\nUsing both of these can really help to make an image sparkle.
\nBUT THEY ARE NOT ALL GOOD!
\nIf they are not done properly, they can make an image worse than if they were not used.
\nSo use them with caution.<\/p>\n
If you are just starting out in astrophotography, do not get hung up about having to take and manage calibration frames.
\nJust get to grips with the basics first.
\nOnly go onto doing these once you have become a lot more confident with imaging and processing.
\nI have found that when using a DSLR, I have managed to produce some great images WITHOUT using either.
\nIn fact most of my DSLR Images are processed with no calibration frames.<\/p>\n
CCD’s and CMOS cameras are a lot different to DSLR’s.
\nThe way they are built and work, means that Dark frames and Flat Fields do become a lot more important when using them.
\nArtefacts show up a lot more easily when using these.<\/p>\n
Let’s look at these two calibration frames in turn.<\/p>\n
Dark Frames.<\/strong><\/span> Where there is dark sky, less photons will be collected. All pixels across the imaging chip will receive a different amount of light. That sounds all well and good. But the way that the camera works frequently plots against us. Here is a single sub taken of the Elephant Trunk Nebula.<\/p>\n This image is a 900 second long exposure using a monochrome ZWO ASI183MM Camera. Look on the right-hand side of this image. Thankfully all these artefacts are extremely easy to remove in the stacking process. To do this, put the cap on the telescope to prevent any light getting in. This will result in an “almost” black image which only contains the hot and dead pixels and, in this camera’s case, that star shape produced by the chip.<\/p>\n Once you have captured both your light images and the darks, everything is now ready for stacking.<\/p>\n In your stacking software, either Deep Sky Stacker, Affinity Photo, or other dedicated stacking software, you can load the light frames that need stacking. During the stacking process, the dark frames will be deducted from the light frames (subs). It is important to remember that the amount of amp glow, or noise in the resulting image will depend on the exposure used. If a longer exposure dark frame is used to stack shorter exposure images, there will be some over-correction and it will be too severe. An example of this can be seen below. The over-correction has resulted in the image now having a dark star shape that has been deducted from the image. So I hope that explains about dark frames.<\/p>\n What’s different about Flat Fields then?<\/p>\n Flat Fields. The image below is of the dark Seahorse Nebula in Cepheus.<\/p>\n The dark nebula can be seen quite clearly, but there are also a couple of very large dust bunnies spoiling this image. These can be removed by using flat field images when the subs are stacked.<\/p>\n A flat field is taken in almost the opposite way to the dark frames. Here’s how I take mine. Take an image that gives you an exposure so the highest peak of the image histogram is about a third of the way up.
\nWhat is a Dark Frame?
\nWhen you take images, we are using the camera to expose a light sensitive chip to any incoming light, where it collects photons.
\nA pixel where starlight is focused onto it, lots of photons will be collected.
\nThere will be quite a high signal and the pixel will become bright, close towards white.<\/p>\n
\nIn this case, the pixel signal will be low and its brightness will be closer to black.<\/p>\n
\nSo there will be pixels at all different levels somewhere between black and white.
\nThese build up an image of the area of sky which is focused onto the surface of the cameras chip.
\nThe longer that the camera is exposed, the more photons are collected by the pixels.
\nThis will result in a brighter image.<\/p>\n
\nDepending on the chip used in the camera, there is quite often some amp glow within the camera.
\nThis can be seen as a brightening on the resulting image.
\nThere may also be bright “Hot” pixels and dead dark ones.
\nThese can create bright and dark speckles in the captured images.
\nThese can all ruin our final stacked image if we did not get rid of them.<\/p>\n
\nThis is known as a light image (Or Sub-image if you are taking lots to stack later).<\/p>\n![\"Exposure-Plus-Star-Light_IC1396A_900.0s_Bin1_H_-20.0C_0002\"](\"https:\/\/www.star-gazing.co.uk\/WebPage\/wp-content\/uploads\/Exposure-Plus-Star-Light_IC1396A_900.0s_Bin1_H_-20.0C_0002.png\")
<\/a><\/p>\n
\nThe chip used in this very popular camera produces a bright star shape coming in from middle of the right-hand side.
\nThe longer the exposure taken, the brighter this artefact becomes.<\/p>\n
\nAll that is needed is to take some dark frames.<\/p>\n
\nThen take a series of images at the same exposure and temperature that you are using to take the images.<\/p>\n<\/a>
\nThis is known as a dark frame.
\nIt is always best to take a number of dark frames to apply to your stacked image.
\nThese will be used later, so that you get an average dark frame image during the stacking process.<\/p>\n
\nThen select the dark frames that need to be used to remove the artefacts.<\/p>\n
\nSo in this particular case, that star produced by the camera chip is effectively removed from the light subs.
\nAs a result you will no longer be able to see the artefacts produced by the camera in the final stacked image.
\nAn example of this can be seen below.<\/p>\n<\/a><\/p>\n
\nSo any dark frames used for the stacking process MUST be taken at the same temperature and exposure as the captured light frames (subs).<\/p>\n
\nThis can result in a reduction in brightness where the artefacts were.<\/p>\n
\n<\/a>
\nIn this image of M110, I took 120 second exposures for my light subs.
\nI then used dark frames of 900 seconds and used those to correct my image.<\/p>\n
\nSo always make sure that the exposures for your dark frames are taken with the same conditions as your subs.<\/p>\n
\n<\/span>When you are taking light images (Subs), there may be different parts of the imaging chip that are slightly less sensitive than others.
\nThis can be caused by manufacturing errors in the chip, or shadows created by dust lying in front of the camera.
\nThese latter shadows are known as “Dust Bunnies” and can be a real pain to get rid of.
\nThere may also be some shadowing around the edges of the image, caused by the restricted field of view created by the scope used.
\nThis is known as vignetting.<\/p>\n<\/a>
\nIt has had the dark frames subtracted from it, which has removed the star and any hot pixels, but it is still showing some problems.<\/p>\n
\nThe most prominent one is towards the top of the image left of centre.
\nThe other one towards the bottom of the image, just left of centre.<\/p>\n
\nThis is normally done in the daytime.<\/p>\n
\nRemove the cap off the end of the telescope.
\nDrape a white T-shirt over the end of the scope, making sure there are no creases that will create shadows.<\/p>\n
\nA flat field image taken for my setup can be seen below.<\/p>\n