Are you using any of the channel calibration or alignment options? I've found they can mess around with the histograms and I leave them disabled and do the colour balance myself using levels.
Definitely look at the histogram of the source images. If they're too far to the left, should be able to stretch OK in post-processing. Essentially, when you're capturing the images you want the histogram to be clear of either end of the scale - too far to the left is underexposed, the right over exposed.
But also bear in mind that M42 is a tricky one to process because of the huge dynamic range, but it's easy to get data because it's bright.
Are the luminance sliders in their default positions when you start.
Sliding the High lower slider to the left from it's default of 50 flattens out the curve until when the slider is fully left the line is a straight diagonal. The defaults are, from the top, 80, 33.3, and 50. Actually, from memory
I think there is a default reset button at the bottom.
raymo
Are you using any of the channel calibration or alignment options? I've found they can mess around with the histograms and I leave them disabled and do the colour balance myself using levels.
Definitely look at the histogram of the source images. If they're too far to the left, should be able to stretch OK in post-processing. Essentially, when you're capturing the images you want the histogram to be clear of either end of the scale - too far to the left is underexposed, the right over exposed.
But also bear in mind that M42 is a tricky one to process because of the huge dynamic range, but it's easy to get data because it's bright.
Hi Dunc,
Should I be using a higher iso for the subs? I don't have guiding so I don't think I can push the exposure time beyond 90secs.
Are the luminance sliders in their default positions when you start.
Sliding the High lower slider to the left from it's default of 50 flattens out the curve until when the slider is fully left the line is a straight diagonal. The defaults are, from the top, 80, 33.3, and 50. Actually, from memory
I think there is a default reset button at the bottom.
raymo
I've been starting out with the default settings, for luminance, but they seems to have little effect on the image even if I flick it to the other extreme. It seems likely that my image capturing is behind the problems I'm having. I'll have a look at the histograms of the subs for some clues.
Thanks again Raymo.
Should I be using a higher iso for the subs? I don't have guiding so I don't think I can push the exposure time beyond 90secs.
I dont, as I find the red mist too distracting at ISO3200, however some other folk here have successfully gotten good results at higher ISO settings. All sensors are not born equal.
90-120 seconds is sufficient to get some good data on M42, just take lots of subs (and darks), and like you have already done, get a bunch of subs of the core with either shorter exposures or lower ISO (I prefer the former as it's quicker!).
For fainter targets you will want longer subs, and for that you will need to practice refining your polar alignment. It takes practice and it's a bit tedious if you have to setup and tear down every night. I can usually hit 3m subs after a while, but I suspect my scope is shorter focal length than yours. If you haven't already, read up about drift alignment...
As I said, I'm no expert, myself I've been very grateful for all the help I've received from fellow IISers both in here and hands-on. A year ago my polar alignment was barely good enough for visual
I dont, as I find the red mist too distracting at ISO3200, however some other folk here have successfully gotten good results at higher ISO settings. All sensors are not born equal.
90-120 seconds is sufficient to get some good data on M42, just take lots of subs (and darks), and like you have already done, get a bunch of subs of the core with either shorter exposures or lower ISO (I prefer the former as it's quicker!).
For fainter targets you will want longer subs, and for that you will need to practice refining your polar alignment. It takes practice and it's a bit tedious if you have to setup and tear down every night. I can usually hit 3m subs after a while, but I suspect my scope is shorter focal length than yours. If you haven't already, read up about drift alignment...
As I said, I'm no expert, myself I've been very grateful for all the help I've received from fellow IISers both in here and hands-on. A year ago my polar alignment was barely good enough for visual
Maybe I'm being a little too tentative with the exposure times - I've sorted the drift alignment so I can keep a star in the crosshairs for 10 minutes now- thats with a 12 mm reticle in a 2x power mate.
I've checked the histograms of many of the subs, and they are very tall and narrow, far to the left of the graph. I read in the nebulosity manual yesterday that the gain of the dslr can be adjusted. I might have a look at that.
Not quite ready to throw in the towel on all this yet!
These histograms below are from the best quality individual sub in my images. The RAW histogram (which I assume is just the total light in the sub) is quite central, but the RGB is way off to the left. These are completely unprocessed.
Do that give anyone some clues to the problems I'm having?
I'm very grateful for the help from everyone. Hope one day I can return the favour in some way.
Edit: I've just added the RGB histogram of the subs all stacked in DSS, with no darks or bias files. Weirdly narrow and centred, but with a straight diagonal dotted line - nothing like Raymo's.
Last edited by chiaroscuro; 24-01-2015 at 09:24 AM.
Thanks for the encouragement. I haven't got photoshop but I can try the GIMP software - it looks extremely complicated with a very unfriendly user interface - but I'll give it a go.
