28 replies to this topic

[bobzeq25]

I understand the issue that the mechanical center and optical center of a mirror may not be coincident.  But I suspect in most RC's that difference is small and inconsequential.  I also own the SkyWave software that gives you accurate collimation without relying on a mechanical center spot.  After using my laser collimation tools, I found that SkyWave could not improve my collimation, at least within the limits that my seeing imposed.
 
-Dan

A lot of owners of inexpensive RCs would disagree with you. Me included.

I started by centering the secondary with a Cheshire. Went through hours of collimation, first with an artificial star, then with real ones.

After that I was in good collimation (at least until I changed the altitude). Went back to the Cheshire. As expected it showed misalignment. As planned, I ignored it.

It's like the AVX mount. People who got good ones just don't understand how frustrated the owners of bad ones are.

[KTAZ]

I'll be taking delivery of my 10" Truss Tube RC in about a week.

 

As noted by Dan, I have also read extensively about these scopes and the vast majority of "horror" stories were from folks that had units with the focuser coupled to the primary mirror. 

 

I am therefore unafraid...

[Pickwick]

I have a custom made Dall-Kirkham (not corrected: it is a planetary killer so narrow FOV and high f ratio F/18): it comes with collimation screws for both the primary and the secondary.

 

All mirrors are made starting from a spherical shape whose radius is half of the intended focal length.

They are then polished to get elliptical->parabolical and eventually hyperbolic shape. So the primary mirror of an RC requires more craftmanship than the one of a DK.

Even more for the secondary mirror where the DK has a spherical shape (so it requires just a QC after machine polishing and maybe some small fine tuning) while the RC requires much more job (and the secondary would deserve a tighter tolerance because it works with a smaller photon beam, so every imperfection gets amplified).

 

So yes the RC optics are more difficult to make and the RC is more sensible than the DK for decollimation.

 

However the corrected DK is not cheaper than the RC due to the presence of the lens corrector and its cost.

There are premium corrected DK in the market: they have a corrected field wider than a pure RC and they are not low cost, for example this one:

 

https://planewave.co...ts/cdk12-5-ota/

 

Of course a corrected RC would have an even better correction and higher cost

[Pickwick]

Can you elaborate on what additional advantages would adding more lenses to CDK would produce?

Just saw this, I suggest you this page to check all different types of correctors for CDK (three of them have 2 lenses, one of them has 3 lenses) and also Cassegrain and RC and the different results they give:

 

https://www.telescop...or_examples.htm