Gianluca67
member
Reged: 11/26/08
Posts: 98
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I have performed a star test of my Megrez 90 after collimating the optics. While I was unable to detect the slightest amount of astigmatism visually even at very high power, the 3 shots in attachment, taken with the SXV guider, show identical oblong stars in intra and extra focal positions. Does it mean astigmatism or what kind of problem?
Gianluca
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LLEEGE
Running out of Oxygen
Reged: 03/03/05
Posts: 9050
Loc: Cloud-chester,NY
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Not astigmatism. Maybe poor cooling. Had to say from the images but it is not astigmatism.
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"Okay! You draw the straws. I'm-a taking the parachute."
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Posts: 25185
Loc: NE Ohio
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Quote:
Not astigmatism. Maybe poor cooling. Had to say from the images but it is not astigmatism.
Yup. Just to clarify for the original poster, astigmatism is when the oblong pattern (or any pattern) flips 90 degrees between the intrafocal and extrafocal images.
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John C
Urban Observatory
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 2700
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Agree with the others that unless the orientation changes, you can rule out astigmatism.
So, what is left?
Do you see any color fringing when viewing bright stars visually? If so, a possible cause of this kind of abberation is that the lenses are wedged or not centered axially.
I would look at a very bright star near zenith (Vega is close enough) to see if ANY fringing is visible. If it is, you likely have one of these conditions.
Also, double check your collimation. Often things can shift a bit even by simply tweaking the lock screws in place.
And check that the retaining ring is tight.
Again, double check for astigmatism by racking focus.
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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
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WarrenS
scholastic sledgehammer
Reged: 03/04/08
Posts: 892
Loc: Orange County New York
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John, do you mean 180 degrees? 90 would flip at a right angle, 180 would flip to the opposite side. Just trying to visual this aberration.
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Warren
Astro-Tech 127EDT
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starrancher
professor emeritus
Reged: 06/09/09
Posts: 582
Loc: Northern Arizona
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90 degrees from inner to outer focus would be astigmatism .
They were gonna name me John but they changed their minds at the last minute & put Dave on the birth certificate ! 
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Posts: 25185
Loc: NE Ohio
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Quote:
John, do you mean 180 degrees? 90 would flip at a right angle, 180 would flip to the opposite side. Just trying to visual this aberration.
Yes, 90 degrees. The horizontal blobs on one side of focus would become vertical blobs on the other side.
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John C
Urban Observatory
A&M/Astreya 76mm F/6 APO
TMB/LOMO 80mm F/7.5 APO
Tak FSQ-106N F/5 APO
Meade 178ED F/9 "APO"
Meade ETX-125AT
C14
Teeter 20" F/3.8 truss Newt w/ServoCat
CI-700, NJP, GPDX/SS2KPC, CG5-GT
ST-10XME, DSI Pro
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Trombone
professor emeritus
Reged: 07/11/05
Posts: 719
Loc: Canadian west (and north!)
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Astigmatism is an 'out of roundness' in the lens - called cylinder in your eyeglasses. Not necessarily at 90 degrees. Can be any direction.
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SV90BV(F) on Gyro DX
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LXD75
Today anyway!
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Posts: 25185
Loc: NE Ohio
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Quote:
Astigmatism is an 'out of roundness' in the lens - called cylinder in your eyeglasses. Not necessarily at 90 degrees. Can be any direction.
In this context, the deformity may at any angle also - but it will flip 90 degrees as it passes through focus.
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John C
Urban Observatory
A&M/Astreya 76mm F/6 APO
TMB/LOMO 80mm F/7.5 APO
Tak FSQ-106N F/5 APO
Meade 178ED F/9 "APO"
Meade ETX-125AT
C14
Teeter 20" F/3.8 truss Newt w/ServoCat
CI-700, NJP, GPDX/SS2KPC, CG5-GT
ST-10XME, DSI Pro
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WarrenS
scholastic sledgehammer
Reged: 03/04/08
Posts: 892
Loc: Orange County New York
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Quote:
Quote:
John, do you mean 180 degrees? 90 would flip at a right angle, 180 would flip to the opposite side. Just trying to visual this aberration.
Yes, 90 degrees. The horizontal blobs on one side of focus would become vertical blobs on the other side.
Thanks! 
