Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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For the last year I was struggling with my Meade SN10 telescope which exhibited severe astigmatism. If you search by my name you'll find MANY topics on the topic 
In order to cure my scope I reglued primary mirror, but it didn’t help. I rebuilt the mirror cell and reglued primary again, it didn’t help. Desperate, I reglued primary and secondary mirrors. Still no luck. Terrible astigmatism was showing its ugly face on my pictures driving me (obsessive pixel peeper) crazy. Two nights ago I decided to try Roddier test and it gave me a depressing 0.2 Strehl showing major cosine astigmatism. But seems like something was going on in my subconscious and I recalled that someone somewhere told me that “both mirror and the corrector plate have some astigmatism which cancel each other, that’s why relative orientation is important.” I decided to change the relative rotation of corrector and the mirror. I pointed the telescope (almost horizontally) at an artificial star at 100 feet or so. The telescope showed nice looking flaring sausage instead of a “star”. I unscrewed mirror cell which was secured to the tube with four bolts. Orientation of securing bolts allowed me to rotate the cell with 90 degrees increments. So I rotated cell by 90 degrees, collimated the scope looked at a star (300x) and almost dropped my jaw. The star was quite round. Not perfect but very close to it. Roddier tested it and voila Strehl of 0.74. Not a super-duper APO but not bad either! I quickly set the autoguider, ccd camera, MPCC took several pictures and here they are small tight stars (astrophotography is much more forgiving than high-mag viewing) with no signs of inverted coma, cross-looking stars and other weird things. Average star diameter decreased by a factor of 2-3. I call this a major breakthrough. Wooohooo!!!
So If you think that your Schmidt-Newtonian is a lemon try what I tried. It really helps
Edited by Dennis Sakva (07/01/08 04:00 PM)
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photonovore
Moonatic
Reged: 12/24/04
Posts: 2472
Loc: tacoma wa
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Bravo!  It is delightful to read a tale of persistence, guided by intelligence, rewarded with success!
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Mardi
4" achromat, ETX-70.
Whitepeak Lunar Observatory Website
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dvb
Carpal Tunnel
Reged: 06/18/05
Posts: 1922
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Congratulations!
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Skywatcher 10" f/4.7 Newt on Matilda
Skywatcher 10" f/4.7 Collapsible Dob
Meade 8" SN f/4
Celestron C8 SCT
Celestron C6 SCT
Skywatcher ED100
Skywatcher ED80
EQ6 Pro "Matilda"
AT Voyager
Skymaster 15x70
Mallincam HyperColor Plus
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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Here are couple of Roddier Results (before/after)
I'm not saying that telescope is a star performer, but it became usefull. Stars with coma corrector are quite tight and round. And there still is room for improvement as I rotated the cell in 90 degrees increments. And everytning in-between was left behind.
Before rotating
After rotating
Edited by Dennis Sakva (07/02/08 03:14 AM)
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dvb
Carpal Tunnel
Reged: 06/18/05
Posts: 1922
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Thanks, Dennis -- while you did not take the short-cut to the end, we have enjoyed your journey and learned from the side-trips!
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Skywatcher 10" f/4.7 Newt on Matilda
Skywatcher 10" f/4.7 Collapsible Dob
Meade 8" SN f/4
Celestron C8 SCT
Celestron C6 SCT
Skywatcher ED100
Skywatcher ED80
EQ6 Pro "Matilda"
AT Voyager
Skymaster 15x70
Mallincam HyperColor Plus
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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Thanks dvb,
I belive that large portion of astigmatism is due to glued mirror and telescope pointing at horison (it reduces further when telescope is pointed up). Coma is due to miscollimation and somewhat off-axis star. Even though the improvment was easily visible through the eyepiece I think that Roddier test is extremely useful tool to numerically evaluate scope performance. I wouldn't use absolute numbers from this test but on relative basis it should work.
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Cyclop_si
super member
Reged: 03/13/08
Posts: 128
Loc: Slovenia
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Have you done Roddier test by yourself? What do you need to do it?
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David
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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Yes, Roddier test is probably the easiest of all tests. All you need is star (real or artificial one) and a ccd or web camera.
More information on test and testing procedure you can find here:
http://tech.groups.yahoo.com/group/roddier/
and some more here:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2478638/page/2/view/collapsed/sb/5/o/all/fpart/1
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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Spent some more time rotating the mirror This time in 45 dergees increments relative to previous best position. Rotating one way yielded strehl of 0.4 (boooo). The other way resulted in 0.8 (IMPROOOVING). The rain made me retreat. Next time I'm going to try more mirror/corector combinations. I LOVE this Roddier test!
Edited by Dennis Sakva (07/06/08 09:01 AM)
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Mert
scholastic sledgehammer
Reged: 08/31/05
Posts: 998
Loc: Spain, Pamplona
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Hi Dennis,
Very nice testing with Roddier, and as you have
stated a most usefull program!
