Expert: Paul Wagner - 11/14/2008

Question
I've been waiting eagerly for your return.Hope you had fun in your vacation.I know you got bored from my questions but I hope these will be the last
1)You explained the Fully illuminated field.So what is the best one?The one you would use for watching planets and deep sky objects.

2)IS minor axis of diagonal the diameter of the mirror and why should it be oval?

3)Is the Focal length of the primary till it reaches the eyepiece or till it reaches the secondary mirror?IS d2 the rest of the focal length from the secondary mirror to the eyepiece?

Thanks alot for your effort

Answer
Hi Hady

I was actually working---traveling through the US and Europe onn business.  

But now I am back and ready to answer your questions!

1.  fully illuminated field is not important for planets, because they are a very small part of the field.  But for deep sky objects, which can be much larger, this is important.  The Andromeda nebula will fill your eyepiece, and that is where the illuminated filed is important.

2.  The diagonal mirror is elliptical because it is placed at a 45* angle to the main mirror.  If you look at the diagonal from the end of the telescope tube, it will appear circular--because of this fact. The minor axis is the shortest dsitance across this diagonal mirror.  

3.  Focal length is from the primary mirror to the focal plane of the eyepiece.  D2 is the distance from the eyepiece to the secondary/diagonal.  You need this information to understand how long the tube must be. When you subtract d2 from the Focal length, that gives you the correct distance from the primary to the secondary.

Paul Wagner