Anthony's Astrophotography

Light, Dark, Flat and Bias Frames. What are these and how do you create and use them to make your images better.

I struggled with these file types for a while when getting started so thought I would write up a small bit of detail about what they are and what they do. I hope this helps explain their purpose and their importance when processing quality images.

Light Frames – Light Frames are the actual images you have taken of celestial objects that contain the valuable data that you want others to see. The key objective is to get enough data (actual photon’s of light) to reduce the Signal to Noise Ratio. All the other frame types are used to reduce or remove the noise in you Light Frames.

Dark Frames – With DSLRs and CCD Cameras, the CMOS or CCD chip is generating noise in the frame during any exposure, the amount of noise generated depends of the exposure time, temperature and ISO speed. To remove this noise from the light frames you use dark frames that contain only the noise generated by the CCD/CMOS chip. The best way to get a dark frame is to take an exposure of equal settings (duration, ISO and temperature) with the lens cap on. Since the temperature is important try to shoot dark frames at the end of or during your imaging session. Take a few of them (about 10 to 20 is usually enough).

Bias Frames – The Bias/Offset Frames are used to remove the CCD or CMOS chip readout noise from the light frames. Each CCD or CMOS chip generates a readout noise which is a signal created by the electronics when reading the content of the chip. It’s very easy to create bias/offset frames: just take the shortest possible exposure (it may be 1/4000s or 1/8000s depending on your camera) in the dark by covering the lens. The bias frames must be created with the same ISO speed of the light frames. The temperature however is not important.

Flat Frames – Flat Frames are used to correct the vignetting and uneven field illumination created by dust or smudges in your optical train. To create good flat frames it is very important to not remove your camera from your telescope before taking them (including not changing the focus). You can use a lot of different methods including using a light box, but I found that the simplest way is to put a white T shirt in front of your telescope and  smooth out the folds. Then shoot something luminous  like a flash, a bright white light, the sky at dawn and let the camera decide of the exposure time (Av mode), The flat frames should be created with the ISO speed of the light frames howevere the temperature is not important.

Dark Flat Frames – Dark Flat Frames are an exposure the same length as the flat frame but with the lens/scope covered up. It helps to reduce the noise in the flat frames. I personally don’t think this is worth the effort but if you want to get really spot on images it can only help.

What next – Once you have your collection of frames you will need some software to help process them, I won’t write a tutorial about how this is done in this blog but it really is as simple as selecting the right image based on frame type and letting the software do what it does best. The software I use is Deep Sky Stacker as its free and does a great job and more info can be found here and the manual here.

I hope that helps explain things a little better.

Cheers

Anthony

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