I am no expert, but to my eye the last test image (post #38) looks very close to being spot on. I have seen more significant star distortions in images taken by a very successful amateur imager using a 20" PlaneWave, as well as in the latest image taken by ESO: https://www.eso.org/...c/news/eso1719/
I reckon a bit more practice with collimation and perhaps refining camera's tilt and you will have a sensibly perfect system
Those images were taken with my QHY163M which has a smaller chip than the G3-16200. I agree that the QHY163 images look decent, but there are still some artifacts around the stars. And when I switched cameras things totally changed (for the worse). That shouldn't happen. Alignment should remain constant regardless of the camera. Even with some tilt internal to the camera, I don't believe that will account for the differences in collimation that I am seeing after switching cameras. Please understand that I don't really know what I'm talking about, but I wonder if the Marcon mirror has different mechanical and optical centers than the CFF mirrors typically do.
The GSO RC optics seem to be acceptable quality. If you use it as an astrograph and not for planetary observations, you don't need high quality optics. The mechanics and stiffness of the telescope are then far more important. If your system has 0.7 strehl, it should be more than enough. For RC's the mechanics seem to be more important than the super high level optics; especially if you have a camera with big pixels and you're limited with the seeing (nyquist criterium). I'm looking at the big GSO rc's right now, but it's not very clear how stiff the telescope is with the new updated mirror cell.
Maybe CFF can offer instruments with the GSO optics but their own mechanics and frame? I wonder how the mirror cell of the CFF looks like and how they counter lateral movements of the primary when the telescope is pointed at different positions in the sky, horizontal, vertical and rapid goto movements come to mind.
I'm not so sure. I believe that CFF guarantees a higher strehl than the GSO mirrors. While I agree that the mechanics of an RC scope is super important, why wouldn't I want the best mirror inside those mechanics? At the risk of putting words in Catalin's mouth, I suspect that he would clearly state that the CFF mirrors are going to be consistently better figured etc than the GSO. GSO makes good mirrors. I learned that in my research. But like anything that is more mass produced it's harder to maintain consistent high quality and it might be a bit more of a gamble with a GSO RC.
Although I suspect the Marcon mirrors are the problem here, I also want to be clear and state that they may not be and there's lots of other things that could contribute to what I'm seeing in images, including my own ignorance. I just don't know. I've gone through collimating so many times I could do it in my sleep.
I kind of disagree on the camera. There aren't a lot of high resolution images (under 1" or less) that have been taken with the 16200 yet and that is a lot bigger chip. In addition to the mirrors artifacts you are seeing could be related to the flattener, tilt (internal or more likely from the focuser), the camera cover glass, filters, or even microlens. I can tell you with the RH200 what looks perfect with on a Canon T3 looks way off on a ICX694 and in turn off on a ASI178. I had to go through two sets of astronmik filters on the RH200 (granted much faster scope) to find a set that didn't cause tilt in the field.
As long as the scope is diffraction limited the strehl ratio (.8) really doesn't matter IHMO especially for deep sky. Every GSO I have seen tested has met that criteria. My experience so far has been that the Celestron and GSO's of the world do a better job on QC than the Bisque and Officina Stellare's probably because they acquire a lot more experience (with a lot more automation).
I believe that CFF guarantees a higher strehl than the GSO mirrors. While I agree that the mechanics of an RC scope is super important, why wouldn't I want the best mirror inside those mechanics?
The price !!! A CFF RC is more than double the price of a GSO RC. I don't know what percentage of the price is the optics and what percentage of the price is the mechanics/frame. If you use a QHY163M camera with very tiny 3.8µ pixels on a location with stunning seeing, then your strehl of 0.94 is important. I plan to use a 16803 chip on an RC, with its 9µ pixels and seeing limited conditions at high focal length, the strehl is not that important since the airy disk is smeared out over an area of 27µ (nyquist creterium). You'll never push the optics to the limit on axis; only a big corrected flat field is important if you use the instrument as an astrograph. Average optics would make no difference than high quality (and expensive) optics. Don't forget that RC's are also designed to be used for planetary observations; then strehl is important off course. It's like buying a ferrari when you only plan to use it inside the city at 50km/h limit. A fiat 500 would be suited better then. :-)
At the risk of putting words in Catalin's mouth, I suspect that he would clearly state that the CFF mirrors are going to be consistently better figured etc than the GSO. GSO makes good mirrors. I learned that in my research. But like anything that is more mass produced it's harder to maintain consistent high quality and it might be a bit more of a gamble with a GSO RC. I can say unequivocally that the CFF scope mechanics are incredible.
Several years ago I did have an AT8RC. Nothing wrong with that scope but the CFF RC is light years more stable. Once I get the mirrors and alignment sorted out this scope will serve me well. The mechanics are probably the primary reason I went with CFF.
This depends on the whole imaging train you plan to use. GSO seems to be able to make repetitive acceptable optics. If I would have a big budget, I would chose without any doubt for CFF. Unfortunately I am not and then you start to investigate what you can sacrifice. The CFF mirror cells seem to be of higher quality than those of GSO, but is this worth twice or even trice the price ? I think things like coating quality on the mirrors is more important then. I havent found reports on that.
Diffraction limited means exactly what it says. You are going to be limited by the Rayleigh/Dawes/Sparrow resolution, seeing, and guiding long before you ever can use all that high strehl ratio once you get a scope of significant aperture .
The mechanics are DEFINATELY worth double the price. I can't stress this enough. The reason to by premium equipment is better mechanical design and large illuminated fields NOT better optics. Planewave/OS/Alluna don't produce better optics than Celestron or Meade but they do produce better mechanics. The adjustments on the primary mirror on a GSO truss are exceptionally rough and very hard to get in exactly the right place. That wouldn't matter as much if the design was more tolerant but a RC is not that design. Edge HD (reducer availability issues, diffraction off low quality corrector glass) or Vixen 260L (huge corrector in front of secondary mirror) definitely have challenges but are definitely easier to get collimation and alignment perfect.
RCs have too large of obstruction to really make use of on planets even with a CCD, the contrast loss vs a lower obstruction instrument (almost 15% vs SCT) is just too much. My poor planetary attempts were always more washed out with the GSO RC vs. C9.25. Great read on why obstruction still impacts CCDs. http://www.astrophot...uction_ccd.html
Edited by akulapanam, 18 June 2017 - 05:01 PM.