To me it still looks like a bit of mis-collimation
present in the images but it is a huge
improvement up to Strehl 0.8.
What is the theoretical value you should be able
to obtain for this scope?
Regards,
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Mert
42º49"N 1º38"W
3" Unitron refractor
6" F12 SW Maksutov,CS2-S
EQ6 + EQMOD 
Webcam with 1.25" nosepiece
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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There surely is some room for improvement. But it started to rain and I had to pack my things. I'm anxious to do more testing next saturday. Good thing that I don't need either clear (I have an artificial star) or moonless skies to do the testing. I don't know theoretical strehl for the telescope, and don't know how to calculate one. But that's a good question. If anyone knows theopretical Strehl for SN scope, please tell
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MtnGoat
scholastic sledgehammer
Reged: 02/18/07
Posts: 932
Loc: Columbia Gorge, WA
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This looks like a very useful test. However, it also looks like a steep learning curve as it appears you need to do a fair amount of detailed manipulation just to get an image you can run the code with.
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"Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use." Mark Twain
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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Hi MtnGoat,
Actually, it's not hard at all. Not even time consuming. I don't do all that complex image manipulations described in other threads. Here is what you need to do.
1. Take 2 short videos of a defocused star. I use ProGuider (free) which allows to capture a region of interest.
2. Stack them in Registax (free), save as 16 bit tiff.
3. Open stacked tiffs in IRIS (you've got the idea how much it costs), crop them with window3 256 and save as FITs.
That's it, you are ready for the Roddier test. The longest part is stacking, which takes several minutes
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snufus
sage
Reged: 09/10/05
Posts: 379
Loc: Round Rock, TX
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Dennis,
You are truly amazing man. You never gave up on your SN and I hope it rewards you for all your hard work. It looks like you are very close. Congrats and thanks for sharing all your experiences.
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Best Regards,
Daniel
Scopes
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Meade 12'' LX200R #1
Meade 12'' LX200R #2
Orion Premium 102ED
William Optics 66SD
William Optics 66ZS
Mounts
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Celestron CGE #1
Celestron CGE #2
Meade LXD750
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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Daniel,
Thank you for you generous comment. I hope that the scope will start performing now. Or else it was a huge waste of time/money.
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 5600
Loc: Islamabad, Pakistan
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Dennis - fantastic experiment and result!
CF
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Learn all about POLAR ALIGNMENT with my Drift Method Tutorial and simulator!! Or visit my Foreign Service Blog!
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Mert
scholastic sledgehammer
Reged: 08/31/05
Posts: 998
Loc: Spain, Pamplona
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Dennis,
Would like to know if you did some more testing!
Any intermediate results showing which way to rotate?
Any clue of which is the theoretical optimum for
this SN?
Can you please extend a little on your artificial
star, what brand, dimensions, led/laser, etc????
I did some Roddier testing with a dirty and quick
"artificial" star and didn't get very consistent
results, this for me is stil a pending job to
get done.
Now that MG version 4 with improved ascom I/F is out
I hope to get the collimation spot on.
Then I will redo all with Roddier and see what I
can obtain.
Keep us informed on your testing please, VERY informative
and usefull! 
Thanks,
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Mert
42º49"N 1º38"W
3" Unitron refractor
6" F12 SW Maksutov,CS2-S
EQ6 + EQMOD
Webcam with 1.25" nosepiece
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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I use PicStar artificial star. I take around 250 frames with BW modified ToUCam and average them in Registax. Results are pretty consistent. They do change a bit when I change collimation or move star off-axis, but that's expected.
I did more testing and got around 0.84 with prevailing spherical aberration, which is most likely due to proximity of the artificial star. Removing spherical from calculation resulted in strehl of 0.91
My friends think I'm crazy. They got tired of my "tesing...","testing...","testing...".  
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blackhaz
Carpal Tunnel
Reged: 05/20/06
Posts: 1564
Loc: Prague, Czech Republic
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Dennis be sure I am not. I watch your threads with a great interest.
I Googled for PicStar and found no relevant results. Can you help finding more info on this thing? I'd love to test my C11.
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Max,
10" Aries MCT AP900GTO, C11-SGT XLT, WO ZenithStar 66ED, 90mm <1Å H-a, <2.2Å CaIIK Solar Achromats
Photos and Projects: http://www.bcsatellite.net/bao/
ČAS Solar Section
Edited by blackhaz (07/16/08 04:51 AM)
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1062
Loc: Kiev, Ukraine
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Hi Max, glad to hear that
I meant PicoStar. Here is the link.
http://www.pulsar-optical.co.uk/prod/rigel/generalaccessories/picostar.html
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