Collimation offset - why?

[Bart]

Hello all

ive never offset the secondary on either of my Dobs 10" and 16". 

I've often thought I got great sharp images of the plants at high mag 250-300 sometimes. 

So I wondered, what am I missing out on. Is it a case of not being as sharp as they can? Or that I'm losing some light and the views could be brighter? Or something else? Thanks.  Barry

[Scooot]

Well I could learn something here too Barry.  I didn't think the secondary was physically offset to one side. I always thought it just looked slightly offset with fast optics. 

[Scooot]

Ah here you go, this explains it. I have learnt something

http://www.skyandtelescope.com/astronomy-equipment/offsetting-your-secondary-mirror/

[John]

On the newts I've owned the offset is built into the way the secondary holder is designed and the secondary is mounted so you don't need to play with the support vanes etc to get it.

 

 

 

 

 

 

[Dave In Vermont]

Our illustrious member here - Astro_Baby - incorporates good images of the offset in regular v. fast Newtonians in her guide to collimation:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Hopefully this will help prevent someone from pulling their hair out fretting over it.

Enjoy!

Dave

[Mr Spock]

Unless you are constructing your own tube/spider it shouldn't be an issue. Manufacturers sort this for you - though it is always good to know how things work

[astronymonkey]

Evening bart,I used to have the same thoughts but my LB16 does a fine job so I presume its built into the design somehow. Cracking scopes though and reasonably portable given the size! Cheers

[niallk]

Hi Barry, good question. I believe that the secondary offset is built into my 250px, whereas in contrast Dave Kriege  recommends centre spotting the secondary on Obsession scopes - many will disagree, but seems to be a pragmatic simplification for visual use.  I  think the tradeoff has to do with fully illuminated field.  I have followed DK's recommendations on my scope, fwiw.

-Niall

[Bart]

Thanks all. I think I can say 'phew' so! 

One less thing to mess around with. 

But just to check, if I look down the focuser with a Collimation cap, I should see something? Probably, maybe. ?

[jetstream]

My 15" has no secondary offset away from the focuser- I chose this when ordering the spider etc. For visual I understand that a primary function of transverse offset is the accuracy it provides with digital setting circles. The offset also plays a role in choosing secondary size and the desired fully illuminated field.

http://www.lcas-astronomy.org/articles/display.php?filename=newtonian_secondary_offset&category=telescopes

http://www.loptics.com/ATM/diagonals.html

 

[Dave In Vermont]

3 hours ago, Bart said:

Thanks all. I think I can say 'phew' so! 

One less thing to mess around with. 

But just to check, if I look down the focuser with a Collimation cap, I should see something? Probably, maybe. ?

The first thing you should notice (this is addressed in the guide from Astro_Baby) is the 3 mirror-clips retaining the primary (BIG mirror). Then you'll notice any offset.

Dave

[mapstar]

The faster the scope the bigger the offset is and many scopes above F5 you probably wouldn't notice much 

Its all to do with the light cone and how the secondary intersects it. I have an F3.4 scope so there is a 9.5mm off set away from the focuser which equates to a diagonal shift of 14mm. 

Here's Nils Olof Carlin's take on it as posted earlier  by Scooot

http://www.skyandtelescope.com/astronomy-equipment/offsetting-your-secondary-mirror/

[John]

I can see the secondary offset when I view through a cheshire eyepiece down the drawtube of my 12 F/5.3 dobsonian.

[andrew s]

As the Sky & Telescope article shows you don't need offset but the cost is then that the optic axis is not parallel to the mechanical axis. As stated above this can cause pointing errors due to the "cone" error it introduces.

One key pint that is often missed is that the secondary mirror need to be centered over the focuser tube accurately. As the secondary mirror has no unique axis it can't be set using either a Laser collimator or a Cheshire eye peice. You need a sight tube or a spot on the secondary mirror. I much prefer the former as the latter is easy to get wrong and causes some light loss.

 

Regards Andrew

[John]

19 minutes ago, andrew s said:

...One key pint that is often missed is that the secondary mirror need to be centered over the focuser tube accurately. As the secondary mirror has no unique axis it can't be set using either a Laser collimator or a Cheshire eye peice. You need a sight tube or a spot on the secondary mirror. I much prefer the former as the latter is easy to get wrong and causes some light loss.

 

Regards Andrew

Good point Andrew. I use the sight tube from the Tectron Tools set for setting the secondary centering.