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Old 09-01-2011, 05:48 PM
mandownsouth (Des)
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That flats and darks question... again!

Hi all,
Just spent the last hour or so reading through the many threads to do with taking and applying darks, lights, bias etc and have to say I still haven't been able to bend my brain around something that apparently is so simple to so many people. Is there a site or a digest that one can be directed to that gives the principles of these skills in the necessary KISS method for newbies like me please?
I haven't ventured into the art of astrophotograhy as yet. Still in the throes of setting up TAL200k on and HEQ5 Pro ultimately in a SkyShed POD and having a Canon 450D just busting to drop its lens off and have a peak in the back of the TAL. When I do get there though I would dearly like to be a little less muddled.

mandownsouth
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Old 09-01-2011, 06:22 PM
gbeal
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Des,
do what virtually all new-comers do then, just shoot what you see and forget (for a while anyway) about the flats/bias/darks.
Yes, for sure your images will be better, but there is plenty of time for that. Shoot now and try the rest later.
Gary
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Old 09-01-2011, 06:53 PM
TrevorW
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Don't loose any sleep over this one I don't and I;ve been imaging for over 2 years

getting a good flat is a task in itself,

if you want an intro using a DSLR then Google, Jim Solomon's Cookbook
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Old 09-01-2011, 07:38 PM
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multiweb (Marc)
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We all pulled our hair out with calibration at some stage. It's ok once you know how to do it but the first time there is nothing trivial about it. But it is worth your learning because calibration will unlock all the details in your data that you wouldn't see otherwise.

Here's a short practical way without going into too many details to get you started.

Let's say you shoot 10x5min subs.

With the scope at exactly the same focus and temperature you shot your subs, plug your scope and you can now shoot 10 dark frames or more of 5min then combine them into a master dark.

Next shoot let's say 40 flats of probably 0.5s or whatever duration will give you approx 1/3 of your camera full well. So if your camera is a 16bit camera, shoot to reach 10k or 20k ADU. Combine them into a master flat.

Finally shoot 80 bias at 0.001s or the shortest exposure your camera will give you. Then combine them into a master bias.

Now you have three masters: 1 dark, 1 flat and 1 bias.

You need to subtract the master bias from your flat.
Then for each or your sub you do the following pixel math:

calibratedSub = [originalSub - MasterDark]/[MasterFlat-MasterBias]

Various programs will do that. I use CCDStack.
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Old 09-01-2011, 10:30 PM
mandownsouth (Des)
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Thanks Guys,
Exactly the stuff I needed. Thanks Gary, I definitely am getting ahead of myself with what I need to know particularly when I have a look at Trevor's suggestion of Jim Solomons Cookbook. And Marc, when I do get somewhere near it I think your recipe brings it all together nicely. I guess the hardest thing with any of these sorts of venturous hobbies is learning the vernacular and jargon with the associated nemonics and then how different the people use them.
All good stuff and vital to learn in order to minimise dissatisfaction.
Thanks again, I'll be back to find out more about some other thing another day I'm sure.
mandownsouth
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Old 10-01-2011, 09:30 AM
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Quote:
Originally Posted by multiweb View Post

Next shoot let's say 40 flats of probably 0.5s or whatever duration will give you approx 1/3 of your camera full well. So if your camera is a 16bit camera, shoot to reach 10k or 20k ADU. Combine them into a master flat.

Finally shoot 80 bias at 0.001s or the shortest exposure your camera will give you. Then combine them into a master bias.
I'm pretty much at the same begining stage, but thanks marc, it does clarify things.
could you explain light frames as well? are they the actual image frames?

what is ADU? "So if your camera is a 16bit camera, shoot to reach 10k or 20k ADU."

"whatever duration will give you approx 1/3 of your camera full well." is this 1/3rd of the exposure time for the image frames?

also, what are the lightboxes used for? i've seen a few black boxes with white insides at the end of a few scopes at snake valley.

for dark frames, is it acceptable to just cover the ota instead of the camera lens opening?

