Quote:
Originally Posted by Gama
Thats why the chap in my post used circles with a strip thru them instead of Rod's method, so he can remove most of the extra diffraction spikes.
I use this method, and its easy and accurate.
Still, Dennis is on a crusade with this one.
Theo.
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Sorry, I read your original post as saying that the circles with strip is Rod's method, I just re-read it and still get that impression.
Anyway, the circle/strip method looks like the spikes line up together in the centre.... exactly where the star is, making it hard(for me) to see when they are nicely crossing each other because the star is in the way.
With the three-triangle that I use, the lining up of the spikes is done away from the star, as you get closer to focus, the lines(which are parallel to each other), will move closer eventually overlaping each other at focus.
Imagine holding clenched fists in front of you with each index finger extended(representing two of the parallel spikes, fists as star), now move you hands closer and closer untill the fingers(spikes) overlap.
Even then though, I find this difficult as its hard to tell when they are exactly overlapping each other.
Pavels method looks great, because it is also done away from the centre, but there is no overlapping, making it easier to see minute movements of focus.
Three-triangle example can be seen here:
http://www.iceinspace.com.au/63-187-0-0-1-0.html
Strangely, the examples there don't show large diffraction spikes when not in focus, but I do get them. (even with curved spider)
Anyway, I guess whatever works for you, works for you.