Question about lightbridge 2ndarys mirrors

[irishbloke]

Just had a couple of hours playing with a friend's lightbridge. One thing seemed different from my skywatcher dob and that was the size of secondary.

His is 12" to my 10" dob. The image of the primary just fits on my secondary, were as the primary image on his had a couple of mm of space the whole way round the primary image.

Is this normal? Does this mean the lightbridges have a greater central obstruction? (not that it makes a huge difference!)

Just curious.

Thanks

Brian

[John]

A 12" F/5 scope will have a slightly larger secondary than a 10" F/5 scope as the light cone that the secondary intercepts will have a slightly larger diameter in the larger scope.

What matters is that the secondary is large enough to fully illuminate a 2" eyepiece but not oversize where it would cause unecessary loss of contrast.

Longer focal length scopes have proportionately smaller secondaries than shorter FL ones which will give a little more contrast - at the price of being somewhat unwieldy - an F8 12" would have a tube over 7 feet long !.

John

[irishbloke]

Thanks John

So it's normal for the primary not to fill the secondary mirror in a colli cap. Mines an f4.7 so that would make mine smaller again. I wasn't sure if the primary was wound to far back or something. It aligned well and got collimation well.

Brian

[John]

If it's working well then don't worry !.

Actually as the focal length gets longer, the size of the secondary can be proportionately smaller, eg: the secondary for a F/6 10" scope can be smaller than the secondary of an F/4.7 10" scope. Sorry if this is confusing but this time it's due to the light cone being longer in the F/6 scope and therefore narrower where the secondary intercepts it.

I hope I've got that right - it's confuses me sometimes as well !.

Having a primary wound back means that the collimation springs are under morecompression which helps hold collimation so it's not a bad thing.

John

[brianb]

So it's normal for the primary not to fill the secondary mirror in a colli cap.

Very much so ... you start to use some of that spare space for objects which are not in the centre of the field of view.

Being a variable star observer, I want an even field illumination, which means having a secondary mirror which is slightly "oversize".

Another way of looking at it is to consider light loss - if your secondary is 10% undersize, you lose the light from the outside 10% of the primary i.e. a 20% light loss. With a typical 25% by diameter secondary, if you make the secondary 10% oversize, the primary obstruction by area increases from 6.25% to 7.56%, i.e. a much much smaller loss of light.

There are definitely effects on contrast of having a large central obstruction, but these are typically vastly overstated.

When I had a 6" f/8 Newtonian, I changed the flat from an adequate 1.25" to a generous 1.75" - not only was any light loss completely insignificant, the planetary image quality actually improved as the new oversize flat was better quality optically.