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Warren
Astro-Tech 127EDT
Celestron Onyx 80ED
Astro-Tech Field Flattener
C8 (circa 1983 Orange Tube)
Atlas EQ-G, Orion SSAG
Canon 135mm F2.8
Canon 40D, Astronomik CLS clip filter
Leica, Minolta binos
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Trombone
professor emeritus
Reged: 07/11/05
Posts: 719
Loc: Canadian west (and north!)
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Yes. Sorry
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SV90BV(F) on Gyro DX
CPC1100
12" LightBridge
Round Table platform
8"F6 dob
LXD75
Today anyway!
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Doug76
Postmaster
Reged: 12/05/07
Posts: 5539
Loc: SE Louisiana, future Texan
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I know you are using a guider, but could this be tracking error? I only ask as you said you didn't see this visually.
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Doug
Truckstop Astronomer
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mooreorless
Just worried
Reged: 07/05/05
Posts: 1821
Loc: Cornpropst Mills,Huntingdon,Pa
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Yea Doug, the in focus picture is oblong as well
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Regards,Steve M
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2926
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This could be a baffle that has got sideways so that it presents an elliptical cross-section rather than a circular one. Check inside the focuser for a dislodged baffle, then up the tube.
-drl
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Gianluca67
member
Reged: 11/26/08
Posts: 98
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I can see no color fringing around bright stars using up to 267x with a Pentax XW 7 and a Televue Barlow 3x. I really can't figure out this aberration. I can exclude poor tracking and there is no problem with the baffling of the scope. I have taken the 3 shots with a WO mirror diagonal. I cannot exclude perfect cooling and perhaps there is still a very tiny amount of misaligment, but it is so tiny to be hardly detectable. Can it be flexure of the focuser, or a problem with the SXV guider?
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Max Lattanzi
sage
Reged: 06/27/07
Posts: 215
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Gianluca,
I assume those are stars at the very center of the field, right...?!
Pls confirm.
Can you post a star test on artificial star? Or on the Polaris?
Just to entirely clear the field from any tracking issue, since even the faintest stars have that shape...
Max
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FLYcrash
member
Reged: 08/29/09
Posts: 95
Loc: Chicago, IL, USA
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OK, this might be silly, as I don't have any experience with astrophotography, but...
...could it be that somewhere in the capture train (from the CCD to our computer screens) the image is getting stretched in one direction? I mean, an oblong point is still a point, so a distorted in-focus image would still show pointlike stars. If you squeeze the images horizontally in your image processing software so that out-of-focus stars are circular, is the result at all distinguishable from perfect?
Just a thought.
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Raman
Young, myopic, non-astigmatic eyes; polycarbonate spectacles; Minox HG 8x33, 10x52 binoculars
Orion XT4.5 (114mm, f/8) Dob
Sirius 25mm, 10mm Ploessls; Vixen 17mm LVW; Orion Deluxe 2x barlow
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2926
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Quote:
I can see no color fringing around bright stars using up to 267x with a Pentax XW 7 and a Televue Barlow 3x. I really can't figure out this aberration. I can exclude poor tracking and there is no problem with the baffling of the scope. I have taken the 3 shots with a WO mirror diagonal. I cannot exclude perfect cooling and perhaps there is still a very tiny amount of misaligment, but it is so tiny to be hardly detectable. Can it be flexure of the focuser, or a problem with the SXV guider?
If the focused stars are small and tight then what you are seeing is the telescope's distorted entrance pupil, for example, from a baffle that is sideways in the tube (as I mentioned).
-drl
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Gianluca67
member
Reged: 11/26/08
Posts: 98
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As it is a cloudy night...I have taken some shots through the Megrez, the guider and the CCD indoor. I think I have discovered where the problem is! It is the SXV guider. If you look at the images you find the same patterns in the lights. Besides, the in-focus image of the eyepiece is distorted. The effect does not appear with the SXV main camera. I will have to e-mail Terry Platt to understand why this happens. I had to reprogram the autoguider a couple of years ago because of a system error in the camera, so perhaps there is something wrong with the file I got from Starlight. At this point I dare say the Megrez is not to blame.
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2926
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Take a picture up the tube through the open 2" focuser - use flash to illuminate the inside of the tube. What is this you photographed? Looks like a UFO.
-drl
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