Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
|
|
Hi Don
The camera white balance is set to daylight.
That file I posted is simply the captured fit converted to tif.
Captured and converted by Nebulosity.
If I use DSS to stack, I choose RGB background calibration to fix that.
Or if I use Neb to stack, I adjust color background (offset).
Later on I will upload a tif that has been balanced by Nebulosity.
--------------------
Arran
My main website
My blog
|
Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
|
|
Hmmm Nebulosity seems to be having trouble balancing the colors.
--------------------
Arran
My main website
My blog
|
Arkalius
scholastic sledgehammer
Reged: 08/03/06
Posts: 878
Loc: Orange County, CA, USA
|
|
Long thread... sheez
Based on the article I'm writing, it would seem that one shouldn't be concerned with how far up the histogram the skyglow gets. Instead, the max exposure time should be that which is just before saturation of the highlights you wish to preserve. A higher skyglow will cause this point to come earlier. You don't need to expose this long of course, and Steve Cannistra's article does a good job of examining what the "minimum" exposure time should be. The article I'm writing hopefully will make a valueable contribution here.
Another large downside to skyglow that I haven't seen explicitly mentioned in this thread (if it's here than I missed it) is that the noise it generates combines with the noise from your target, reducing the overall max SNR you can acheive in a single subexposure.
To me, the answer to the question "how long should I expose for" would generally be "as long as you can while keeping the target steady and without overexposing the highlights you want to keep". Of course, there may be some limit of practicality when dealing with a faint target in really dark skies, and there is a point of diminishing returns.
--------------------
-Arkalius
11" Celestron SCT on Orion Atlas EQ-G
Celestron 100ED Refractor
8" Zhumell Dobsonian Reflector
|
Qkslvr
Pooh-Bah
Reged: 06/23/06
Posts: 1316
Loc: NE Ohio, US
|
|
Quote:
Long thread... sheez
.....
Another large downside to skyglow that I haven't seen explicitly mentioned in this thread (if it's here than I missed it) is that the noise it generates combines with the noise from your target, reducing the overall max SNR you can acheive in a single subexposure.
In a single exposure yes, but as your sampling rate increases I'm not so sure.
I think maybe what we're doing is becoming more applied digital signal processing than photography, and not that that should scare anyone away, but we rarely talk about sampling rates, and duty cycle, and how do we detect a single photon than arrives once every 10 seconds, and how is that different than detecting a photon that arrives once every 10 minutes?
Quote:
To me, the answer to the question "how long should I expose for" would generally be "as long as you can while keeping the target steady and without overexposing the highlights you want to keep". Of course, there may be some limit of practicality when dealing with a faint target in really dark skies, and there is a point of diminishing returns.
Agreed!
--------------------
Mike
Onyx 80ED/N8/CG-5/40D
|
Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
|
|
I have reprocessed my different stacks of data so many times now, that I have come to a decision on my future imaging!
I can clearly see that the data from the dark site is CONSIDERABLY better than I get at home.
Just one hours of data is so much better than many hours I get at home, it is well worth travelling for it.
The differences being I can stretch to get the faint detail with out causing a ton of noise to be introduced. And also the colors are so much better.
And that is with only one hours of data VS many hours.
The real test will be when I can get 4+ hours at my dark site. At that point I will truly know what the differences in sky quality will yield.
From my home I can produce very nice images, but I guess I am a perfectionist, and I am never truly happy with them. Travelling will enable me to get the images that I have always wanted.
Time will tell though, I need more data to be totally sure about this!
--------------------
Arran
My main website
My blog
|
DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
|
|
Hi Samir,
Quote:
Pity, because the way human knowledge expands is by the tiny, incremental understandings proferred by millions of individuals.
I couldn't agree more, but observations and measurements are at least as important as theories in the process of expanding knowledge. Typically observations come first, then theories to explain the observations, then measurements to confirm the theories. If the theories don't conform to the facts they need to be revised or abandoned.
The work that you, Cannistra and others have done is important, but in some cases I think it has been mis-characterized. You can obtain a figure that represents the minimum exposure time needed to reach the point on the sky glow SNR curve where longer exposures produce little improvement in the sky glow SNR, and that's good. But that point does not necessarily represent the optimal exposure time, because it doesn't consider the subject's brightness.
The way I estimate the exposure time needed with my equipment and in my location for an object I haven't attempted to photograph previously is to make a guess, based on experience, and then make some trial exposures. I examine the trial exposures closely, with very strong stretching if necessary, to determine if I am capturing any signal from the faintest parts of the subject that I desire to capture, disregarding the noise, and using a linear gamma if it helps the visualization. If I can't see any detail at all I know I must expose longer. When I do begin to see the detail I'm after (again disregarding the noise), I examine the position of the sky glow hump on the histogram and make a judgement (based on my experience) about whether I will have enough range left above the sky glow to produce a decent image. Sometimes the answer is "No", so I don't waste time trying to photograph that object from my light polluted site. If the answer is "Yes", then I determine if there is a reduced ISO and correspondingly increased exposure time that will allow me to achieve a better compromise between total number of subs and exposure time per sub. This last step is important because even if I already know the optimal exposure time to minimize noise in the sky glow, the SNR of the subject is a different matter. Once you get past the read noise, shot noise becomes the major contributor to noise in the subject, and the SNR due to shot noise increases with illumination, which increases with exposure time - and the SNR of the subject relative to shot noise continues to increase with added exposure time, with no upper limit short of infinity.
