There are several different kinds of image frames that we deal with in astrophotography:

• Light Frames - When we take an image of a deep-sky object and record its light, we call that a light frame.

  The term "frame" is a holdover from the days of film where each piece of film in a slide or negative with an exposure on it was called a frame. A motion picture shot on film is made up of many individual frames. This term has carried over into digital photography where each exposure is now called a frame.

  If you shoot a bunch of frames and stack them together to equal a longer image, each individual frame that is part of the total exposure is called a "sub-frame", or "sub-exposure", or just a "sub" for short.

There are three basic kinds of calibration frames:

• Dark Frames - A dark frame is a picture of the camera's thermal signal and bias current. By subtracting a dark frame from a light frame, thermal current signal and bias signal are removed.

• Bias Frames - A bias frame is a zero-second exposure that records the bias signal, a low-level charge that is present in the sensor in the form of a fixed signal.

• Flat-Field Frames - A flat-field frame is a picture of an evenly illuminated subject used to correct for vignetting, uneven illumination, dust on the sensor's cover glass, and uneven pixel response.

Mouse-Over

Hold your mouse cursor over the image to see a comparison between the image before and after a dark frame has been subtracted to remove the thermal signal. Calibration removes the hot pixels and color blotches from the light frame. This is a 500 percent enlargement from the original to make the thermal signal more visible.

In this example, the thermal signal in the form of hot pixels and color blotches has been removed from the light frame by subtracting a dark frame.

Image calibration can be used on JPEG images, but it is more accurate when done with high-bit Raw images before the data is interpolated to create color. For example, you may have a single hot pixel from thermal signal in a Raw image. But when it is interpolated, the single hot pixel may be merged into the surrounding pixels. It is better to remove it before this happens.

If you are shooting single frames and recording them as JPEG files in the camera, you can use long-exposure in-camera noise reduction to let the camera take a dark frame and automatically subtract it from your light frame.

If you are shooting multiple light frames to stack to improve the signal-to-noise ratio, you are better off shooting separate dark frames and subtracting them later.

Use that clear dark-sky time to record more photons of signal, rather than to shoot the back of a lens cap to record a dark frame. Dark frames can be shot later on cloudy nights.

Shooting Light Frames Use Raw file format. Use ISO 1600 for 12-bit cameras and ISO 400 or 800 for 14-bit cameras. Shoot exposures long enough so that the sky signal overwhelms the read-out noise in the camera, but not so long as to overexpose the deep-sky object. Use the histogram to determine the correct exposure. Shoot with in-camera noise reduction turned off if you are planning to calibrate later with dark frames. Shoot with in-camera noise reduction turned on if you are not going to use separate dark frames. Shoot as many light-frame sub-exposures of the deep-sky object as you can. Aim for at least an hour's worth of total exposure time for most deep-sky objects. You'll need more exposure for fainter objects, or objects shot under brighter skies. More exposure always produces better images. Average the light frames together to improve the image's signal-to-noise ratio. Usually this is done automatically in an astronomical image processing program.

Dark Frames

A dark frame records the thermal signal in a camera for a given ISO, temperature and exposure time.

A dark frame is made by covering all openings to the camera, such as by removing the camera from the telescope and putting the body cap on. You should also cover the viewfinder eyepiece opening. Shoot the same exposure you used for your light frames, at the same ISO and temperature.

Thermal signal also has true noise associated with it called thermal signal noise, or dark noise. This dark noise is actually added to the light frame when a dark frame is subtracted, increasing the overall noise in the image.

The amount of dark noise that is added to the lights in calibration can be reduced by creating a master dark frame made from a bunch of individual dark frames.

Most astronomical image processing programs, such as Deepsky Stacker, Nebulosity, or Images Plus will automatically average all of these dark frames together and create a master dark frame. The master dark is then subtracted automatically from each individual light frame.

Shooting Dark Frames Use Raw file format. Turn off in-camera noise reduction. Shoot the dark frames at the same ISO, exposure and temperature as the light frames. Shoot as many dark frames as you can, with 9 or 16 as a minimum. The more, the better. Average the dark frames together to create a master dark frame. Usually this is done automatically in an astronomical image processing program.

DSLRs, Heat, and Dark Frame Temperature Matching

All digital cameras generate heat while they are being used. As this heat builds up in the camera, the thermal signal also increases. The thermal signal will be different for each frame in a long exposure run.

Likewise, it will be different for each dark frame in a series of darks taken to create a master dark.

The dark frames are supposed to match the light frames in temperature so they have the same thermal signal, and it can be removed in calibration, but the temperature is constantly changing in a continuous exposure run.

If the temperature is not exactly the same, artifacts in the form of dark or light pixels can be left behind after calibration.