I've tried my hand with deep sky stacker so far.
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Old 10-01-2011, 10:24 AM
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could you explain light frames as well? are they the actual image frames?
Yes - your data images.
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Originally Posted by alistairsam View Post
what is ADU? "So if your camera is a 16bit camera, shoot to reach 10k or 20k ADU."
ADU is a measure of the brightness of a pixel in your pic. If you have a 16 bits camera the full well is 65536.
Quote:
Originally Posted by alistairsam View Post
"whatever duration will give you approx 1/3 of your camera full well." is this 1/3rd of the exposure time for the image frames?
20000 ADU would be about a third of the total illumination your camera is capable of. Anything above 65K will be 'burnt out' (i.e. true white). So you need to set the illumination or time you exposure to reach that maximum level in your flat.

Quote:
Originally Posted by alistairsam View Post
also, what are the lightboxes used for? i've seen a few black boxes with white insides at the end of a few scopes at snake valley.
I use a small A4 EL sheet for convenience but you can use a light box as well. Anything that gives you a relatively uniform illumination across the aperture of your scope. Many people shoot sky flats as well at dusk or dawn.

Quote:
Originally Posted by alistairsam View Post
for dark frames, is it acceptable to just cover the ota instead of the camera lens opening?
Yes - you can cover the scope but I wouldn't shoot darks in daytime in case of light leaks. Do it just after your imaging session or before while is still dark(ish). If you have a temp controlled camera then you can do it later in a dark room and build a library of darks.
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Old 10-01-2011, 10:58 AM
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Quote:
Originally Posted by alistairsam View Post
what is ADU? "So if your camera is a 16bit camera, shoot to reach 10k or 20k ADU."

"whatever duration will give you approx 1/3 of your camera full well." is this 1/3rd of the exposure time for the image frames?

also, what are the lightboxes used for? i've seen a few black boxes with white insides at the end of a few scopes at snake valley.

for dark frames, is it acceptable to just cover the ota instead of the camera lens opening?

I've tried my hand with deep sky stacker so far.
ADU = analog to digital units. Basically when you take an exposure photons strike various pixels on your sensor. When a photon strikes a pixel an electron is released and stored in a well (one for each pixel). When you finish the exposure the electronics in your camera read the pixel values (count the number of electrons in the well of each pixel) and produce an image based on these values (e.g. brighter areas = more electrons in each well). This conversion of the analog signal (photons releasing electrons into the well) is converted to a digital signal via the analog to digital converter present in your camera. So in a nut shell 1 photon = 1 electron in the well = 1 ADU. You can see the ADU count if you open the information window on prgrams like MaximDL or CCDstack etc. They will be given as average, max, min values etc.

Now using a mono CCD as an example it is fairly easy to understand what is going on. These camera's have a analog to digital converters that are capable of delivering images with 16 bit depth. This means it can produce 65535 shades of grey. Pixels with low or 0 ADU counts will be black (or close to) whilst pixels with 65535 ADU counts will be white. Now not many chips are actually capable of producing this depth as the well size varies for different types. A general rule here is that small pixels have small wells whilst large pixels have large wells. What this means is if you have a CCD chip with 5 x 5 um pixels chances are the wells will be full before the 65535 ADU value is reached (e.g. it may only have 45000 ADU well capacity). This means they become saturated more quickly and limits the time you can expose for. Different chip manufactures generally state well depth as part of the specifications for each chip and this is useful to know. DSLR camera's only have 12 - 14 bit converters (less dynamic range e.g. fewer shades of grey).

Now darks are easy to take and as you have mentioned it is fine to just put the cap on the scope and shoot away. But different types of cameras have different methods. Your dark frames should be at least the same exposure duration as your light frames (The actual images you take). Some sources state they work better when you expose your darks for 5 x as long as your lights. Darks are also temperature dependant and if there is a difference of a few degrees between the temp that the darks and lights were shot at they probably won't work that well. It should then follow that if you are using an uncooled DSLR that the darks must be taken at the same time as the lights (e.g. on the night). I use a cooled mono CCD camera which can be set to an exact temperature so it is possible to take one set of darks and store them in a library which can be used to calibrate all images taken for at least a year afterward. My max exposure is about 15 mins so I took 10 dark frames at -20 degrees for 75 mins each and combined them into a master darkframe. I use the master dark to calibrate all my pics even when the exposure is only 5 mins or even as little as 30 secs.

