darks and flats ???

[Pete Presland]

just love all the images on here simply stunning ! but what are darks and flats?

[steppenwolf]

Darks are images taken with the lens cap or dust cap on the telescope for the same exposure time as the image subframe - they don't capture light, they capture the 'noise' generated by the heat of the sensor. These are subtracted from the image frames thus removing the unwanted noise from the image.

Flats are images taken through the telescope at exactly the same focus position as the image subframes but with the telescope pointing at an evenly illuminated surface. These are used in the calibration process to compensate for pixel to pixel variations in sensitivity of the sensor, to remove dust halos and to aid in combating the effects of vignetting as they capture a sample of light but without any image detail. 'Dividing' these images into the image frame removes these unwanted features.

There is a third type of frame called a BIAS frame as well. A Bias frame is a subframe taken for the shortest exposure time possible with your camera, with the front of the telescope sealed closed to exclude all light. The Bias frame is used during calibration 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.

[Merlin66]

In any digital image (called a "light") there's always some sort of noise or interference in the image. generally the electronics can cause speckles in the image.

By taking an image with the same exposure as the usual target "light" exposure, but with the cover on the lens gives a "dark" ie no light, which records the same noise pattern as the "light" - by subtracting this "dark" from the "light" it removes the noise and gives a better image.

Likewise all optical systems cause some distortion - the edges of the image may be dimmer than the centre of the image (vignetting) - caused by baffles, adaptors etc etc. By making a short exposure of a uniform white surface ( white T shirt - empty blue sky at sunset etc) this records the variation across the image and can be used to correct the "light" in the same way as a "dark"

What you end up with, hopefully, is a uniform image from centre to edge with minimum annoying noise.

Hope this helps,

Ken

[Glasspusher]

Excellent explanation, clear and concise.

John

[Pete Presland]

thanks that was quick new it would be

i am think about taking the plunge and buying a dslr and having a go at some dso's but it all seems a bit daunting at the moment.

so is the proccessing done in registax like i would for my planetary webcam images ?

i am guessing no, its going to be much more complicated

[steppenwolf]

You can use Registax, especially for Solar System images and I used to process deep sky images too when I started out and was using a DSLR but there are better programs available for Deep Sky work now and the excellent and free Deep Sky Stacker (DSS) is well worth a try and it will process these calibration frames for you 'automatically'.

[abumuhannadh]

thanks bunnygod1 for asking this.

i have been for a few days wondering what those are. but since i have no intention for DSO photography i have been shy to ask.

thanks all for the nice and clear explanation. you guys rock.