Central Obstruction Diameter

[bazza]

Not sure where to put this so the Lounge seems best!

Central Obstruction Diameter

How do I work this out & what exactly does it mean?

I've got an awful feeling, I should know this!

Another Q: Maxim wants the Obstruction as a percentage of Diameter?

& does it make a difference if a Focal Reducer is fitted?

Thanks all.

Barry.

[GazOC]

Just measure the obstruction caused by the secondary mirror holder with a ruler and work out its pecentage of the aperture of the scope. A focal reducer won't make any difference.

If you have a 8" (200mm scope) scope and you measure the obstruction caused by the secondary mirror holder at 40mm then the percentage that maxim wants is 20%.

HTHs

[barkis]

Forgive me for butting in, but I just want to say the measure should be taken across the minor axis of the secondary.

I know full well you meant that Gaz, but am was not sure Bazza would have. If you do Bazza, then apologies to you both.

Of course if Bazza measures the diameter of the actual cell, then my interference is superfluous altogether.

I am reaching for my coat right now.

Ron.

PS. I hope your scope is not the SCT. Baz.

[bazza]

Thanks both - Ron it is the SCT!

Me thinks the Central Obstructive Diameter of the 10" LX200R is 90mm which makes it about 35% in my book.

(I'm dying to be corrected!!)....PLEASE!!

Barry.

[GazOC]

That sounds right Barry.

The obstruction on a SCT is circular, otherwise as Ron said, the minor axis is measured.

[Astroman]

Well, even with a Newt, the effective obstruction of the secondary is circular, with a tiny, tiny straight line contributed by the spider.

[bazza]

So what would be the Central Obstruction Diameter of my William Optics Zenithstar ED80II APO be?

Can't find any info on the William Optics website & how could I possibly measure it?

Thanks for all the help on this.

Barry.

[James]

There shouldn't be a central obstruction in the Zenithstar ED80II APO!

[bazza]

Now I feel stupid!! :shock:

Barry.