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Adam1234

Which to trust most: collimation cap, cheshire eyepiece or laser?

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Hi all, so following on from my previous post on help with collimation, I have taken advice and bought a collimation cap which is much easier to use, and I can actually see the mirror clips (which I couldn't with Cheshire).

Below is a picture of my efforts using the collimation cap, please let me know if this looks good...

 

20200214_190343.thumb.jpg.a7d151342eed0dd00a133a54d5bd6535.jpg

But... if I then put in the cheshire, it then looked like this:20200214_190003.thumb.jpg.498262f8c88eac0aca6be1c394aa9319.jpg

 

However... what the cheshire shows seems to depend on how I tighten up the screws on the eyepiece holder and i can put it in again or apply some movement to the cheshire and end up with this:

20200214_190044.thumb.jpg.2ce5a97e35a1bafafaae7c254a190c2a.jpg

Then if I put in the laser it also shows to be off, but again the laser shows up in a different spot each time putting it in and if I move it slightly or change the way I tighten the screws.

So the question is, which method is correctly showing how my mirrors are aligned? Is the view through the cap correct? And is my collimation ok?

 

Thanks Adam

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Welcome to the world of collimation!

Every instrument should agree if the instruments are accurate. Try pulling the cheshire back out a bit to see the clips. Your correct about tightening the focuser screws vs sec alignment. In loose focusers I note how the laser/crosshairs go and tighten in that order.  This function of collimation centers the collimated light beam in the exact centre of the eyepiece. If off you might notice a sharper lunar view off center for instance, depending on the speed of your scope.

Note that the cheshire primary function is NOT affected by focuser slop but un barlowed laser primary collimation is. You can use your accurate laser for sec collimation and then the cheshire for primary alignment.

Works for me down to f3.8...

Gerry

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After mucking around with lasers, caps, cheshire, i always found that they don't really agree with each other, some may disagree with my method but, it saves your hairline. Using the cheshire, i will check to see the secondary is entered and, looks like a nice circle, chuck the cheshire in the door after that. Then i'll just align the primary with a cap, once i'm out in the field ill use a high power eyepiece to fine tune the out of focus star into a neat circle both plus and minus ends of focus. If things look great to you, don't fuss anymore, i have lost sleep over collimation, its not worth it. Your first image with the cap looks pretty darn good to me, i would check it under good seeing with a high power eyepiece, it is the ultimate test.

Edited by Sunshine
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38 minutes ago, Adam1234 said:

Hi all, so following on from my previous post on help with collimation, I have taken advice and bought a collimation cap which is much easier to use, and I can actually see the mirror clips (which I couldn't with Cheshire).

Below is a picture of my efforts using the collimation cap, please let me know if this looks good...

 

20200214_190343.thumb.jpg.a7d151342eed0dd00a133a54d5bd6535.jpg

Thanks Adam

You're barely off on primary collimation.  I generally can't perceive any difference between being in that state and tweaking the center dot to the exact center of the O-sticker with an f/6 scope, so I'd call it good enough for most observing.

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Best tool bar none for setting up the secondary is a Concentre. I now consider it an essential for setting up a newt. Using a Cheshire will get you somewhere near but not spot on. Once you’ve used one you just can’t go back to a Cheshire.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5506_TS-Concenter-2--colimation-eyepiece-for-Newtonian-Telescopes.html

After the Concentre then I use a Hotec laser. Surprising how much having an accurately positioned secondary helps with the colimation.

Edited by johninderby
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I use a cheshire eyepiece followed by a star test. They tend to agree whereas the laser collimator that I have (well collimated) does not. I don't use the laser much now.

 

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Hi . I use a good old collimation cap to set up my reflector.

But a good old "Star test"  will show you the real test of how well your scope is adjusted

 

 

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1 hour ago, Sunshine said:

 I have lost sleep over collimation, its not worth it. Your first image with the cap looks pretty darn good to me, i would check it under good seeing with a high power eyepiece, it is the ultimate test.

I definitely lost some sleep over it the other night, I was up until about 11 PM attempting and failing to collimate, and was miserable the whole of the next day due to the lack of sleep and the frustration. At least I'm getting somewhere now though. 

 

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I was just looking at the 1st photo that you posted in this thread. The collimation there looks goon enough to be getting most of the performance potential from the scope I would think.

I think the problem with collimation is, I think, many of us suffer to some degree with forms of mild OCD and we agonise about getting it "perfect" when scopes that are "near enough" will still show very nice image.

 

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Collimation is a complex process. What you need and how to do it depends on what stage you are at. 

The aim is to get all the components lined up on a common optical axis. This includes the focuser, secondary mirror and primary mirror.  The secondary does not have a unique optic axis and nor will a focuser if the tools or eye pieces can be offset with a locking screw or band.

Even a star test cannot tell if the secondary is aligned properly. 

The more you understand the process the better it gets.

I used a sight tube to align the focuser and center the secondary under it.  Then a Cheshire to align the primary followed by an auto collimator for fine tuning the secondary.

