Secondary Mirror Questions...

[morls (Stephen)]

Hi,

I'm starting to think further about my 12" project, and have got to the point of looking at the secondary mirror and focuser. My primary is a GSO 12", which is currently being recoated. The GSO website lists the aperture as 304mm, which comes out as 11.97" and the focal length as 1500, which makes it an f4.93.

My main interest for this scope is deep sky, not planetary.

Using "Newt for the Web", I'm trying to find the best size for the secondary. I want to get a high quality mirror, so I'm looking at Antares Optics. It seems there are 2 options - 2.14" and 2.6". Ideally I'd like to go somewhere in the middle, but anyway...

If I go for the 2.6" (66mm), and a low-profile focuser, I get the following:
22% obstruction
Angular FOV for 100% illuminated area = 0.794º. Diameter = 20.66mm
Angular FOV for 75% illuminated are = 1.7176º. Diameter = 44.7mm

If I go for 2.14" (54.4mm):
18% obstruction
Angular FOV for 100% illuminated area = 0.2594º. Diameter = 6.75 mm
Angular FOV for 75% illuminated are = 1.2258º. Diameter = 31.9mm

So, I'm thinking the 2.6" is the better option.

What I'm wondering is:

If I have a large 100% illuminated field, does this mean I am 'wasting light' if it exceeds the field stop of my largest eyepiece?

As always, any advice would be greatly appreciated.

Cheers

Stephen

[Satchmo]

Hi Steven

For an F5 scope the light drop off is fairly gradual. A 6mm fully illuminated field is standard if you want to minimise your central obstruction. A 1/12 wave Antares should be adequate . The optical system is only good as its weakest link- so I assume your mirror star tests well and is worth pursuing a smaller central obstruction.

[morls (Stephen)]

Hi Mark, thanks for the reply,

I haven't done any star tests on this mirror. I bought it as a 12" 1/12 wave mirror that needed recoating. I took it to Isaac Qin, who said he couldn't tell if it would be usable until he removed the old coating. He's done this, and says it's good, so he's going ahead with the recoating.

I've thought of going for the Antares secondary assuming the primary is 1/12 wave. I'm not sure exactly what this means, but Isaac said 1/12 wave is very good, so I'm hoping this will be a very good mirror, especially with Isaac's coating. I'm not sure how to test the mirror, so it's all a bit of a gamble...In any case, I don't want to put in an inferior secondary if the primary turns out to be very good.

Cheers

Stephen