Flat Frames

Flat Frames

Pete's Pages

Back to Camera page
Dark Frames
Bias Frames

Flat Frames

Dark frames & bias frames are both designed for correcting unwanted noise in the CCD chip. Flat fields are designed to correct problems in the optics i.e. dust, dirt, internal reflections & even vignetting. Because of this every time you change the optical system you should take another flat field. This means even if you just refocus or change a filter.

To take a flat field.
The flat field must be taken at the same temperature as the dark, bias & image frames. The time should be as short as possible whilst at the same time getting the correct saturation level. For the correct saturation level the flat fields average value should be approximately one third of the maximum saturation level of your camera. i.e. My ST7 ant bloom camera will hit saturation at about 20,000 to 22,000 units so the flat field's average value should be around 7,000 units. To take a flat field you need an evenly illuminated surface. There are several ways of doing this. The simplest method is to get a piece of white card & position it parallel to the telescope. Then shine a light onto the card. Take several images with different exposure times to get the right average saturation level. The light can be a porch light or any low powered light. The card can be exchanged for a blanket over the washing line or the inside of your observing dome. I use the back of my roll-on roll-off observatory. I attached one of those screens you show film slides on to the back wall. Then around the door I attached eight eight watt bulbs. In front of these bulbs I put a baffle to stop the light from shining directly onto the screen. The light goes from the bulbs and is reflected off the baffles and the observatory walls, which I have painted white.

What does a flat field do.
When you take an exposure any dust or dirt in the optical system will show to some extent on the image. A speck of dust on the mirror will be out of focus and appear on the image as a doughnut shaped smudge. Generally the nearer the dirt is to the camera the darker the smudge will be. Vignetting (a gradual darkening towards the outside of the image) may also show up if the optics have this problem. Because the image you are taking is even (that means the image reflects the same amount of light across the whole of the image.) any imperfections in the optical system will be recorded on the flat field image. These imperfections will also be on the final astronomical image. To remove these imperfections we divide the final astronomical image by the flat field. This is different to dark & bias frame processing because we are trying to flatten the background not remove hot & cold pixels.

Take 2 pixels that receive the same value of light.

Pixel 1.
Image (1000) + extra value caused by light reflecting inside the telescope (200) = 1200 1200 \ flat field value (400) = 300

Pixel 2.
Image (1000) - light reduction caused by dirt on mirror (400) = 600 600 \ flat field value (200) = 300

The internal workings of the camera can also create localized hot spots, which will alter the sensitivity of the pixels in that area.