Er... The smaller cap in the telescope tube cap

[Breakintheclouds]

Okay, I almost feel a bit silly asking this, but what exactly is the smaller aperture in the middle of a Newtonian telescope cap used for? Clearly it lets in less light, but what's the effect of this? Is it essentially a crude moon filter, or does it actually change the aperture and so alter the f/ ratio?

[Doc]

As you say it changes the F ratio by reducing the aperture. It's called am aperture mask, it will add more contrast to planets etc as it removes the central obstruction out of the equation.

[Merlin66]

I don't know why they still do this!

It's a throw back to the 1960's when you used Herchel wedges and ND filters to view the Sun. This reduced the aperture to a "safe" size!!!

Nowadays as a means of reducing light it's sometimes used as a moon filter ( with reduced resolution!) I suppose you could fit the aperture with a piece of Baader solar film and still use it for the Sun.

[Brinders]

I suppose you could fit the aperture with a piece of Baader solar film and still use it for the Sun.

That's what I did with my TAL 150P, and it works a treat.

Brinders

[The Warthog]

That's what I did with my TAL 150P, and it works a treat.

Brinders

Also what I do.

[hoobz]

I originally thought it was a little hole to put ones finger in to gently pull the main cap off lol !!

I soon found out from this forum that thats not the idea haha !

[astropete]

it can be used for solar observation, as it reduces the light input to the main mirror, as said its a light stop. BUT still use a solar filter OR ELSE.

[Sam]

I originally thought it was a little hole to put ones finger in to gently pull the main cap off lol !!

I soon found out from this forum that thats not the idea haha !

Ha ha - that's what I use it for on mine:D

[jimhaleysscomet]

Since it is not "off axis" it does not make a very good aperture mask. On a reflector I would just ignore it.....