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Old 18-08-2020, 03:50 PM
bluesilver (Peter)
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What is mean by the term Darks and flats in Astrophotography?

Hi, I am just starting out in Astrophotography, using a Modified full specrtum Canon EOS 600D, Skywatcher evostar 150ED, AZ-EQ6 mount.



Have read many times that people are talking about darks and flats, No matter how i try to research this up i can't seam to find out what this actually means.

I have figured it out that it has something to do with the processing part, at least that is what i think it is.

Or do i actually have to capture something else as well as the image i am working on?

​​​​​​​So basically if i am say for instance photographing the Eagle Nebular, Say i take 20 photos of it, Do it hen need to take more of it at a different setting or something, or is this term darks and flats only to do with the processing stage?

I am only just staring out with this so please keep it simple for me to start with if possible.

Thanks.
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Old 18-08-2020, 04:31 PM
RyanJones
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Hi Peter,

A perfectly legitimate question. They are both “ calibration “ frames. I’ll break it down for you into the darks and the flats, what they are, how to capture them and why we use them.

Firstly Darks;

A dark is an image that you take with your camera with the same ISO setting and the same length as your subs, ie: 180sec @ iso 800. The only thing that is different is that you have the cap on the front of your telescope so that the sensor sees no light. The idea behind this is that when your camera takes long exposures, the sensor gets warm. This inherently causes some pixels in the sensor to falsely read a signal. These are called hot pixels. Also during this time electrical signals are traveling through the sensor causing the pixels to read these signals. This is called “ read noise “. The idea behind taking these calibration images is that your processing software is able to subtract these signals from your images so that you are left with much less “ false signal “. These should also be taken at the same temperature ( as much as is practical ) as your images because the noise will increase through higher temperatures so you are trying to capture the same noise levels as your images are capturing to be able to accurately remove it.

Flats ;

These images are essentially taken to again calibrate your final image but this time it is for errors in the image arriving at your sensor. These can be dust motes, light scratches on lenses but most often vignetting. This causes by a curved image being recorded on a flat sensor or other optical anomalies that cause the center of an image to have a different level of illumination compared to the edges. Flats must be taken by evenly illuminating a white sheet or piece of paper ( some use specifically made light boxes ) across the front of your telescope and taking an image only long enough to capture a complete illumination. You can do this by simply adjusting your exposure speed down until your camera shows the correct exposure. These are not temerature dependent as such but they do require the same focus. Just bare in mind that focus of your telescope does change through temperature variance. Again, any faults in illumination, dust or imperfections can then be corrected by your processing software.

I hope this has cleared things up for you a little. There are other calibration frames and there is a more technical level to all of this but as you said you’re a beginner I thought I’d just run the the basics of what, how and why.

Cheers

Ryan
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Old 18-08-2020, 06:22 PM
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leon
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Wow Ryan, although i did not ask the question that was a fantastic explanation on both, "why take dark's and flats" I too got some good information out of your answer, and I'm sure Peter will as well.

Thanks for sharing that.

Leon
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Old 18-08-2020, 11:25 PM
Startrek (Martin)
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Ryan,
Excellent explanation !!
Can I add some further information for Peter in relation to Noise and Signal to Noise ratio (SNR) in Astrophotography which I think is important to mention and relevant to the topic of “darks”

Noise and SNR (Signal to Noise Ratio) in Astrophotography

Photons are the fundamental particles of light

Signal are photons from the celestial object or DSO together with nuisance Skyglow and Dark current ( noise )

Noise comprises of Read noise and Shot noise

Quality of images are dependent on the Signal to Noise ratio expressed as
SNR = Signal / Noise

Signal is made up of photons directly from your image target
Noise is target signal + Skyglow signal + Dark signal + Read noise square root

“Skyglow” is basically a build up of luminance in the night sky caused by natural and human sources ( light pollution, reflected light off the moon , star light etc.. )

“Dark current” ( Dark Noise ) is a thermal signal produced by the cameras electronics which subtracts the good signal ( photons ) The higher the temperature of the cameras sensor the higher the thermal signal the higher the dark current or dark noise

“Read noise” is the noise produced by your cameras sensor electronics. A lower read noise has the ability to pick up weaker signals and differences in signal levels

“Shot noise” is the fluctuations in the number of photons detected by the sensor which includes both photons from your object and skyglow. This variation in the number of good and bad photons from each exposure reduces your SNR


Ways to increase Signal

* Stack more images ( more Target Signal )
* Longer exposures
* Lower Focal Ratio or f ratio ( more photons per pixel for same time exposure ) Eg: 8” f6 newtonian telescope vs 4” f4 newtonian telescope using same camera
25% less photons are captured by the 4” scope but 3 times the photons hit each pixel on the sensor
* Quantum Efficiency ( QE ) of Camera ( higher the better ) Low is around 40% and high is around 80%
* Cool your sensor
* Use filters in urban areas

