Secondary mirror offset for a newtonian reflector

[Eric Watkins]

I’m just in the process of re-building an 16″ F6 Newtonian reflector.

With regards to the secondary mirror offset.

Is the spider and mirror mounting centrally placed and the mirror offset applied when  gluing the secondary mirror to the flat?

If I’m using the telescope for deep sky work imaging  and planetary imaging where the imaging area is relatively small,

i.e. 12mm x 15mm for the deep sky work, is the offset that important?

Thanks, Eric

[Martin Mobberley]

Hello Eric,

>Is the spider and mirror mounting centrally placed and the mirror offset applied when gluing the secondary mirror to the flat?<

Yes. See the pic of the front end of Mike Sidonio’s Orion Optics AG12 which clearly shows the arrangement. This is roughly f/4, so an extreme case:        

http://www.pbase.com/strongmanmike2002/image/132998032

To be honest, at f/6 you won’t notice a lot of difference, assuming the secondary is big enough, especially on such a modest imaging circle. Minor vignetting can be flat-fielded out.

Place your eye at the distance where your CCD sensor will be. Does the secondary show you the whole of the primary with a bit to spare? If so, fine.

For planets, assuming the telescope is collimated and cooled down, seeing conditions are the biggest factor by far. Everything else is pretty trivial!

Martin

[Eric Watkins]

Martin,

thank you, clarifies that matter for me.

This is going to be a trial run for a 20-22″ F:5.5 Newtonian, Surrier Truss mounted on a Paramount ME for CCD deep sky imaging.

Cheers Eric

imaging

[Richard Miles]

Good luck with the project, Eric

[Author]

Posts