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Ledge1962

Collimation secondary mirror position

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Hi all,

I am new to collimating and have read a lot of conflicting posts, but I have given it a lot of tries. Can someone advise if there should be this much secondary mirror showing  in the view please? The black pipe is the chimney reflecting in the primary. It has been a frustrating few days so any advice/comments on this picture would be greatly appreciated. 

Thanks

George

mirror position.jpg

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I'm struggling to see where your secondary edge is although you look close. Can you hold a piece of paper between the secondary and primary and take the photo again?

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Just now, Moonshane said:

I'm struggling to see where your secondary edge is although you look close. Can you hold a piece of paper between the secondary and primary and take the photo again?

Hi. will this be any different from the regular collimation we do using the laser colimation? 

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I think you are seeing so much secondary is because your focuser is racked to far in.

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Looks good but the only way to be 100% sure is to do it with an artificial star, pinhole in front of a flashlight. Defocus a little so you see two or three rings around the central dot (at high power). When the dot is centered collimation of the mirrors is perfect.

Checking the centering of the mirror mark relative to tubes, vanes and mirror clips is not foolproof because of mechanical tolerances. However, relying on the diffraction pattern guarantees you'll have perfect adjustment of the mirrors relative to each other, not relative to other things that have no role in forming the image.

Remember it's collimation of the optics you want, not collimation of mechanical parts. For the same reason lasers are a good approximation but they are useless for fine tuning because they tell you nothing about where the central dot is in the actual star image. 

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2 hours ago, Ledge1962 said:

a lot of conflicting posts

Hi. It maybe a good time to review common myths surrounding this process. HTH.

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21 hours ago, Moonshane said:

I'm struggling to see where your secondary edge is although you look close. Can you hold a piece of paper between the secondary and primary and take the photo again?

 

19 hours ago, paul mc c said:

I think you are seeing so much secondary is because your focuser is racked to far in.

The picture is the same when the focuser is fully in or out

19 hours ago, Ben the Ignorant said:

Looks good but the only way to be 100% sure is to do it with an artificial star, pinhole in front of a flashlight. Defocus a little so you see two or three rings around the central dot (at high power). When the dot is centered collimation of the mirrors is perfect.

Checking the centering of the mirror mark relative to tubes, vanes and mirror clips is not foolproof because of mechanical tolerances. However, relying on the diffraction pattern guarantees you'll have perfect adjustment of the mirrors relative to each other, not relative to other things that have no role in forming the image.

Remember it's collimation of the optics you want, not collimation of mechanical parts. For the same reason lasers are a good approximation but they are useless for fine tuning because they tell you nothing about where the central dot is in the actual star image. 

I did the star check and got a perfect black circle dead centre of a grey circle, but never saw rings in it. I will try again and see if I can snap a photo.

2nd mirror.jpg

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Thanks all for taking the time to respond. I am worried that the secondary mirror shows more of itself than other peoples photos do. Hopefully I am over reacting, but after spending $5k (NZ prices are wicked) I would like to get it right, or return it if it is not... The sky here is awesome during the summer.

Of note, I did try the laser collimation and it said it was good.

 

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21 hours ago, Ledge1962 said:

Can someone advise if there should be this much secondary mirror showing  in the view please?

Yes, the amount of secondary mirror/secondary mirror holder showing is fine.

There are only two rules for collimating the secondary mirror. 1) It needs to be circular when looking down the focus tube and 2) This circular aspect needs to be central in this view. As @Moonshane suggests, if you didn't do it while adjusting the secondary mirror, a piece of paper behind the secondary mirror and blocking off the primary mirror helps to see the secondary mirror shape and position.

Looking at your picture, if I was going to be fussy then perhaps it's not quite central? It appears to my eyes that there's a bit more distance from the top of the secondary mirror to the focus tube edge than the bottom. I don't know how critical this is only that if I saw this in my telescope I would try to adjust it.

A word of warning though - sometimes a small adjustment done badly on an almost perfect setting can throw it out completely and you have to start again. I suppose it depends on how easy it is to make the adjustments on your particular telescope. 

On the other hand you can always try a star test with the telescope as it is and see what the views look like.

This is many people's go to reference for collimating a Newtonian reflector:  

Astro Baby's Collimation Guide.pdf

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Hi Alacant.

a very important omission sigh... it is the Orion xx14g. I should have started with that. 

The Star Test is shown below. The seeing is pretty poor tonight but this is what I managed. I do not see any other rings (posts mention several), but there is traces of the spider vanes if zoomed in. 

I also snapped a shot of the primary showing the black dot of the cap inside the donut, but i am not sure about the offset of the secondary mirror or if it is tilted properly.  If zoomed in on the picture, you can see the fucuser tube off to the upper left slightly. Not sure if this means the secondary is still off so any suggestions on how I recentre the focuser (or if I even need to)  would be appreciated.

 

star test.jpg

primary mirror.jpg

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You don't need to adjust the focuser. It can appear in the view if it is screwed in far enough. It does with my scope anyway.

With regard to the star test and seeing rings: you need to use a very high magnification, so use your shortest focal length eyepiece and a Barlow. I use 300 to 400x magnification. It might also help, if the magnification is too low, to only slightly defocus the view of the star. You can get the view in your picture if you defocus too much. If the magnification is small then the ring pattern maybe quite small.

Edited by David Levi

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3 hours ago, Ledge1962 said:

Orion xx14g.

Ah, fine. I'd recommend a Cheshire sight tube with cross hairs to get as near as damit each time you setup. Otherwise, I'd say you're good enough to go for visual. It should be real treat:)

 

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3 hours ago, Ledge1962 said:

The Star Test is shown below. 

 

star test.jpg

 

This is not a star test, but a wildly out of focus telescope. To do a star test you start with a perfectly in focus star exactly in the centre of view and defocus by a tiny amount, a couple of mm focuser travel at most. 

As for your initial query about the amount of secondary visible, I suspect this is because you have used a collimation cap instead of a sight tube. The walls of the sight tube will narrow the view as seen in other photos. A sight tube is the correct tool for collimation of the secondary mirror. 

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Thanks all, your assistance and patience has been awesome. I underestimated how much was involved in collimation as I have been using a 10" sct. 

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