Tonk
Post Laureate
Reged: 08/19/04
Posts: 4356
Loc: Leeds, UK, 54N
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Some of you might know this trick.
If you are a "unguided" astrophotographer like me, and over do the exposure length for your polar alignment accuracy, then all is not lost.
If your stars are slighly oval try this in photoshop:
1) Duplicate the image (ideally fully processed) as a new layer
2) Set the duplicate layer blending mode to "darken"
3) Then apply Filters -> Other -> Offset and nudge the vertical and horizonal offset values until you get round stars. If the values are greater than +/-3 in either direction it might not be possible to be subtle enough
4) As a final tidy up - use the colour range selection tool to select dark background sky from the original layer and paste the selection on top of the second layer. This limits artefacts in the dark background areas
Heres an example - original oval stars on left, rounder ones on right
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Televue 85, GM-8/Gemini, Canon 40D (unmodded), Canon 450D (modded w/Astronomiks clip-ins - UV/IR, OWB)
Coronado SM60/BF10, Baader Herschel Wedge
Leeds Sky Clock Ripon Sky Clock
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jmX
super member
Reged: 04/23/09
Posts: 166
Loc: Orange County, CA
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Thanks for the tip, I hadn't thought of that.
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Jon
C6N + CG5
Skywatcher Equinox 80 + CGEM
http://jmx.ls1howto.com
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bryansay
super member
Reged: 02/25/09
Posts: 111
Loc: Georgetown, TX
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Good Tip.
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Bryan Sayler
Celestron C9.25; Orion ED80
HEQ-5; Orion ST-80 w/starshoot
Nikon D80; Canon 40D Hap Griffin Mod
Empty Wallet 
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s58y
Postmaster
Reged: 12/12/04
Posts: 5509
Loc: Eastern NY
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Interesting technique.
In my case, when shooting through some narrowband H-alpha filters, the bright stars can be elongated by a few pixels, and dimmer stars by less than one pixel.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, TV102iis
Old camera lenses: 800mm f/5.6, 180mm f/3.4
AP900, Barndoor tracker
http://www.pbase.com/s58y
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waassaabee
Carpal Tunnel
Reged: 11/26/07
Posts: 2710
Loc: Central California Coast
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Yet another fabulous tip from Tonk!!
You da man!
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Gary
34N 120W
-My kingdom for blue squares!-
WO Megrez 90FD/TV 0.8x FR/FF
AT8RC
mini Borg 50/Q-Guide/PHD
CGEM
Canon 350D Hap Griffin Baader mod - o.o
My Friend Flickr
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lawrie
Carpal Tunnel
Reged: 01/31/06
Posts: 1744
Loc: Okanagan Valley
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Great tip, thanks.
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Clear Skies
Lawrie
Ultima 8
Atlas EQ-G
ZenithStar 80 FD
DSI Pro - Pro II
Canon 350D
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Nils_Lars
Post Laureate
Reged: 01/04/08
Posts: 3443
Loc: Santa Cruz Mountains , CA
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Cool stuff Tonk , ive used the star rounder plug in a few times.
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Erik
Orion Atlas Self Hypertuned (EQMOD)
Orion ED 80
Williams Optics VII reducer
Celestron 8" SCT
Orion Starshoot Autoguider
PHD guide
Canon 400D Hap Griffin Mod w/Baader filter
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Stilleto CVF and Bahtinov mask
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http://www.flickr.com/photos/31986095@N05/
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Tonk
Post Laureate
Reged: 08/19/04
Posts: 4356
Loc: Leeds, UK, 54N
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Quote:
ive used the star rounder plug in a few times.
Well now you know how it works and frankly its so easy and reliabable to do.If the oval offset gets too large you can quickly see if its beyond repair
I was playing around last night trying to figure this out and had got as far as using darken blending as the basis of thinning the oval, but a simple translate of the duplicate layer didn't have the effect I wanted.
After some googling I found out how to use the Offset Filter from an Indian astromony site to get the effect I wanted. The way they talked about it on the thread I found, it appeared to be an established technique, but I can find little else on it.
Any one else kknow of any references to this method?
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Televue 85, GM-8/Gemini, Canon 40D (unmodded), Canon 450D (modded w/Astronomiks clip-ins - UV/IR, OWB)
Coronado SM60/BF10, Baader Herschel Wedge
Leeds Sky Clock Ripon Sky Clock
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nickatnight
Carpal Tunnel
Reged: 04/14/08
Posts: 1751
Loc: Santa Clarita, CA (LA Suburbs)
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Great tip Tonk. It worked well on the image you posted. It may help take care of a bit of PEC I get to while guiding.
