General Question Regarding Image Capture and/or Processing Techniques

[aeroman]

I have enjoyed learning PixInsight and have been generally pleased with my initial results.  In some of my online research, I came across an image captured and processed by Roger Clark that made me wonder if my image capture methods, image processing, or both need some improvement.  He is addressing removal of light pollution in his processing techniques.  I know Roger uses a hybridized workflow, so I won’t go into that at all, but he posted an image of Rho Ophiuchi that he was able to pull out quite a bit of color, nebulosity, and very little noise from a short capture time.  The image in question can be found at:
http://www.clarkvision.com/articles/astrophotography.image.processing2/

He used a Canon 6D and a 100 mm F/2 lens for the capture with 9 light frames at 1 minute each.  I don’t believe he listed the ISO, but I believe he generally uses ISO 1600.

I recently began photographing parts of the Milky Way this summer.  I drive to a dark blue zone 45 minutes east of Castle Rock, Colorado for my imaging sessions.  I have a Canon 5d M2 and a Rokinon 24mm F1.4 lens.  I typically shoot 10 light frames at ISO 800 for 300 seconds each at F2.8, with 2 corresponding dark frames.  While my ISO is lower and my F Stop and ISO will require longer exposure time than Roger’s, I should still have more useful data to pull out the color and nebulosity out of Rho Ophiuchi; however, I have not been able to do so.  There is air glow and some light pollution from a distant city I have to deal with in the image noted below.

I have attached a jpg of a single RAW file that I am working from as a reference.  I have also attached a jpg of the final processed image through PixInsight.  I feel I did a relatively good job removing the air glow and light pollution, but I feel that I’m not quite getting the nebulosity and color that I should out of Rho Ophiuchi.  Perhaps I am wrong and it is processed as well as can be expected given my starting point, although I admit the image to me is a bit overcooked.  I am wondering if my image capture time is too long and should go for shorter subs because of air glow and distant light pollution which may be washing out the image, or if my processing is more of the issue.  I know that is a broad question, but I feel others that have imaged similar objects with much more experience processing with PixInsight will be able to give me advice on what I should focus on to improve my final images.  I have reworked this image about 6 times over the last few months using various workflows and processing programs.  There are just so many variables that I’d like a little bit of expert opinion.

Thanks in advance for any advice you can give!

[ngc1535]

Hi,

I will let others (more expert) comment on some of your questions- but I just wanted to put something out there for you. I have read a few articles that show for many DSLR cameras that lower ISO values do not equate to a less noisy image. Indeed there appears to be an optimization of noise for longer exposures (i.e. not typical daytime stuff) with higher ISO settings. So you might want to consider taking more high(er) ISO images that might initially seem noisier. (because you are seeing more sky values)..but when combined might do a bit better for low S/N objects.
I wouldn't worry about comparing results with Mr. Clark- there are so many variables in both acquisition and processing. Usually better to optimize your own system and make it internally "consistent" with the best system performance results. Here is a link with a Canon 5D graph (suggesting indeed the 1600 value)

http://dslr-astrophotography.com/iso-values-canon-cameras/


-adam

[aeroman]

Thanks Adam, I may take a second look at that.  I actually did do some testing at night, but not with astro images specifically.  I took shorter length photos of houses across the street at night at different ISO's, but changing the exposure time to have equivalent EV.  For each ISO setting, I made sure I took enough photos so that the equivalent total exposure time was the same, and then stacked and processed images using various forms of averaging to compare how my camera performed at different ISO's, but ensuring I was always capturing the same amount of actual light.  I can't quite remember the settings, but it was probably something similar to this:

A single image at 400 ISO for 8 seconds.
Two images at 800 ISO for 4 seconds each.
Four images at 1600 ISO for 2 seconds each.
Eight images at 3200 ISO for 1 second each.
Sixteen images at 6400 ISO for 0.5 seconds each.

Again, as appropriate, I stacked the multiple images and did appropriate combines for each.  I then looked at the color and dynamic range for each.  At least for this style of testing, the two images at ISO 800 looked slightly better than the single image at ISO 400, but the images at 1600 to 6400 couldn't really compare with the images shot at 800 ISO for DR and color.  That is how I ended up at 800 ISO for all my current astrophotography.  I did this test awhile ago before I knew some additional factors that come into play, which is the dark current noise that goes up significantly when comparing 30 second dark frames versus 300 second dark frames.  I'll do some testing again to see if things improve at 1600.