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Dyptorden

Another Dob Collimation question

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Hi,

Last night I "collimated" my telescope (I am the third owner :), and this is my first telescope/collimation process ), using only a home made collimation cap.I made great progress I might say, since the donut was almost to the "middle" of the Top-Right quarter. Yet what is not right at the moment is that the focuser's reflection is a bit to the Bottom-Right quarter (as you can see in the attachment). Since it's reflection is to the right, can you confirm please that the secondary is moved too far from the primary? Also I don't find an explanation for the reflection being to the bottom.

Any ideas/advices for moving the reflection to the left and to the top would be of help.

Thank you.

P.S.: Sorry for the lack of drawing talents :)

post-40094-0-85459700-1420016280.png

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Astro Baby site might be down at the moment... or maybe it's not accessible from my location.I will try later.Anyway, what I have done until now is based on other collimation tutorials.I'm only missing something regarding that focus' reflection that is "moved" to the Bottom-Right in the secondary mirror. A direct answer would also be greatly appreciated :) .

Thank you,

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Hi Dyptorden the cross in the picture looks like the spider holding the secondary mirror in place, if this is the case then you should NOT use this cross for anything when aligning your optics, it is only there to support the secondary mirror. Pretend it is not there.

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Indeed, that is the spider.So, according to what you say, should I consider that the telescope is perfectly collimated although the reflection I speak of, is not well centered?

See the new attachment with more details.

post-40094-0-64899200-1420031898.png

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Are you using just a collimation cap or a cheshire or laser?

The best way to do the alignment of the secondary is by blocking the ota with a white piece of paper and then putting a contrasting coloured piece of paper in front of the focuser inside the ota 

When you have done this take a photo and upload it. If the gap around the secondary is even and the circle looks round then you are done with that. 

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Astrobaby's guide explains the slight 'offset' common to some makes of faster Newtonians.  My 12" Dob (F 4.9) collimates with a slight shift to the right of centre like your picture.

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Astro Baby website works now, and I saw the offset part.My ratio is 5.9 ... the Newts are considered slow starting with 6.That's why I am not 100% sure it's the offset.

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Astro Baby website works now, and I saw the offset part.My ratio is 5.9 ... the Newts are considered slow starting with 6.That's why I am not 100% sure it's the offset.

It can seem a little confusing I agree but if you dont try you wont learn, try a more basic guide without all the detail first. Loads of videos on it.

What tools are you using to collimate and why are you collimating? is there a problem or are you just checking?

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Well I just bought this telescope, and I am the third owner. Since I didn't know much about the collimation when I bought it, now I wanted to check it, and according to my collimation tools ( tools = collimation cap ) the secondary was too deep towards the primary and the donut was in the top-right corner.Now it's almost perfect.

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You may still see some offset, at the end of my guide I do caution against being over picky with collimation.  Most modern scopes will never show collimation a'la  the perfectly geometrical type drawings you may see.

Lost of things can serve to create slight errors in the view you are seeing, iffy focusers, slighty untrue edges to mirrors (some secondaries will struggle to show perfectly round) and a host of other ills.

If I were in your shoes I would say all is well and star test.

It may be your secondary needs to go up the tube slighlty.  Given most of these scopes have oversized secondaries I would suggest your collimation is most likely ok and you will not be suffering from image clipping/dimming at the edges at all.  Take the scope out and give it a whirl - my bet is its ok.

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Thers a guide at the end of my tutorial, dont worry about much other than checking if when centred on a star you get some even diffraction rings. Centre the scope on a bright star, polaris is good. Then slightky defocus. If collimation is ok you should get a perfect polo mint shape which is even all round. Really excellent airey disks to check optical quality are hard to get unless the sky and atmopshere are perfect.

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I find it a shame that so many guides to collimation never mention that fast Newt's - F/5  and such - will show an offset of the secondary when perfectly collimated. Your's does, and that's a rarity, sir. Pity.

Keep up the good work!

To A Cloudless New-Year,

Dave

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Thank you too, it is nice to hear it from two persons, now I can sleep well... in the first day of this new year :D

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About secondary offset appearance of slow versus fast scopes, the attachment show the difference. The offset is approximated by M/(4*F) where M is the secondary mirror size (minor axis) and F is the F-ratio.

The secondary mirror shadow as it appears from the focuser side will shift by the same relative amount. The percentage shift is (100/(4*F)). As you can see, slow scope will show smaller offset and it is not not as dramatic as many might think.

Jason

post-5330-0-61763600-1420153769.png

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Jason, those are very interesting comparisons there, old duffers like me may have had even slower newts in the past and thats probably why some collimation guides always show concentric circles, at f12 the circles probably do look concentric at least through the sorts of basic tools available years back when people were limited to colli caps.

Also older scopes tended to have somewhat smaller secondaries whixh would of course make it just that bit harder to see any offset.

Its good to see you posting again and giving such excellent help and advice.

Mel

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Hello Mel,

Optical illusion has a lot to do with how conspicuous the offset will appear for slow Newtonians. For example, if the 2" focuser drawtube has a shinny edge, the offset will be clearer. If the secondary mirror size is closer to the size of the focuser drawtube, the offset will also be clearer. My point from the last post is to emphasize that the secondary mirror offset will appear for all Newtonians regardless of F ratio but the offset appearance will differ based on many factors. Many seems to take it for granted that only fast scope will have an offset which is untrue.

Thanks for your wonderful collimation guide. It helped 1000s of beginners.

Jason

post-5330-0-86126000-1420224797.png

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