[Dennis]

Hi Matt

Okay, let’s have a look. We’re looking at 3 things here.

Sky coverage per pixel in arc secs (resolution based on seeing).
The focal length of our telescope.
The pixel size of our ccd chip in microns.

Starting on the centre line (focal length), we know the C9.25 has an fl of 2350mm, so we place an imaginary ruler, horizontally, between 2000 and 3000, where we estimate 2350 to lie. Let’s drive an imaginary nail through the ruler at this 2350 mm mark so the ruler will pivot on this nail.

Next, we would like to do some deep sky imaging of say, NGC253. So, we look at the left hand curved line (sky coverage per pixel) and see a grey block labelled “Deep Sky”. You will note the grey block starts at 1.5 arc seconds and finishes at 2.5 arc seconds. What this is indicating is that these are the typical seeing conditions we might encounter when imaging e.g. galaxies. 1.5 arcsecs is good seeing whereas 2.5 arcsecs is average seeing, where a typical star on a ccd would bloat out due to the seeing.

Next, we pivot our imaginary ruler around the virtual nail so the left hand side of the ruler dips down until it lands in the middle of this grey bar, on say the number 2 for 2 arc secs.

When we have done this, the right hand side of the ruler will have moved up and will cut the right hand curved line (pixel size) at the 24 tick, just below the grey area labelled TC-241 which is a Texas Instruments ccd chip with pixels of around 26 microns.

So, the nomogram is indicating that the optimum size of a pixel for our C9.25 at 2350mm focal length to give us 2 arc sec is 24 microns, when imaging deep sky objects.

Now, if we were to fit a focal reducer to the C9.25 giving an effective focal length of 1480mm, we would pull out our nail and refit the ruler at the 1500 mark which would indicate an optimum chip size of around 15 microns for deep sky objects with 2 arcsec seeing.

However, if we want to image planets and hi-res parts of the moon, the 2 arcsecs doesn’t cut it and if we look at the left hand “seeing” line we note that for Planetary imaging, we would like to resolve around 0.5 arcsecs per pixel. As our ruler pivots so the left hand edge comes to rest at 0.5, the right hand edge slides down to between 5 and 6 microns, which happens to be the pixel size of the ToUcam (5.6 microns).

I think that this is based on the Niquist sampling theory, which simplistically says that to resolve a single piece of data, to need to sample it at twice the resolution (or frequency for a signal). If your pixels are too small, then you are over sampling and maybe using say, 4 pixels when 2 would give you all the detail that is able to be resolved.

If you under sample by using a ccd with pixels that are too large for the fl, then stars begin to look like squares rather than nice round dots.

Hope that helps.

Cheers

Dennis