Stacking Stars



(Raw shot of the brightest object that had been in the sky on Feb. 13th.)

I don’t know why I keep hoping that I’ll be able to do something new for the first time and have it come out right. I should know better by now.  Anyway, I’ve been wanting to do astrophotography for years, and recently I decided to actually see what I could accomplish. Initially, I did have an 8″ Newtonian telescope, but only a cheap pocket camera that I couldn’t get rigged in front of the eyepiece. Now, I have the CoolPix, but no telescope. So, the best I can manage is to set up a tripod and just accept what I can get.


(Stacked result.)

The first step was to learn more about stacking. Ian Musgrave has a decent tutorial on this. The basic idea is simple – stars are shy and hate sticking around for people with cameras.

Well, maybe not. There are two big problems with taking star shots at night – the lack of light, and the annoying tendency for the Earth to rotate. These two things work against each other if you have a fixed tripod (instead of a telescope with a motorized mount). To get enough light, you need long exposure times. But, the longer the exposure, the more streaking you get from the shot. One way to get around this is to limit exposures to 30 seconds (or less), and take LOTS of photos one right after the other.


(Raw shot of the big dipper.)

I’m told by a sales clerk at Bic Camera that my particular CoolPix can’t support a remote trigger, that I need a more expensive camera. And, after fiddling with the settings, I know I can’t save photos in RAW format (I’m stuck with jpeg compression). Finally, I can do manual focus with auto exposure, or vice versa, but I want manual focus AND manual exposure. So, the only choice is to take lots of notes, fiddle with the photos at home, and then try again until I get it right. So, this is what I did:

1) Use a tripod. The bigger and heavier, the better.
2) Select Programmed Auto. This lets me change ISO and activate interval timer.
3) ISO (originally ASA) indicates how sensitive the digital camera sensor is to light. According to Photoxels, the higher the ISO number, the better the picture will look in low light. I kind of got this mixed up in my head, and I used ISO 100 for some of the shots, and 200 for a few others. Regardless, the exposure times automatically set to 30 seconds, which was what I was counting on. What’s important to keep in mind is that at higher ISO values, there’s going to be more sensor noise. I’m going to have to play with this setting to determine the best trade-off between light sensitivity and noise.
4) The CoolPix has a continuous interval timer, but it’s only available in certain programmed modes, not in the full Auto mode. Fortunately, one of those modes is Programmed Auto. In continuous interval, I can specify down to 30 second periods, but this seems to be 30 seconds from the end of the last shot, and not from the start of one shot to the start of the next. Since the shots take 30 seconds each, it’s closer to 6 minutes from the beginning of shooting to the end, if I only want 6 photos of a particular target.
5) Press the trigger and get away from the camera. Sensitive tripods can pick up jiggle from vibrations in the ground if you pace to close to the camera. Either way, Programmed Auto uses the same focus and exposure settings from the first shot through the subsequent ones, and turns off the display monitor between shots to save battery.


(Stacked result, plus color correction.)

That night, it was cold, maybe close to freezing. I’m pretty sure that’s why I got so many “stuck pixels” in the shots. These pixels are ones that don’t turn off when there’s no light hitting them. Generally, if the sensor has a problem with stuck pixels, you can correct for this by taking a couple practice photos with the lens cap on. I didn’t actually do this until the next day, when I was at home and the camera had warmed up. By that time, NONE of the pixels were stuck and I couldn’t cancel anything out in the stacking process. I’ll know better next time.

Another reason for taking the practice photos prior to, or immediately after the other pictures is that if you do have sensor noise, it can be averaged out when you do the stacking.

The idea behind stacking is two fold – remove stuck pixels, and some noise, from the raw photos; and, create a final image that is the summation of the light from the individual shots you took.

In Gimp, step one is to open your first raw photo. Then, drag in your practice shot (the one when the lens cap was on). Open the Layers toolbar, and select the practice shot and set the mode to “Subtract”. This will cause the stuck pixels to cancel themselves out, as well as remove some of the sensor noise (since the noise is random, you may want to repeat this step with more than one practice photo.) Use Layer->Merge Layers, then save the file back to hard disk. Repeat these steps for every raw photo.

Next, open the first corrected photo in Gimp, and drag in the second photo. This time, set mode to “Addition”. This will cause Gimp to increase the brightness of the total shots. You’ll also find that the Earth rotated enough that the stars in both shots don’t line up. You’ll need to zoom in and use the Layer Move tool to align the second photo with the first one. Then drag over the next few corrected raw shots one at a time and repeat the same mode and alignment steps. Ian Musgrave suggests using 5 photos, so that’s what I did.


(Raw shot of Orion’s Belt.)

Because I’m in the city, and I was only a few yards from some street lights, I had light pollution to deal with. It’s not a visible issue in the raw shots, but it does add up to be significant after 5 photos. Additionally, you may notice some streaking in a few of the photos. The more you zoom in on a star, the greater the impact the Earth’s rotation has on the shot for longer exposures. If you have a 30-second exposure, and a fixed tripod, you’re going to get streaking. The only way around this is to reduce the exposure times, which means messing with the ISO sensitivity, and the manual exposure settings. I don’t have a good handle on this yet, so I need more practice.

Also, the more you zoom in, the greater the impact of the Earth’s spin will be. By this I mean that the stars at the center of the photo may not move as much as the stars at the corners. You can try rotating the photos before stacking them, but this is very fiddly. And the amount of spin is going to depend on where you are and where you’re pointing. Being at the north pole and pointing straight up is not going to rotate the stars the same way as being at the equator and aiming the camera to the northeast. If you have a good camera with a remote trigger, taking photos in quick succession with short exposure times will reduce the amount of spin you get between shots. The alternative is to get a good software app that can automatically align the photos for you. (I don’t have this kind of an app yet.)


(Stacked result plus color correction.)

One temptation is to use Gimp’s Color tool to adjust the black levels of the finished stacked shots to get rid of some of the gray and make the pictures a bit clearer. This is kind of self-defeating, because you’re removing the light that you got from doing the stacking.


(Orion’s Belt closer up, raw.)

If you look closely at the stacked photo below, you’ll see a number of fainter blobs. These are what you want to be seeing with all the stacking you’re doing. These fainter blobs only show up in the camera if you stack many shots on top of each other. You’ll lose these if you color-correct the finished image by making the gray more black.


(Stacked result without color correction. Notice that the belt is showing some streaking.)


(Handle of the big dipper, raw.)


(And, stacked.)

Anyway, this is what I got after my first outing. You may ask why I didn’t experiment more when I had the chance. Well, first, there were some clouds during the night, and I had to shoot around those. Second, it was cold and I had been outside for a full hour; I’m not sure how much longer the battery would have lasted. Third, and more significantly, the authorities don’t like it when people stand outside in a park in the middle of a city at 2 in the morning, with a camera. I really need to find a photography club, so I can go with them out to the countryside. Or, for that “safety in numbers” thing.

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