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nickatnight
Celestron CPC 9.25" GPS
Lumicon 2" LumiBrite Diagonal
Meade s5000 18mm UWA, Meade 6.7mm UWA,
Parks G.S-5 10mm, Celestrom 40mm Plossl
Parks 2x Barlow, Meade Tele-Extender
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Lumicon UHC Filter and Deep Sky Filter
Canon Rebel XTi (prime)
Canon Powershot G9 (afocal)
Canon EOS T-Ring and Parks T-Adapter
Lumicon Univ DigiCam Adapter
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KevinUK
Vendor (DSLR-AstroMod)
Reged: 08/22/07
Posts: 720
Loc: N 51'53 W 00'25
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Yes, I found out about this method a while back, had some success but I prefer to actually get my guiding to produce round stars, although at times even this fails 
My main concern is that in translating the frames there no doubt is some detail blurred or lost in the target area due to geometrical transformation applied when stacking.
OTOH, if you don't autoguide, then yes as long as you keep the movement to less than a few pixels this is a easy way of correcting oval star shapes or tracking errors.
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DSLR AstroMod
DSLR filter removal and replacement packages
http://www.dslrastromod.co.uk
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Tonk
Post Laureate
Reged: 08/19/04
Posts: 4356
Loc: Leeds, UK, 54N
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Quote:
My main concern is that in translating the frames there no doubt is some detail blurred or lost in the target area due to geometrical transformation applied when stacking.
That detail is already lost due to the tracking error. Darken blending is a exclusive method picking the darkest pixel from the corresponding points in the two layers. Thus it doesn't make the loss of detail any worse - that has aready been done. All it does is repair teh asthetics of the star shape
I will point out that my example above is only a tiny crop from a larger mosaic, so when the full size image is on screen, the error (blurring due to tracking error) is comparable to the pixel resolution of my monitor. Use it with care and judge carefully - but when the error is minor it can "save" an image until you can get round to doing it again better
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Televue 85, GM-8/Gemini, Canon 40D (unmodded), Canon 450D (modded w/Astronomiks clip-ins - UV/IR, OWB)
Coronado SM60/BF10, Baader Herschel Wedge
Leeds Sky Clock Ripon Sky Clock
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Alex McConahay
super member
Reged: 08/11/08
Posts: 131
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>>>Any one else kknow of any references to this method?
See Scott Ireland: Photoshop Astronomy (p 141 ff.) for a slight variation of this and other methods of what he calls "Star Shaping."
One thing I noticed using this method is that large areas of nebulosity (or brighter background skies) can pick up streaking. A way to minimize this is to move only one pixel at a time. That is, Copy Layer, Change blending mode to Darken, Filter:offset (or "free transform" and the arrow keys is what I use--same idea but more control--or maybe it is just personal preference), then merge down, and do it again and again as necessary.
A better way to minimize this--(a long story coming up):
I was at the Riverside Imaging Workshop just before PATS last year, and Tony Hallas was getting ready for the last part of his presentation, which was answering questions from the crowd.
We were supposed to write our questions on a piece of paper, and hand them in, and then go off to break. When we came back, Tony would have chosen the most popular questions and gotten an answer for them.
I went up to him at the beginning of the break, and asked him to tell us how to fix stars that are trailed like this. His informal answer was immediate: "Just reshoot." That was funny enough for me, who runs around in a much more mundane world of struggling astroimager than Tony resides in. I get so many star trails from my less than perfect equipment and sloppy technique than he does. I simply could not reshoot. And reshooting would not produce the good stars anyway!
The funny thing, though, was that out of about fifty questions that came in on the during the break, about a third of them were the same question about elongated stars.
So, I apparently have lots of friends in my mundane world.
Alex
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Stardaug
professor emeritus
Reged: 08/03/08
Posts: 531
Loc: Ontario, Canada
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You're just full of good ideas eh! Thanks a bunch for this tip - I'll definitely give it try next time.
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"Keep looking up!"
Shawn / Ontario, Canada
Celestron CPC800 SCT w/XLT & GPS // Skywatcher Equinox 80ED F6.25 500mm APO // Canon Rebel 350XT unmodded // Milburn EQ Wedge (a Meade model modified to fit my CPC)
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Gary Honis
Vendor-DSLR Mods
Reged: 12/15/04
Posts: 220
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Another method to correct star shapes and referred to earlier is the free Photoshop Plug-in "Multiple Star Repair 16-bit" by James Ryan and available on his web site at:
http://www.grekalova.com/photo_Astroplugins.html
Click on the link : "Multiple Star Repair BETA.8bf". I use it on all of my astro images since they usually have stars in need of repair, especially in the corners.
I apply the multiple star repair filter on a duplicate layer of the original image and then use a layer mask to just paint the fixed stars over those needing correction.
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Gary Honis
DSLR Modification Service
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Javier1978
sage
Reged: 02/12/09
Posts: 227
Loc: Buenos Aires, Argentina
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Good tip. Thanks for sharing!
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6" f5 Sky Watcher Reflector
Eq3 mount (Dual Axis)
Unmodded Canon 300 D
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