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Book - Star Testing Astronomical Telescopes

PostPosted: Sat Apr 22, 2017 4:52 pm
by Ngent
Anyone got this book? Is it any good?


Star Testing Astronomical Telescopes

It sounds like it could be what I need to set up my RC.

Re: Book - Star Testing Astronomical Telescopes

PostPosted: Sat Apr 22, 2017 7:11 pm
by silentrunning
Hi Nigel

Most of whats in there probably won't be applicable to the AA 6RC as the primary problem with this scope is the fact that the mirror and the focuser are joined together. Following what should be an easy collimation strategy for these scopes will fail if the mirror, main baffle and focuser are not aligned to begin with, it's near impossible to work out what is going on if these aren't aligned. Since non alignment of these attached components is a manufacturing issue you have two options, either to separate the focuser from the mirror arrangement as I did with my twelve inch or come up with a strategy for aligning that takes it into account, this will require a tilt ring. I've think you have read my strategy for doing this.

When it comes to star testing it is essential that you do this without any other components in place such as the FF/FR, make tiny adjustments to the screws and most importantly ensure that the star you are collimating with is at the centre of the field. If the star is not centred you will mistake inherent aberrations - chiefly atigmatism with RCs - with miscollimation and once again you'll be on a hiding to nothing!

The standard collimation process for RCs can be found at various web sites although it probably won't work for this scope. The book won't explain what I have discovered but it may be an interesting read!

If you really want to dig into optics and telescope design I would save your money and check out these sites ... F_CONTENTS

Good luck.

Re: Book - Star Testing Astronomical Telescopes

PostPosted: Sat Apr 22, 2017 9:13 pm
by Ngent
Thanks John.
I have indeed followed your strategy and it made a lot of sense - and I ordered a tilt plate!
I still see two issues however:
  • the physical and optical centres/axes may not coincide
  • I have inadvertently changed the separation between the two mirrors
I am unsure of the best way to deal with either of these issues, but one of the many RC calibration process I have seen relies on iteratively removing the on-axis coma and balancing the off-axis astigmatism with the aid of star tests. I have also read that the spherical aberrations (which are affected by the mirror separation) can be assessed with star tests. Sooo, I was sort of hoping this book might help. However I am not keen to spend £35 if it doesn't, and until someone can recommend it I think I will wait.

Re: Book - Star Testing Astronomical Telescopes

PostPosted: Sun Apr 23, 2017 11:24 am
by silentrunning
Hi Nigel

you are right as there are no lateral adjusters for the spider vanes or main mirror arrangment it may prove impossible to get the physical and optical axis to coincide - this is where you have to rely on the manufacturing tolerances. I had the 12" adaptor made with lateral adjustment possible to overcome this problem. I wouldn't worry to much about the mirror separation unless, as you point out spherical aberration is apparent once all else has been addressed!