Focus points ?

[bmitchell82 (Brendan)]

okay, im a fella that likes to Know before i go ahead,

I Know for a fact that a cannon 350D comes to focus in a 8" SW Newt with a focal length of 1200mm, and a mirror width 203mm giving a F# of ~5.9

will the same camera come to focus in a 10" SW Newt with a focal length of 1200mm and a mirror width of 254mm giving a f# of ~4.7

In my mind the f number should only affect the FOV and how fast you can capture things not the focal point as the focal point is 1200mm. I might be wrong though but i thought ide post the question and see what the response is.

[sheeny (Al)]

G'Day Brendan,

The problem is not so much to do with the f ratio or focal length of the scope but the way it is built. Just becasue you can achieve focus in one scope doesn't necessarily mean you can achieve focus on another scope.

Many people have to "shorten" their Newtonian scope to achieve focus with a camera... usually this means mounting the primary mirror a little higher in the tube to move the focal point further out of the focussing tube.

Al.

[bmitchell82 (Brendan)]

I have heard of this, but there has to be some way of "knowing" where it will come to focus in the draw tube. i would have thought that there would be something that i can do :S becase in any case i would prefer not to start hacking and slashing at my ota

[sheeny (Al)]

Originally Posted by bmitchell82

I have heard of this, but there has to be some way of "knowing" where it will come to focus in the draw tube. i would have thought that there would be something that i can do :S becase in any case i would prefer not to start hacking and slashing at my ota

Ah, yes, there is.

The procedure is to use a frosted glass surface against the end of the focus tube to determine when the focal point is level with the end of the tube. Then measure the distance that the tube is out of the focusser.

Next, put your camera on the scope and adjust until the camera is in focus and again measure how far the tube protrudes from the focusser. The difference will tell you where the focal plane of the camera is relative to the end of the focus tube.

Now you repeat the frosted glass part of the experiement again on the new scope, and make sure you have enough adjustment before full in/out focus.

You can use a piece of 3M Magic tape on a glass slide for the frosted surface, or possibly even grease proof paper if you can hold it flat and taut across the focus tube.

Al.