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A question about aperture and imaging?


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Hi all

I have been wondering again, this has prob been asked before.

If you image with a lense that changes focal length, but at a constant F4.

At 200mm FL @ f4 , your aperture is now 50mm

At 70mm FL @ f4 , your aperture is now 17.5mm (is this correct?)

So with a large CCD chip, at 200mm f4, say your FOV is the whole of the Rossete nebula.

Now, if you change FL to 70mm f4, and use a smaller CCD chip, and have a similar FOV as the above, will less aperture, mean you will get a lot less detail in the final images?

Thanks in advance.


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The aperture stays constant, there's a lens assy which moves to change the magnification...a smaller CCD may have less pixels so you simply have a smaller picture as the rest of the frame is lost. You can simulate this using software like Starry Night or AstroPlanner

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Re-reading this...

To get the best image quality, generally get the largest aperture optical elements as this makes the airy discs as small as possible.

CCDs are more sensitive the larger the pixels are.

You will sacrifice image quality using a small lens with a sensor which has small pixels, hence why a webcam with a tiny lens always looks softer than a DSLR on a DSLR lens - or even better a telescope.

I hope this helps?

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