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Need help understanding dark, flat, bias files.

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Thread started 20 May 2010 (Thursday) 23:19   
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dmitrievich
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Ok so yesterday following the DeepSkyStacker tutorial on here I took my first astrophotograph :D

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http://farm5.static.fl​ickr.com ...25550527_a0ff26c97d​_o.gif

Now naturally, I want to take better pictures!

So while I wait for a good tripod/remote shutter release to get here i want to brush up on some other stuff.

What are the dark, flat, bias files that you can use with stacking images and how do they work? I couldn't find a good tutorial, but maybe I missed it on here.

Thanks!

Post #1, May 20, 2010 23:19:41


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DonR
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The Bias frames are very short exposures made with no light reaching the sensor. With a DSLR, they are taken at the fastest possible shutter speed and lowest possible ISO, with the body, lens or telescope capped. There purpose is to remove the read noise that is present and roughly the same in every frame. You should take a large number of Bias frames (~50), and DSS will average them to create a master Bias frame, which it will subtract from every other frame used in the image processing. You don't need to take new Bias frames very often, just once or twice a year, because the read noise doesn't change much over the life of a DSLR. The Bias frames also contain the "Offset" signal, an elevated baseline added to every image by the camera electronics. With my Canon 350D, the Offset signal is 255 counts out of a maximum range of 4,096 counts (12-bits). So subtracting the Bias frame from every other shot removes the Offset signal as well as the read noise.

Dark frames are made at the same exposure time and ISO as the subject light frames, but with no light reaching the sensor. They should also be made at approximately the same temperature as the light frames. Normally they are interspersed in the imaging session, or taken at the end of the imaging session. Their purpose is to remove the thermal noise from the light frames. Normally you want between 10 and 20 dark frames during each imaging session, and if you use multiple exposure time or ISO values for the light frames during a session, you must acquire sets of dark frames to match the exposure and ISO of each set of light frames. The master Bias frame is subtracted from each of the dark frames before they are combined to create a master dark frame.

Flat frames are short exposures made through the imaging optics with the camera pointed at an evenly illuminated field. They record uneven field illumination due to vignetting caused by the optics and due to partial obstructions in the light path such as dust particles on the sensor, and they are use d to remove that uneven field illumination from the light frames. Flat frames can be taken by pointing the camera at the twilight sky, by covering the aperture with a smooth, translucent, light colored material such as a t-shirt, or by covering the aperture with a specially constructed "light box", and making 15 to 20 exposures in Av mode. They must be taken with the camera in exactly the same orientation relative to the telescope as used for the light frames, so they are usually taken at the beginning or end of each imaging session. ISO isn't critical but most people use the same ISO as used for the light frames. The master offset frame is subtracted from each flat frame, and then the flat frames are averaged to make a master flat frame. The master flat frame is divided into each of the light frames, after the master Bias and master Dark frames have been subtracted from the light frames.

Applying the Offset, Flat and Dark frames to the subject frames is called calibration. A few months ago I made a photo documentary of the calibration process on my PBase site, and you can find it here:

http://www.pbase.com/d​treed/how_its_doneexternal link

Click on the images to read the descriptions.

Don

Post #2, May 21, 2010 07:35:57




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David ­ Ransley
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The Help File also comments on the files you are talkng about, but my question: Dark Flat Frames?

Dark Frames and Dark Flat Frames

The Dark Frames are used to remove the dark signal from the light frames (or the flat frames for the Dark Flat frames).

With DSLRs and CCD Camera, the CMOS or CCD is generating a dark signal depending of the exposure time, temperature and ISO speed (DSLR only).

To remove the dark signal from the light frames you use a dark frame that contains only the dark signal. The best way to create the dark frames is to shoot pictures in the dark (hence the name) by covering the lens.

The dark frames must be created with the exposure time, temperature and ISO speed of the light frames (resp. flat frames). Since the temperature is important try to shoot dark frames at the end or during your imaging session.

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master dark or master dark flat.

