Thanks all for the sharing of the post, tis much appreciated. I've received a few questions about the images and figured I'd take a moment or two to answer them to the masses even though I've responded directly already. The first question I'll address here was actually the most recently received, but for a bit of background on my process, I decided to answer it first.
What is image stacking, and how does the PhotoShop action work?
Essentially – image stacking is layering a series of images together so that you can then process them as a set. Because I shot from a tripod, and the illumination on most of the foreground objects stayed the same throughout the shoot, the sides of the houses, the chimneys, the trees, all have a consistent illumination. The sky is mostly black, except for where the stars are in any given shot. In each shot, they move just a little bit, not doing much more than adding a bit of blur to the star rather than a discernible trail in any given image. By stacking 683 of them, that lets the stars move through their arcs, the process pulling the brightest pixel from the column of images. Now, PhotoShop doesn't have to actually stack all 683 at once, you start with a new PS document and fill it with black. As you run the automated batch process against your folder of images, PhotoShop grabs each one, and copies it into the "new" file, and then applies a lighten action to get the brightest pixel at each location. So, after processing the one image, the "processed" image looks just like the first one… from here, the changes are more subtle. The action closes that first image and works through each successive image, merging the new layer into the previously processed image and brightening fairly small areas. Over time, this yields the streaks.
Why 683 images? Why not a 3 hour exposure?
Okay – so that is a shortened version of the question, but the essence is the same. Why do folks stack rather than using longer exposures or a very long exposure?
I've not searched for an answer but a few quick reasons come to mind. First off, early digital cameras had more noise the longer the sensor had to gather data to process, so to avoid this extra noise, you had to keep your exposure times short. Even now, there are settings in DSLRs that enable or disable long exposure noise reduction on images; so there is still some noise generated by a longer exposure.
Another practical reason is that most cameras' longest exposure time (excluding bulb) is 30 seconds; but again this changed with the advent of intervalometers (such as my canon TC-80N3) – devices that allow you to set exposure time, time between exposures, and number of exposures, greatly simplifying time lapse photography. There are even firmware builds (such as Magic Lantern unified, available on several Canon bodies) where countless additional features can be added to your DSLR. Without one of these devices (or a firmware modification), it is easy (as I did, see yesterday's post) to set the camera on continuous shooting and just slide the latch to lock the shutter into the "shoot" position… the camera dutifully shoots till it runs out of battery or storage.
Test shots are much easier to evaluate in shorter exposures. You can always do the math and calculate how a longer shot needs to either have the aperture cranked closed, or iso lowered, or both; but why not just go ahead and do 25 or 30 second exposures?
Flexibility in post processing pops to mind as well. I is fairly easy for me to process 180 or so images:
It nets a very different result. I could also skip sections of images to add some gaps… hmmm… spelling out a secret message in morse code anyone?
So, did you actually capture any meteors?
Yes, I think… well, one… maybe. How's that for decisive? LOL Well, based on info discussed in yesterday's post, I think I ended up with one meteor, and one satellite. Both came from the lower portion of the image towards the upper portion… both were caught in multiple frames. The first item I saw was what I think was a meteor:
(click to enlarge)
and what I think is a satellite – tho it may be a faint meteor. It is a longer trail, but the trail is pointy at the end, so it may have been a more glancing blow to the atmosphere – but while it is faint – the trail goes all the way into the tree… so perhaps tis a satellite after all:
(click to enlarge)
There are gaps in both due to them being captured in two frames each. There is a slight pause between closing the shutter and re-opening it. I got "lucky" both times – LOL
If anyone can give me a bit more info on meteor vs. space junk, I'd appreciate it.
All for now – keep those questions coming!
Thanks for reading!