Anonymous
Unregistered
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Okay, so i've tried my hand at some longer exposure (albeit short) constellation shots. I have horrible sky glow after about 15 seconds and wonder just how much information i can get out of these images. Also, could someone explain how they accomplished what they did with this? Just curious if i have anything useful.
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Emanuele
Lord Pentaxian
Reged: 11/19/03
Posts: 4895
Loc: Miami, Florida
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Chris,
here's my shot at it, but I'm just starting out, like you, so we need some kind of expert here!
 I suggest you visit this grea great webpage here:
http://www.astropix.com/INDEX.HTM
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http://www.backyardskies.com
TEC140 #393 --- Vixen VC200L w/FeatherTouch --- Takahashi EM400
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Anonymous
Unregistered
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Thanks, now maybe i can sum this with the other images i have, and reprocess it.
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Emanuele
Lord Pentaxian
Reged: 11/19/03
Posts: 4895
Loc: Miami, Florida
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You`re very welcome.
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http://www.backyardskies.com
TEC140 #393 --- Vixen VC200L w/FeatherTouch --- Takahashi EM400
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4344
Loc: Pleasanton, CA
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Emanuele:
Cold? You don't KNOW cold yet... ha ha ha. Go visit Ottawa in January...
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http://www.siliconvalleyskies.com
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Emanuele
Lord Pentaxian
Reged: 11/19/03
Posts: 4895
Loc: Miami, Florida
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lol I know Suk, I'm getting used to the word for now!
PS: Suk did I do a good job on that image or am I missing something? I need a word of an expert!
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http://www.backyardskies.com
TEC140 #393 --- Vixen VC200L w/FeatherTouch --- Takahashi EM400
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4344
Loc: Pleasanton, CA
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I think you took the black level down too low and lost some stars. You can use the "curves" dialog to fix the contrast. I'd show you an example but my computer committed suicide this morning and I'm slowly restoring everything...
Suk
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http://www.siliconvalleyskies.com
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Anonymous
Unregistered
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Oh dear, i know the feeling Suk...my gateway occasionally gives me "hiccups" that can take a day or two to figure out!  Good luck with that...and i guess i'll give my hand another try at manipulating the images i got and i'll post the end result. If anyone wants, i'd love to see how this should look, and how you got it...
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Emanuele
Lord Pentaxian
Reged: 11/19/03
Posts: 4895
Loc: Miami, Florida
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Thanks Suk, and I'm sorry for your PC 
I'll try another run on that widefield!
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http://www.backyardskies.com
TEC140 #393 --- Vixen VC200L w/FeatherTouch --- Takahashi EM400
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ralon73
sage
Reged: 04/29/04
Posts: 223
Loc: 50.89N - 6.02E, Netherlands
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Hi Chris,
I took the liberty of processing your image a little too.
I just did 2 small and easy adjustments in Photoshop.
1)Setting a new black point and 2) adjusting RGB channels somewhat (seperately).
Maybe not the best you could get out of this image, but it's quick and easy.
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Losmandy G11
Orion UK 10" f4.8 SPX
TMB 80/480
Canon 20D
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4344
Loc: Pleasanton, CA
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Well, since everybody's having a whack at this, I'll go too.
I first set the black and white points, then adjusted the red mid tone slider (all in levels) to get rid the of the red cast and skyglow.
Then applied the curve as shown.
Cheers,
Suk
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http://www.siliconvalleyskies.com
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Emanuele
Lord Pentaxian
Reged: 11/19/03
Posts: 4895
Loc: Miami, Florida
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ok Suk you beat us all! 
How do you know what curve to apply to the image?
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http://www.backyardskies.com
TEC140 #393 --- Vixen VC200L w/FeatherTouch --- Takahashi EM400
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Krytoss
sage
Reged: 09/10/04
Posts: 382
Loc: Toronto, ON
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We in Toronto are looking towards a nice, cold winter. Hows it going to be in California? 
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- Robert
Skywatcher 5.1" Newt / EQ6 / DSI / Dogged Persistence to make it all work
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Anonymous
Unregistered
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Not sure...right now we are still having 90 degree days pretty much constantly. The only clouds i've really seen were last sunday when it rained a bit. It'll probably be a typical 50 degree winter, unless they decide El Nino is having a comeback! 
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Anonymous
Unregistered
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playing wit Suk's shot  We all know he's one of the best
Edited by madchef (09/26/04 02:15 PM)
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StormKat
super member
Reged: 09/29/03
Posts: 144
Loc: Oakland, TN
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Suk is the man! Now create us a video so we can see all the steps 
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-StormKat---
Orion 8" SkyView Deluxe
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4344
Loc: Pleasanton, CA
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OK, here's step 1, which is simply to look at the histogram. Photoshop CS has a really nice feature where you can see each channel individually.
The top right shows the overall histogram. The funny double hump is not natural (but no fault of the photographer!) and reflects the fact that all channels are not accumulating signal at the same rate.
The lower histograms show each channel individually. Note that:
- red channel is offset a bunch, hence the overall red tone
- all channels are offset due to skyglow/chip heat
- for the green and blue channels there's a lot of space to the right of the "lump" - those are the stars - the space indicates that it was safe to take an even longer shot
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http://www.siliconvalleyskies.com
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Emanuele
Lord Pentaxian
Reged: 11/19/03
Posts: 4895
Loc: Miami, Florida
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Ahhhh, nice beginning Suk! Thanks for taking the time to show us where to begin! Have you ever thought about writing a small booklet about astrophotography?
 
EDIT: Now question is: how do you subtract a Darkframe?
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http://www.backyardskies.com
TEC140 #393 --- Vixen VC200L w/FeatherTouch --- Takahashi EM400
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4344
Loc: Pleasanton, CA
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Step 2, adjust the black point using levels.
Don't adjust using the "RGB" channel, adjust each channel individually so that the "hump" starts at the left hand side of the histogram.
The picture shows the blue channel being adjusted and the histogram windows show "before" and "during adjustment".
You can also see at top right that the overall histogram now has a natural "one hump" shape.
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http://www.siliconvalleyskies.com
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4344
Loc: Pleasanton, CA
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Step 3, adjust the white point. In the previous step there was no astonomical data you were throwing away, just skyglow and heat. In this step you're actually throwing away the brightest parts of the picture in order to expand the "hump" and make fainter stars visible. Since a star is just a dot, throwing away some data and making the dot bigger is no problem.
Again, adjust each channel separately, striving to keep the "humps" similarly shaped in each channel. There are more detailed and accurate methods involving measuring mid-tone greys, but for that go buy Jerry Lodriguss' book 
I typically drag down the white point slider so that the mid-tone slider (the one in the middle) is right at the right hand side of the "hump". That gives about half the tonal range of your shot being "hump" and the rest stars. I find that too much more than this and you star highlighting a LOT of noise.
As you adjust the individual channels, watch the background to make sure you don't get any unnatural color casts.
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http://www.siliconvalleyskies.com
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