Collimation 'models' at different f-ratios

[A McEwan]

Hi all,

In light of an incoming f5 12" Dob, I'm curious about the different 'models' of collimation and how they shoul be applied to different f-ratio Newtonians.

I've collimated my 8" f6 to the standard or classical(?) model where everything is concentric with no offset, and it is quite easily done.

The 'offset' model looks a bit trickier to i9ntroduce and maintain, especially for someone who hasn't used it before. Is is required for an f5 scope?

If I were to collimate using the standard model instead, what difference would I see at the eyepiece?

Thanks,

Ant

[Astro_Baby]

You can't in fact collimate a fast scope with all the circles concentric. What happens is that you either have concentric circles but the secondary is not centred to the focuser OR you have a secondary centred to the focuser and offset showing.

Don't be over concerned about offset, this is something which baffles a lot of people but in practice it's actually very simple. Centre the secondary under the focuser as you would with your existing scope, align the secondary to the primary (the bit where you get all the primary mirror clips showing), then a dust the primary to get the centre spot aligned.

The offset will be taken care of in the procedure, all that happens is there is not complete concentricity of the view through a Cheshire. Take a look at my collimation guide here

Astro Babys Guide to Collimation

The final few pics show you the difference in view through the Chesire, in all other respects the procedure is the same whatever F ratio the scope is at.

Hope that's of some help.

[A McEwan]

That sounds very reassuring. So basically the offset is built-into the scope because of the nature of the mirrors (f-ratio) and the laws of physics? Nice!

Great guide by the way AB!

Ant

[sixela]

Yup. Don't worry about offset. Simply centre the secondary's outline from the vantage point that makes the primary reflection almost as large, point the reflected focuser at the centre of the mirror (the most precise methods use a centre spot plus either a sight tube or combo tool with cross hairs or a laser collimator), and point the optical axis of the main mirror back at the centre of the focuser on the focal plane (with either a Cheshire or a barlowed laser).

Any offset is implemented automagically.

The only reason to care about mechanical offset away from the focuser is if you have digital setting circles and have a need to have the optical axis end up parallel to the tube axis (or rather, perpendicular to the last rotational axis of the mount).

[A McEwan]

It's also very reassuring to have Sixela say it's easy and automagic! Phew... <heaves sigh of relief>

Thanks for the good news!

[Astro_Baby]

you did not trust the queen of collimation then

[A McEwan]

Oh I certainly did AB - but I've also seen lots of Sixela's Newtonian related posts around and it's always nice to see the two top experts agreeing with one another!

<phew...>

[Astro_Baby]

actually I had the same hassles coming back to astro after more than25 years. Back then all scopes were slow and a collicap was the standard way of doing it. The F5 scope absolutely refused to be concentric as is their way but being a muppet and out of practice it took a lot of fiddling, reading and endless 'help' posts to Jason Khadder on CN before I got it sorted

When I was happy I did the guide so other muppets like me would have a reliable simple guide to it all rather then the massively technical guides I had struggled with. Vic Menards guide is a good read but I mus confess a goodly part of it went over my head

I'm really more of a meddler than theoretician

[A McEwan]

Consider me a "slow" Newt to "fast" Newt muppet too then!