Quote:
Originally Posted by Rodstar
Mark,
The mirror is two inches thick. What thickness of mirrors do you use? I am surprised that you would think there might be vertical mirror bend on the horizontal axis from such a thick mirror, or that this could so significantly affect the Strehl. Can you clarify?? 
I am aware that Strehl is only one measure of the quality of a mirror, and does not disclose other potential failings in the mirror, such as astigmatism or a turned edge. I'll let you know how the mirror performs in real life when I have a decent night's observing under my belt. In the meantime, I am not going to anticipate any negatives unless they become apparent. |
Hi Rod ,
I wasn't intending to cast any doubt on the quality of your mirror. 20" mirrors regardless of thickness bend about 1/2 wave with axis *horizontal* regardless of whether being 1.6" or 3" thick. ( In answer to your question I use 1.6" as they perform perfectly on an 18 point cell and cool faster than 2" ). This mirror bend is almost gone by the time you are pointed 15 degrees above the horizen. It is with axis *vertical* that thicker mirrors need less float points. But ( horizontal ) thats not how you use your scope, and the wavefront quality from the sky near the horizen would be many waves anyway.
I responded in an 'educational' sense when I saw you make a comparison about your mirror not being 'quite as good as Mike Sidoneo's 12" , by comparing the Strehl ratio's. From memory Mikes mirror being a lot more rigid, allowed the opticians to leave the Astigmatism component 'switched on' in the output. Makers of large mirrors usually subtract it out to show a high Strehl number and more indicative of the good spherical aberration correction) and use judgement and experience to make that call. That is the sense that I meant there was artistic licence involved. A proper analysis of a large mirror to extract the true astigmatism component would probably cost more than the mirror. So to keep mirrors affordable you have to trust the optician and their reputation.
Strehl is probably a bit too mis-understood as a `golden bullet' and it is just not possible to make comparisons of different size mirrors on the basis of Strehl alone, as it is usually derived in basic interferometry by fitting polynomals, and doesn't really cope or take account with small period zoning, roughness , astigmatism , and turned edges. Phase shifting interferometry is better at those things but I'm not aware of any opticians to the amatuer industry using it.
OMI certainly know what they are doing, it wouldn't really matter what documentation they supplied, they have a great reputation. I've only looked through John Bambury's 18" but there were certainly no isssues there and yours won't be any different I'm sure
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Hope this clarifies your questions and proved educational.