how is focal length measured on a reflector telescope?Is it the length of the tube or is it the legnth from the primary mirror to the secondary mirror?
2007-09-06 15:11:57 · 8 answers · asked by Anonymous in Science & Mathematics ➔ Astronomy & Space
I need to know where to put my tape measure and where to read it.Do I run it down to the mirror and read it at the secondary or what?
2007-09-06 15:42:57 · update #1
It is distance from the primary mirror where an image would form if there were no secondary. Normally that is a few inches past the secondary. In the case of a Newtonian where the secondary is a flat mirror, it is close the the length of the tube. In a Cassigran it is more complicated because the secondary is not flat. In that case the secondary changes the focal length, usually to something several times the length of the tube, but it can vary depending on the exact curvature of the secondary. In that case you need to know the effective focal length, which is the primary focal length multiplied by the amplification factor of the secondary. That amplification factor would be approximately the ratio of the diameters of the primary and secondary. So if the secondary is 1/4 the diameter of the primary, it probably has an amplification factor of 4. Then 4 times the tube length would be about the effective focal length. But that is all rough estimates. Exact measurements are harder to do.
2007-09-06 15:21:28 · answer #1 · answered by campbelp2002 7 · 1⤊ 2⤋
Geoff is right of course.
If you are building your own scope, you want the focal point of the mirror to fall about 1/2" outside the focuser with the focusing tube racked all the way in. This assures that even the eyepiece requiring the most in-travel will come to focus.
So, you need the distance from the surface of the primary to the surface of the secondary plus 1/2 the tube diameter plus the tube thickness plus the distance to 1/2" above the focuser = Focal length.
But there is a very simple solution for your problem. Do a google search for a program called "Newt for Windows" it is a freeware program where you just plug in the parameters and it will compute everything you need. A simple program for designing Newtonians.
One unknown in all this is the ACTUAL focal length of your Mirror. If from a good source, the focal length stated will be very close. If not, you might have to bench test the mirror to get the exact focal length.
Due to production tolerances some discrepancy between the stated and true focal length can exist.
Adolph
2007-09-07 03:40:18 · answer #2 · answered by Adolph K 4 · 1⤊ 0⤋
The focal length is a measure of where the light from the mirror comes to a focus. The secondary mirror is placed at this point to reflect a proper image to the eyepiece.
2007-09-06 15:34:34 · answer #3 · answered by Twizard113 5 · 0⤊ 1⤋
It is neither of these. It is the distance from the surface of the primary mirror to the centre of the secondary mirror PLUS the distance from the secondary mirror sideways to the focal plane (in the focuser where the eyepiece is).
2007-09-06 15:58:02 · answer #4 · answered by GeoffG 7 · 3⤊ 0⤋
There is no phsyical thing you could measure with a tape measure that will yield the precise focal length of the scope, but the total length of the tube itself on a netwonian is a fairly good approximation.
2007-09-06 16:09:50 · answer #5 · answered by Arkalius 5 · 0⤊ 0⤋
The focal point of a Newtonian telescope is at the end of the eyepiece tube. The image is deflected 90 degrees by the secondary mirror.
DO NOT AT ANY TIME TOUCH THE PRIMARY MIRROR WITH ANYTHING!!!!!!!!
2007-09-06 15:52:36 · answer #6 · answered by Anonymous · 0⤊ 1⤋
its a simple formula its times 35 so 3 x 35mm is eqiv to 105mm 2x35=70 1x35=35 10x35=350 they are a little approximate though a
2016-04-03 07:59:58 · answer #7 · answered by ? 4 · 0⤊ 0⤋
It's measured by the curve of the mirror. How convex it is.
2007-09-06 18:23:17 · answer #8 · answered by Anonymous · 0⤊ 1⤋