Good news for those night shooters who are using Macs. Until now the only choice we had for stacking shots was an action in Photoshop. The widely used Startrails.app was limited to windows which has always been a major nuisance to say the least. Photoshop did a wonderful job but was slow and tied up memory which limited what other tasks could be done simultaneously.
So what about the good news? Well, Markus Enzweiler has developed Starstax and had kindly made it a free program. It is currently for Mac and Linux only but apparently he is working on a Windows supported version. Pulled from the site;
"StarStaX is a fast image stacking and blending software, which is developed primarily for star trail photography. It allows to merge a series of photos into a single image, where the relative motion of the stars creates structures looking like star trails.
StarStaX is available as Freeware for Mac OS X and Linux."
I tried a few stacks and immediately fell in love with it. What you notice right away is the incredible speed. Stacking with Startrails.app or Photoshop takes a while and can be frustrating but StarStaX seems to do the the job instantly. The quality is the same as a stack done in Photoshop using the the same blend mode of "lighten" to get the job done. So if you are a Mac or Linux user I highly recommend StarStaX.
Photoshop Stacking action
Why stack? There are a few reasons why I choose to stack most of the time. The trouble with one long exposure is to keep the correct exposure you have to stop down. If you are trying to catch as many stars as possible this is definitely not ideal. If you want an hours worth of stars in your frame you are going to have to stop down to f/11 or so and shoot at an ISO of 100 which is not going to pick up many stars but would give you a correct hour long exposure. Noise would be visible and if you make a mistake with your foreground lighting well you have just wasted an hour. With stacking you generally have to only worry about 30 seconds. This allows you to max out your aperture and bump that ISO up seeing that not much noise is going to show up after 30 seconds especially with new cameras. With wide apertures and high ISOs you can pick up an incredible amount of stars and go way beyond what you can see with your own eyes. Another huge benefit is you can do a frame of foreground lighting at the beginning and again at the end and include the better one in the final stack.
I recommend stacking for star trails and highly recommend StarStaX for those of you doing it on Macs. Stay tuned for a detailed tutorial on how to shoot star trails. The tutorial will cover shooting single exposures, stacking multiple exposures and shooting with film. Follow me on twitter for that and other updates.