|K3's Astronomy - Telescopes|
|"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4|
This article shows a basic principle of prime focus astrophotography. It can help beginners to find a focusing position of their camera if eyepiece focusing position is known.
In prime focus astrophotography the camera is mounted in the focal plane of the telescope's objective (the objective can be mirror or lens) without any additional lens. The following picture shows a difference between normal watching through scope (using eye) and prime focus:
As you can see in the picture, when you are watching through a scope using eyepiece, the distance between the main objective and eyepiece lens is fO + fE . When using a CCD camera at prime focus, the CCD chip plane must be identical with the main objective focal plane - i.e. the CCD chip distance from the main objective is only fO . This means, that your scope focuser must enable you to move the camera a bit inwards - towards the main objective.
Let's suppose that we have a Newtonian with focal
length 1000mm and we use 10mm eyepiece for visual
watching. When the telescope is focused on a very far
object (all astronomy objects are very far) - the
eyepiece lens is at 1000mm + 10mm = 1010mm distance
from the main mirror (distance in our case is length of
light ray from the objective to the eyepiece lense). When
we want to use the CCD camera at prime focus, then the
CCD chip must be positioned 10mm inwards in comparison
with eyepiece position - i.e. the distance of CCD from
the main mirror is 1000mm.
Computer generated images, real images, drawings and texts are property of the author and may not be reproduced or used without permission of author.
Last Update: 10.06.2006