secondary mirror

[Curt]

Hi
I'm just wondering if I can use a 3.10 secondary mirror in my 16 inch lightbridge. Or will l be losing light if l do. Or is it a bit of a trade off between obstruction and light gathering.
Cheers.

[Robh (Rob)]

Hi Curt,

From what I can find, the standard secondary seems to be 3.5 inches or about 8.9cm. Let's say it is 0.5 cm oversized all the way around. That means 7.9cm is in use. The area of light lost at the centre is negligible compared to the area of the primary mirror. Maybe only 1%.

The secondary you want to use is 3.1 inches or about 7.9cm. You would be extremely lucky to have all the light fall on the mirror. The area of light potentially lost at the edges of the primary is far more significant than that above. If your light cone was offset by 0.5cm from one edge of the secondary, you would lose the outer 1 inch off the side of the primary mirror. Roughly 10% of your light.

Better to lose a bit of light at the centre of the primary than at the edge where the area is larger.

Regards, Rob

[Curt]

Thanks Rob
That makes perfect sense to me now. I was getting lost in all the information from the internet to make a decision. I had an offer to buy a nice piece of glass. Guess l will have to knock back the offer.
Cheers Curt.

[glend (Glen)]

The secondary in the GSO 16" is 88mm.

[Curt]

Thanks glen.
I'm pretty sure they are the same as the lightbridges.

[MattT]

There is a design program called Newt of Web. Google it and give it a try. It will give you all the info you need to know…hopefully.
Matt

[Curt]

Cheers Matt.That is a very comprehensive page. I have just saved it under my favourites.
Cheers for that.