Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
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I went to a dark sky site last night for the first time, and made some comparisons from my home site.
My home site sky is 4.25 - 4.50 mag, and the dark site is 5.25 - 5.50.
Each shot is 7x10 minutes, and conditions on both nights were excellent.
The first image is the stack from DSS with no processing.
The second shot has quick basic processing. I set the black point on each, then did the same curve stretches.
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Arran
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gatsbyiv
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Reged: 03/29/09
Posts: 67
Loc: Whitehouse Station, NJ
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I find posts like this to be enormously educational as a novice. Nothing speaks more clearly than an example. (It also makes me feel better that I can't coax any detail out of M101 from my light polluted skies!)
Someone should compile all of these A/B examples that have been posted on CN (LP vs. non-LP skies, 80mm vs. 102mm shots, F7 vs. F10, etc...) into a book.
Thanks for posting this, Aza!
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Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
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I was shocked, as I had never been to darker skies before.
Previously I had 12 hours of M101 data, but this single hour of dark sky data is of about the same quality!
Being in the UK, I dont get many oppurtunites for imaging. So it seems to me, it is worth the hours drive to get to the darker skies.
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Arran
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7808
Loc: Franklin, Ohio
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Hi Arran,
Thanks for posting the very informative images. I had the same experience this weekend imaging M81, once from the house and once from our clubs dark sky site. The dark sky site was so much better that it makes the 45 minute trip there seem trivial (except for the time and energy it takes to pack everything up). Maybe I should be focusing on ways to keep everything ready to go at a moments notice rather than on ways to cut through the lp in my neighborhood. 
Patrick
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Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
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Yeah I know what you mean Patrick!
As I dont have a permanent set up at home, I feel as if I should make the trip each time I image in future.
However being out on your own in the cold with nothing to do, is a bit different from sitting indoors and checking things every once in a while 
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Arran
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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Hi Arran,
Your images are interesting, but don't really tell the whole story. The fact is that since the images from the dark and not-so-dark sites have the same total exposure times, the amount of signal you have in the galaxy is the same for each. The difference is the amount of signal in the sky glow, but the galaxy signal sits on top of the sky glow, so it can be extracted. It takes more stretching, and the room left for stretching is different because of the higher baseline, but all of the details you captured from the dark site are there in the image from the not-so-dark site as well.
Dark skies are a distinct advantage, obviously, but not a reason to defer imaging until you can make the trip IMHO.
I edited your second JPEG (with GIMP because I'm currently on a Linux PC, and I'm not very familiar with GIMP) to demonstrate the fact that all of the details are there in both images. With some careful manipulation to optimize both (and starting with RAW or TIFF files), it would be difficult to tell the two apart.
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Don
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turbo399
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Reged: 12/21/07
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Loc: Michigan
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Nice images. That is a very interesting comparison. I have just moved from a light polluted city to a country location. I would be all set if the weather was was an equal improvement.
Terry
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Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
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Hi Don
You have shown that very well! For me though, it was so much easier processing the dark sky image. I could get the image looking good with a few stretches. To get the same detail showing in my LP image, I had to make a right mess of the rest of the image.
It seemed to me that the detail was much more forthcoming, without having to butcher the image. I would like to see what you could do with the full 12 hour stack I have!
To me the one hour image is on par with the 12 hour image I have.
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Arran
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Patrick
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Reged: 05/16/03
Posts: 7808
Loc: Franklin, Ohio
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Don,
That's interesting too. Did you try to do the same processing on the dark sky version? Given what you've said, what are the advantages of dark skies?
Patrick
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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Quote:
Hi Don
You have shown that very well! For me though, it was so much easier processing the dark sky image. I could get the image looking good with a few stretches. To get the same detail showing in my LP image, I had to make a right mess of the rest of the image.
It seemed to me that the detail was much more forthcoming, without having to butcher the image. I would like to see what you could do with the full 12 hour stack I have!
To me the one hour image is on par with the 12 hour image I have.
Hi Arran,
You're exactly right, it is much easier to process images where the brightness of the sky glow is low in comparison to the subject. But the real advantage of dark sky sites comes with being able to increase the exposure times, thereby catching more photons, while keeping the sky glow under control. If you had shot the dark site images at 20 minutes instead of 10 minutes, the sky glow brightness might be about the same as in 10 minute exposures from the light polluted site, but you would have captured twice as many photons from the galaxy. Then you would be faced with more difficult post-processing to separate the subject from the sky glow, but the results would be very noticeably better.
I think it's a shame when beginners get the idea that they can't do AP from home because of the light pollution. In all but the worst conditions, that simply isn't true. And the skills you can gain in post-processing by dealing with lots of sky glow in your images will pay huge benefits when you ARE able to get to a dark site.
I suggest buying a book on astrophotography image processing, and if you use Photoshop (there's really nothing better for post-processing IMHO), perhaps buying some pre-made tools like Noel Carboni's Photoshop actions for AP. But if you have the patience and inclination, you can do everything Noel's tools do with nothing but Photoshop out of the box. Obviously Noel did
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Don
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Atlas EQ-G
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jmasin
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Reged: 12/22/08
Posts: 489
Loc: Murphy, TX (DFW)
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Arran,
Thanks for your post. Undoubtedly pulling detail out of the soup is harder
I'm surprised you were able to get 10 minutes from your "LP" sky. Were you using a filter on the LP sky and no filter on the dark site? You don't mention any camera settings. I can say from my home site 2 minutes (ISO1600, so 4 minutes ISO800) nearly completely saturates the frame with LP. Any more than that is a near washout. I'd love to get 10 minutes out of my home site!!