Ways to reduce Noise
* Stacking ( increases signal )
* Dark frames ( removes thermal signal eg hot pixels etc.. )
* Dithering ( removes fixed pattern noise or dark current noise )

Clear Skies
Martin
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Old 19-08-2020, 06:52 AM
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Hi Peter
Do you have stacking software?
If not download Deep Sky Stacked...it is free..you will notice that in the software you load your light frames, your dark frames and your flat frames...It stacks the light frames and uses the dark and flat frames to remove noise and blurs in the optics caused by dust etc.
Alex
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Old 19-08-2020, 07:57 AM
bluesilver (Peter)
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Thanks heaps for the basic run through, Makes it very easy to understand the basics and what they are, at least for myself it dose anyway.

The darks make sense now you have mentioned it that way, I could be on the wrong track here,
But with the darks, and please correct me if i am completely wrong here, But do i only really need the one image of the darks, or do i need to take just as many images of darks that i take of the Eagle Nebular, So i i am taking just say 20 images of the Eagle Nebular, do i also need to take 20 dark images?
The same question would also be for the Flats.

It kind of reads to me that the Flats are basically like doing a white balance, This is what i have to do with my full spectrum modified Canon EOS 600D to get the colour balance correct and eliminated all the unnecessary reds.
But again, i could be completely wrong here also.

I see that Alex also mentioned Light frames, can't say i have heard of this, but thinking it through, are the light frames the images of the Eagle Nebular that i am working on?

I have yet to download Deep Sky Stacked, just going through the process on what i need to do in the way of capturing images.

I would guess that after the images have been stacked, there is a program also used to process them?

I have found that Autostakkert 3 and Registax 6 are used for Planetary, and have to say thank you very much to Martin for providing me the instructions on how to use these, appreciated.

Very much appreciate all the information provided, makes understanding the basics so much easier.

Thanks.
Peter
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Old 19-08-2020, 08:33 AM
Startrek (Martin)
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Peter
I’ve been using my unmodded Canon 600D for over 3 years now in my 6” and 8” newtonian reflector telescopes ( I chose not to mod the camera as I believe modding let’s way to much red Ha into the image and washes it out with red but that’s only my opinion and choice , each to their own )
To answer some of your questions -
I usually take 15 darks ( dark frames ) for up to a total of 1 hour of exposures or subframes (subs) or light frames and 20 darks for everything beyond 1 hour
The 1 hour could be made up of 1 minute to 5 minute long exposures

Since you have a Canon 600D I strongly recommend you purchase the E book Beginners Guide to DSLR Astrophotography from Astropix ( Jerry Lodriguss) the guru of DSLR astrophotography. It covers the A to Z of DSLR Astrophotography including Camera settings ,white balance , darks , flats , technical info everything etc... I bought it 3 years ago and answered most of my questions

Last night I took 50 x 2 minute exposures ( subs ) and 15 x darks on M16 Eagle Nebula. Yes your light frames are the light exposures You take to capture an object !!

I use Deep Sky Stacker DSS for stacking frame and have a procedure for stacking using a DSLR

I use Startools V1.4 and V1.5 for final processing and again I can send you a basic procedure, I can’t remember how much it cost but it wasn’t expensive

Cheers
Martin
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Old 19-08-2020, 09:02 AM
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Hi Peter
We call our photos all sorts of names so it can be confusing,...

Photos = "subs" = "light frames" = "captures" and there maybe more.

Flat frames are to pick up dust ( mainly) in your system.

I second Martin's advice re StarTools but also Photoshop can be handy..if you don't have photo shop download Gimp it's free and very powerful these days ... with Gimp you can play with brightness and contrast, ..a few things really...but you can get it even before StarTools to have some control over your images.
If I could suggest for your next session. Address polar alignment, if you can't get it right back off exposure times until no trailing. Get focus right..do you have a Mask? But take time to get to as best you can..take a caputure, sub or light frame and look hard at it ..blow it up to max and them take another with further adjustement until you are sure its as good as you can get..I used to take 15 minutes when I used a dslr with no blow up feature...again polar...start... maybe try one minute..if you get trails, drop to 45 seconds..mmm still there try 30...20...even 15 if that is what it takes....take say 50 ( as many as you like really) then put on your lens cap and take 10 " darks" ( more it you like)...even try to take a couple of flat frames...although I still don't bother ..one day I will. I would take hundreds and select only the best..takes time but its the littlecthings that make images improve.
Go thru your light frames and make sure they are decent, be prepared to delete the ones that show an obvious problem..a cloud made it dimmer or a plane flew by...then stack them all in Deep Sky Stacked.. lights, dark flats...in DeepSkyStacker you can adjust colour and brightness of the final image but it is best not to adjust that image at all in Deep Sky Stacker( but if you don't have startools or gimp you can at least do some adjustments)..
Take your final image into Gimp...adjust brightness and contrast ..generally see what you can use..just for starters and so you feel like you are doing something...or better still to Startools.
In Gimp try the dodge and burn tools... I love them they selectively brighten or darken specific regions so you can fix a burnt out spot or darken a dust lane for impact. With StarTools you really do not have to adjust the settings much...if you get it I am sure we can give you a very simple work flow to get you started...
How are you at getting a good polar alignment? It all turns on that.
Alex

