danner
super member
Reged: 02/27/07
Posts: 141
Loc: Bay Area, Cali
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on the outer edge of view but sharp in the middle out to 85% of the field of view? I take it to mean the focus is not sharp on the outer edge. What causes that? Astigmatism?
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XT8
Custom 10" f/4.5 Dob
Nagler's dot the i and cross the t.
 Coe
 Montebello
 Coyote
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THESKY
scholastic sledgehammer
Reged: 02/16/06
Posts: 928
Loc: SOCAL
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It is called curvature . . . usually caused by your EP.
Astigmatism is when you make the star image out of focus
the form is elliptical (it should be perfect circle). And when you rack the focuser in and out, the ellipse inside focus is 90 degrees to the outside focus.
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10x42L IS
12" DSH
3.5, 5, 7,
10, 14, 20,
30, 40
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danner
super member
Reged: 02/27/07
Posts: 141
Loc: Bay Area, Cali
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Thanks.
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XT8
Custom 10" f/4.5 Dob
Nagler's dot the i and cross the t.
Coe
Montebello
Coyote
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KWB
Postmaster
Reged: 09/30/06
Posts: 9268
Loc: Westminster,Co Elev.1646Meters
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I disagree. To me it means a view with noticeably less
contrast and definition/sharpness. That's a simplistic
answer. Astigmatism and FC are different matters. I don't
equate them to "soft".
Kenny
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Kenny
"Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe." - Albert Einstein
Edited by KWB (04/24/07 03:24 AM)
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4968
Loc: Zuni Mtns, NM
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Your eye piece may be contributing.
Might it be coma? coma is a natural abberation of the parabola. Hard to tell from the description.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14711
Loc: Cumberland, R I , USA42N71.4W
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It could be a number of things. It could also be several things happening at once.
WHAT DO ABERRATIONS LOOK LIKE
ON_AXIS
Generally, not all aberrations are seen on-axis. The on-axis image created by aberrations is as follows:
curvature not seen on-axis.
coma not seen on-axis.
If the stars seem never to focus to a fine point, but at best remain slightly bloated and are still circular, and it cannot be focused out it is probably spherical aberration.
If the star seems to not come into sharp focus it may be astigmatism. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint. Passing in and out of focus will show the out-of-focus image to flip orientation by 90°.
Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point. Sometimes seen as a small circular band of color arounnd a finely focused star point.
OFF_AXIS
As for how you notice stars in the outer fov, the image created by aberrations is as follows:
If the star seems slightly bloated, but it can be refocused down to a finer point, then it is curvature.
If the stars seem flared as wedges with the point towards the middle and the flared wedge toward the outer edge, then it is coma. It cannot be focused out.
If the stars seem slightly bloated but are still circular, and it cannot be focused out it is spherical aberration.
If the star seems slightly bloated and slightly elongated than it is astigmatism. Astigmatism will show elongated images off axis. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint.
If you have both astigmatism and coma, the off axis images will look like comets with an oblong axis, or like seagulls.
Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point.
Lateral CA will produce color fringes around all bright objects when viewed slightly off axis, but you should see a different color towards the lens center than you see towards the lens edge.
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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csa/montana
Wild Spirit
Reged: 05/14/05
Posts: 40146
Loc: montana
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Edz: Thanks for posting this great explanation. This would be a good post to "sticky" at the top of the forum, so it doesn't get buried.
Carol
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Carol
AstroTech 16" Dob (Thanks ASTRONOMICS!)
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werewolf6977
Lord High Smasher
Reged: 12/15/03
Posts: 8429
Loc: Hanover, Ohio
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I'll go ahead and make it sticky now
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Pete
6" Apogee/LXD55
Starhopper 6" Dob
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WO Zenith Star 66 Patriot Edition
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csa/montana
Wild Spirit
Reged: 05/14/05
Posts: 40146
Loc: montana
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Thanks Pete! Edz explanation is the best I've seen to explain all of this. This will give us something to refer others to read.
Carol
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Carol
AstroTech 16" Dob (Thanks ASTRONOMICS!)
Vixen 80MF/AstroTech Voyager
Masuyama's 7.5, 15, 25W, 35mm,
Pentaxes; 5XW, 7XL, 10XW.
