Yet another thought -- With my SCG I can change the scale depending on an extension tube's length without an eyepiece. That is because this type of telescope changes focus internally, rather than having a fixed focus like a refractor. But mostly I use a field flatener / focal reducer with tubes of a particular length. Hope that makes sense. -- Joe ------------------------------ On Sun, Aug 11, 2013 8:51 AM MDT Erik Hansen wrote:

...

are not the function of telecompressors and teleextenders to change the scale of photographs? I am no photographer but I thought that was what Joe was originally referring to.

Actually Joe, no, you can't change magnification in the direct objective

...

configuration.

The prime focus of any telescope focused for infinity is fixed. This is the focal length of the objective, or system in the case of a compound telescope such as a Cassegrain. Moving the camera further from the prime focus by means of extension tubes just puts the image out of focus. In other words, the camera chip and prime focus of the objective must coincide. Moving a focuser does not move the image.

You CAN increase the image size by moving the camera further from an eyepiece when used in a projection system, number 3 in the illustration.

Note that the primary image is in the same location relative to the objective in all three configurations. The camera moves to accommodate the shifted focus, dependant on configuration.

The secondary mirror in your SCT is actually a reflective Barlow. It does the exact same thing in your telescope that a Barlow does in a Newtonian or refractor. It changes the focal ratio of the objective, lengthening the EFL of the sytem. While the light cone of your primary mirror is something like f/2, the light cone from your secondary mirror is that of an f/10 system.

The small tube in the illustration is supposed to depict a focuser drawtube or camera adapter. It is not important to the diagram.

Your SCT focuses by changing the spacing between the primary and secondary. Your prime focus moves according to the distance of the object under view. The closer the object, the further from the back of the telescope the image will be formed. But for objects at infinity, the focus is unchanged. The same thing happens with other telescopes; the focal plane moves further out, the closer the object under view is. But again, it's always in the same place for sky objects which are optically at the infinity position.

On Aug 10, 2013 5:24 PM, "Joe Bauman" <josephmbauman@yahoo.com> wrote:

In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube. -- Joe

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