Why

[dark knight]

I'm curious as to why we need to take more than one dark and one flat. Can we not just take one of each and just duplicate them in PS4, or is it so that there is some randomness in their generation?

You can tell I'm new to the dark side can't you....

[arad85]

To try and smooth out the resulting image from random noise in the camera. Just like you take more than 1 light, you need to do the saem with darks/flats/bias etc...

[LukeSkywatcher]

I just let my 450D take my darks etc for me. Once i get into stacking images i guess i will start doing everything manually. I believe that duplicating images and stacking doesnt work because you are only stacking the same data ontop of itself and this wont improve the image. So by that i guess different frames contain different data and improve the overall image when stacked.

[dark knight]

Thanks Arad

[Catanonia]

All darks produce random noises from the electical static in the camera that gets onto the sensor.

Lights also have random noise, but the true data is always in the same place.

So stacking lights reduces the random noise, then applying the darks removes the rest as well as hot pixels.

If you only add one dark, your are actually adding noise to your stack as the randomness has not been removed.

My web site has an article on all this under the Basics section.

[Space Bat]

Hey Cat,

Just taken a look at your site....looks very promising..

I have learned again something new - which is the noise issue on the DSLR which I have enabled as I thought this would help...and yes it takes a long time to process the image...

Will see what I get when this is disabled.....

Chris

[Catanonia]

Hey Cat,

Just taken a look at your site....looks very promising..

I have learned again something new - which is the noise issue on the DSLR which I have enabled as I thought this would help...and yes it takes a long time to process the image...

Will see what I get when this is disabled.....

Chris

glad it is helping some people. I thought would try and explain things in a simpler way than you normally see. Yes noise reduction is on by default and will double your imaging time. Switch it off and keep a library of darks vs rough temperature.

Hopefully will be adding more to the site soon.

Cat

[themos]

Here's an analogy I used before:

Imagine light falling on your sensor as bricks fall on a lawn. A strong signal will make a nice pile of bricks. A weak signal will be a single brick somewhere. Unfortunately, your sensor will produce random bricks all over the lawn that do not correspond to anything out there. But they will look just like a weak signal, one brick

If you take the same frame and duplicate it, you will get all the bricks doubled up, signal and noise.

If you take a second frame, your weak signal will throw a brick on the same place but the noise will throw a brick at a random place. If you add the two frames, the weak signal will have two bricks and the noise will be only one brick high ( but in more places, ok).

If you do that 20 times, the signal will stack up neatly, the noise will just tend to fill the lawn with much lower stacks. The more frames you add, the bigger the difference and the more your signal stands out.

[LukeSkywatcher]

I'll never think of anything but falling bricks now when i'm out in the garden.

[Darth Takahashi]

Its mathematical, noise reduces as a function of the square root of signal, therefore, 16 darks will reduce the noise 4 fold etc... simply copying the same dark 16 times does nothing since the same random noise pattern exits in all the darks.

So if you capture 'x' photons then +/- sqrt('x') photons are noise

I know, its a pain to have to waste all this time taking dark but there's no way around it :-(

Neil.

[RobH]

I know, its a pain to have to waste all this time taking dark but there's no way around it :-(

Neil.

Actually, although it is certainly best to dark subtract, many of us don't bother.

The reason for this is that unless you are lucky enough to have a mount that tracks perfectly, there is likely to be a bit of image shift between subs.

If you use a stacking algorithm like median combine, SD mask, or sigma clip, any hot pixels will be rejected in the stacking process, removing the need for darks.

If you have a mount that tracks perfectly, you can 'Dither' your subs, which means that between each sub, the mount shifts by a pixel or two, thus allowing the stacking routines mentioned to remove any unwanted noise.

Always subtract bias and flats though, bias being really easy to do, and the set will last you for quite a while....I redo my bias frames every 6 months to take account of any sensor degradation.

Cheers

Rob

[dark knight]

Thanks peeps. Rob what are the benefits and pitfalls of each of the algorithms you mention and when would you use them. I like the idea of not having to take darks, but not sure which of the 3 to use.

Carl

[Psychobilly]

I knew there was a reason why I didnt take darks I just couldn't articulate it quite so well

DSS also has options under the cosmetic tab for processing Hot and Cold pixels which I guess is done on a sub by sub basis before stacking...

However...

If your sensor suffers from I will use the generic term "glow" then darks are definitely worth taking...Otherwise I much rather use the time to acquire useful signal...

Billy...

[Psychobilly]

What are your current darks like ... resize and post one up..

My Nikon D200 suffers from ampglow so darks were essential with it as do my 350D, KM5D and D50 ... the 1000D onthe otherhand doesnt so its become my main imaging camera...

I did post a link to a "1000D first darks" thread I did a range of practical tests on my 1000D to see how it performed...

Billy...

[RobH]

I forget where I saw it recently, it may have been a post by Dennis 'roundycat', but a test was done between each of the 3 sigma reject type of algorithms to see what the best was in terms of noise, and SD mask came out tops. This is the one I use.

Some programs don't have this option...I remember that Autostar suite doesn't, but I think that most have median combine.

Cheers

Rob

[dark knight]

Thank for all the info guys, I'm all for saving time which can then be used for obtaining more signal/data. Peter I will post one if I ever get the chance to uncover the equipment (note to self, I need an obs) but since it's a D80 I have a feeling I will suffer the same problems you have encountered with other Nikons.

Once again cheers for all the advice and explanation, just what i needed...

[narrowbandpaul]

i have commented on this issue before, in these threads. Take a look, whats going on in your darks, and when you average etc, is all in here:

http://stargazerslounge.com/imaging-discussion/99178-sony-a350-its-own-darks.html

http://stargazerslounge.com/imaging-discussion/95491-darks-continued.html

cheers

paul

[dark knight]

cheers Paul and everyone who has shed light on this topic.