Calculating magnification as seen by webcam

[trj]

I've taped a webcam (raw CCD, no lens) to a x2 barlow on a scope with a 1200mm focal length.

The image produced is 640x480 and the CCD is about 5mm across.

I know that I can calculate the magnification for a regular eyepiece as scope focal length / eyepiece focal length. Does this apply in the same way for the webcam?

Is the magnification 2 * (1200 / 5) = 480 ?

[Capricorn]

No.

Magnification is a term that basically says: What you see with this objective and this eyepiece will be/appear to be X times bigger then with you eye alone.

With a webcam your eye is not in the optical system, so there can be no comparitive.

What happens is that what you are pointing at has an angular size - Orion/Plaides are both about 1 degree

That forms a relationship with the focal length of the scope and the image formed by the objective is approximately:

Scope focal length times the tangent of angular size of the object.

So that is the size on the image formed in mm if the scope focal length is specified in mm, the image being formed at the focal plane of the objective.

The Barlow should simply take this image size and multiply it by 5 or whatever.

So you end up with an image of X by Y millimeters on the webcam chip.

[themos]

For digital imaging, the relevant measure is "image scale" and it tells you how many arcseconds are covered by one pixel. The Philips ToUCam Pro has 5.6 micron pixels and that translates, in your scope of effective focal length 2400mm, to 0.48 arcseconds per pixel. The 640x480 pixel matrix comes out at a size of 3.6mm x 2.7mm which covers 5.1 x 3.8 arcminutes.

[tamasflex]

I've taped a webcam (raw CCD, no lens) to a x2 barlow on a scope with a 1200mm focal length.

The image produced is 640x480 and the CCD is about 5mm across.

I know that I can calculate the magnification for a regular eyepiece as scope focal length / eyepiece focal length. Does this apply in the same way for the webcam?

Is the magnification 2 * (1200 / 5) = 480 ?

See if it helps link below :  http://t7.hu/156y

In your case :

CCD = ~ 5 mm diagonal, 

Image diagonal (640x480 pixel) = 211 mm ,

Lens focal length = 1200 mm. 

Magnification is equal to: 

M = 1200 / 250 / (211 / 5) = 202.56 times

Ps. 

Webcam eyepiece focal length equivalent :

f eq. = 250  /  211 / 5  = 250 / 42.2 = 5.924 mm

[ronin]

As was previously said there is no magnification.

The scope simply produces an image of Xmm size on the chip.

You have said:

Magnification is equal to: 

M = 1200 / 250 / (211 / 5) = 202.56 times

Which if normal convention is followed should come out as 0.113744 not 202.56, the bracketing is incorrect.

However all that is produced is a simple image of so many mm which is dependant on the angle subtended by the object and the scope focal length. Magnification refers to a change in the angle subtended by the object as the light passes through tne optics of the scope, in this case the angle subtended is unchanged so the term Magnification does not apply.

[Langy]

It's an area we all fall in the first time. This EP gives me 50x and this gives 100x so what do I get with a webcam?

As said when imaging it's all about what portion of the sky the image covers.

To give a slightly different reason to this.

You webcam has a minute little chip which when displayed on different devices could have a different output ratio. For example viewing on a mobile or laptop screen the image will look a different size even when scaled to 100% as they have different pixel sizes for their screens. To show an extreme if you printed your image, most printers print at 300 or 600 dpi. So your 640 x 480 image now appears on a piece of paper at 1" wide but displays on the screen several times bigger. The image hasn't been magnified on scaled different due to the output of the device displaying it.

That is why when imaging magnification is not put in the mix.

Now I know that doesn't help too much what you can do is just take a simple image with the webcam compare it to what you see in the EP. For example when I use my xBox webcam with no Barlows it just about fills the frame with the moon, that's equivalent to my 10mm EP which is about 65x magnification. So that simple scale just gives a different perspective as to what you will record on the webcam compared to seeing in an EP, The recorded image can only be scaled and known as how much sky it covers.

Hope that helps with a different view.

[Tommohawk]

Forgetting the sums for a minute, if you want to see what the image will look like, try this:

http://www.12dstring.me.uk/fov.htm

I found it quite useful  - hope it helps.