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Peter,
A spot diagram shows you how a point source of light, like a star, gets smeared over the image plane of a scope (i.e. where your eye or a CCD sensor goes).
Attached are a couple of examples. The first is a TOA-150 with no flattener. At the top right is the scale (0.1mm = 100um). This is useful to compare with your camera pixel size. If the spot is large in relation to the pixel size then you'll get bloated stars and loss of resolution. The legend on the right shows the colours that represent different wavelengths. The spot diagrams themselves show how the light of each of these different wavelengths behaves. There are three spot diagrams in the first example. The first one is on-axis, and it's a lovely small spot. The second one is 11mm away from the axis and you can see it gets splattered about quite a lot more. The third spot diagram is 22mm from the axis (it would be right in the corner of a sensor with a 44mm diagonal) and without a flattener you can see it looks pretty ugly! Not only does the point get spread out, but different colours are affected differently which will cause colour fringing.
The second diagram is a TOA-150 with a flattener. At 22mm it still looks pretty good. They show additional spot diagrams all the way out to 44mm, equivalent to an 88mm diagonal sensor which is huge! Tak claims a 90mm image circle with this configuration.
Hope that helps...
Cheers,
Rick.
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