How to figure prime focus

[Kirkus (Kirk)]

Sorry for all the newbie questions.

Is there a way to figure out what magnification a particular scope is at prime focus?

My 10" has a focal length of 1270. So I know if I'm using a 4mm EP the magnification is 317.5x. However, if I'm using my web cam at prime focus, how do I know what magnification that is? Is there a way to figure this out?

[iceman (Mike)]

With a webcam, or any imaging device, magnification just isn't relevant. I know a lot of people want to know it, it makes it easier for them to picture in their head, but for photography, focal length is the key factor for determining of the resolution and image scale.

But, if you still want to know, most people argue that a ToUcam is equivalent to a 6mm EP, so you can work it out from there.

[Barrykgerdes]

I have found the same results with a Meade LPI and DSI 1. The FOV of the chip at Prime Focus is approximately equivalent to what you see with a 6mm plossl in the same scope.

Any camera with a chip that has the same physical size as the TouCam will have the same FOV. If the chip is say twice as wide the rule of thumb eqivalent would be what you would see in a 12mm occular.

Barry

[Kirkus (Kirk)]

So, going to focal length, just to be sure, if my focal length is 1270 and I'm using a 2x barlow, my focal length is increased to 2540, right? And a 3x barlow would increase it x3, or 3810.

[Barrykgerdes]

Originally Posted by Kirkus

So, going to focal length, just to be sure, if my focal length is 1270 and I'm using a 2x barlow, my focal length is increased to 2540, right? And a 3x barlow would increase it x3, or 3810.

Yep that's right and the FOV will be 1/2 or 1/3 on the TouCam or equivalent to 3mm or 2mm occulars. The ultimate resolution will always be the pixel count on your camera (and the quality of the rest of your set up).

Barry

[Kirkus (Kirk)]

Very cool. Thank you!

[[1ponders] (Paul)]

Just to throw a spanner in the works here . If you are using a ToUcam with 640 x 480 pixel chip and 5.6 micron pixels the "magnification" will be higher than another camera using a 640x480 pixel chip with 7.6 micron pixels by an approximate factor of 7.4/5.6 = 1.3x ie the larger pixel chip will be reduced by about .75x

Ie. 600mm scope using toUcam = approx 100x. 600mm scope using larger pixels = approx 75x

[Barrykgerdes]

Yep! That is true also.

That is why I said the same physical size and was specific about the actual imagers which have chips with approximatly the same area not the number of pixels or their size in the chip. If the chip is 1/4" square it will occupy the same area of the focal plane irrespective of the pixel count and have that portion of the FOV falling on it. I only refered it to a 6mm plossl because it gives a view of the approximately the same area of the focal plane as occupied by a TouCam chip, a Meade LPI, and a Meade DSI 1, I haven't checked any other imagers .

However the actual size of the image as a portion of the sky in arc minutes will depend on the focal length of the object lens or mirror. Short focal length will give a wide field, long focal length a narrow field.

Barry

[Geoff45 (Geoff)]

There is a lot of confusion about "magnification" If you look at a typical image of the moon at prime focus it will be about a centimetre or two. Since the real moon is about 3000 km in size, there is no magnification, in fact quite the reverse. The image is much smaller than the real thing. Magnification makes more sense when you are using an eyepiece, because then you are comparing the apparent angular size you see through the eyepice with the real angular size that you would see if you looked at it directly. Keep both eyes open and compare the image through each.

When you make a picture, things get complicated again. Obviously an A3 picture is twice the size of an A4 pic of the same object, so is twice the magnification. It also depends how far away you hold the picture. It looks smaller as you move it further away.

Hope this has confused everybody.
Geoff