on the forum. Is there any qualitative way of stating how being out of collimation affects visual quality at the eyepiece? If the scope is slightly out how much is resolution affected? How much is luminance affected? Any pictures on the web depicting out of collimation views to in collimation views? Is the importance of perfect collimation being overly emphasized?


This is the way I think about collimation of a Newtonian:

- There are a number of adjustments, the two final adjustments that are checked and adjusted each session are the "focuser axial alignment" and the primary mirror tilt adjustment which is the most critical adjustment.

- Focuser axial alignment simply means that central axis of the focuser is pointed at the center of the mirror. The tolerance on this is about 3% of the aperture, for an 8 inch scope, this is about 6mm, this is a relatively easy adjustment. If one is using a coma corrector, the tolerance is tighter.

- The primary tilt: The size of the diffraction limited region (sweet spot) depends on the focal ratio of the mirror:

D = 0.022 x F^3 mm.

The diameter of the sweet spot is related to the cube of the focal ratio so slower scopes have more lax tolerances than faster scopes:

The numbers look like this:

F/4 = 1.4mm
F/5 = 2.75mm
F/6 = 4.75mm
F/8 = 11.3mm

You want the alignment of the primary to be a small fraction of the diffraction limited region, clearly F/4 is more of a challenge than F/8. With good tools both are doable...

So what does this all mean when looking through the eyepiece: In a fast scope, F/4 is fast, you better be right on the money because the sweet spot is small. With a slower scope, F/6-F/8 you can get by with more, the sweet spot is larger and not so dramatic.

Jon

#17 frito

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Posted 15 July 2013 - 11:32 PM

Jon has given good sound technical advise. collimation is important and becomes much more critical the faster the scope is regardless of what type of scope it is.

ok vs great/perfect collimation can result in large differences in image quality in my experience provided seeing conditions allow the higher accuracy. how accurate you should collimate is really dependent on the F Ratio of the scope as Jon's chart shows above. F/6 and below it becomes something that should be done pretty precisely to get the most out of your scope, at slower focal ratios its becomes much more forgiving and thus has less impact unless its really out of whack in alignment.

#18 FlorinAndrei

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Posted 16 July 2013 - 12:13 AM

I think the page that REC is referring to is linked below. It shows simulated images of the moon, Jupiter, and Saturn at varying levels of miscollimation.

http://legault.perso....fr/collim.html

Brian


Legault's site is excellent. :waytogo:

Bottom line is: don't rationalize it. Just learn how to achieve the best collimation you possibly can, and practice until you do it quickly and very well. Spend some time learning the process, so then in real life you can collimate the scope in 1 or 2 minutes - and do it very very well. Like others have said, an f/8 will cut you some slack, whereas an f/5 will not. Either way, try and do a good job.

The best scope in the world, and the best eyepieces, aren't worth jack squat if the whole system is miscollimated, or is not at thermal equilibrium. Fine tuning the scope always comes first.

#19 T1R2

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Posted 16 July 2013 - 12:31 AM

I refuse to look through anything that isn't perfectly collimated...If you see me commin at a star party, watch out and hide your scope if it ain't, cause if I get to I'll be like ..."hey man, can I tweek that for you?"...lol

#20 Jon Isaacs

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Posted 16 July 2013 - 04:58 AM

I refuse to look through anything that isn't perfectly collimated...If you see me commin at a star party, watch out and hide your scope if it ain't, cause if I get to I'll be like ..."hey man, can I tweek that for you?"...lol


Perfect? no such thing... within necessary tolerances? Possible.

Jon

#21 mclewis1

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Posted 16 July 2013 - 08:44 AM

Clearly there are different levels of precision in performing collimation for different folks with different scopes in different uses.

If you're primarily viewing DSOs you can certainly be a bit less critical but if you are working at the limits of your scope's capabilities you will find that accurate collimation does make a difference. With higher magnification viewing (small DSOs, double stars, or solar system objects) collimation becomes more important. There are plenty of examples of folks who were disappointing with their scope's high magnification performance only to change their minds after having the collimation properly done. A common quote "it's like a new scope now".

Your scope type also has an impact. You can be a little less critical with a longer focal ratio Newtonian and not notice much but an even longer f ratio SCT will often show really obvious optical benefits from fine tuning the collimation. As mentioned above, the shorter f ratios are more sensitive to (and therefore benefit more from) accurate collimation.

However, if you are imaging with just about any setup accurate collimation makes an obvious difference, especially if you have other optical elements in your optical path (Barlows, focal reducers, field flatteners, etc.). One of the worst case setups is aggressive focal reduction on an SCT which will clearly show up virtually any miss collimation.