Some people have terrible trouble getting good flats. As Marc has stated you should aim for 1/3 to 1/2 full well depth of the chip which is why it is useful to know what it is. To get good flats you need to have a totally uniform light source which is why many people use a light box to do these. My light box has a varible voltage setting so I can increase/decrease light intensity to get my 1/3 full well. When using mono camera's with filters, each filter will let different amounts of light through (e.g. luminance = lots of light so settings are low for a short exposure whilst SII only lets in a little light so high settings for a longer exposure to achieve 1/3 saturation) so it is useful to be able to adjust for different filters no only for exposure time but also light intensity. For a one shot colour camera (e.g.DSLR) you only need to set the light intensity once.

Basically all you need to do is take a heap of bias shots (20 - 40) with the camera/scope covered at the absolute minimum exposure time your camera is capable of and combine them into a master bias frame. You then point your scope at a uniform light source (e.g. light box) and take subs until you get between 1/3 - 1/2 ADU saturation. Once you have found the setting take about 10 - 15 subs (they are usually only 2 - 6 sec long) and combine them into a master. You then use these along with your master bias and dark frames to calibrate each light sub (your actual picture). Once this is done you can stack your light frames to make an image. With my camera I have found an ADU value of about 23000 works best for flats. Flats always need to be taken on the night as the dust, focus, temp etc must be exactly the same as your image whether you have a cooled CCD or not. You should always calibrate your subs before you do anything else (e.g. deconvolution, debayering, stacking, noise filter etc) it is the very first step in processing.

Whilst darkframes will remove uneven reponse from individual pixels (e.g. hot pixels) flat frames will remove uneven lighting across the image (e.g. dust on the lens/filters/focal reducers/chip and vignetting). If done properly the result is a much cleaner image (better contrast and detail) to start your processing.

Hope this helps

Mark

Last edited by marki; 10-01-2011 at 12:49 PM.
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Old 10-01-2011, 11:51 AM
TrevorW
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Thanks Mark that's an easily understood explanation of the process
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Old 11-01-2011, 09:45 PM
mandownsouth (Des)
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Mark, Trevor says he has gained from your input. Let me tell you though I have taken a giant leap as a result of the info. Brilliant stuff thanks,
mandownsouth
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Old 12-01-2011, 05:26 PM
gbeal
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Des,
like the lads have said, try it, if you feel you want/need. It can come later, sure it is important, but right now the last thing you need is more learning, so get some imaging done and shoot the darks/flats/bias frames if and when you need/want.
Perhaps let us know what software you will use to process with, as Marc says master frames (dark/flat/bias) are handy, but many packages allow you to point to the folder with all the frames in and it does the rest, masters are not mandatory.
Above all, keep trying and asking, I certainly am.
Gary
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Old 12-01-2011, 07:01 PM
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My max exposure is about 15 mins so I took 10 dark frames at -20 degrees for 75 mins each and combined them into a master darkframe. I use the master dark to calibrate all my pics even when the exposure is only 5 mins or even as little as 30 secs.
Mark
Thats very odd, makes no sense to me
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Old 13-01-2011, 07:54 PM
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Thats very odd, makes no sense to me
Standard practice--shoot darks longer than your maximum light frame exposure, then if you take varying length exposures (say for a bright cluster, or the inner part of the Orion neb), you can use the same dark for all the different exposure length lights by scaling (eg in CCDStack). If you do this it is essential to take bias frames.
Geoff
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Old 14-01-2011, 01:35 AM
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Originally Posted by ghsmith45 View Post
Standard practice--shoot darks longer than your maximum light frame exposure, then if you take varying length exposures (say for a bright cluster, or the inner part of the Orion neb), you can use the same dark for all the different exposure length lights by scaling (eg in CCDStack). If you do this it is essential to take bias frames.
Geoff
Thanks Geoff, I forgot about scaling, its been so long since I took a pic . I always apply the master bias when calibrating in CCDstack. Once you setup the drop box menu's they automatically fill each time and after a while you forget what you have done. I just press calibrate all these days .

Mark
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Old 19-01-2011, 08:24 PM
mandownsouth (Des)
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Thanks chaps...every little bit of wisdom helps. Thanks again to those that have contributed in whatever manner.
mandownsouth
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