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Secondary size ??

Started by Lucifigus, 04/15/2002 05:43AM
Posted 04/15/2002 05:43AM Opening Post
I have a used, but newly acquired 12.5 f/6 dob. It has (to the best of my abilty to figure) a secondary size of 2.6". It occurs to me that this may be larger than necessary, but finding easy to understand information on making an absolute determination on secondary size seems difficult.

NEWT (the software) indicates I can get a full field illumination with a secondary of 1.83". Other designs of similar size have used 2.14".

My question is...What is the smallest secondary I can use for a fully illuminated field for a UO 40 mm Konig (AFOV 70*)? Is this different from what would be considered to be the optimum size for the same result?

Any help will be appreciated.

Posted 04/15/2002 08:31AM #1

I'm sure you don't want a FULLY illuminated field in a 40mm Konig unless you're only going to do photography exclusively. Even a 2.6" diagonal will have some dropoff in illumination at the edge of a wide true field.

Selecting a secondary size unfortunately requires you to make compromises. Do you want excellent planetary performance or are you exclusively a DSO observer? I'm interested primarily in planetary targets and globular clusters, so I tend to prefer a secondary under 20% of the primary size, and I'm more than willing to accept a drop of 50% or even more in illumination at the edge of a wide true field to get it.

It depends on the focuser, specifically the distance from the axis to the focal plane, but I think I personally would go with 2.14" in this case. That's a 17% obstruction, and going lower is not likely to help planetary views much (and might hurt them if the edges of the secondary aren't as good as its center). It also probably gives about 60% or more illumination at the edge of the focal plane accessible to a 40mm UO MK-70 Konig. IMO, that's fine.