Re: trouble focusing with a vaiable camera adapter

On Fri, 9 Feb 2007 19:11:22 -0000, "Callum" <no@xxxxxxxx> wrote:

Hi there

thank you for the reply.

I have just tried this:

connecting the 1 1/4" insert to the t-mount.
connecting the t-mount to the d70
putting the insert into the lens mount on the telescope & pointed the
telescope at pollux.

I can see the star as a crisp point in a 25mm eyepiece, but I can only see a
blur through the camera, which starts to get sharper as I focus down - but
before I get a crisp point through the camera, the focus movement reaches
the lower limit - which seems to be the same problem I have with the
variable setup and the use of a lens in the adapter.

Any ideas where I am going wrong?

This isn't an uncommon problem when imaging with a Newtonian. When you
use an eyepiece, the proper focus is obtained when the focal points of
the objective and eyepiece coincide, which happens behind the eyepiece,
down inside the focuser tube. To allow the secondary mirror to be as
small as possible, it is desirable to also make this focal point close
to the secondary, so focusers are often rather low profile. Of course,
when you are using a camera there is no way that you can get the sensor
down inside the focuser, so even with the drawtube all the way in you
end up with the sensor behind the focal plane, and with an unfocused
image.

One solution is to switch to afocal imaging, using an eyepiece, a
special adapter, and a lens on your camera (as you have been
attempting). But that is usually not a very good solution, since it
tends to be mechanically unstable, is likely to unbalance the scope, and
presents all sorts of interesting optical aberrations. It is usually
reserved as a method for obtaining high magnification for planetary
imaging, where only a small central field is of concern.

The best solution is to relocate your primary mirror. The mirror cell is
probably held in place by three screws through the tube wall. If you
provide three new holes an inch or two further up the tube, and
reposition the mirror, you will push the focal plane far enough outside
the focuser that you can use a camera. Of course, you may also have a
focuser that is very long, and replacing it with a low profile focuser
may work. In any case, you are looking at some sort of simple
modification of your telescope.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

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