Luckily this problem can be solved by clever software, such as Images Plus, that examines the lights and the darks and adjusts the calibration to account for differences in temperature between the lights and the darks.

Some people think they can keep the temperature down in their cameras by pausing between frames. While this is true to a degree, you are better off gathering more photons to increase the signal-to-noise ratio in your images. Investigations into this question have proven that you are better off shooting light frames without pause while the sky is clear. Do not use clear dark-sky time to shoot dark frames.

Dark Frame Library

Since dark frames can be shot on cloudy nights, it is easy to create a dark frame library of master darks at different temperatures.

If you also shoot bias frames, master dark frames can be scaled down to different exposure times (see Bias Frames below). So all you need is a master dark frame at the ISO and longest exposure you normally use, and a range of temperatures, say for every 10 degrees Farenheit (5.56 degrees Celsius) that you shoot under.

Optimum Use of Clear Dark Sky Time Nights when the sky is clear and dark are precious. Since there is no substitute for gathering more signal to improve the signal-to-noise ratio in an image, this clear dark-sky time is best spent in shooting light frames, recording photons, and gathering signal. Dark frames can easily be shot on cloudy nights.

Bias Frames

Bias Frames record the bias signal, a low-level charge that is present in the sensor in the form of a fixed signal and that is recorded when an exposure is taken.

Since bias is included in every frame, it is also present in dark frames. So when you subtract a dark frame, you are also subtracting the bias. If you are only going to subtract dark frames shot at the same ISO, exposure time and temperature as your light frames, you do not need to shoot bias frames.

So why shoot bias frames at all?

Bias frames can be used to create scalable master dark frames. Thermal signal scales with time. So if the bias is removed, software can simply scale a dark frame of one exposure to match a light frame shot at a different exposure, as long as the ISO and temperature are the same. This is very convenient. Most astronomical image processing programs will do this in the image calibration step if you include bias frames and indicate they should be used for scaling the darks.

Scalable dark frames work the best if they are shot at a longer exposure than the lights and scaled down. So shoot your darks at the longest exposure you think you will use.

Shooting Bias Frames For most astrophotography, you can get away with not shooting bias frames if you subtract dark frames because the bias is included in the dark frame. If you really want to do it the right way, shoot a series of bias frames and make a master bias frame. Exposures for bias frames are very short so it doesn't take much time to shoot them. Use Raw file format. Cover all openings including the camera eyepiece. Use the shortest exposure that the camera can take. Use the same ISO and temperature as the dark frames. Shoot a total of 9 to 16 frames at a minimum. More is better. Average the bias frames together to create a master bias frame. Usually this is done automatically in an astronomical image processing program.

Flat-Field Frames

A flat-field frame is a picture of the optical system's peculiarities in illumination, such as vignetting, uneven illumination, and dust shadows. It also records sensitivity variations between individual pixels in the sensor, which are usually small, but present nevertheless.

When a flat-field frame is divided into a light frame, it removes these problems.

A flat-field frame is made by photographing an evenly illuminated subject, such as the cloudless sky at twilight, or a light box, or by placing a white T-shirt doubled over the aperture of the telescope and pointing it at an evenly illuminated extended light source such as a blank wall.

While flat-field frames can remove vignetting, they cannot compensate for the reduced exposure and resulting lower signal-to-noise ratio in the sections of the image where vignetting is present. This is why it is better to locate the source of the vignetting and correct it in the optical system.

Shooting Flat-Field Frames Use Raw file format. Use an evenly illuminated, fairly bright light source such as: A light box A double white T-Shirt in front of the lens or scope The twilight sky opposite the Sun A large evenly illuminated flat sheet or foam-core board Use a low ISO. Expose on manual at the camera's suggested meter reading and expose to mid-histogram, or shoot on autoexposure with aperture value mode. For camera lenses shoot at the same f/stop you will use for the light frames. Focus the scope or camera lens on a star or planet. Shoot with all components of the optical system the same as for the light frames: filters, Barlows, telecompressors, adapters, etc. Shoot 4 to 16 frames total at a minimum. Do not change the orientation of the camera in the scope after shooting the flats. You can, of course, re-aim the scope at the deep-sky object to shoot the light frames. Median combine the flat field frames. Usually this is done automatically in an astronomical image processing program.

Once you have progressed past the beginner stage in your astrophotography endeavors, you will want to stack multiple frames to improve the signal-to-noise ratio in your images, and you will want to calibrate them. This will produce the highest quality images.

Image Calibration - The Bottom Line Image calibration can correct for problems such as thermal signal, vignetting, and dust shadows in long-exposure astrophotos. Stacking multiple dark frames to create a "master" dark frame for a given temperature and then subtracting the master dark from each individual light frame is the optimum method for removing thermal signal. Flat-field frames can also reduce vignetting and remove dust shadows.