a collision of fortunate events…

[ravnostic]

I’d have to calculate how long I could leave my wide-angle open without getting startrails

600 divided by focal length for APS or 35mm.

/1.6ish if you’ve an APC sensor. I delayed, due to lack of prep, my visit to the galaxy this week. Will do so tomorrow. I happen to like the shot, chupa. Dark Lane goodness.

[chupathingie]

Windows XP. PS3. (or GIMP). (or Irfanview).

OK, it’s easy for windows since you have things like Deep Sky Stacker (that’s just one, for instance). They have tutorials for using their software. If you don’t know how to navigate directories and the like from a DOS prompt or don’t want to do things the hard way, stick with one of these types of programs. All of the photos I’ve posted here have been done the hard way (because I’m a cheap bastard using Linux).

If you want to do it the hard way, which really helps in understanding what’s going on, there is a fairly minimal toolkit you need.

The GIMP for final minor adjustments only. Whenever GIMP gets GEGL fully implemented that will change. You could actually do the entire process in GIMP if it could work in a 16bit/channel color space. GIMP is 8-bit, and during processing that makes a huge (and immediately apparent) difference in the final image quality. All of the combining, noise subtraction and levels adjustments really need to be done in 16bit. So far I like RawTherapee for doing the visual adjustments, and it also know dark frames and flats (more on those later).

Since GIMP can’t handle the depth, you need a copy of Image Magick and align_image_stack. There may be others, but these are the ones I found and so I use them. Both are available for windows, the latter as a simple .exe file that runs from any directory after you place a reference to it in your PCs path. Both are command-line tools. You don’t need to see what’s going on while they work so it works fine.

OK, so now you need image data. Bear with me because some explanation is needed first. The reason you stack and how you stack comes about because what you’re trying to do is defeat the flaws of the imaging process; you want to be able to reject the data that’s in the way and reinforce the data about the object you want a picture of. So what’s in your way? Camera noise, light pollution and atmospheric turbulence are pretty obvious, but dust on your sensor and vignetting are things you also want to correct.

To correct the things that degrade your image data, you’re going to get images of them. Since you can’t get an image of your object without those interfering factors (wouldn’t that be easy), you come at it from behind. Start with noise, it’s the most obvious. There are 2 ways to deal with noise. One is stacking and taking an average of all of your images (that’s what we’re here for) and the other is to take pictures of the noise (dark frames) of the same duration, ISO and camera temperature. This is time-consuming, but really has to be done. So snap the lens cap on and let the camera take about 20 frames in the dark. If the temperature is expected to change a lot during your shoot, do 1/2 your lights, then your darks, then the rest of your lights. The more lights you can swing the better, 20+ starts to look smooth but 40 or more will really give you a smooth final.

OK, enough theory… here’s how you stack: Convert all of your raw files to 16 bit/channel TIFF files (of course you shot in RAW) and separate your darks from your lights into 2 folders, “darks” and “lights, of course. Use imagemagick to average all the darks together. In Linux the command looks like:

Quote:

convert -average *.tif darkframe.tif [/unquote]
Check the manual for the windows equivalent, it should be similar if not identical. Just open a run dialog, navigate to your folder of darks and type it in. WHen it’s finished, you will have a file named darkframe.tif that you will use to clean up your individual files before you do anything else to them. Copy this file over to your lights folder.

Now you want to use imagemagick to subtract the dark frame from each of your lights; the resulting files will then be aligned and stacked. So navigate to your lights folder now and type:

composite -compose minus lightframe#.tif darkframe.tif correctedlightframe#.tif

Again, check the manual for the windows version of the command composite and how it’s options are structured. If you do this right, you’ll have files named so that wildcards work in the following steps.

/screeching halt.

Yaknow, I thought I had done this write-up before… here it is.

[chupathingie]

Dark Lane goodness.

Thanks, rav. I had no idea that dark nebula would show up so well, but that background is pretty bright… you should see the full-sized version, lack of darks really ruined it! 😆

[ravnostic]

You can p.m. me the full sized version…I won’t tell!

