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Originally Posted by casstony
I've seen dramatic changes in the star test between a fully cooled and partly cooled scope. You can think you've got a lemon then check again at midnight or so and see very even images.
If the ronchi lines are straight with just 2 or 3 lines showing, and no astigmatism is visible when you roll through focus, you're probably somewhere better than 1/4 wave which is all you can expect from a mass produced sct. I'd be happy with that scope.
Tony
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Hi Tony
Thanks for the input. I have a Stellar Technologies International, Series IV 'Stiletto' Focuser for Film and Digital Imaging – see
http://www.stellar-international.com/
This is a modular system that uses a Ronchi-grating to allow you to quickly achieve pinpoint focus using a star. I think the grating in mine has 300lpi?
I have only used it a couple of times with my Pentax *istDS DSLR and the lines have appeared nice and regular as well as parallel, inside and outside of focus, which seems to indicate satisfactory optics.
Cheers
Dennis