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Casse vs Newt construction difficulty

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#1 rgpalmer


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Posted 21 September 2018 - 08:48 AM

My ATM experience to date is all Newts.  So a simple question.


Is a Casse more complicated for an ATMer of average ability and shop set up than a Newt?



#2 siriusandthepup


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Posted 21 September 2018 - 10:42 AM



Not tremendously more - but that is dependent on the particular Cass design you pick.


Easiest Cass design - the Dall Kirkham. The spherical secondary makes the collimation requirements easier and the secondary mounting less critical. Primary to secondary spacing is very important.


Next we have the Classical Cass. Much more critical on the colimation and secondary mounting, again super important on the pri - sec spacing.


If you are asking this question then it is not necessary to cover other more sophisticated designs, such as the Ritchey-Chretien. Another level up.


If you are making the optics yourself, then double that yes - the optics are another level up of difficulty in making compared to the Newt optics. You must make two mirrors to both high spec and close to design numbers. Example -  it doesn't matter much if your Newt parabola is an inch plus or minus on the focal length. It matters on the Cass, as the primary and secondary are a matched set and must be close to the design to work well. A quarter inch variance on the focal length can make a devastating difference, depending on the Cass design. Missing the design spacing, pri - sec, by a quarter inch can also be fatal to performance.


If you want to do one - start with a DK.


and good luck!

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#3 ed_turco



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Posted 21 September 2018 - 01:40 PM

I'd stay away from two mirror systems.  They require far more than twice the work.


As for Dall Kirkhams, they are afflicted with large amounts of coma away from the center of the field, and while you may not notice this much at your first peek, you will notice it once you have more experience under your belt!

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#4 Steve Dodds

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Posted 21 September 2018 - 02:20 PM

Don't let him scare you away from Dall-Kirkham's  If you keep it above f/12 coma is minimal.

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#5 macdonjh



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Posted 22 September 2018 - 07:32 AM

I assembled a classical Cassegrain using a mirror set I found on the internet.  It was a terribly frustrating, but ultimately rewarding project that took nearly forever.  I really like my driveway scope and am glad I didn't buy a Mewlon now (that was my fall-back plan).  There's a photo posted in the "post a picture of your home made scope" thread", #297 or something like that.

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#6 turnerjs085



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Posted 22 September 2018 - 09:02 AM

This may not be the thread for this, but is it possible to reduce the coma of a dall kirkham by adding a stop just a bit smaller than the primary near the radius of curvature? I seem to remember something in that vein being done with schmidt cassegrains, creating a long tube scope with the corrector and stop at the ROC and the secondary about halfway down the tube.


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