Magnification at the prime focus of telescope

[clop]

Hi,

I'm pretty sure this is going to be a stupid question with a simple answer. My understanding is that the sole job of a telescope is to collect light and focus it to an eyepiece, and that it is the eyepiece that magnifies the image, not the telescope. So does that mean that if I mount my SLR camera with no lens at the prime focus of my telescope, the image it records will be unmagnified? If so, how does one predict the size of an object on the film/sensor and the width of the field it can see?

clop

[cjl]

Nope.

If you use it prime focus, then the telescope will act as a large camera lens. A very, very high telephoto lens. A 10 inch F10 scope has a focal length of 2500mm, and a 4 inch F10 is still just over 1000mm. It will simply act as an eyepiece with the focal length that it has (compared to 200-500mm for most camera telephoto lenses).

[clop]

Nope.

If you use it prime focus, then the telescope will act as a large camera lens. A very, very high telephoto lens. A 10 inch F10 scope has a focal length of 2500mm, and a 4 inch F10 is still just over 1000mm. It will simply act as an eyepiece with the focal length that it has (compared to 200-500mm for most camera telephoto lenses).

OK right, thanks. So my 8" SCT f/10, having a focal length of 2048mm, will magnify things around 7 times more than my 300mm telephoto lens? What focal length is considered unity magnification? Can I divide the field of my 300mm lens by 7 to get the field of my telescope?

clop

[Tog_]

OK right, thanks. So my 8" SCT f/10, having a focal length of 2048mm, will magnify things around 7 times more than my 300mm telephoto lens? What focal length is considered unity magnification? Can I divide the field of my 300mm lens by 7 to get the field of my telescope?

clop

According to sky mapping program, a 2048 fl with a 35mm camera will give a field just a bit bigger than 60 by 40 arcmin.

When I hold my camera up to one eye and nothing up tp the other, and adjust the zoom until the images are the same size, it's just about 70mm for the focal length on the lens. From this I assum 70 to be 1x, giving the camera an effective Fl of 70mm when being used as an EP. I have no clue about that accuracy there.

[Bojan]

It is important to understand the fundamental difference between "magnification" of the telescope, and the "scale" of the photograph.

Magnification of the telescope is the number which tells you how much bigger the object appears in the telescope, compared to how it is seen by the naked eye, and it is the ratio between focal lengths of the objective (mirror) and eyepiece.
This number does not have dimension.

Scale of the photograph is the number with dimension and it can be expressed in, for example, how many degrees are there per 1cm, or inch on the photo... or, how many arc-seconds are there per one pixel.
This number will be dependent on the (effective) focal length of the telescope and the size of the pixel (or the unit of length you want to use).
I used here the term "effective focal length" because the "scale" is applicable both to focal photos (when sensor is placed in the focal plane of the objective) and to afocal pictures (when the photo is taken through the eyepiece of the telescope).
The scale of the camera is not constant over the whole area of the sensor (or film), and for the objects near the centre (optical axis) it can be expressed as arctn(d/F) degrees per unit of length, where F is the focal length of the objective lens (expressed in length units) and d is the length unit (cm, inches…. )