Why isn't a secondary mirror visible in your fov?

stupid question warning!

I just finished my first dob, and I know i should know this. think i did at one time but...took it for granted until I had a pop quiz today: a friend of mine asked me over a beer how my scope works. I drew the usual newt diagram showing incoming light, the primary parabola and the focussing property of that,, the reflection and the 45° secondary. then he asked the confounding question. Why doesn't the secondary block out the center of your view? I struggled to convince him that it doesn't, and ultimately couldn't prove it, or draw it out, but just left him with "trust me, it's not visible. it does impede contrast, but it's not a hole in the middle of your view. but i'll have to get back to you as to why exactly though."

so how do you explain this and/or is there a site you can point me to that explains this?

darn those "simple" questions!
Tom.

It is just waaaay out of focus. You are normaly focusing on something that is close to infinity, the secondary is only few feet away. Try this: take a single hair or a very fine string, hold it with both hands and get it closer and closer to your eye, at some point you won't see it anymore because you can't focus your eye close enough. Hope it helps.

oren.

Oren's explanation that it out of focus is a good one, I will expand a bit on it a bit.

If one think's about it a bit, one can see that each part of the exit pupil, the beam leaving the eyepiece, essentially contains the entire image. If you block off half the light entering the scope with board, the intensity of the image is reduced, the resolution is reduced, the exit pupil will be a semi-circle rather than round but the field of view and overall image will be unchanged.

If you look from a distance at the exit pupil of an obstructed telescope such as a Newtonian or SCT, you will see the bright round pupil with a dark spot in the center, that is the shadow of the secondary mirror. Normally you do not see it but in certain circumstances it is visible. This happens when viewing at low magnifications during the day or with the moon. Your eye's pupil is closed down and about the same size as the hole in the center of the exit pupil. Thus if your eye is centered, all you see is the dark hole.

The hole can be considered an exit pupil of it's own and the central obstruction has a "focal ratio" of it's own. Imagine an F/5 Newtonian with a 20% central obstruction. The focal ratio of the secondary obstruction is 20% of F/5 or F/20. The diameter of the exit pupil with a 40mm eyepiece is 40mm/5 = 8mm and the diameter of the dark hole/shadow is 40mm/20= 2mm.

If your eye is only open to 2mm, then all you may see is that dark shadow.

At higher magnifications or when your eye is appropriately dilated, there is not a problem.

Hope this helps

JOn