[bk1000]

Ok, I’m curious. How does one do a 33 hour exposure? The shutter must be closed during daylight hours, right? Would this not be a multiple exposure?

[ProtectOurFreedom]

Good catch!

They use what's called "Stacking software." Stacking minimizes the ISO noise that accumulates in one long exposure. You effectively take a lot of long exposure shots (probably several hours) and the software "stacks" the many exposures into one. It automatically matches control points in the photo to align the many photos into one and does all the calculations needed to get the merged photo right. It is really amazing software.

It's widely used for nature photography. The slick thing is you can create a huge false but very real appearing depth of field. For small subjects like insects, you set your focus point for the initial shot, change the focus point (a few thousandths of an inch), take a shot, etc. Software controls everything - changing the focus point and taking the shots. You can stack thousands of images this way to get photos that are impossible due to optical physics limitations. Here's an example:

One popular tool is "Zerene Stacker." Here is a Gallery of Images taken with Zerene Stacker.

[Kirkwood]

Frame stacking. It is not a multiple exposure in the strictest sense. A multiple exposure is when you open and close the shutter multiple times without advancing the image frame.

[Swordmaker]

Ok, I’m curious. How does one do a 33 hour exposure? The shutter must be closed during daylight hours, right? Would this not be a multiple exposure?

Depending on the time of year in the Arctic, it’s not a problem, when the sun does not rise above the horizon. Similarly, taking a 33 hour exposure of deep space is also never a problem. . . If you have a stable platform from which to take it.