understanding spot diagrams.

[pmrid]

Could someone help me to demystify these diagrams?
peter

[RickS]

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.

[pmrid]

Thanks Rick. That makes sense now.
cheers
Peter

[Shiraz]

Good topic Peter

You should also consider that a spot diagram is primarily a design tool that lets the designer visualise the effects of the various compromises inherent in the design process. It generally represents only the geometry of a mathematical representation of a system composed of perfect optical components and does not provide complete information on how a real system may perform.

to get to reality you need to:
1. add in aperture diffraction effects - basic geometric ray tracing does not include diffraction
2. add in practical depatures from perfect geometry (fabrication/figure imperfections, mechanical misalignment, variable optical characteristics of glasses etc.)
3. add in environmental effects (thermal effects in the optics, atmospheric seeing etc.)

ie, spot diagrams must be treated with caution when they are used as marketing tools - they provide some info on the likely performance of a system, but by no means the whole story. As a simple example of their limitations, any Newtonian telescope will produce an infinitely resolved single point as its on-axis spot diagram, but of course no real system can do that and all Newtonians are certainly not equal.

a pretty good summary of where the spot diagram approach fits in is presented on the ZEEMAX website:

http://www.radiantzemax.com/kb-en/Knowledgebase/What-is-a-Point-Spread-Function

regards Ray

[pmrid]

Thanks Ray. The Zeemaxarticlewas very helpful, as was your comment. It seems that in real-world conditions, spot diagrams are like advisory speed signs - indications at best.
Peter