Once collimated try it in different orientations to see if it stay collimated (the auto collimator is very sensitive). If it shifts too much find out what is moving and strengthen it.

If all this is too much trouble the advice in the posts above is fine.

Regards Andrew 

 

Edited by andrew s
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However... what the cheshire shows seems to depend on how I tighten up the screws on the eyepiece holder and i can put it in again or apply some movement to the cheshire and end up with this:

What you need is a centring adapter, it fits to the focusser and the cheshire by an expansion method so you are not relying on 2 bolts to push the Cheshire to one side.  

Carole 

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2 hours ago, carastro said:

What you need is a centring adapter, it fits to the focusser and the cheshire by an expansion method so you are not relying on 2 bolts to push the Cheshire to one side.  

Carole 

I'll look into that, thanks Carole

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In that any and all eyepieces are pushed over to one side when securing with thumbscrews, then it would follow that the Cheshire, cap, or laser should also be pushed over to one side, the same side, when collimating.

That will ensure that the centres of the 1.25" eyepieces will correspond to the centres of the secondary and primary mirrors.

You can certainly use a centring-adaptor, but the adaptor will be required when using the 1.25" eyepieces as well, if you want everything spot on.  It's not so much an issue with 2" eyepieces, as those are at the lowest powers.  Rather, it's at the higher and highest powers where the collimation needs to be precise.

Edited by Alan64
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One thing to bear in mind is that the tolerance for collimation error is greater the slower the scope.

For example, in an F/8 newtonian the so-called "sweet spot" for collimation is 11mm in diameter around the optical axis whereas for an F/5 it shrinks to just 2.8mm. The upshot of this is that fast newtonians need to be collimated that much more accurately if they are to be able to achieve their theoretical performance potential.

More on this here:

http://web.telia.com/~u41105032/kolli/kolli.html

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When I had my reflectors a 150p and a 200p both F5 scopes (fast) I had a laser threw that in bin I used a Cheshire/Sight tube never had a problem with mine using it always well collimated. 

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I bought a laser collimator for my 200P dob and after checking the laser for accuracy I completed collimation in about 10 minutes, maybe I was just lucky.

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12 hours ago, Alan64 said:

In that any and all eyepieces are pushed over to one side when securing with thumbscrews, then it would follow that the Cheshire, cap, or laser should also be pushed over to one side, the same side, when collimating.

That will ensure that the centres of the 1.25" eyepieces will correspond to the centres of the secondary and primary mirrors.

You can certainly use a centring-adaptor, but the adaptor will be required when using the 1.25" eyepieces as well, if you want everything spot on.  It's not so much an issue with 2" eyepieces, as those are at the lowest powers.  Rather, it's at the higher and highest powers where the collimation needs to be precise.

those pesky thumbscrews if using stock focuser adapter pinches the eyepieces/collimator  I changed my 150pds to the adapter fitted with compression ring get's me a better collimation also my eyepieces don't get marks on them from the thumbscrews

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On 14/02/2020 at 19:23, Adam1234 said:

However... what the cheshire shows seems to depend on how I tighten up the screws on the eyepiece holder and i can put it in again or apply some movement to the cheshire and end up with this:

Three words for you...

Howie Glatter Parallizer!

One of the best astro things I've ever bought

 

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Not sure what some members above meant by star testing in this thread. Do they mean "checking" or "collimating" with a star test? 

In my opinion, I wouldn't suggest the latter, particularly with manual driven mounts. Regarding the former, the procedure can be rather complex as it is easy to get errors due to other factors which don't have anything to do with misalignment. Of course, if one knows how to star test, the feedback given by this tool is incredibly useful in order to understand what does not work properly in a telescope.

 

Regarding the importance of collimation, well Suiter's book on star testing offers some quantitative data on this subject. Of course, a slightly miscollimated telescope still works. Said this, if one pays for premium optics and is happy to have an "okay-ish" collimation, the same person should reflect that some okay-ish optics with excellent collimation could probably give the same results in terms of view quality.

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35 minutes ago, Piero said:

Not sure what some members above meant by star testing in this thread. Do they mean "checking" or "collimating" with a star test? ....

 

In my case, I use the star test to check that the collimation is good following adjustments that I might have made using a cheshire eyepiece. I agree that trying to actually collimate using a star test can be a trying process.

 

 

 

 

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I'm curios about how everyone does the star test? The only accurate way I know of is using very high mag that needs VG seeing to do. I don't think using the secondary shadow is that great a method.

Edited by jetstream

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6 hours ago, jetstream said:

don't think using the secondary shadow is that great a method.

I agree.  You're only 'eye balling' the shadow essentially, unless using a camera to get an image of the defocused star which you can overlay concentric rings onto? I dont like to be wasting precious clear-sky time with star collimation, much better to get it sorted inside in poor weather.

 If I'm out observing and the image looks rubbish I'll probably defocus on a star to see if anything is wrong but i'm usually looking for thermals rather than collimation

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