effect of reflector secondary size

[Bender]

I bought a s/h Orion Optics SPX 200 f/4.5 some time ago. The original owner bought it with a larger secondary for ap (73mm instead of 63mm).

Me being a perfectionist, I wonder now if the larger secondary will have a noticeable effect for visual use. Or wouldn't there be a big difference, especially considering the never perfect London suburb sky.

[NGC 1502]

As long as the scope is cooled and collimated, the visual views will be limited

mainly by the state of the atmosphere. Any theoretical problem caused by the

larger secondary would be very hard to see I think.

It's easy to get bogged down in optical theory. Some worry about loss of contrast

from a larger secondary, others worry if the whole of the secondary is used and the

edges are not good enough..........!

I'd just relax and enjoy the views.

Regards, Ed.

[Bender]

Thanks for putting my mind at rest Ed. Yes it would be sometimes better not to read to much and simply go out and enjoy the views.

Unfortunately the weather leaves me with to much time lately

[brianb]

Oversize secondary - wider field of view without vignetting (+/- essential for visual variable star work, very useful for imaging with a medium / large chip) - very small reduction in contrast, may possibly be noticed on planets in exceptionally good conditions but really nothing to worry about

Undersize secondary - light collected by the outer part of the primary mirror will not come to the eyepiece / imaging chip i.e. aperture will be reduced. This will a much more serious effect on image quality than a slightly oversized secondary and furthermore will be an issue in all viewing conditions.

[Moonshane]

hi there

I hope Brian can answer this related question?

I was beginning to worry that my 25mm secondary was a little small but running the date I have through NEWT it appears to confirm I have it right based on my own understanding of the outputs below?

I can change the secondary to 24mm but not 23mm before I get to the point where the 'secondary too small to get 100% illumination' goes to YES so it seems my system is pretty spot on for planetary?

I can certainly see the whole of the primary (just) at the focus point so I was working on this assumption too?

cheers

Shane

sixinchf11innewt.pdf

[sixela]

I thought we discussed it in another thread. "Die hard planetary" would be with the focal plane a lot closer to the tube so that you could have a 13% central obstruction, but whether it's worth it to change anything is, of course, quite another matter.

The solution to making the "secondary is too small" etc. item toggle to YES later in Newt --so that you can use a smaller secondary-- is to have a shorter focuser and/or focuser travel punched in Newt as well.

[Moonshane]

cheers Alexis

true re the other thread - I always like to get 2nd, 3rd, 4th etc opinions on everything before I decide on changes. I think all things considered I should follow the advice which seems to be that for visual it's unlikely to make a real difference and as Brian said, I should just get on with it and enjoy the view