optimizing secondary size and position for F4 reflectors

[alistairsam]

Hi,

I have an 8" F4 OTA which I bought second hand and noticed that the primary was moved up by around 8cm to achieve focus for DSLR's.

First thought that came to mind was that there might be vignetting as the light cone boundary may be outside the secondary boundary.
how do i measure the size of the secondary?

I ran the figures through the newt for windows program for two scenarios, one with a 70mm secondary and one with a 50mm secondary.
output attached.

but what would happen if I used a smaller secondary, moved the mirror back and moved the focuser hole upward so vignetting is avoided, along with a very low profile home made focuser, say 20mm. that would have the least central obstruction, but what other effects would that have. i don't understand why the difference between angular field of view with a smaller and bigger secondary. can someone please explain.

primary objective behind this is to maximize light gathering from an 8" F4 for astrophotography.

thanks.

[DavidU]

I find this free software very good at showing all the design figures for Newtonians.
http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm

[MrB]

Hi Alistair (or Sam?)

The secondary size is the minor axis of the elipse.

[alistairsam]

Hi,

Dave, I am using that program, Newt for windows. thanks.

what i wanted to know is how is the optimal distance from the primary determined. is it based on the diameter of the light cone that would hit the secondary and would that need to be the same as the 2" focuser, or can it be reduced by moving the secondary further up, thereby reducing the light obstruction.
but what is the effect on the resulting angular field of view?

the footprint of a ccd chip in a dslr is a lot smaller than a 2" eyepiece.
so if the secondary is moved up the tube enough to cover the light cone and cover the area of the ccd chip?
i just wanted to understand the concept behind where the secondary is positioned and what the limit is on how far up the tube it can be moved.

[alistairsam]

Hi,

could someone confirm if I can use the following on an 8" F4 scope
50mm secondary, 32mm low profile focuser.

my current setup is

63mm secondary, 50mm focuser. mirror is moved up by around 9cm.

i ran both through newt for windows and got the following results

63mm secondary, 50mm focuser
-vignetting of 75% ray at front aperture - yes
-obstruction by diagonal - 32%
-angular field of view for 100% illuminated area - 1.527 degree

and for

50mm secondary, 32mm low profile focuser
-vignetting of 75% ray at front aperture - none
-obstruction by diagonal - 25%
-angular field of view for 100% illuminated area - 0.7074 degree

can someone pls clarify what the practical implication is with reduction of angular field of view for imaging or visual?

i'm thinking of using a 50mm secondary, low profile focuser and moving my
mirror back.

can the reduction in obstruction by 7% be related to say contrast or limiting magnitude?