Let me clarify. For color cameras, which is preferred, the ASI224MC or the ASI290MC?

And maybe why.

Thanks.

Jeff

It depends on your scope system.

**Ideally, you want a system f = 5* pixel size (in um).**

So if you have a C8 (f/10) with a 2x barlow (=f/20), you would want a camera with a pixel size of about 4.

The 224 has 3.75u pixels, the 290 has 2.90u pixels, so based on this the 224 might be a better fit.

Ok, I'll bite. Where did this criteria come from and what is the down side of going to a larger multiplier than 5? You mentioned greater over-sampling. What is that and what are the pit-falls of having that too large?

All help is appreciated!

Jeff

It comes from sampling theory: if you do not want to undersample, you need a pixel spacing no larger than half the resolution (Nyquist-Shannon theorem).

So we need to solve the following:

- angular sampling < angular resolution / 2

- angular resolution = 1.22 * wavelength / diameter (it's one of several definitions of the resolution)

- angular sampling = pixel size / focal length

- focal length = f * diameter

When you solve the whole thing, you get: f > (pixel size * 2) / (1.22 * wavelength)

Using for example the shortest visible wavelength (~390 nm), this yields f > 4.2 * pixel size (in um)

In practice, you could use whatever f value above this lower limit. However, the more you magnify, the lower the signal intensity. So you will need a larger gain (and then get higher noise) or longer exposure time (and the risk to have blurring due to atmospheric effects / vibrations ...).

In summary, 5 times the pixel size in um is close to the "ideal" value, and for bright targets having a higher factor is not a problem.

Edit:

Using color cameras complicates the calculation due to the Bayer matrix... It's probably better to use slightly larger f values in that case.

**Edited by SwissCheese, 21 March 2017 - 01:51 PM.**