#22 orion61

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Posted 16 July 2013 - 09:46 AM

I refuse to look through anything that isn't perfectly collimated...If you see me commin at a star party, watch out and hide your scope if it ain't, cause if I get to I'll be like ..."hey man, can I tweek that for you?"...lol

AMEN!
I use nothing less than 70X per inch on a Focused Star to collimate mine.
Looking through even a slightly Mis-colimated scope will
be about the same as turning a set of optics with 1/8th wave optics to 1/2 or 1/6th Wave!
That is also why I do not use Bobs Knobs.
They are a great idea, and a real time saver in the dark.
In my case, they just havent given me the performance I want.
Having those knobs RIGHT under the corrector cover, gives me Willies, in case of an unexpected bump and resulting shattered corrector!
I used to have to tweek collimation every couple weeks or any time I traveled.
I took the BN's off my C8 and it has been 6 months since i have had to do a critical collimation.
You will find that after you begin training your eyes to see finer and finer detail, you will demand closer collimation.
Especially with Double Stars!
Good Luck

#23 ensign

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Posted 16 July 2013 - 11:42 AM

Since this is a beginner's forum and since I am relatively new to the hobby (I've been observing for around 6 years), I would like to add that beginners should not let the issue of collimation intimidate or discourage them.

As a beginner, there seems to be an ocean of knowledge required in this hobby. Since, as a beginner, you're unlikely to be pushing your scope anywhere near its limits, collimation is not a really big deal unless your optics are grossly out of alignment.

I started out with a F4.7, 10 inch Dob that I had made an attempt at collimating. Unbeknownst to me, the focuser was loose, sloppy and misaligned. So my attempts at collimation, fledgling though they were, did not come anywhere near properly aligning the optics.

The primary mirror was so filthy, you could see the crud all over it just looking down the tube in the daylight.

Yet, for the first year or so, I had a blast hunting down and viewing most of the Messier objects. And to my less experienced eye, I think perfect or near-perfect collimation would have been lost on me. Ten inches of aperture will cover a multitude of sins. :grin:

Fortunately for me, about a year into the hobby, a friend with a good deal of experience with collimation spent an afternoon in his shop with my scope dealing with all the issues and turned it from a fair-to-middling performer into an outstanding scope.

I have a much better grasp of collimation now, and having more eyepiece time, appreciate better the difference between ho-hum performance and excellent performance.

FWIW - my advice to newcomers to the hobby: collimation is something you should eventually know something about, but don't stress about it in the beginning.

#24 Jon Isaacs

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Posted 16 July 2013 - 02:19 PM

I use nothing less than 70X per inch on a Focused Star to collimate mine.



How often does the seeing support the 1000x you seem to find necessary to collimate your C-14? There are few places in the world where such seeing ever exists.

It's worth understanding to understand is that there is no such thing as a "perfect" anything, a perfect mirror, a perfect lens, perfect collimation. When one collimates a scope, the goal is to be within tolerance...

In practical terms, an F/4 Newtonian that is miscollimated will be quite apparent at the eyepiece because the tolerances are close and the effects are soon seen. If the primary is misaligned so you are looking slightly off axis, about 8 degrees at 200x in a 10inch F/4, you are no longer in the diffraction limited region.

On the other hand, in a 10 inch F/6, things are considerably better, at 200X the you would have to be misaligned by about 18 degrees. These are small angles... 2.4 minutes of arc for the F/4, 5.4 minutes for theF/6.

Jon

#25 jrbarnett

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Posted 16 July 2013 - 04:49 PM

The answer is, "it depends". It depends on the design and focal ratio of the telescope, the nature of the targets being observed, and the overall optical quality of the telescope. Collimation is more sensitive and a given degree of miscollimation will have a greater impact on the view in a telescope with faster optics (SCTs are fast -the primaries are usually f/2; larger Dobs are usually fast too). Collimation is more sensitive with lower quality optics. All defects "stack". To enjoy reliably high quality images, your scope should deliver diffraction limited performance at the eyepiece. Defects that move the scope away from diffraction limited performance add together. A scope miscollimated a little bit but with otherwise perfect optics will produce better quality images than an identical scope with the same degree of miscollimation, but with lower quality optics. However, there's another side to optical quality. A scope with really poor quality optics could be so bad that with or without perfect collimation, it never achieves near diffraction limited performance. For such a scope collimation won't matter much, and you'd be better off chucking it or upgrading the optics if possible.

Target choice, too, dictates how stringent your collimation needs to be for best results. Everyone will notice moderate miscollimation in an SCT on a planet. Turn that same SCT to a DSO like the Omega Nebula, and you might or might not notice the effects of miscollimation. For dedicated lunar, planetary and double star observers, collimation is really important for getting the best performance possible on those targets. Your collimation can be a little sloppier if primarily you observe DSOs.

- Jim


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