Post #3, May 21, 2010 08:43:02


DRH

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David ­ Ransley
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Help file agan:

Bias Frames (aka Offset Frames)

The Bias/Offset Frames are used to remove the CCD or CMOS chip readout signal from the light frames.
Each CCD or CMOS chip is generating a readout signal which is a signal created by the electronic just by 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 create with the ISO speed of the light frames. The temperature is not important.

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master bias/offset frame

Post #4, May 21, 2010 08:45:33


DRH

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pitrow
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Awesome descriptions, thank you! I've been wondering the same things.

Could we get this as a sticky?

Post #5, May 21, 2010 12:18:04




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DonR
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The Dark Flat frames are optional, because the amount of noise in the Flat frames is negligible, since their exposure times are usually less than 1 second. Subtracting the Bias frames from the Flat frames essentially produces the same results as subtracting Dark Flat frames would. If you use them, you must subtract the master Bias frame from them before combining them into a Master Dark Flat, and since the Dark Flat frames are very nearly the same as Bias frames (being for example 1/20 second dark exposures vs. 1/4000 second dark exposures for the Bias frames), doing this subtraction results in the Dark Flat frames containing essentially no signal or noise at all.

One supposed advantage of using Dark Flat frames is that they eliminate the need for Bias frames. Technically, since the Dark frames contain the Bias signal and so do the Dark Flat frames, subtracting them from the light frames and Flat frames, respectively, removes the Bias signal without the use of separate Bias frames. So some astrophotographers acquire Dark frames and Dark Flat frames and skip the Bias frames. In the past I did this too, but I have found that I get better end results when I use Bias frames - not dramatically better, but subtly better IMHO. I guess that the master Bias frame, since it is made from a large number of exposures (50 or more), contains a better representation of the Bias signal than the Master Dark and Master Dark Flat frames. And since the Bias frames are quick and easy to acquire and don't need to be refreshed often, it's really no trouble to use them.

I believe that most DSLR astrophotographers use Bias frames and skip the Dark Flats. Others use Dark Flats and skip the Bias frames, as I did in the past. My current process is to use both Bias frames and Dark Flat frames. Like the Bias frames, Dark Flats are quick and easy to acquire, and using both doesn't add a lot to the processing time.

Don

Post #6, May 21, 2010 17:00:57




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DonR
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David Ransley wrote in post #10220932external link
Help file agan:
Bias Frames (aka Offset Frames)
The bias frames must be create with the ISO speed of the light frames.

I know that this came from the DeepSkyStacker manual, but I disagree with this statement. The purpose of Bias frames is to capture the read noise and the offset signal, which is independent of ISO. ISO-related noise is captured in the Dark frames, so there's no need to capture it in the Bias frames also.

CCD imagers capture Bias frames with 0 exposure time if their camera is capable of doing so (not possible with DSLR's, but using the shortest possible exposure time is close enough), and with the CCD gain (analogous to ISO in the DSLR) set to minimum. This captures only the offset and read noise.

With DSLR imagers I believe the common procedure is to capture Bias frames at the lowest possible ISO and exposure time, so as to capture only the read noise and offset as nearly as possible.

David Ransley wrote in post #10220932external link
Take a few of them (between 10 and 20 is usually enough).

Also quoted from the DSS manual, I disagree with this too. I had a discussion with the author of DeepSkyStacker (Luc Coiffier) on another forum a while back, about the advantage of using Dark Flat frames, and whether Bias frames were needed if Dark Flat frames were used. Luc's opinion was that while Dark Flat frames theoretically eliminate the need for Bias frames, there is still an advantage to using both, if the Master Bias frame was made from a relatively large number (50 or more) individual Bias frames. The advantage according to Luc, as I mentioned in a previous post in this thread, is that a Master Bias frame made from a large number of exposures contains a more accurate representation of the read noise. This point could be argued, since subtracting a perfect Bias frame from an imperfect single light frame, for example, leaves just as many imperfections as if the Bias frame were equally as imperfect as the light frame.

