ISO1600 Short Exposure Imaging

or.. astrophotography made easy :)

How ISO affect sensitivity to light

The mount that I use, a GPDX, was at one stage not very smooth in its operation. This was probably due to the fact that the lubrication in the mount was pretty old. As soon as the temperature was near, or below, freezing - the mount got very tight and difficult to guide during long exposures.

During this time I had bought a new digital SLR camera, a Canon 10D, which presumably featured pretty good quality and low noise images at ISO1600. This suddenly gave me the chance to do some imaging even though my mount was in pretty bad shape. Basically trying to take many exposures of the same object with high sensitivity ISO setting ans short exposures. The short exposures would enable me to use the mount without having to re-lube it.

An image taken during 1 minute @ ISO200 will show twice as much detail (e.g. on a nebula) when taken with the same settigs and lens, compared to ISO100. The higher the ISO the more noise you will get in the image so one will have to take short exposures to counter the noise effect that builds up during the exposre.

If I were to choose the ISO setting for a astropicture, and if the tracking/mount wouldnt be an issue, I would probably go for ISO100 since the quality is higher.. However, when the mount and tracking is an issue and I thus have to take shorter exposures I can increase the ISO and get a similar result in terms of how much light I gather in the image.

Here are some examples:

1 minute @ ISO1600 = 5 minutes @ ISO100
1 minute @ ISO1600 = 4 minutes @ ISO200
1 minute @ ISO1600 = 3 minutes @ ISO400
1 minute @ ISO1600 = 2 minutes @ ISO800

The sensitivity to light is thus 5 times as high @ ISO1600 than @ ISO100

Making many short exposures count..

The reasons for making many short exposures instead of one long are many. By combining more than one exposure of the same lenght one will give the computer that combines the pictures data that has been aquired at two different times. This data can be used to see if there are parts of the image that only apear on one if the iamges. A speck or artifact that apear on only one image is probably not something you want and the computer can take that away for you.

One can also utilize many exposures to increase certain aspects of the images when the program is combining the images for you. You might for instance want the program to value light parts of the images higher than dark parts - making light parts stand out more in the picture than if it was one single exposure.

Most importantly thoug, and the main reason for this whole test, is the fact that you will need a less accurate mount and tracking if you take a short exposure than if you take a long one. The reasons for this are many. The most obvious fact is related to the earths spin which will make stars move on your image unless your counter spin your camera and lens. Countering the spin of the earth is pretty difficult since you need to align your mount to spin with the same speed as earth as well as along the same axle as earth. The shorter the exposure the less time the stars can travel accross your image. Another simple reason is that, the less time you expose the less issues can happen. You will cry far less if you happen to bump the mount during an exposure of 30 seconds than if you do the same at the end of a 30 minute exposure.

There are some fancy algorithms that indicate pro's and con's related to combining many exposres but if you dont have a very good mount or dont have an autoguider - you will want to take as short exposures as possible, while still getting something to show on your image - regardless of any algorithms in the world

For this sceanrio lets assume that 3 images of 1 minute gather as much light as one single 2 minute exposure. By saing this, even if it might not be exactly true, I am basically making you aware of the fact that you might loose some sensitivity to light compared to making a long exposure but its far better to catch a little less light than getting star trails on your picture.

Digital cameras make all this possible since you dont pay for each image.. and you dont have to manually scan the pictures into the computer in order to combine the images, you simply download them from your camera memory.

So, we have now established some simple facts for us people with inaccurate mounts.. We can increase the ISO in order to catch up to five times as much light as a low ISO picture (ISO1600 compared to ISO100). The shorter the exposure the less likly that we get star trails or problems during the exposure and we can add many short exposures and get them to act like one longer exposure.

Ok, I get it..

I have now taken the time to describe the case so I dont have any time left before bedtime other than to show an image that I took with the inaccurate mount at ISO1600. There is a total of 40 images of 30 seconds each combined into one single image. The total exposure time is then 20 minutes (but we know we loose some compared to one single exposure of 20 minutes. The ISO of 1600 means that we would had needed a total of 100 minutes at ISO100 to capture the same ammount of light.
If we assume that we loose 30% by combining instead of taking one single image that would mean that we have an image that is similar to one single exposure of 66 minutes @ ISO100. That is not so bad considering that we only took 30 second exposures that need far less accuracy than one single 66 minute exposure would need.

And.. here is the result.. I will try to provide some more step by step info later on.. might also add some comparisons to longer exposures at lower ISO.. I hope you have enjoyed!