FoucaultFringes

March 7, 2011:  A Client's Story, Part 2 - Resolution

You may remember an "In the Shop" entry from last year regarding a 32" mirror and 6" minor axis flat that I tested.

I am happy to report that the owner has reassembled his telescope following the refigure and recoating of both mirrors, and has tested the telescope both at his home and at the Winter Star Party.

To try to quantify things a bit, using my Foucault zonal measurements, before the refiguring the mirror had a (best case) peak-to-valley wavefront error of about 0.9 waves.  After the rework, the peak-to-valley wavefront error was indicated below 0.1 waves according to the tests.  This mirror is smooth and is a good figure of revolution, so the estimated wavefront errors are very close to what should be measured across any axis.  The flat was improved from ~1/2 wave to ~1/8th wave.

Not convinced by the numbers?  Based only on his experiences at home, he wrote this short report which should help to remove any doubt.


"I had a chance the other night to make some initial observations with the 32" 'premium' mirror and accompanying 6" flat that you refigured for me.  I only had a few hours before the rising of the waning gibbous moon to observe a few objects.

Using the 32" scope at WSPI am absolutely STUNNED by the visual improvement over the old optical performance of the scope.  I had expected quite a difference, but I guardedly didn't expect this magnitude of improvement.

All traces of the severe undercorrection of the primary mirror have been eliminated in the star test, and all aspects of visual observation are greatly improved.

There is so much more detail to see in all objects:
I realize these are not 'challenge objects', but these are the objects that I can compare and contrast to a higher accuracy based on observational experience.

Before refigure, I found myself struggling to find an exact focus point due to the large area of longitudinal aberration caused by the undercorrection.  'Best focus' was obscured by scattered light from the correction error.

The focus is now so sensitive that I find myself wanting a high gear reduction focuser greater than 10:1.  Focus suddenly and definitively snaps, impressive.  The initial views through these optics are every bit as sharp as my smaller excellent 12.5" scope.  Finally, no compromise at larger aperture.

What amazes me the most, is that these observations were made under average at best seeing conditions.  The views on this 'average' night far outperformed any night at any location, star party, etc.. that I have been to with this scope.  It is a stark reminder of how much the optics, not the seeing or other variables, were limiting the views.

I look forward to no longer having to spend my nights blaming seeing or some other variable/excuse for the poor performance of these optics.  Thank you so much for saving me from sliding into the depths of SALAD (spherically aberrated large aperture denial)!

I would like to say to other large aperture owners out there, if you are not pleased with your 'premium' large aperture optics performance and are tired of eliminating thermals, seeing, collimation, and structural support, and making nightly excuses for the performance of your "premium" optics - there is a remedy!

Thank you for figuring/refiguring large aperture optics that perform without compromise or excuses.
"


That pretty much says it all.  

I had the opportunity to use the 32" telescope
at the Winter Star Party last week.  Monday night was clear and reasonably steady.  I uncovered my own 20" f/3, collimated it, and headed for the 32".  As it ended up, I did not use my 20" f/3 for that whole night!  I was having too much fun "borrowing" photons from the 32", and the views were some of the best I have ever seen through an eyepiece.

Above is one of my photos of the owner enjoying a view on this evening.  I'll never forget this night of viewing, because it was the first time I have ever seen such a large instrument perform like it truly should thanks to a combination of good optics, proper collimation, and steady, fairly dark skies.

Highlights were a stunning Saturn early Tuesday morning, showing more color and contrast than in any instrument I have ever seen, many faint moons, the Encke minimum, and spokes/structure in the rings!

Early Tuesday night I saw ten stars in the Trapezium in M42, and previously I had not ever seen more than six.  I think more would have been visible if the wind had not been shaking the telescope.

On Thursday night, the Eskimo had a mouth at 1400x, despite strong winds and seeing that was not as good as is possible in the Florida Keys.

Here are some more of his words:



"What a conclusion to the story! Having this scope with it's now superb refigured optics, thanks to Mike, has re-energized my passion to observe. It is providing views with more clarity and detail than I could have imagined and has so far met and exceeded my expectations.

Some of the observations made with this scope from the WSP will forever be imprinted in my mind. Looking at the trapezium Tuesday evening and seeing G, splitting the H star(at 1.6" separation), and seeing fleeting but definitive glimpses of the I star for a total of 10 stars in the trapezium, was awesome. Considering these are 15-16 magnitude stars(depending on what information source
is cited) surrounded by four bright stars and a beautiful nebula, this is quite an exciting observation. Just imagine, it required the 36" lick refractor to be built before the G star and H double were discovered by Barnard.

The Owl nebula was a very fine observation with the dark "eyes" staring back at me unfiltered. The central star, and others, easily visible with direct vision.

The Eskimo Nebula at 1380x showed amazing structure of the inner gas ring including the oval loop(mouth) feature that I have only seen in photographs. The outer wide ring presented faint, intricate lateral striations. Why stop at 1380x? That was the limit of my eyepiece collection!

The view of Saturn around 2 or 3 AM on Tuesday morning was the absolute highlight of the whole week for me. I could have packed up the telescope Tuesday and left satisfied. The wind calmed enough to take advantage of razor sharp views of the planet. The planet was crisp with multiple tiny moons skirting the rings. The vividness of the creamy brown,yellow, white and gold colors of the gas bands on planet was something I have never experienced before.  The cloud bands on the globe were well defined. The ring detail was absolutely jaw dropping as the Cassini and Encke division were both easily visible. I have never seen the Crepe(C) ring so well. It had a very dark brown/ruddy color and appeared to have a coarse texture. Lateral color variations(spokes), light and dark yellow/brown were noticeable in the outer portions of the rings themselves. My description cannot do the experience justice. What a sight!!

Many other nebula, globulars, clusters, galaxies, and planetaries were observed. Each with intricate details to be seen with newborn sharpness, crispness, and color present.

I couldn't be happier with the visual results after the refigure. I had a wonderful WSP despite the wind. I felt fortunate to have the
opportunity to observe/compare the same objects through superb 20", 25", and 32" instruments and see the true resolution/detail advantage that quality large aperture provides. I'm looking forward to my next observng session.

Great work Mike!!"



The moral of the story is - with good optics, aperture rules.  It delivers more light, greater resolving power, and greater perceived contrast due to the extra light.

I hope this example gets more observers interested in serious aperture.  There are a lot of bad, large instruments out there that give the good ones a bad name.  I believe that, right now, there are far more bad instruments than good ones.  However, once you see a good one your whole perspective changes.  I hope to improve them, one by one, and show people what they are truly capable of.

Finally, I thought I'd end with a rather symbolic photo - the owner starting at the rising summer Milky Way on a warm night in the Keys.  Scorpius is tempting the observers, rising just over the truss poles and observing ladder, climbing out of the wind-swept Gulf.

Pondering the rising summer Milky Way

As the year goes on, he will get to explore this region of the sky, densely populated with beautiful and interesting objects.

Should keep him busy for a while!



Please check back for future installements of "In the Shop".

Mike Lockwood
Lockwood Custom Optics

Home page