beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 508
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Hi everyone,
When I checked my SCT 8" collimation tonight for the first time, I found it to be out of wack to the left. I was able to bring it back to center, however, I'm not able to get a sharp star image. Where do I go from here?
Thanks
Beachchairbill
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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rough collimation is performed with the scope out-of-focus, looking at a donut.
fine collimation is performed with the scope precisely in-focus, centering the visual disk in the midlle of the visible diffraction rings.
I've always found it necessary to perform a fine collimation adjustment.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Bob Griffiths
Postmaster
Reged: 10/10/05
Posts: 6592
Loc: Frederick Maryland
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Yep it sounds like you now have to do a final fine collimation with the star slightly out of focus so you can see the airy discs.
I use a 4 mm eyepiece (500x) that I keep just for doing my final collimation .... you do not want to see a Donut ....
Bob G
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CPC1100
Nexstar 8i + GPS & Rays Brackets
Denk S1 power switch
Orion 100 mm Refractor
Meade LXD 55 ...AR-5 127 mm Refractor
Exploradome Observatory S.I.E. (Smiling Irish Eyes)
Gerbring Heated Motorcycle clothing in the winter
39*21'03" N
77*28'12" W
The sky over my head....
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Ouranos
sage
Reged: 11/07/06
Posts: 427
Loc: Illinois
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Hmmm. Thanks for posting beachchairbill. I spent almost two hours this week working on collimation as well (C8). What a chore. Collimate, look at image, refind star because I touched the scope and even though it was locked down it moved, collimate, refind star, etc, etc. Collimation, Frustration, Exclamation. How can you see an airy disk when the star is in focus? And, how big do you make the donut when collimating? Do you have to see in the center of the donut, the star in order to collimate? B/C using Polaris - I don't. And, is it a matter of bad seeing when the star image is, for lack of a better word, "boiling". When I was looking at the donut, it was like water was pouring over the top of the image. Is that bad "seeing"? There is a job here - if someone played it right. Moving around the country collimating telescopes!!! I would pay to have someone do it.
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East/Central Illinois
Orion XT10
Celestron C8
AstroTech 102ED
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1826
Loc: Central Texas
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Getting a usable Airy disc requires good seeing and a scope that's pretty well cooled to ambient.
From your description, it sounds to me like your scope was not adequately cooled.
Do you have tracking with your C8 ? If so, you are making too great an adjustment if you lose the star out of the eyepieces' FOV. Try using less power (a wider FOV) get the collimation good, and proceed to higher powers and finer adjustments. The star should decenter from the FOV each time, but not leave the field completely.
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David
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Ouranos
sage
Reged: 11/07/06
Posts: 427
Loc: Illinois
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Thanks, David. Yeah, it was cooled - I think. I had it out for about 90 minutes before I started. Although, initially, I did see a "plume" in the ep that disappeared after about an hour of struggle. I am going to plug the tracking in next time. I was using a Baader Hyp 8 and a TMB 6mm ep. It was not a very rewarding experience. The three screws on the secondary mirror require an allen head wrench that I eventually lost in the grass. At least I now know what it looks like when it is severely OUT of collimation.
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East/Central Illinois
Orion XT10
Celestron C8
AstroTech 102ED
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 2703
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David is correct in that if you move the collimation screws too much, the star will go out of the field.
In general, it takes a VERY small amount of motion to make a BIG difference at the eyepiece.
When you turn a screw, you needed really "Turn" it at all. Mostly, you are moving it in VERY small increments. By that, I mean a couple of minutes of motion (Imagine the screw is a minute hand on a clock.. We are talkking about the difference of the minute hand between 12:00 o'clock and 12:03!
The idea is to move the screw JUST enough that you can see how it affected the image. This will keep you from over-correcting.
You STILL need to RECENTER after EVERY movement of the screw. The reason is that the SCT has pretty pronounced coma, and if you start with the star at the center of the field and move it, the decentering that results is NOT always mis-collimation, it is the COMA from the target star not being in the center of the field.
After moving the screw, MOVE THE TARGET STAR BACK to the center of the field to CHECK it FIRST before moving it again.
Center, check, adjust, recenter, check.
If you fail to do this, you will be chasing collimation because you are trying to adjust for coma. Alays check with the target star right at the center.
Maybe you were doing this already, but I have watched people struggle with collimation because the were not re-centering on every pass.
And when have the technique down, you can tweak collimation in couple of minutes. So, don't stress. Little movements and centering.
Regards.
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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)
The night sky is my mistress. She seduces me away from all other lovers.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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Quote:
How can you see an airy disk when the star is in focus?
That's is the only time you CAN see the Airy disk.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Ouranos
sage
Reged: 11/07/06
Posts: 427
Loc: Illinois
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Eddgie, Thanks for the info and encouragement. It really is appreciated. Forgive me for not making it clear, I was losing the star not b/c of collimation per se, but because the OTA was moving just enough to move the star out of site from the highpowered ep.
Edz, now I am really confused. I thought the airy disk was the donut like apparition that occurs when you focus/defocus. Apparently not. I made the assumption it was an "airy" meaning "light, gauzy, transparent" disk. Brother.
pt
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East/Central Illinois
Orion XT10
Celestron C8
AstroTech 102ED
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sang33ta
scholastic sledgehammer
Reged: 07/08/08
Posts: 767
Loc: UK
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The star has to stay in the center of the eyepiece. I find it easier to collimate on a non-moving light like a TV mast.
