Expert: Paul Wagner - 6/30/2008

Question
QUESTION: Hey,
Hope you had fun in your vacation.
First, I don't understand the fully illuminated field and what is the difference between half and quarter inch fully illuminated field and what are the pros and cons of each thanks alot.
Second, I don't understand what's d2(the secondary-focal plane distance)in  this link
http://bi-staff.beckman.uiuc.edu/~melockwo/diagonals.html
Thanks again

ANSWER: HI Hady

Thanks for the note!  Yep--vacation was a lot of fun...

Here's the deal on secondary mirrors.  They have to capture the light from the primary, and direct it to your eyepiece.  But how big do they have to be?  It's always a compromise.  If they are really large, they capture ALL the light from the primary. But they block more incomging light going TO the primary, and increase difraction.  If they are very small, they capture only a very small field of view...and the edges of the image in your eyepiece will look dimmer, as if they are fading out.  

The easiest way to imaging this is to see the focal path of the telescope like a large cone that runs from the primary mirror to the eyepiece.  But it doesn't end in a point--that would mane that you are only looking at a very tiny image.  In fact, the cone ends in a focal plane that needs to be about 1/2" across--that's the full width of the image that you want to be able to see with your eyepiece.  

If your secondary mirror is too small, it will fully illuminate the center of this image, but some of the light for the edges of this image will miss the mirror, because it is too small.  

The article you sent as a link is excellent, and I have used it to measure the secondary mirrors for some of my scopes.

d2 in the equation is the distance between the secondary mirror and the focal plane of the eyepiece.  YOur mirror will have a total focal length, and during the design of the scope, you should divide that into two sections:  the distance between the primary and the secondary, which is d1, and the distance between the secondary and the eyepiece, which is d2.  Obviously, the close you get the eyepiece to the secondary, the smaller secondary mirror you can use--because you are intersecting the cone of light closer to the peak--and it is smaller there.  But that also means that your telescope will have a longer tube...

If you draw this on a piece of paper, it will be much clearer.  

Paul Wagner


---------- FOLLOW-UP ----------

QUESTION: if i am going to illuminate 1/2 inch only why should i use for example  a 1.67 inch secondary mirror?
how do i know the d2 and how do i decide it?

Answer
H Hady

You have calculate d2 from your telescope plans...how long is your focuser, and how far away from the center of the optical tube will it be?  That's your d2. Most focusers will list this distamce..the you have to add 1/2 the diameter of the primary, plus an inch or so for the clearance distance around that mirror.

Your secondary mirror could be smaller, but remember that the light path is a cone....as you get closer to the primary. the cone gets larger.  If you use a drawing to illustrate this, you will see how it works.  So you could use a 1/2" secondary mirror--but the it would have to be placed exactlt at the focal plane---not in the middle of the tube...

PW