CCD Chip & Pixel Size

[moondog]

Small chip size CCD's (say 640x480) give high magnification for astro images. The larger chip size CCD's (say 3000x2000) give lower magnification, but a wider field of view.

So.... assuming that the pixel size is the same, would a 640x480 crop from a 3000x2000 image produce the same quality image as a 640x480 image from a small chip size CCD?.

Comments welcome !

MD

[Kaptain Klevtsov]

If all other factors are equal, yes.

The "other factors" are, chip sensitivity, noise (lower with a cooled chip) and bad pixels (usually lower in an astro CCD camera than a webcam).

Kaptain Klevtsov

[moondog]

Thanks for that KK. It seemed logical to me, but I wanted confirmation.

Being on holiday in NZ and looking at the night sky regularly, I think I would be better with a larger chip CCD when and if I end up moving out here.

Knowing that if I crop an image from a large chip CCD, I can still get good magnification and reasonable image quality for smaller objects is useful. I can then use a single CCD camera for all my DSO imaging instead of a small chip CCD camera and a DSLR camera. I would still of course use the webcam for the moon and planets.

8)

MD

[anthony]

Please feel free to correct, but I thought that the magnification part of things is just dependant on the focal length of your scope.

The size of the chip dictates how much of your target you can fit in.

As KK says, given the same pixel size a crop from a larger chip will give the same field of view if the crop has the same dimensions as the smaller chip.

Now, smaller pixels should allow you to get better resolution and so you can enlarge that cropped area more before image quality suffers. (However, smaller pixels are better at revealing tracking errors and there will probably be reduced sensitivity.)

Don't know if i put that across well or not

Anthony

[moondog]

Anthony

In talking about magnification and CCD's you really need to consider the CCD camera as an eyepiece. The smaller CCD chip being similar to a 5mm eyepiece and the larger one to a 30mm. That's what I mean about greater magnification with the smaller chip CCD camera.

The current small chip size CCD camera that I have has a pretty average 7.8 pixel size. The larger CCD chip size camera that I'm looking at has the same pixel size as my smaller camera. Therefore a crop of the large CCD image would provide the same resolution as the same size image taken with the smaller chip CCD camera.

MD

[Kaptain Klevtsov]

I think we're talking "field of view" here, rather than the wierdly abstract magnification thing.

Check this out and tell me the magnification:-

An image of M31 printed out on a 5" X 4" sheet so that it fills the sheet.

To work out the magnification, lets say that M31 is 250,000 light years across, how much do we need to magnify it to fill the page?

5X? 10X? You might find that you actually have to shrink it quite a bit.

Using any 'scope, M31 has a bigger dimension of 3 degrees ish. If we use a 'scope with a focal length of about 100mm, a webcam chip will be the correct size to accomodate this FOV. Using a CCD with twice the size of the webcam chip, with four times as many pixels, there will be projected onto the chip a view of M31 with room to spare round the edges. The 'scope will produce the same size image, so for the same pixel size, it will cover just the same amount of pixels, it's just that you have more pixels to play with.

Kaptain Klevtsov