Central Obstruction

Ok, I need some verification here.

I'm trying to make a spreadsheet demonstrating the increase in light-gathering ability as a scope's primary mirror increases. I'm thinking I should be concerned with the area of the secondary blocking the light rays as opposed to the diameter of the secondary. I'm I missing something?

The area will give you how much 'area' of light is being blocked by the secondary. But as the secondary grows, diffraction becomes a factor -- basically as the diameter of the obstruction increases, a larger percentage of light is diverted into the 1st and 2nd diffraction rings instead of being focused inside the airy disc -- this is aside from area blockage, a contrast-affecting factor which increases with secondary diameter. Typically the area-coverage impact of the secondary obstruction is itself not an issue unless the secondary gets especially large.

Ok, I need some verification here.

I'm trying to make a spreadsheet demonstrating the increase in light-gathering ability as a scope's primary mirror increases. I'm thinking I should be concerned with the area of the secondary blocking the light rays as opposed to the diameter of the secondary. I'm I missing something?

Here's my thinking:

- The effect of secondary on the light through-put is related to it's area.

- One needs to consider the reflectivity of the coatings when comparing scopes for through-put. This is pretty much a gray area as manufacturers are likely to hype the numbers.

- The main effect of the secondary is on the contrast, planetary contrast that is. The math is apparently complicated but a linear approximation is popular.

- I am not sure how useful such calculations are. Aperture buys not only light but also contrast and resolution. The total effect is significant but difficult to put a number on.

Jon

Thanks everyone for their comments. Gives me a lot to think about. It's going to be more complicated than I thought.