Confused

[gtis]

Hi all

Just being looking at some excellent astro photos but I am confused

About what is meant about lights darks flats and bias

Neil

[Marky1973]

Hi gtis

In a very basic nutshell

LIGHTS - these are your actual imaging subs - so the pictures of the actual target

DARKS - are images taken with the lens cap on and are mapped onto your images in the stacking process to remove noise from the final image

BIAS/FLATS - are calibration frames to sort out vignetting and dust bunnies that might affect the picture

They are all used to combine into a final image that is, hopefully, full of imaging data and devoid of noise and artifacts from the sensor/imaging train.

Still battling with them all myself, (and my knowledge is limited) but check out the book Making Every Photon Count - if you are thinking of getting into imaging it is the best start.

[steppenwolf]

From my own book:-

Light Frames

Light frames are the actual images of the night sky that you capture. You normally take a whole sequence of them.

Dark Frames

When the final image of the sequence has been taken you will have a set of images that we will call ‘LIGHTS’ for convenience. You will now need to take a set of ‘dark-frames’ which we will call DARKS for convenience. A dark-frame is an image of the same exposure length as the LIGHTS taken with the front of the telescope sealed closed to exclude all light – in other words an exposure of nothing! I say ‘nothing’ but when you have a look at the DARK image you will see that in fact you have actually captured some data after all. The image may well look like the ‘noise’ that you get on a television screen that has no antenna attached to it but this image contains something very useful indeed - a sample of the ‘noise’ generated purely by the sensor chip and its attendant amplifier. Additionally any ‘hot’ pixels which show as bright dots will also be present in the data. If you take several of these dark-frames and average them together in your stacking software, you can produce a ‘master’ image representing your ‘average noise’. If you now subtract this noise data from your real object images, you will remove a lot of the noise component from them leaving you with a much ‘cleaner’ image. This process is known as ‘Dark Frame Subtraction’.

Bias Frames

There is one other sequence of images that you must also take but these can be taken at any time prior to final processing of your images and these images  are called BIAS frames. A BIAS frame is an image taken for the shortest exposure time possible with your camera with the front of the telescope sealed closed to exclude all light as for the DARKS. The BIAS frames are used to compensate for pixel-specific offsets in the sensor chip and camera electronics during the readout of the data and its subsequent saving to the memory card or hard drive. It is normal to take at least 20 BIAS frames but because of their short exposure, these won’t take that long and unlike the Dark-frames you can store them for use later with images from subsequent sessions. Again, these frames should be averaged together in your stacking software to produce a ‘master’ image representing your ‘average BIAS’. This BIAS master is also subtracted from your LIGHTS in a similar manner to the DARKS in a process known rather imaginatively as ‘BIAS Frame Subtraction’. This frame may at first glance look like the DARK frame but it contains a very different kind of data.

The processes of DARK frame and BIAS frame subtraction are jointly known as ‘Calibration’ but there is a third process that can be included in the calibration process involving ‘FLAT frames’ and I will be discussing these later as they are taken in the same way for both a DSLR camera and an astro-CCD camera.......

Flat Frames

Earlier, I mentioned FLAT frames and these frames cause more difficulties for imagers than any other kind of frame as producing them is not an exact science. Many imagers don’t take them at all and if your optical and sensor surfaces are spotlessly clean and dust free and your system doesn’t suffer from vignetting then you may well be able to avoid taking them too.
    Vignetting is the reduction of image brightness at the edges of the frame caused by the lens design or an obstruction in the light path. A flat frame will correct for variations in pixel to pixel sensitivity, the effects of vignetting and will remove ‘dust doughnuts’ which are shadows of dust particles on the optical and sensor surface that are cast onto the sensor and show as dark rings in your images.
    A FLAT is an image taken at exactly the same orientation of the camera and the same focus position as the LIGHTS but with the telescope/lens pointing at a completely diffuse but evenly lit surface. The overall illumination should be at between 30% and 50% of the full well capacity (saturation level) of the sensor elements but it is vital that the whole image surface is equally illuminated. Somewhere in the region of 20 FLATS should be taken and these should be stacked using a Median combine method. The FLATS themselves must be BIAS subtracted using your BIAS frames and for longer exposures DARKS should also be subtracted before the FLATS can themselves be used for calibrating your LIGHTS where they will be divided into the LIGHTS.
    If you are using a one shot colour astro-CCD camera or a DSLR camera, when you produce your master FLATS, you should apply a ‘boxcar’ filter to the individual FLAT subframes to smooth the data and destroy the Bayer matrix before applying the FLAT to your LIGHTS. Carrying out this procedure will help to ensure that the FLAT doesn’t impart any colour cast to the finished image.
    The FLAT will contain some of the ‘faults’ that will impair the finished quality of your images and will give a good representation of the dust particles in the light path as well as showing any vignetting that may be present. The larger grey ‘doughnuts’ indicate the presence of dust on an optical surface in front of the sensor but the darker, smaller patches indicate dust on the sensor itself. The vignetting introduced by the optical system is clearly evident and it can be seen that the brightness of the light cone falls off significantly towards the edges of the frame and this will be evident in your LIGHT images too where the objects towards the edge of the frame will appear dimmer. Applying a FLAT frame to your images will very effectively correct these errors although it will not remove them entirely.
    There are several methods for taking flat frames ranging from illuminated LCD panels and light boxes at the sophisticated end to imaging a cloudy sky through a white ‘T’ shirt at the low-tech end. I have yet to find the ‘perfect’ guaranteed to work method but when I do take them, I use a method known as ‘twilight flats’ where the sky at twilight is used as a diffuse source of light. Sudden changes in light levels should be avoided, however, the illumination at this time of day can be pretty uniform but care must be taken to ensure that no light from stars is captured. To avoid this eventuality the telescope can be moved a small amount between exposures so that a slightly different area of the sky is imaged in each frame.
    Taking FLATS is not a trivial task as the camera must not move in either focus or orientation between the taking of the LIGHTS and the taking of the FLATS and this can be a real inconvenience but the added hassle will result in better quality images with less chance of vignetting and the removal of dust doughnuts. With regard to the problem of dust, you should still do everything you can to exclude any dust from the inside of your equipment in the first place.

All this subtraction of BIAS and DARK frames and division of FLAT frames might sound complicated but many processing programs will carry out these tasks for you automatically although you can do the work manually if you want to.