Lights, darks, flats, bias

[Dom1961]

As you know, I'm wanting to start astrophotography, I will be starting with milky way shots, what are darks, lights, flats and bias? And will I need them for what I will be doing?

Thanks

[swag72]

Lights: Your regular exposures

Darks: The same exposure length as your lights and the same ISO (if using a DSLR) and temperature (difficult with DSLR but easy with astro cooled CCD) - With the lens cap on so that they are all dark except for any hot pixels on the chip. 

Bias: The fastest exposure time that your camera can do with the lens cap on. This will show your electronic read noise.

Flats: Images taken against an even light source (people often use light panels) without changing ANYTHING in your imaging train. So focus, ISO and temperature remains the same. You then change the exposure length until you get an ADU of about 20-55k (or until your histogram is between half way and 2 thirds of the way across to the right if using a DSLR) - These are used to remove imaging train imperfections, such as dust and vignetting.

Ideally you want a number of these. I use about 30 darks, 100 bias and 50 flats.

You add them into your stacking programme when you add your lights and they are called calibration frames.

Hope that makes sense

[Alien 13]

A basic introduction to the various types is given here http://www.rawastrodata.com/pages/typesofimages.html.

When you are shooting Milky Way type shots on a fixed tripod (less than 30 seconds) you can probably manage without darks or bias frames but flat frames will still be useful to control some of the vignetting from wide angle lenses.

The biggest issue with wide field AP is sky gradients and this is something that flats cant correct for but there are other processes that can handle that.

Alan

[Dom1961]

Ahh, okay that helps a lot, thanks!

[Knight of Clear Skies]

When starting out it's probably not worth worth taking darks if the sky is still clear, more light frames and practise is more worthwhile. Darks become more important with increasing sub length and at higher temperatures.

I always take dark bias as they are time cheap, but don't usually bother with dark frames. Here's an example image without darks,  I use dithering and kappa-sigma stacking to reduce noise. With the short unguided subs i'm currently taking it seems to be sufficient. For Milky Way shots a tablet or laptop could be used as a flat panel by displaying a white image.

[Vox45]

I always take dark bias as they are time cheap, but don't usually bother with dark frames.

Not the first time I come across this term (dark bias) and it always confuses me. I know of only 3 types Dark/Bias/Flat

In DSS they also list something called [dark flat] and [offset/bias] so are these the same thing as Dark/Bias/Flat ?

[Knight of Clear Skies]

Yes, dark bias is the same as offset/bias. They are minimum length exposures taken with the lens cap on and an eyepiece cover as well on a DSLR to stop any light leaking in that way. They are for removing read noise from the stacked image. Maybe I should just use the term 'bias'.

[Alien 13]

The issue I used to have was matching the temperature of Lights with Darks and Bias get it wrong and the image ends up with more noise, if you are only shooting 15 or so subs at 10 seconds (fixed tripod) you can use the cameras inbuilt dark noise reduction which pretty much guarentees a good match but doubles the total exposure time.

Alan

[Vox45]

No worries ! In french they use PLU for flats (Plage de Lumière Uniforme) so, being somewhat bilingual adds to my confusion sometime

Still not sure why in DSS they list in the file menu [Dark], [Flat], [Offset/Bias] and then this extra one called [Dark Flat] ... I'll go read the manual

[Alien 13]

No worries ! In french they use PLU for flats (Plage de Lumière Uniforme) so, being somewhat bilingual adds to my confusion sometime

Still not sure why in DSS they list in the file menu [Dark], [Flat], [Offset/Bias] and then this extra one called [Dark Flat] ... I'll go read the manual

DSS.JPG

The highlighted file is used to remove the bias noise in flat frames that have an exposure of many seconds not realy needed with a DSLR in AV mode as this shoots with the fastest available shutter speed.

Alan

[Vox45]

The highlighted file is used to remove the bias noise in flat frames that have an exposure of many seconds not realy needed with a DSLR in AV mode as this shoots with the fastest available shutter speed.

Alan

Ha ok ! Thanks

I would have called it "Bias Flat" instead but who am I to criticize

[StevieO]

Interesting thread, this.

I have a question: does a dark frame not also contain the read noise from the camera as well as the hot pixel data, and does applying a dark frame and also a bias frame not, in fact,  subtract the read noise twice, giving some weird 'minus noise' result?

Cheers

StevieO

[Pompey Monkey]

Interesting thread, this.

I have a question: does a dark frame not also contain the read noise from the camera as well as the hot pixel data, and does applying a dark frame and also a bias frame not, in fact,  subtract the read noise twice, giving some weird 'minus noise' result?

Cheers

StevieO

Absolutely.

That is why bias is (or should be) only subtracted from the flats. That way, subtracting the darks from the lights corrects for read noise and hot/cold/thermal noise. Then, dividing the dark corrected lights by the corrected flats accounts for noise (dust bunnies, hairs, vignetting) in the optical train.

At least I think that's how it works

[Owmuchonomy]

To add to the excellent posts above Kappa Sigma stacking as mentioned by Knight of the clear skies also removes those annoying satellite trails that make an appearance on your light frames!

[dph1nm]

I would have called it "Bias Flat" instead but who am I to criticize

No I think they really mean a dark frame taken with the same length exposure as the flats. However, I can't see the need for this with modern cameras - maybe 10-15 years ago, perhaps.

NIgelM

[Pompey Monkey]

Absolutely.

That is why bias is (or should be) only subtracted from the flats. That way, subtracting the darks from the lights corrects for read noise and hot/cold/thermal noise. Then, dividing the dark corrected lights by the corrected flats accounts for noise (dust bunnies, hairs, vignetting) in the optical train.

At least I think that's how it works

Gaaah! Now I'm not convinced by my own explanation... lol

[Jokehoba]

Hi,

There's lots on the web about the technicalities of pre-processing. I found Craig Stark's hangouts very informative. For example, https://www.youtube.com/watch?v=BNWGYZ7MH7g. About 14mins and 21mins in he shows some equations which help explain why we do all this pre-processing.

John

[ollypenrice]

Flats won't work properly if they don't have their own dark frames subtracted. They will tend to over-correct. The good news is that a master bias or a set of bias frames will be effectively identical to specially made 'darks for flats' so just use a bias for this purpose.

Olly

[Knight of Clear Skies]

Flats won't work properly if they don't have their own dark frames subtracted. They will tend to over-correct. The good news is that a master bias or a set of bias frames will be effectively identical to specially made 'darks for flats' so just use a bias for this purpose.

Olly

I'll give that a try thanks Olly. I've been taking some flats and have had exactly that problem.

[ollypenrice]

I'll give that a try thanks Olly. I've been taking some flats and have had exactly that problem.

If it persists you might also try shorter flats exposures. Yves and I had a long battle with this issue at one time in the 14 inch.

Olly