I totally agree on the light/darks. Thus far, the more I have of each, the better the final result. Sadly, I’m typing this at 7:19 am local, when I really should be sleeping already, if I’m to go out tonight. I’m lacking motivation, for whatever reason, could be a lot of things…but I hope to get up around 2pm, and start packing.

[chupathingie]

Heh, it’s funny that I get looked at like some crazy homeless guy when I snap the lens cap on and start taking a bunch of darks. It’s just not an intuitive concept to most folks that a bunch of pictures of nothing can actually serve a real purpose. But then, most folks who know something about photography never have to be concerned with taming noise since modern DSLRs are very clean with short exposures under daylight or artificially lit conditions.

My next outing is going to involve shooting flats as well, I think. I haven’t figured out how to apply flats with my basic toolkit yet, but RawTherapee has tool panels for both darks and flats, and can apply all your tweaks identically to any number of files which would really make the way I’m doing things more an exercise in spartan processing techniques. I’ll make sure I post findings when I get a chance to dig into that side of things. I will still need to use the command line to average my images together and register them for stacking, though, as I don’t really wanna bother with trying to make some of the existing stacking programs run under WINE.

One thing that the “commercial” packages do very well (in addition to just being easier) is use better algorithms for stacking. A straight-up average is pretty good, but there are other routines that combine the pixels statistically; rejecting any data that strays too far from the rest of the pack. This gets you cleaner finals with fewer subs.

[chupathingie]

You can p.m. me the full sized version…I won’t tell!

heh, might as well let everyone see what happens when you don’t have all the pieces you need….

That’s Antares bottom center, the globular cluster M-4 center right. Rho Ophiuchi is up above about a frame and a half. 100% crop looks presentable, normally. The m42 and horsehead shots I posted were 100% crops from the same image, for instance. The purple stars are because my UV filter was off (I need a set of AR UVs badly), my existing filters are not AR and I get prominent ghosts.

No darks=no image, for the most part.

[ravnostic]

Makes a world of difference, huh? I did find that in Deep Sky Stacker, playing with the algorithms makes a huge difference in the result.

[chupathingie]

Astrophotography I think is mostly noise control, at least from the processing side. Tracking/guiding is a bit pricey, but just about any DSLR bolted to just about any lens will show you the sky better than most expect.

I found this in the stack of subs tonight (I have yet to touch the fisheyes that I actually have darks for). It’s 30 seconds at ISO3200 and 24mm f4. Look at the trailing in 30 seconds on the “original” size, then look around in the lower part of the image and notice the hot pixels and the noise floor. Those are what you need to minimise or filter out (rav knows this, I’m directing this more towards Yugo and anyone else who wants to try stacks). Now compare that noise with what you see in a normal <1sec exposure. Now imagine what that noise looks like after 4 minutes. That's what I've got for the fisheye shots. I'm going to try to stack them, but there is rotating landscape in the FOV so I don't know if my alignment tool is going to lock on the stars or the landscape or simply lose it's mind. I'll find out, I guess. At the least, I have a good set of darks that I can use to apply noise reduction to single subs.

[ravnostic]

Here’s hoping the software doesn’t get lost. It would be interesting to see how it blends the terrestrial elements in the stack.

[chupathingie]

I’m hoping the terrestrial just gets blurred and it locks on the stars. If not I’ll load them up in another app and try to apply manual control points. The kicker is that the time I spent taking subs was when the side of my planet was facing a relatively boring stretch of sky 😆 I may drop that lens on the old 300D and simply lay it on the ground over the summer at some point and let it catch the milky way passing overhead. I won’t get 180 degrees, but it will still get most of the sky at 6.3 megapixels while I concentrate on longer focal lengths with the hi-rez body.

[ravnostic]

The ‘if not’; lens distortion may spoil the opportunities. I might suggest Microsoft’s “ICE” for blending; it seems that I have had good results, though I hate to admit it…

[Author]

Posts