Last edited by xelasnave; 19-08-2020 at 09:16 AM.
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Old 19-08-2020, 09:05 AM
bluesilver (Peter)
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Thanks heaps again for helping clear those questions up for me, makes a bit better sense now.
I did happen to just come across this video on darks, Flats that i found was very informative, not sure if anyone else has seen it, but helps a lot in explaining a few things.
https://www.youtube.com/watch?v=UmmTuIEW94M
Appreciated.
Peter
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Old 19-08-2020, 09:21 AM
bluesilver (Peter)
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I guess i am working a tad in front of myself, as i have only just got myself a AZ-EQ6 mount, been using my goto Dobsonian.
So just got to wait on some clear nights and shift work to align to give it all ago, bit of luck it will be this weekend or next week.
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Old 20-08-2020, 07:12 AM
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The_bluester (Paul)
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I generally have not used the number of light frames to determine the number of darks and flats to use. For both I would normally consider 20 to be a bare minimum. Having a cooled cam I have the advantage of being able to control the sensor temperature where you can't with a DSLR, so I pick my temperature and shoot a dark library of the exposure times I expect to use, 100 shot sat each exposure time, then I generate a master dark from those. I do the same for flat frames, I use a flat panel so it is easy to control the light input, so I shoot 100 flats to make a master flat, and dark-flats to go with them.

Per the information above, darks characterise your sensors dark current (The electrons that each pixel well will accumulate in a given exposure time) including hot pixels (Pixels which accumulate electrons faster than the rest of the sensor) so that the repeatable unwanted signal can be subtracted from your light frames. Shooting at least 20 averages out the readout noise, which is more or less random.

Flats characterise the whole system for vignetting or dust motes, or cold pixels, which react less to incoming light than the rest of the sensor. 20 at least means that the readout noise that is random in nature is averaged out of the master flat. If you use flats of more than a second or so you want to shoot dark flats (Which are just dark frames at the same exposure time as your flats and used to create the master flat) to eliminate dark current and hot pixels from the flats.
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Old 20-08-2020, 06:50 PM
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And don't forget the 'dark flats' or 'bias' frames ...
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Old 23-08-2020, 12:04 PM
bluesilver (Peter)
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Hi again,
I am hoping to give all this a go this coming week if the skies stay clear enough.
Just some basic questions in regards on the sites to download these programs from.

I just want to make sure i am going to get them from the correct place as i am sure there are some that are not the best.

So these are the programs i am looking at:

Registak 6 : ( It tells me on this page it is Version 6.1.0.8 ) https://www.astronomie.be/registax/

Autostakkert 3:
( It tells me onthis page there are two versions, Autostakkert 3.1.4 64 bit version
and also Autostakkert 2.6.8 32 bit version )
https://www.autostakkert.com/wp/download/

Deep Sky Stacker:
( It tells me on this page that it is version 4.2.3 )
http://deepskystacker.free.fr/english/download.htm

Startools:

A little confusing on where to actually go in this site, as it tells you there is a free trial program, but you can't save your work, not sure if that would be of much use if you can't save or print out your end result,

There is a buy option:
https://www.startools.org/buy

and also a download option:
https://www.startools.org/downloads

I am guessing everyone would be using the paid option as you can save your work?

So all up, really looking to find out if these sites are the correct ones to download these programs from,
And also if the versions are the correct ones?

With Startools, am i best to just send them an email?

Thanks.
Peter.
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Old 23-08-2020, 03:52 PM
Startrek (Martin)
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Peter
All locations are ok
Yes of course Startools you can save your work
Warning you need to check your computer processor and ram to see if it can operate and satisfies Startools requirements
Martin
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Old 23-08-2020, 04:21 PM
bluesilver (Peter)
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Thanks for the reply,
Sorry i meant to say in the Startools trial version, in the trial version it says you can't save your work.
You would need the paid version to save your work?.

Thanks again
Peter.
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Old 23-08-2020, 04:44 PM
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Merlin66 (Ken)
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Peter,
If you run a Win10 64 system then you need the x64 packages.
I use Astroart for all my pre and post processing.
It has a free download to check out all the features. Stacking/ darks/flats etc etc.
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