What is meant by 'Darks' Bias' and 'Lights' for astro photography?

[K9Lewis]

well the title says it all really.

As I'm interested in photography, I would like to make use of 'DSS'

Not a lot of kit...Canon 40D, 5D mk1 35mm / 50mm / 70-200 f4 photo tripod only....my scope is a home made dob +Jessops 80mm.

Can anyone help?

Lewis

[badgers]

Hi Lewis.

Astrophotographs have a number of issues.

1) Some pixels on the camera sensor have biases and are slightly more or less sensitive to light than others, or are permanently on, or permanently off (hot/dead pixels).

2) The hotter the sensor on a camera gets the more some pixels on the sensor misbehave and give misleading results.

3) In pretty much any optical system you will get not perfectly even illumination of the camera sensor as the light passes through all the optics.

Also Light pollution in the sky or light leaking into the telescope can cause your images to be brighter or darker in different regions.

Bias Frames are taken to allow DSS to know which pixels are biased in general... (Point 1) You take a lot of these and they get averaged.

Dark Frames are taken to factor in thermal noise as the sensor heats up or cools down (Point 2). These are taken with the cap over the telescope at the same temperature as your main imaging

Flat Frames are shots taken of an evenly illuminated white field (e.g. a clean white T-shirt over the telescope). These are short shots to work out how even the illumination is (Point 3).

Your Light frames are the normal shots you are taking of a target you want to capture.

DSS will use the Bias, Dark and Flat frames to calibrate the Light frames, i.e. remove bias, thermal noise and illumination gradients, hopefully leaving you with a much better picture.

You normally want to take multiple light frames of a longish exposure and stack them to increase the signal to noise and get a nice astrophotograph.

Sadly the Dob and the Jessop are not great for imaging, but the good news is that your Cameras and Lenses are probably a reasonable starting point.

With an equatorial mount you could mount your camera and lens and take wide field shots of larger nebulae.

You would want a fast refractor or newtonian or similar to get in closer to deep space objects.

If you want to take seriously long exposures you need more expensive mounts and probably another small telescope to help guide the mount and keep the images on target.

Hope that helps!

Anton

[K9Lewis]

Thanks very much Anton. I think I have a bit of an idea now. The 'Lights' are the only real exposures of what we're photographing, while the other shots give DSS info about the sensor / lighting and noise?

Lewis

[badgers]

Exactly

Bias shots are easy, you take a few hundred of the fastest exposures your camera will manage and store them.

Darks are fairly easy, you cap the telescope and take shots of the same exposure length as your lights and roughly the same temperature. You can bank these by temperature too, especially if your camera is cooled to a constant temperature.

Flats can be more tricky, but you'll find lots of tips on here about different ways to take them. I use a laptop screen and some A4 paper, not ideal but it works fairly well.

Starting off, the most important will be Darks as software can remove some of the gradients that flat frames remove, but in the long run you should really be doing all three types.

Anton

[K9Lewis]

Thanks again Anton...I will give it a try (I gather I'll best be using the 35 f2 or 50 f1.8, as I do not posses an equatorial mount yet).

Lewis

[badgers]

I've had nice results on a Canon 40D with the 50mm f1.8 lens on a tripod, stacking 15-30 second exposures at ISO800 or 1600.

You would want a remote shutter cable, or at least USB controlled shooting software.

Anton

[Novarius]

There is also a great magazine out at the moment in WH Smith or local Newsagents, Complete Guide to Astrophotography (by Sky at Night magazine). It's £7.99 and worth it for beginners (like me). Does have info on Darks, Flats, Lights and Bias frames (although the above is good info), ideal and mostly free software to use to get the most out of your shots, loads of useful information, very good magazine for reference. (Thanks again to Aenima for the heads up).

http://buysubscriptions.com/Product/complete-guide-to-astrophotography

[badgers]

I've not seen the Sky at Night book, looks good!

Also "Making every photon count" is a great guide too from Steve Richards right here on SGL.

And all the nice people in the imaging forums here have helped me with so many things in the last year.

[K9Lewis]

Thanks again, Anton & Novarius...I'll definitely be buying that mag soon.

Lewis