Types of images that need capturing

When capturing images it’s not as simple as pointing the camera to the night sky and releasing the shutter.  The complications come from the exposures be much much longer than daytime photography.    The images are very dark in comparison and it’s important to try and extract every last bit of signal so removing noise is key.   This is why capturing images can be tricky.  There are several types of image frame that needs to be captured.   I’ll do through them in this post.  With my equipment, we can remove some of the variables which can make this tricky. As the imaging camera is cooled, the temperature of the sensor remains constant, this means that the noise level of the image remains constant too.  The electric focusser will allow me to put the telescope back into a specific position without any issues. Meaning that I can take flat frames later if desired – although they don’t take long to do at the time.


So what are the types of image that I capture during a session?

Light frames

Light frames are the images of the target.  These are the raw images, complete with noise from the telescope imaging system and other factors too.   The more of these images that I capture, the lower the signal to noise ratio.    However, there is a problem of dimishing returns for this…   Let’s say I take 1 image it will have noise.  if I take a second, I can combine both and halve the noise.  if I take four images, it’ll half again.  each time I double the amount of images, I half the noise.

However, when a image can take 5 mintures.  Taking 64 of them, will take 320 mins (over 5 hours)  But will only have halve the noise of 32 images which took 160 mins (just about 2 hours 40 mins)


Dark frames

Dark frames are images taken with the camera sensor completely shut off from light.  ideally, there will be an equal number of these to the light frames.     What they do is record only the noise from the camera sensor.  This includes bad pixels.  The fortunate part of this is that it doesn’t matter what the target is, the dark frames can be used again.  Most astrophotographers will create a dark library to save time and effort.  Later on.


Bias frames

Just like dark frames, these can be created ahead of time.  What they do is record the minimum values of each pixel on the camera.   Taken under the same conditions as dark frames, the only different is that the exposure time is as short as it’s possible for the camera to take.  Again, this can be saved and used over and over for a given camera.


Flat frames

Flat frames are images of a flat light source.  There are several ways to take these, I use a light panel, other people use the T-Shirt trick, its up to you which you use.  The idea is that you take a photo, a good exposure of the light panel through the optics of the telescope.  The important thing here is that the images are taken with the same focus as the light frames.  What the flat frames will do is record an imperfections in the imaging train.  Things like vignetting, dust particles and so on.    This can make a huge different to an image.


Dark Flat frames

These are dark frames, just like before, but this time they are darks that are for the exposure time of the flat frames.


All these kinds of images can be combined together to calibrate the light frames and remove alot of noise.  The lucky parts is that the bias, darks and dark flats can be produced and stored in a library.


One final thing to remember, is that when using a monochrome camera with filters, you will need to capture a set of lights, flats and dark flat frames for each filter.    The reason for the need to capture the dark flat frames is that the exposure time will be different for each filter.

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