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Question regarding focal lengths and webcams.

Simms

By Simms
in Imaging - Discussion

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Simms

Simms

Posted
Posted

OK, struggling to get my head around the concept of webcam/ccd imaging.

What kind of focal length multiplier is added to the scope when using a webcam (say the popular Phillips one).

When shooting with my DSLR (a 1.3x crop factor or my fullframe) I am obviously imaging a lot of sky at once - an image I took of the whirlpool galaxy last month showed the galaxy quite clearly however it took up around 5% of the entire image. Now cropping afterwards isnt too much of a problem however I have plenty of megapixels to crop into - my worry with using a webcam will be that the subject will be small, but the limited resolution will not allow me to crop in at all.

Now, Barlow lens aside, what size, or what percentage, would (for example) the moon be on the sensor/final image on a webcam? Or the Orion Nebula even?

My scope has a focal length of 750mm.

Hope that makes sense.

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Simms

Simms

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AFAIK the webcam acts as a 6mm or similar EP.

750 / 6 gives you about 125X magnification which should more than fill the sensor with the moon?

Heh, maybe thats how I should of fielding the question. Exactly the answer I was looking for.. Thanks!

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Ant

Ant

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It is the focal length of the scope that decided how big the object is...

The chip size determines the size of the image - not the image scale.

So M51 will be the same size ion the dSLR chip as the Webcam chip... there will be less "space" around the outside of M51 in the webcam.

Adding in a 2x barlow will mean that the focal length doubles to 1500, m51 will be twice the size and the FOV will be a quarter of what it was (because both width and height have been halved).

Search for a small but off free software called CCDCalc - this will let you visualise what I've just said with lots of different scope lengths and chip sizes.

Hope that helps.

Ant

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Simms

Simms

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It is the focal length of the scope that decided how big the object is...

The chip size determines the size of the image - not the image scale.

So M51 will be the same size ion the dSLR chip as the Webcam chip... there will be less "space" around the outside of M51 in the webcam.

Adding in a 2x barlow will mean that the focal length doubles to 1500, m51 will be twice the size and the FOV will be a quarter of what it was (because both width and height have been halved).

Search for a small but off free software called CCDCalc - this will let you visualise what I've just said with lots of different scope lengths and chip sizes.

Hope that helps.

Ant

I understand that thanks - however I was also applying the logic that - for example - when using a standard 50mm lens on a DSLR with a 1.6x crop factor sensor then the FOV you actually have will be closer to using a 80mm lens on a fullframe - this is due to the crop (50 x 1.6) - so I was assuming that the same principle applies to using a smaller sensor (webcam) on a 750mm lens (my scope) - so all I really wanted to know was using the webcam sensor will `crop in` to the view offered by my scope and give me an imaging circle equivalent to a longer `lens`.. Which basically you have confirmed for me..

Thanks for the link. will check it out!

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Revs

Revs

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^ wot ant said :D

The pixel size on a SCP900NC is about the same as my 1000D, at about 6µm. So the the resulting image will be about the same size in pixels.

You can add a barlow, or just use a diagonal to give a slight increase in size. Like wise you can use extensions tubes.

There are plenty of pics here to give you an idea.

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