Focal plane

[martindale]

Hi,

Just started to take a few photo's with my 300mm f4 newtonian.

Tried prime focus and eyepiece projection using dslr and spc900 and got some OK images.

Regarding prime focus, I'm now trying to get my head around the mechanics and relationship of the scope's optics and the focal plane. Never seemed an issue before.

I assume:

1. the the focal length of any optic is that point where light entering from infinity is focused to a single point

2. the focal plane is an area across the light cone where light from the target image is focused

3. as the target image is never at infinity, light from this is always focused to a single point beyond the focal length

Where is the focal plane for any given optic arrangement in relation to the focal length of those optics?

Can I then calculate It's physical size?

By knowing this, I would I then be able to choose a camera with a chip size to best utilise my optics?

Just one more thing, as adding a telescope to a camera, and using prime focus, is the same as adding a telephoto lens to a camera.

My scope has a focal length of 1200mm, will this give me an effective magnification of 24x ?

Cheers in anticipation

[GlassWalker]

The focal length is defined from a specific point to the focal plane, which for a newt would be the primary mirror. To estimate the location focal plane, you need to travel from the primary mirror a distance equal to the focal length. For more complex designs, the reference point doesn't have to coincide with any physical optic and may even be outside the physical design!

As for targets not being at infinity, there comes a point where practically speaking it is at infinity. For astro use, infinity is close enough!

For calculating the size of the focal plane, I presume that means area, this is very complicated and can't be worked out from focal length alone. Best thing to do is to see what the manufacturer claims, if anything. Unless you have a massive budget, you're going to be limited in sensor size anyway.

Magnification in this context doesn't have much meaning for prime focus.

[martindale]

Glasswalker, OK, just about got it.

It seems to me that what you are saying is, there is a focal plane, but as I don't have infinitely deep pockets, it's there and don't worry about it, that I can live with. And I get the reference point being physically outside the scope, etc.

Going back to magnification,,,

I take an image of jupiter,say, using prime focus on dslr, it would be nice to know how much I could magnify that image to the sensor (optically) to get within the 300x to 400x range?

I bet there's no easy answer

LOL

[Bizibilder]

If you download this: http://www.newastro.com/book_new/camera_app.php it will calculate the fov for any camera/scope combination that you can enter - it also produces a nifty picture of what you should see on the camera chip. There are preloaded cameras and scopes so you can have a good "play" to see what you can achieve.