That's the great thing about this hobby. Everyone has their way of doing things, but the only people who are 100% sure that they are doing things the right way are the people who really don't understand at all.:)

Cheers,

Don

Post #7, May 21, 2010 17:30:28




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David ­ Ransley
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Hi, Don, you say:"Dark Flats are quick and easy to acquire".

I assume that the process to capture them would be the same as a flat frame, but under dark conditions? We place a cap on the lens and take some short exposure shots?

David

Post #8, May 22, 2010 06:36:16


DRH

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DonR
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Exactly, David. They should be taken at the same exposure time and ISO and roughly the same temperature as the Flats, so once you have taken the Flats, all you have to do is cover the telescope (or lens) to take the Flat Darks. The position of the camera doesn't matter, of course, but most people take them right after taking the Flats. Fifteen or so is enough and they are less than 1 second exposures typically, so it goes really fast.

Don

Post #9, May 22, 2010 07:57:28




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VegasBoz
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I've been searching and reading for 3 days now about this and never found an answer to these questions.. until this post! PHEW!!! I was about to post up my own question with exactly these questions! Then I found this one as I clicked the 'talk about' forum. I even went to two bookstores last night to find books on astro-photography but didn't have any luck.

Thanks for helping out n00bs like us. I'm printing out these instructions and now I'm headed out later tonight to try this out. Wind be damned!!!(forecast calls for 30mph gusts in/near Vegas)

:cool:

Post #10, May 22, 2010 11:43:13 as a reply to DonR's post 3 hours earlier.


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David ­ Ransley
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Thanks, Don

I re-stacked one sequence I have and excluded the Dark Flats to see the difference. I will support you on the little effect it has.

My problem is building the flats - my current idea is to take a reflector and place it in front of the camera, but what is the best way to get it white? Maybe a flash, or should you use a torch or something? My challenge - evenly illuminated field.

Any tips?

Post #11, May 22, 2010 13:27:11


DRH

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DonR
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Yes, David, getting an evenly illuminated field is a challenge. I have success every time when I am able to set up by sunset. Between 10 and 15 minutes after sunset, if you point the camera at the southeast sky about 45 degrees above the horizon, you will have a nice, even field illumination. Too early and the setting sun may cause a gradient; too late and stars may show up. Set the camera to Av mode and fire away, 15 to 20 shots, then cap the lens or telescope and take the same number if you want dark flats. You could also do this in the morning twilight, but I never have. I assume 10 or 15 minutes before sunrise aiming at the southwestern sky would work.

Another way that works pretty well is the white t-shirt method. Put a double layer of white t-shirt over the lens or telescope and point it at the sky. The sky should be clear or evenly overcast, and of course don't point it anywhere near the sun. I have used this method several times when I forgot or wasn't able to take twilight flats.

The other way I have done it is with a piece of translucent plastic film that I salvaged from a defunct LCD TV. It's frosted, and not clear enough to see through, but light does pass through, and I use it like the t-shirt, pointing my telescope at a clear or evenly overcast daytime sky.

I assume all of these methods could be problematic if you're shooting wide fields with a wide angle lens, because it may be difficult to find a big enough piece of sky that's free of gradients.

It's not necessary to adjust the white balance of the flats if you use DSS, Iris of other good astrophotography software. They calculate the grayscale equivalent of the master flat, but do it without de-bayering the RAW file first - something that Photoshop and other non-astro specific software can't do. Twilight flats always come out sky blue, and so do t-shirt or diffusing film flats unless the sky is overcast. It's not a problem.

BTW, I assume you're shooting in RAW mode, and not converting the RAW's to TIFF, JPEG or other RGB format before sending them to DSS. This is very important. The flats, darks and bias frames can't be applied accurately unless the images are non-debayered RAW files.

Don

Post #12, May 22, 2010 20:43:20




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dmitrievich
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Don thank you!

It makes more sense now, and I'll try it out when I get the chance!

Post #13, May 22, 2010 22:49:44


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David ­ Ransley
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Yes, I am usng RAW and I understand the T-shirt method better. Never thought of doing the flats earlier in the process when the sky is more evenly lit. Will do that for sure.

Post #14, May 23, 2010 04:23:43


DRH

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