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Hioptic 152mm f12.5 Maksutov, CG5-AGT, Meade Super Plossl Set, Casio QV-2900UX
Edited by sang33ta (11/04/09 08:56 PM)
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3460
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In... Out... are you talking inside of focus/outside of focus?? Can you explain your pic's a bit clearer??? Thanks!
Wes
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sang33ta
scholastic sledgehammer
Reged: 07/08/08
Posts: 767
Loc: UK
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yeah, you do a test of focus in and focus out to make a doughnut. You aim to get the black circle in the middle and a doughnut with equal thickness all the way around. If you have air currents the dougnut will look spikey and shimmer 
You can see some more peoples collimation shots here... http://www.cloudynights.com/photopost//showgallery.php?cat=564&thumb=1 the MN61 has baffles and a slight inside collimation error and the Nexstar 8 shows a little tube current still present.
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Hioptic 152mm f12.5 Maksutov, CG5-AGT, Meade Super Plossl Set, Casio QV-2900UX
Edited by sang33ta (11/04/09 09:29 PM)
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EdKiefer
super member
Reged: 04/09/07
Posts: 158
Loc: NY, LI, USA
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these 2 sites will help you understand .
http://www.asterism.org/tutorials/tut14-1.htm
http://legault.perso.sfr.fr/collim.html
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Celestron C-8
Canon XSi
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beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 508
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Hi everyone,
Sorry for the delay, however, the weather in NYC is not great tonight and guess who won the World Series again.
Once a New Yoker always a New Yoker - Lets Go Mets.
EDZ, when you get back to focus and the bright star that I'm looking at is centered and I can not get it to become as sharp as what it looks like through my Canon 15x50 IS, what is the next adjustment? I have three screws - turn them clock wise - counter clock wise - try one at a time, etc. The star that I'm using for collimation is Sirius.
Thanks everyone for your suggestion and comments.
Beachchairbill
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beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 508
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Hi everyone,
Sorry for the delay, however, the weather in NYC is not great tonight and guess who won the World Series again.
Once a New Yoker always a New Yoker - Lets Go Mets.
EDZ, when you get back to focus and the bright star that I'm looking at is centered and I can not get it to become as sharp as what it looks like through my Canon 15x50 IS, what is the next adjustment? I have three screws - turn them clock wise - counter clock wise - try one at a time, etc. The star that I'm using for collimation is Sirius.
Thanks everyone for your suggestion and comments.
Beachchairbill
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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No real need for good seeing. With a star centered, I dither the focuser very slightly through focus and watch the dancing blob expand and contract. If it expands symmetrically, it's good. If it expands toward ones side, I adjust the secondary to shift the image in that direction, recenter the star and try again.
Using this method you can tell if the scope is collimated just about every time you focus when viewing at higher magnification. When you get the hang of this, it just takes a quick look to see that most peoples scopes are not well collimated.
Works on Newts too.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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zawijava
sage
Reged: 10/06/07
Posts: 224
Loc: Wells, Maine 04090
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Quote:
Eddgie, Thanks for the info and encouragement. It really is appreciated. Forgive me for not making it clear, I was losing the star not b/c of collimation per se, but because the OTA was moving just enough to move the star out of site from the highpowered ep.
Edz, now I am really confused. I thought the airy disk was the donut like apparition that occurs when you focus/defocus. Apparently not. I made the assumption it was an "airy" meaning "light, gauzy, transparent" disk. Brother.
pt
A common misconception, actually the Airy Disk is named after George Biddell Airy
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Lone Mountain Observatory
Andover, Maine
Takahashi Mewlon 180
Giro III w/Sokkia Tripod
Losmandy GM8 w/tripod
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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Here's a photo of a precisely focused double star showing the diffraction pattern of both stars. The extent of the Airy disk is to the middle of the first dark ring. here you see the first diffraction ring surrounding the Airy disk of each star.
I fine tune my collimation adjustment by centering the bright central disk within the center of the first ring seen here. Must be done at absolutely precise focus and at very high power, maybe at least 250x in my C5.
Airy Disks Separated
Adjusting collimation to on out-of-focus donut will almost never show a perfectly centered Airy disk. That's why the out-of-focus step is called rough alignment.
Follow the links posted above to Theirry Legualt's page on collimation. I've been using that method for 10 years.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Lane
Carpal Tunnel
Reged: 11/19/07
Posts: 1515
Loc: Frisco, Texas
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I can see the airy disk with my scopes on some nights and that is by far the best way to get precise collimation, but when I cannot see it due to environmental conditions I can still fine tune the collimation. I defocus a star in the center of the field slightly and watch for a small flare or spike to one side. Then I adjust the collimation knobs to eliminate that spike. Basically I am just looking for the star to become round as I go in and out of focus. This works very well except when the stars are dancing all over the place due to serious turbulence.
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Mounts: CGEM, ORION SIRIUS, AT Voyager/motorized
SCTs: C6, C8, C9.25, C11,
Refractors: TV Pronto, Orion ED80
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pgrunwald
member
Reged: 08/09/09
Posts: 42
Loc: Albuquerque, NM
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Replacement for above link: http://www.astrosurf.com/legault/collim.html
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Celestron CGE 1100 (XLT), Telrad
Celestron Celestar Deluxe 8" with NGC-Max and Encoders (Sale Pending)
Meade ETX-90RA with JMI Encoders, MotoFocus, MotoDec, RedDot
Orion Mini-Giants 9x63
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