If someone can come up with calculations to make that process less subjective and to eliminate the need for experience, it would be very beneficial. But there are so many objects with such a wide range of brightness and contrast, the task seems quite formidable. In the mean time, I'll be taking pictures whenever I can.
--------------------
Don
----------
Atlas EQ-G
Orion 8" f/4.9 newtonian
Orion 127mm Mak-Cass
Orion Skyview Pro mount
Orion 80mm guide scope
Canon Digital Rebel XT
Meade DSI
Philips SPC 900 NC webcam
http://www.pbase.com/dtreed/astrophotos
|
groz
Pooh-Bah
Reged: 03/14/07
Posts: 1071
Loc: Duncan, BC
|
|
Quote:
As for the subject of the picture, I think most of us would consider that the subject of interest is invariably the faintest tendril of any nebula or galaxy.
hehe, BINGO. I think this just goes to prove, different opinions on various things come about from DIFFERENT OBJECTIVES. In your case, the objective is the very faint details hiding down near the 'sky fog'.
On my wall, I have a poster, it's a hubble shot of M51, came in a skynews mag a couple years ago. When I'm processing my M51 stack, I'll often look at that poster, and compare to what I see on my screen, keeping in mind that I have neither the resolution, nor the clarity (seeing plays a big part here) of the hubble shot. Bottom line, my C8 + canon 350 is NOT a million dollar sensor strapped into a billion dollar telescope, mounted in a trillion dollar location.
But, the thing that really sticks out to me, when looking at that poster, the amount of detail in the vicinity of the galaxy core, as well as the detail in the faint tendrils. So, in my attempt to produce that kind of stuff with my modest gear, I _know in advance_ there's limitations I have to deal with.
a) If I expose to long, the gradients in the core start to wash out.
b) If I expose to short, I dont take advantage of the full range of the sensor.
c) I have a camera with relatively high read noise compared to dedicated astro equipment, so I need to take lots of frames to deal with that statistically.
d) The 12 bit a/d in my camera will _never_ achieve the detail of equipment with higher bit counts in the a/d, but I can indeed fake it down the road by using hdr techniques.
So, everything becomes a compromise, and, I try to adjust the compromise to reach _my_ goals, which turn out to be different than your goals. You are after the maximum detail in the faint stuff, prepared to sacrifice the brighter cores. I'm after the maximum detail in the cores, and prepared to sacrifice a bit on the dim end. You have higher end camera equipment (and telescopes), so you can push envelopes out of reach with my equipment.
And this is likely why discussions become passionate, folks are _assuming_ that everybody has the same objectives. In the end, i think we are all applying the same theory in general, but, working toward different goals, so we end up treating the equipment limitations differently.
|
Arkalius
scholastic sledgehammer
Reged: 08/03/06
Posts: 878
Loc: Orange County, CA, USA
|
|
Quote:
c) I have a camera with relatively high read noise compared to dedicated astro equipment, so I need to take lots of frames to deal with that statistically.
Actually, based on the math, it is the length of the subexposure, not the number of them, that has the greatest effect on read noise contribution.
Quote:
d) The 12 bit a/d in my camera will _never_ achieve the detail of equipment with higher bit counts in the a/d, but I can indeed fake it down the road by using hdr techniques.
Actually... the lower bit depth essentially amounts to a greater level of quantization noise, and, so long as you work with a program that will give you 16 bits of depth, this noise can be combatted the same way other noise can, with more integration time. At least, that's how it seems to me. It's hard to prove it algebraicly because quantization noise is hard to quanitfy statistically.
--------------------
-Arkalius
11" Celestron SCT on Orion Atlas EQ-G
Celestron 100ED Refractor
8" Zhumell Dobsonian Reflector
|
denjo
professor emeritus
Reged: 11/25/05
Posts: 506
Loc: Creve Coeur, IL
|
|
Arran,
I took a shot at processing your 13hr one. I have never processed a photo with anywhere near that kind of time in it and all I can say is WOW!!!
Now I know why you guys put so much time in your shots. That thing has tones of information in it. More than I know how to bring out and it was a joy to work on, THANKS for posting it.
My version has more blue than most people like but I go mostly by how it looks to my eye than whether or not its evenly balanced color.
Thanks again, this was fun.
--------------------
Coopatory Observatory
Astro Tech 111 APO
Atlas Mount
W/O 80mm EDII F/6.8
Orion Starshooter Guider
Black Knight Binoviewer
Canon 20Da
|
Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
|
|
Thanks Denjo, you have done a great job on the image!
I love to see how each person interprets the data 
--------------------
Arran
My main website
My blog
|
Bart Declercq
sage
Reged: 01/21/07
Posts: 200
Loc: Haaltert, Belgium
|
|
I've processed your 13 hour M101 tiff, using Photoshop CS2 and Carboni's tools, I include two versions, one of the full image, but also one slightly larger, as I feel your image deserves the larger image scale.
Mostly playing with levels, using Shadows/Highlights and finishing touches using Curves - I removed a slight color gradient with Carboni's tools, then finally some sharpening, denoising and a high-pass filter overlay.
|
Bart Declercq
sage
Reged: 01/21/07
Posts: 200
Loc: Haaltert, Belgium
|
|
And the larger version
|
Aza
sage
Reged: 04/08/08
Posts: 203
Loc: UK
|
|
Very nice job Bart! I think thats the best version of the data I have seen
Thanks for taking the time to process it!
--------------------
Arran
My main website
My blog
|
|
13 registered and 12 anonymous users are browsing this forum.
Moderator: Charlie Hein, knuklhdastnmr
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 1564
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
[Re: 
Edit
Reply
Quote