14mm Meade 4000 UWA
TV Panoptics; 22, 35
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FebStars
scholastic sledgehammer
Reged: 02/02/06
Posts: 826
Loc: MA
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Thanks, Ed Z.! Great explanation!
Tom
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Tom H
Orion ED80
Celestron 8
Televue Genesis 500mm
Orion EQ-G, LXD75 Mounts
"The cure for boredom is curiousity. There is no cure for curiousity."
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werewolf6977
Lord High Smasher
Reged: 12/15/03
Posts: 8429
Loc: Hanover, Ohio
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Ed's the man!!
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Pete
6" Apogee/LXD55
Starhopper 6" Dob
Spaceprobe 130EQ
Black C8 OTA
WO Zenith Star 66 Patriot Edition
Sun Pak Pro 7500 Platinum Edition
8X42 Bushnell H2O Porro
7X35 Tasco
10X50 Nikon Actions (Type 7)
15X70 Skymasters
Dell Inspiron Dual Core 531s
"Science without Religion is lame, Religion without Science is blind" A.Einstein.
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karim
super member
Reged: 11/09/05
Posts: 129
Loc: HB, California
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I must be lucky, I have never experienced any of these symptoms. I have a C8 and use
13mm T6
9mm T6
7.5 mm Parks golden Series
2x Parks Barlow
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http://www.eyepiecereviews.com
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mnpd
professor emeritus
Reged: 06/28/05
Posts: 646
Loc: Nashville, Tennessee
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Speaking non-technically, to me a "soft" view means you are turning the focus knob on an out-of-focus object to bring it into focus. Before you get to that crisp image we all like so much, the focus starts moving out-of-focus the other way. Optics, atmosphere... different causes.
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Tom
12" f/5 Orion 12XTi
8" f/6 Orion 8XT
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Dylan Gladstone
scholastic sledgehammer
Reged: 08/05/07
Posts: 988
Loc: Connecticut, USA
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I think I've experienced all of these with my eyes if I take off my glasses. 
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Orion SkyView Pro 127mm Maksutov
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lymorkiew45
professor emeritus
Reged: 12/30/07
Posts: 735
Loc: In the dark, and way out there...
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Me too...clear skies 
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Jorgen
Starfinder 16 dob: Zambuto Optics! 
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Toryglen
member
Reged: 01/04/09
Posts: 30
Loc: From Glasgow to Canberra
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Quote:
It could be a number of things. It could also be several things happening at once.
WHAT DO ABERRATIONS LOOK LIKE
ON_AXIS
Generally, not all aberrations are seen on-axis. The on-axis image created by aberrations is as follows:
curvature not seen on-axis.
coma not seen on-axis.
If the stars seem never to focus to a fine point, but at best remain slightly bloated and are still circular, and it cannot be focused out it is probably spherical aberration.
If the star seems to not come into sharp focus it may be astigmatism. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint. Passing in and out of focus will show the out-of-focus image to flip orientation by 90°.
Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point. Sometimes seen as a small circular band of color arounnd a finely focused star point.
OFF_AXIS
As for how you notice stars in the outer fov, the image created by aberrations is as follows:
If the star seems slightly bloated, but it can be refocused down to a finer point, then it is curvature.
If the stars seem flared as wedges with the point towards the middle and the flared wedge toward the outer edge, then it is coma. It cannot be focused out.
If the stars seem slightly bloated but are still circular, and it cannot be focused out it is spherical aberration.
If the star seems slightly bloated and slightly elongated than it is astigmatism. Astigmatism will show elongated images off axis. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint.
If you have both astigmatism and coma, the off axis images will look like comets with an oblong axis, or like seagulls.
Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point.
Lateral CA will produce color fringes around all bright objects when viewed slightly off axis, but you should see a different color towards the lens center than you see towards the lens edge.
edz
excellent !!
concise, simple, and informative 
now, what can be done to fix all of the above ???
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Ibanez Jem 7V, Washburn N4 Relic, Washburn N4 Paduak, Washburn N4 Black Ash (#18 of #25 worldwide), Line6 Flextone III
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sxinias
member
Reged: 10/04/08
Posts: 48
Loc: Greece
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I too think that Edz's excellent posting ought to be a sticky.
joe
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