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16" dob

todd8137

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Moonshane

Moonshane

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which 16" truss dob is it?

a secondary circle is easy to put on - just put one on?

the secondary might need the odd tweak but not a great deal, maybe nothing.

the primary will need a tweak each time but so does my solid tube 16"

it's really nothing to worry about.

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jetstream

jetstream

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How about cutting out a template (paper) the exact size of the mirror,then fold it in 4 carefully-unfold,center is at the junction.Put little hole in paper at its exact center,carefully place on mirror and Sharpie the hole in paper.Then place center dot over the marker dot.

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todd8137

todd8137

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Hi all thanks for your comments it's a Oldham optics mirror set ,and it's a wooden affair will get a image ,all points taken in a will say this it's a lot bigger than a thought maybe two big eeeek had a quick look tonight as we had a small clear patch me and a friend stuck it together and to says it's not colaminated ( spell checker strikes again) i thought the orion neb and a few clusters looked good in my sct ,but boy o boy I nearly cried the views just where literally out of this world pardon the pun

I will try the clear plastic method ,and the paper one ,also I it did not come with the normal donut .

Pat

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Ptarmigan

Ptarmigan

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Hi, interesting thread, I've just been reading up on collimating and spots and things.

I have come across various websites saying that the mechanical/physical centre of the mirror is maybe not the optical centre for purpose of collimating everyting to be on the optical axis.

Is this a problem ? Any data on how far these spots may diverge ? Does it matter much ?

Is it a hangover from 'the olden days',, are modern production processes less liable to such an offset ?

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Ptarmigan

Ptarmigan

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here's one

Drool !

Moonshane >

Ok on the "middle" ! Me too but that was when I was building a 6" f8 in days of yore before fancy lasers and cheshires and whatsits, yep, just lined things up more or less concentric,

6" then was BIG :)

but now at the f ratios of these monsters should we be concerned about these wee details ?

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Moonshane

Moonshane

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Drool !

Moonshane >

Ok on the "middle" ! Me too but that was when I was building a 6" f8 in days of yore before fancy lasers and cheshires and whatsits, yep, just lined things up more or less concentric,

6" then was BIG :)

but now at the f ratios of these monsters should we be concerned about these wee details ?

collimation is a means to and end and needs to be good enough to give nice sharp images at the eyepiece. other than that don't worry too much, just enjoy the scope. I don't worry about things I cannot measure/affect/see at the eyepiece  :smiley:

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Ptarmigan

Ptarmigan

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collimation is a means to and end

I cannot measure/affect/see at the eyepiece  :smiley:

Couldnt agree more : !

But :) hehee

If we are going to go to the bother of marking the centre to aid us I just wondered if there was a way of marking the optical one you see.

If there isnt, or if it isnt significant, ( cos the webpages I've seen never address this) then I'll go back to sleep !

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todd8137

todd8137

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When I had the 12" I used a Cheshire every time I have read the pro's and cons on both going to have a ride to Rothervalley optics ,in the morning and see what they have a telrad would be nice as well might have to hit the wanted section ( I no these are getting hard to get at the min )

Pat

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John

John

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Ok on the "middle" ! Me too but that was when I was building a 6" f8 in days of yore before fancy lasers and cheshires and whatsits, yep, just lined things up more or less concentric,

6" then was BIG :)

but now at the f ratios of these monsters should we be concerned about these wee details ?

The collimation "sweet spot" around the optical axis at F/8 is a comparatively generous 11mm wheras at F/5 it's dropped to 2.8mm and knowing where the optical axis is seems somewhat more important !

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Moonshane

Moonshane

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The collimation "sweet spot" around the optical axis at F/8 is a comparatively generous 11mm wheras at F/5 it's dropped to 2.8mm and knowing where the optical axis is seems somewhat more important !

the section at the end of this article suggests how to test your optical vs physical axis position http://www.skyandtelescope.com/howto/diy/3306876.html

I have never bothered though as I say as the results at the eyepiece are good enough for me.

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