Luke
GIMP is one of the best pieces of open source software out there, there is an abundance of information available and it's very useful for post-processing astro images if you don't have Photoshop.
Nice to see another ED120 user out there.
Your images are looking fine, try not to be discouraged by minor problems here and there. Unfortunately I can't comment on the green issue you are having, I'm not a DSS user and haven't done any astro imaging with DSLR apart from the occasional moon shot here and there.
Gain is ISO with your DSLR. I use ISO1600 almost exclusively with my 1100D because it seems to be the sweet spot for the signal-to-noise ratio. Besides changing the gain, the only way you can increase the light in your image is by taking longer exposures.
The histograms represent the distribution of the pixel values across the whole image. What the RGB histogram is showing is that the majority of the pixels are low in value or close to zero (black). This should be the same as the histogram you can see if you press the Display button on the camera while reviewing the image you just took on the camera. What you want is to get the histogram shifted away from the left hand edge so there's clear flat line between the left hand edge and where the histogram ramps up.
If you're drifting visually for 10 minutes at that magnification, you shouldn't have any problem pushing the exposure with your DSLR.
I'm not sure about the diagonal line in DSS though, I've never seen that behaviour.
The raw file should be just a more accurate representation of what's in the JPEG. The histograms shouldn't differ all that much...if they do, then I'd be worried what the camera/converter is doing to the raw data
Gain is ISO with your DSLR. I use ISO1600 almost exclusively with my 1100D because it seems to be the sweet spot for the signal-to-noise ratio. Besides changing the gain, the only way you can increase the light in your image is by taking longer exposures.
The histograms represent the distribution of the pixel values across the whole image. What the RGB histogram is showing is that the majority of the pixels are low in value or close to zero (black). This should be the same as the histogram you can see if you press the Display button on the camera while reviewing the image you just took on the camera. What you want is to get the histogram shifted away from the left hand edge so there's clear flat line between the left hand edge and where the histogram ramps up.
If you're drifting visually for 10 minutes at that magnification, you shouldn't have any problem pushing the exposure with your DSLR.
I'm not sure about the diagonal line in DSS though, I've never seen that behaviour.
Should 30sec at iso1600 be enough? The sensor on the 70D is supposed to be pretty good.
The raw file should be just a more accurate representation of what's in the JPEG. The histograms shouldn't differ all that much...if they do, then I'd be worried what the camera/converter is doing to the raw data
Is it possible that the file transfer into the Canon EOS software is altering the data? Maybe I should try loading the images straight from the SD card into DSS?
Should 30sec at iso1600 be enough? The sensor on the 70D is supposed to be pretty good.
It's hard to say, as it will depend on your scope. From the look of your earlier histogram, try doubling the exposure time. You may be able to get away with less, but you will pick up more faint nebulosity the longer you expose. Experiment
Quote:
Originally Posted by chiaroscuro
Is it possible that the file transfer into the Canon EOS software is altering the data? Maybe I should try loading the images straight from the SD card into DSS?
No, it shouldn't make any difference. Regardless of how you transfer the RAW files to your computer, raw files are raw files. So long as you select those raw files in DSS itll take care of it from there as it has its own raw decoding routines.
On my laptop I use Canon software to import images from the camera, and on my desktop I don't, so as Dunk said ,that doesn't seem to be an answer.
What you have posted is a workable image for DSS, so exposures and ISO settings are irrelevant until you have solved your histogram problem.
raymo
Last edited by raymo; 24-01-2015 at 05:34 PM.
Reason: more text.
Thanks Russ, much appreciated.
I've been using gimp but it can only handle 8 bit tiffs and doesn't handle CR2 files. It was a case of lose info in the finished stack file or convert prior to stacking. Gimp works well and is easy to use, I just couldn't find a permanent fix for handling the input of files.
Now it's too easy, stack the raw files and straight into photoshop.
Sorry to revive this thread, but I've run out of ideas.
After spending the weekend going through every sub, picking the best by looking at the image and the histogram, I then used the Canon EOS software Digital Photo Professional to tweak the RGB histogram which improved the image quality substantially. I kept a copy of unmodified subs too. All were ISO 1600 at 30 secs, with matching darks and bias frames.
Then I stacked the unmodified subs - poor result seen below, and stacked the modified subs - little improvement in image. Neither could I do much to improve them in Startools or nebulosity.
The same problems seem to be there, the main ones being a green cast, poor colour and contrast overall, which I can't seem to improve with processing.
Both stacking sessions took about 10 hours each, using median sigma clipping on the subs and bias frames as recommended by DSS and youtube tutorials.
I still though get a left sided RGB histogram, and the diagonal dotted line is always diagonal! Never a sigmoid curve as it is in any other images I've seen online of the final stacked file.
Below is the final stacked image of both sets of subs, and one sample image of the modded subs I used, which looks so much better than the stacked images!
