Darks Flats and Bias as I understand them

Before we begin on the files themselves, lets start with a little background information. When my camera is attached to the telescope, there is no aperture adjustment. It is fixed. The only adjustment options i have are ISO and Exposure time. ISO is a measure of light sensitivity. In the days of film, different coatings produced different light sensitivities, but higher ISO films produced a grainier end product. Now it's just a matter of setting the sensitivity of the chip. Exposure time is... well... the time you expose the chip to light. Usually, unless i am dealing with a fairly bright object, i will set the ISO at 1600 and adjust the exposure time depending on the results i get in the histogram. That is not a hard and fast rule. There are several factors that figure into my settings. But that is a whole other blog. For the purpose of this blog, let's say i've decided i want one hour of exposure on the target. In this example, i'm going to set the ISO at 1600. To get an hours worth of light, i'll set the exposure time (shutter speed) to 60 seconds, and plan on taking 60 photos. In the stacking software, these photos are called "Lights" because they contain the light data that will actually create the photo. But sometime in the process i will need to shoot Darks, Bias and Flats. The following is my explanation of  Darks, Flat and Bias as i understand them.

Darks
The chip inside your camera is defective. Yup, sorry. They all are. There are pixels on the chip that will be ON when they shouldn't be. It's just the way it is. During normal photography this isn't much of an issue. But in astrophotography it makes a big difference. Temperature also has an effect on the chip. Turning pixels on depending on how hot or cold the chip is. To battle this, the photographer will take a series of Dark frames. there's no mystery to it. It's just a matter of taking a few photos with the lens cap on at the same ISO, Temperature and Exposure time as the photos. The pros will take Darks before, during and after they have taken their Light frames. But i just do this for fun. As the target photos are taken, the chip heats up inside the camera. I assume that there are more defective pixels as the camera heats up. I could be wrong, but this seems to be working for me. So i take all the Light shots i want, 60 in this case. Then after i am done, i just pop the lens cap on the scope and take 5 Dark frames at the same ISO and Exposure time. I'm not sure how many darks frames i should actually take, but i know if i don't feed SOME into DeepSkyStacker, the results are less than desirable.

Bias
There is a certain amount of electronic noise generated inside the camera when the shutter is activated. This noise has a effect on the chip, causing even more pixel naughtiness. To counteract this, the photographer will record this noise in Bias frames. Many people argue that the bias noise is already recorded on the Dark frames. I don't know. What i do know is that after i have taken my Dark frames, i can change the Exposure time on the camera to the fastest time (in my case 1/4000) leaving the lens cap on, ISO the same and shoot five Bias shots pretty quickly. Does it help much? I don't know. But it's easy, so i will continue to do it.

Flats
Dirt, dirt, dirt. It's all about dirt. Flats are used to record the dirt within the optics path so it can be removed from the photos. I take Flats when it's handy, but i must admit that i am bad about not even collecting them. The idea is to have the same camera/scope setup and use some even flat light to record the flat frames. What i usually do, when i think of it, is to cover the end of the scope with a white t-shirt (during daylight hours) and point the scope at some empty part of the sky. I'll usually take five shots using the same ISO and Exposure settings as the Light frames. In this example ISO 1600, Exposure time 60 seconds. I don't fret about it. My pics will never make ASTRONOMY magazine. I'm sure that as my skills improve, there will come a time that i wouldn't even dream of processing a photo without Flats. But i'm not there yet.

So there you have it. No mystery to it. Don't worry too much about what to do with the files once you acquire them. I just load them all into a program called DeepSkyStacker and it does the required work. I should probably also mention that i use a camera control program. BackyardEOS is an excellent camera control program and well worth the affordable price.

ok, that's it!

clear skies...