shadow of central obstruction - eyepiece vs. SLR camera

How come that through our 5 SCT we get to see the shadow of the central
obstruction visually,
but not through our SLR digicam at a much lower magnification ?

At the eyepiece at 30-40 times magn. the shadow begins to be annoying
visually,
but at the camera at 2-3 times lower magn. there is not any trace of such.

Regards
Pete

On Sun, 11 Sep 2005 09:26:25 +0200, "Topstuck" wrote:

How come that through our 5 SCT we get to see the shadow of the central
obstruction visually,
but not through our SLR digicam at a much lower magnification ?

At the eyepiece at 30-40 times magn. the shadow begins to be annoying
visually,
but at the camera at 2-3 times lower magn. there is not any trace of such.

A focused telescope produces a cylindrical bundle of rays out of the
eyepiece. The diameter of that bundle increases as the magnification
decreases. This diameter is called the exit pupil. A telescope with a
central obstruction (such as an SCT) has a shadow in the middle of the
exit rays- that is, the exit bundle isn't a fully illuminated cylinder.
The diameter of the shadow also increases as the magnification
decreases. In the day, when your own pupil is constricted to a small
size, it can be smaller than the shadowed zone, and that shadow will be
very obvious. That's why you need to look off-axis when using an SCT at
low power in the day- to see around the shadow.

When you make an afocal camera image (that is, with the eyepiece in
place and using the camera lens) you probably won't see the shadow
because the entrance pupil of the lens is much larger than the entrance
pupil of your eye. The camera is seeing the entire bundle of rays coming
from the scope.

Usually, when you use a DSLR you image with no eyepiece and no camera
lens. In that case you will never see a shadow, because the sensor is at
the focal plane of the objective (technically, you aren't even using a
telescope at that point- a telescope requires both an objective and an
eyepiece).

I'm not sure what you mean when you compare magnifications, since the
term doesn't mean anything when imaging. I'm assuming you mean that when
making an afocal image, you can use a much longer eyepiece than you can
when viewing?

_________________________________________________

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

Hi Chris.

Thanks for your reply.
You´re right, the terms magnification aren´t comparable sizes
when comparing images with the information that reaches the eye.
But yes, the eyepieces usable within imaging can be of much longer focal
lenght
than with the eye, thus giving a much wider field of view.

Regards
Pete

I'm not sure what you mean when you compare magnifications, since the
term doesn't mean anything when imaging. I'm assuming you mean that when
making an afocal image, you can use a much longer eyepiece than you can
when viewing?




"Chris L Peterson" skrev i en meddelelse
...
On Sun, 11 Sep 2005 09:26:25 +0200, "Topstuck" wrote:

How come that through our 5 SCT we get to see the shadow of the central
obstruction visually,
but not through our SLR digicam at a much lower magnification ?

At the eyepiece at 30-40 times magn. the shadow begins to be annoying
visually,
but at the camera at 2-3 times lower magn. there is not any trace of such.

A focused telescope produces a cylindrical bundle of rays out of the
eyepiece. The diameter of that bundle increases as the magnification
decreases. This diameter is called the exit pupil. A telescope with a
central obstruction (such as an SCT) has a shadow in the middle of the
exit rays- that is, the exit bundle isn't a fully illuminated cylinder.
The diameter of the shadow also increases as the magnification
decreases. In the day, when your own pupil is constricted to a small
size, it can be smaller than the shadowed zone, and that shadow will be
very obvious. That's why you need to look off-axis when using an SCT at
low power in the day- to see around the shadow.

When you make an afocal camera image (that is, with the eyepiece in
place and using the camera lens) you probably won't see the shadow
because the entrance pupil of the lens is much larger than the entrance
pupil of your eye. The camera is seeing the entire bundle of rays coming
from the scope.

Usually, when you use a DSLR you image with no eyepiece and no camera
lens. In that case you will never see a shadow, because the sensor is at
the focal plane of the objective (technically, you aren't even using a
telescope at that point- a telescope requires both an objective and an
eyepiece).

I'm not sure what you mean when you compare magnifications, since the
term doesn't mean anything when imaging. I'm assuming you mean that when
making an afocal image, you can use a much longer eyepiece than you can
when viewing?

_________________________________________________

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