Star Shots

vonDrehle · Aug 8, 2007

I wasn't really sure where to put this thread so I hope this is ok. Recently I have been experimenting with some Starscape and Startrail shots. As of right now I think I am coming along fairly well with the starscapes (ie short exposures) but when I try and get some star trail shots lets just say it is always interesting. I am currently using a 5D for these shots because I wanted to make sure I could actually get something to come out and not waste the film. Posted below are a few samples of the shots I have taken. As you will notice the last shot is where I really need help, and I have no idea what happened to it.

F-Stop: f/4
Exposure Time: 83 sec
ISO: ISO-400
Focal Length 24mm

F-Stop: f/4
Exposure Time: 177 sec
ISO: ISO-400
Focal Length 24mm

F-Stop: f/4
Exposure Time: 73 sec
ISO: ISO-400
Focal Length 24mm

F-Stop: f/9
Exposure Time: 2929 sec (~49min)
ISO: ISO-400
Focal Length 24mm

Thanks in advance for any help,
Chris

Garbz · Aug 9, 2007

LOL no idea what went wrong there. But the 5D won't give you any comparison to film that will be useful to you.

The reason is three fold. a) Digital cameras are affected by noise. The longer the sensor is reading the more the noise is amplified. This doesn't happen with film and you can easily produce a 12 hour exposure without any noise problems.

b) Film receives a colour cast if it's exposed to low-light for long enough. This often tends to green / blue. This is why taking an hour exposure with film will give completely different results to a digital camera even if you lock in your whitebalance to Daylight.

c and most importantly) Film stops following a logarithmic exposure rating beyond a few seconds. This had a technical term but I forgot what it is right now. Basically it means if you take a picture at 8 seconds, and then try again at 16 with all settings held constant you will NOT get a 1EV increase in brightness. If you really want to use the 5D to get a base exposure this is something technical you want to read up on. Download the film manufacturers datasheets which often has compensation tables to help figure this out.

astrostu · Aug 9, 2007

That last one looks weird. In general, it's a noise issue which is helped by your high ISO. You need to be using a much larger aperture and lower ISO so the stars will be brighter and less noisy. It looks like you also had a plane or satellite come through.

As to why there's a large dark "finger" coming across, I have no idea. It could be the noise characteristics of the camera, but I doubt that. It could also be there was something nearby blocking it somehow and you didn't notice.

shorty6049 · Aug 9, 2007

i'm guessing that the dark bar that showed up is caused by sensor heat. Usually it shows up as purple fogging at the corners and gradually overtakes the entire image, but maybe because the exposure was so long, it just overtook the image completley and produced that strange artifact...

Garbz · Aug 10, 2007

Oh btw to do this digitally and nearly noiselessly without spending a lot on custom cameras for the job. Consider taking a series of photos. One 30 second exposure every 31seconds should cover it. And then take the resulting photos and stack them in an image stacking program.

shorty6049 · Aug 10, 2007

registax is sort of the tool of choice for most astrophtographers but theres a learning curve to it .... theres another one but i cant remember what its called, i've also heard you can do it in photoshop but PS really isnt made for that kind of stacking (CS3 stacks feature works though)

vonDrehle · Aug 10, 2007

Alright thanks everyone. I am going to try out some shots with my film sometime soon and I will see how they turn out.

astrostu · Aug 10, 2007

Registax only works if you have a Windows machine. Granted, that may not be an issue for you, but if you have a Mac, it is. There's a program called "Keith's Image Stacker" that I sometimes use, but it is sometimes difficult to figure out what's going on.

Also taking multiple exposures as a substitute for one long exposure is not the same thing. The read noise inherent in the sensor's electronics is not dependent upon the length of time the sensor is exposed. Hence, unlike other noise, it will NOT diminish with multiple exposures.

Plus, if you want to stack many exposures at full resolution and 16-bits, you will need A LOT of RAM and a fast processor. When I did it for about 30 images, it took up 6 GB and 7 hours. Granted, I was going through dark-subtraction and flat-fielding, but an 8.2 Mpx image in 16-bit TIFF is 45 MB. Multiply that by 30 and so on ... you get the idea.

Oh yeah, and, if you want to do this with film, you can't really digitally add the images together unless you go through a scanning process, which I think defeates the purpose.