Thinking about the impact of LP is straightforward if you think about a single pixel example.
At a LP site, say a pixel receives 1 DSO photon per minute, and 0.5 LP photons per minute. For a two minute exposure you have 2 DSO photons and 1 LP photon that you have to differentiate between.
At a dark site, say a pixel receives 1 DSO photon per minute, and 0.1 LP photons per minute. A ten minute exposure gives you 10 DSO photons and 1 LP photon. A much easier distinguished difference.
Like Don says, going to longer exposures is the real gain of dark site. Above example is way oversimplified because there may be some fainter details of the DSO that send the same photons/min as the LP. Those may not be distinguishable from the LP for any reasonable amount of integration time.
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Cheers,
Jon
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Edited by jmasin (04/27/09 01:24 PM)
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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Quote:
Don,
That's interesting too. Did you try to do the same processing on the dark sky version?
Hi Patrick,
The real advantage of dark skies is the ability to shoot longer subs. Longer subs always increase the ratio of signal to noise, but under light polluted skies not only do longer subs require more skill in post processing, but the dynamic range left over after the sky glow is clipped is smaller, so less stretching is possible before artifacts start to show up.
If there were a site where there were absolutely zero light pollution, you would still need to shoot multiple subs and combine them to reduce noise, but if your subs were long enough the stacked image would need no stretching.
Quote:
Given what you've said, what are the advantages of dark skies?
Patrick
No, I didn't try stretching the dark sky image. With these small JPEGS it would be futile. If Arran can post the stacked 16-bit images somewhere it would be interesting to see how much can be made of both of them.
I would expect somewhat better results from the dark sky site, but not dramatically better, since the total integration time is the same in both.
Cheers,
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Don
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Atlas EQ-G
Orion 8" f/4.9 newtonian
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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Quote:
I'm surprised you were able to get 10 minutes from your "LP" sky. Were you using a filter on the LP sky and no filter on the dark site? You don't mention any camera settings. I can say from my home site 2 minutes (ISO1600, so 4 minutes ISO800) nearly completely saturates the frame with LP. Any more than that is a near washout. I'd love to get 10 minutes out of my home site!!
Me too! I would have to go down to ISO 200 to get decent 10 minute exposures from my home site. That would actually be better from the noise perspective than 5 minutes at ISO 400 or 2.5 minutes at ISO 800, due to the fact that longer exposures have higher SNR than shorter ones with the same exposure value (time x ISO). But the lower ISO with longer exposures would mean getting much fewer subs in one night, and potentially running into problems with tracking/guiding and vibrations.
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Don
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Atlas EQ-G
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Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
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Very interesting!
About the settings:
Both images were ISO 800.
I used a CLS filter for the LP shot, and none for the dark shot.
I have some experience in processing already, and use photoshop along with the actions and some plug ins.
However I was unable to make the 1 hour LP image anywhere as good as the 1 hour dark image. In fact it was onyl with 8 hours or so, that I could get the LP image as good as the 1 hour dark shot.
Here is my final processed image, which is 12 hours of LP shots plus the 1 hour dark shot.
I would love someone to have a process of my data to show me how much better it can be!
Where can I upload a 50mb file?
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Arran
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7808
Loc: Franklin, Ohio
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Quote:
I would expect somewhat better results from the dark sky site, but not dramatically better, since the total integration time is the same in both.
I don't doubt what you're saying, but a lot of what I've read (and experienced) is that light polluted skies require magnitudes (pun intended) more integration time than dark sky sites. Samir Kharusi in particular states that as sky fog goes up, the integration times required to achieve the same SNR as the dark site would go up by the difference in the stellar magnitudes.
The problem with more integration time and shorter exposures (reduced for sky fog) is that a lot more subframes will be required. More subframes equates to a lot more post processing time. I've noticed a huge difference in my post processing time going from a 12mp camera from an 8mp camera, let alone adding a huge stack. I'll have to get a new computer to handle it. Of course, post processing by itself is not that big a deal...just get it started and go take a lunch break.
Don't get me wrong...I want to be able to image from my back yard, but I'm just not sure if it's worth the effort.
Patrick
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10" f/6 Truss Tube Newtonian
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Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
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Just found somewhere to upload the tifs, will pass on the links shortly for anyone that wants to have a play.
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Arran
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Mike Clemens
Post Laureate
Reged: 11/26/05
Posts: 4275
Loc: Wasilla, Alaska 61N
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Simple, take 400 - 800 exposures !
<ducks>
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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Hi Arran,
That is a really great image! You did a very good job of post-processing obviously, and I'm not sure I could make it any better.
Where I have gained the most experience is in processing images with substantial light pollution, and I would love to have a go at yours.
I think there are some free sites that you can upload files to so that others can download them. Perhaps someone else will read this and provide the info. If not, I may be able to make an FTP site available temporarily.
Cheers,
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Don
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Atlas EQ-G
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Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
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Mike, it was seeing your M101 that really made me think something must be wrong for me! So I headed to the dark skies
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Arran
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Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
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Thanks Don
I am currently uploading my 2 comparison stacks, plus the full 13 hour stack.
I would be interested to see what people make of them!
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Arran
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