View Full Version : My first stacking
1st of May 2010 (Sat), 04:47
This is my first post, and I only had my Canon 350d since Easter. I am in a place that has some light pollution and had been taking photos at 1600 iso and 30 second exposures. After reading about stacking and looking at photos on Flickr, I noticed that I can get less noise by doing 800 iso and 6 second exposures and stack plenty of them together. Last night, I took 8 lights and 1 dark. I can certainly see why stacking is better.
The following is the stacked image:
According to astronometry at Flickr, it is a picture of Cassiopeia. Unfortunately, some of the finer details are missing from the scaled photo on Flickr. I used DSS for stacking.
1st of May 2010 (Sat), 10:44
I have now uploaded my second stacking photo, but in a different part of the night sky. I think for more stars, I will need to find a place without light pollution.
1st of May 2010 (Sat), 11:27
Stacking is only the first step in post processing all that image data you collected. Hopefully you also shot Dark Frames for each Light frame you shot, in addition to Bias/offset frames, and Flats and Dark Flats. Usually when someone first starts astrophotography with a DSLR and learning to stack their images they tend to ignore those. They are however important to a good image.
After stacking with DSS how did you save the image and what other post processing did you do, and with what processing software?
After stacking the image will look very dark. You must save it for further processing in something such as Photoshop.
I typically save my images to 16 bit Tiffs, with changes embedded, Not applied! I don't use DSS to make any color or luminance adjustments. I save all that work for better software more suitable to the task.
After stacking and saving the stacked image to whatever file name of your choice, you will need to open your image in Photoshop and start the post processing with a histogram stretch. You do this with Levels and Curves adjustments. Doing this will bring out the data you're not seeing in the DSS Stacked image.
There is much more to it than just stretching the histogram to manipulate and enhance your image data, but that's a start. You will want to be careful not to over stretch it clipping the dark points too much, A mistake many of us make. Space is dark but it's not black!
Further processing will include things like Noise Reduction, and, maybe a little localized Curves and Tones Adjustments to emphasize certain areas and bring out more detail.
1st of May 2010 (Sat), 12:32
Thank you for your reply, I really appreciate it. I have kept the original RAW files that I used. For stacking, I used the default settings in DSS and actually applied the changes. However, as I am using Gimp for any post processing, I had to convert the Tiff into png format using Faststone Image Viewer. This is because Gimp does not yet support 16 bit files. I did not carry out any post processing on the current file, but after reading your reply, I had a quick play about with it. I am very surprised at how many stars do come out after quickly playing with the levels settings, although it also shows that there are problems with light pollution near the bottom of the image. I think I will do more post processing once Gimp 2.8 is out as it will support 16 bit tiffs natively. In the meanwhile, I will need to learn more about image processing rather than blindly playing with the sliders.
1st of May 2010 (Sat), 13:36
Nice, I got my camera yesterday and did some stacking, total time of this stacking was only 2:30 minutes. I was playing with the setting, and don't know what dark, flat images are, im reading about those now.
Ill get the pictures up in a little bit.
1st of May 2010 (Sat), 13:52
Great job on your first go at stacking.
Tim offers some excellent advice. Also be aware that 6 second exposure contain minimal data. While stacking these will help with noise, they fall very short of the exposure length needed to capture adequate signal to resolve the good stuff(signal) you're after. I'm assuming you don't have a means for tracking so your exposures are limited by movement as well as light pollution, thus the short exposures. I'm only mentioning this because as you stretch your images to pull out more stars you'll find there won't be all that much more to pull out, simply because there isn't that much content in each subexposure that were combined in the stack.
As an example, if you stacked 100 six-second exposures you would expect to see the what looked like a 600 second or 10 minute exposure. Although 10 minutes is not that long in deep sky imaging, it would be enough to capture quite a bit of a bright object like say the Orion Nebula(M42). But with only six second subexposures the resulting stack would not contain enough data in total to look like let's say a 10 minute exposure taken using a tracking mount under fairly dark skies*. Yes the noise would be less but so would the signal contained. So you see there's a point of lack-of-return in minimal exposure just as there is a point of negative return from light pollution and noise. If or when you are set up at some point to track during exposures and extend your times you'll be able to find the maximum return point for your camera, mount and skies. There's also an option for a light pollution filter to help with your location, they really do help.
I hope that wasn't info overload and I hope it helps in some way. You're off to a great start, keep 'em coming.
* There is no substitute for dark skies. Makes every part of this hobby better, especially the end results.
1st of May 2010 (Sat), 16:28
Thank you all for the advice. I have done a rough attempts at playing with the levels and I finally managed to upload what I managed. The following is the second stacking attempt with adjusted levels. I was very surprised at how many stars were visible.
I can only imagine just how much better it would look with dark skies. Unfortunately, summer is coming here in the UK and in my area, the sky does not get black from mid May till mid July, just darkish blue. The bright big star near the middle is Capella.
1st of May 2010 (Sat), 19:43
Wow, you did great, that's a load of stars for under one minute.
2nd of May 2010 (Sun), 00:12
I need some help, everytime I stack my file is HUGE! how can I down size the file without hurting the image quality?
2nd of May 2010 (Sun), 10:07
When I stack an image with DSS, the autosave.tif created by DSS is usually very large, around 140 Mb or more. (Size of file depends on size of stack(how many subs used)). However, when you save it to a 16bit tff (With Changes Embedded), the size of the file saved is about 40% smaller. E.g. that 140Mb file would be around 84Mb. You don't want to convert or reduce it until after you have completed post processing.
Once you have completed all your post processing in Photoshop to your satisfaction, then you can reduce the image size if you wish. Some people will save a full size copy of the image, and save another reduced in both size and bit depth for sharing on the web. That image you save for the web to a 8 bit Jpeg will reduce the size significantly. A 85Mb image tiff image can be saved as a 650Kb or smaller Jpeg.
If you use Photoshop "Save for Web and Devices" it can reduce the amount of memory space used quite significantly.
I can take a image and reduce it to 1024x791, and from 16 bit to 8 bit then save it as an uncompressed jpeg and it will be around 650Kb to 700Kb in size, but when I save that same 1024x791 8Bit image for the web and device it reduces it down to about 56Kb and still looks just as good.
Are you having troubles with your computer working such large image files?
2nd of May 2010 (Sun), 11:43
No, I was just trying to upload them to the internet.
2nd of May 2010 (Sun), 14:54
No, I was just trying to upload them to the internet.
Yep, too large for uploading to the Internet. For that I generally reduce the size to 1024 on the longest dimension, and as a 8 bit jpeg. Only after I am finished in Photoshop.
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