Just bought an 18-200mm lens; had just a few minutes to spare, so
popped the 1100D onto a tripod, quick focus, and then 9 x 15 secs
at 18mm and 1600 ISO @ f/3.5 [wide open, I wanted to see how bad it's aberrations would be wide open.] I've cropped it because of buildings at
the bottom of the image. Included a single frame so that newbies can see
what stacking does.
Thanks to all of you that either missed me, or were glad that I had seemingly disappeared.
raymo
P.S. please forgive the horrendous noise in the images.
Good stuff raymo! I've got some image data from the (hot) weekend that I've just started looking at and hopefully later I'll post an example of what it does to the noise, even from a warm night.
Hi Chris, yes, static tripod. Being as the celestial equator passes pretty
much through the centre of the image, at 18mm focal length you can't
get much more than 15 secs before star trailing occurs. I used Deep Sky
Stacker, and cropped and downsized for the web in Photoshop.
raymo
That's very interesting. I tried stacking 3 30s exposures but as the end result was worse than the indivdual frames I suspect I got something wrong somewhere. I'll try again sometime, clouds permitting!
Thanks
You don't say what focal length lens you used, or where in the sky your target was, but if you had significant star trailing, the end result could be worse than a single image.
raymo
So here's a demonstration of the noise behaviour when stacking frames from my 1100D...
Top Left = 2m frame at 6-8C
Top Right = 2m frame at 16-18C
Bottom Left = 15x 2m frames at 6-8C, stacked
Bottom Right = 50x 2m frames at 16-18C, stacked
No other adjustments applied to any of the images (other than flipping!) and cropping at 100%, I haven't even colour balanced the stacked images. The noise is actually worse in the raw images in front of me but the jpeg compression seems to smooth some of it out!
You can see that the 15 frame stack does a nice job of stomping on the noise compared with the raw frames. The 50 frame stack does an even nicer job of smoothing out the noise, but note how much noisier the "warmer" frames were to start with...many more brighter pixels lurking there. I even think the 50 frame stack has more detail in it, but I haven't yet processed it fully.
The blue halos are from my scope, it's a little doublet
So here's a demonstration of the noise behaviour when stacking frames from my 1100D...
Top Left = 2m frame at 6-8C
Top Right = 2m frame at 16-18C
Bottom Left = 15x 2m frames at 6-8C, stacked
Bottom Right = 50x 2m frames at 16-18C, stacked
No other adjustments applied to any of the images (other than flipping!) and cropping at 100%, I haven't even colour balanced the stacked images. The noise is actually worse in the raw images in front of me but the jpeg compression seems to smooth some of it out!
You can see that the 15 frame stack does a nice job of stomping on the noise compared with the raw frames. The 50 frame stack does an even nicer job of smoothing out the noise, but note how much noisier the "warmer" frames were to start with...many more brighter pixels lurking there. I even think the 50 frame stack has more detail in it, but I haven't yet processed it fully.
The blue halos are from my scope, it's a little doublet
Hi Raymo. My shots were widefields of the Milky Way around Crux. I'm more puzzled now because I can see the difference in your stack but to my seemingly uneducated eye the "best" shot in Dunk's example is the top left! The others strike me as having had the contrast reduced and the brightness turned up. Exactly why I thought my stacking experiment resulted in a worse image. I am viewing them on a cheap Toshiba laptop. I doubt if it's helping. What qualities should I be looking for?
Cheers
Hi again Chris. The top left image shows moderate speckling [noise],
and top right shows more speckling [noise], due to the ambient temp being 10 degrees higher, hence the need for cooled cameras.
The bottom left stack of 15 subs image is indeed lacking in contrast, and the 50 stack image even more so, but the speckling [noise] has been
reduced in the 15 stack, and further reduced in the 50 stack. The lack of contrast can easily be remedied whilst processing the images. Star field images [ like mine] don't suffer much from lack of contrast; all I did to it was increase the colour saturation a little. The brightness can also be adjusted to your liking during processing, as can the background sky colour, which should not be black.
All in all the bottom right image is the best, because it has the potential
to be turned into a good finished product. Removing [or reducing] noise
using software such as Neatimage is quite difficult, and often results in a plastic like look.
In short, you thought stacked images looked worse because stacking
is only the first step to a finished product. I hope this helped. I'm no
digital expert, by the way.
raymo
Following on from raymo - and I'm just a beginner - the noise is your biggest enemy! The fainter details of nebulae and galaxies require steps in processing that stretch the data, effectively brightening pixels within a range of values to make them more prominent. Then there's usually some sharpening function applied to tease out finer details.
Both of these processes bring out noise just as readily as they bring out details, so the less noise in your data when you start, the better it will look out the other end.
The stacking process increases the signal to noise ratio of the data by statistical calculations. Noise tends to present itself randomly in the data whereas the real signal occurs with much less variation in value (intensity), so the software is able to discern the real signal from the (random) noise.
One thing that seems to happen is that the colour data tends to get washed out and needs to be teased out again.
Star fields are tricky...and I haven't figured them out yet...
Chris, it just occurred to me that your poor stacked image could be due
to something being done wrong. Did you use DSS ? and if so, did you navigate your way through it by using the help feature, or did someone
help you with it, or did you just do what seemed right? Forgive me if I sound patronising, but I have no idea of your knowledge of stacking software.
raymo
No worries Ray. I used DSS. It was a while ago now but I think I used an online tutorial and a bit of guesswork where the tutorial used an older version of the software. I may have just been looking at the result from the wrong perspective. I think I was expecting sharper stars etc rather than less noise. I'll bag some more data when my inclination coincides with a clear sky!
I've been meaning to experiment with the Mak with RA drive I now have for a while now. I think M42 is going to be my next AP project. It's been too damn cloudy up here lately.
Cheers
A couple of final points Chris. Stacking does not sharpen images at all, it
just improves the signal to noise ratio, as Dunk said. It is important to
check all subs, and only stack the good ones, as one or two poor ones can seriously affect the final result. Elongation of stars due to wind, or a tripod knock can ruin the whole stack. M42 is one of the hardest DSOs
to image well, owing to it's huge dynamic range; it requires some processing expertise to stop the bright core from being totally washed
out. I would suggest starting with something else.
raymo
Raymo & Dunk, thanks for the clarifying that stage of AP for me.
I was going to try M42 as I get an ok view of it from my place. I get an ok view around the SCP too. How about something around there? The SMC, LMC, 47Tuc? Not for beginners?
Cheers
The SMC and LMC are actually good candidates, and show great improvement with stacking even a few frames. Ditto 47 Tuc, but
you have to keep the exposures short to avoid burning out the core.
Start with around 8-10 secs.
raymo
