I have found that taking darks just after your flats really helps. You should correct your flats with the darks in the usual way.
Think about it the dark for your light removes the bias as well. The flat still has the underlying bias DSLR banding pattern. You are then modulating all your light data with this artefact.
Try it!
It is really most noticeable with really faint data and extreme stretching.
Bert
Below is a crop from the corner of a one second exposure (dark) at 100 ISO and 22C stretched to show detail.
spot on Bert, even if the flats are only short exposures. Longer exposures >1 sec should be done anyway for noise removal of the flats otherwise the noise from the flats will be divided into the lights. Removing the bias from the darks is crucial if you want them to perform correctly.
I take my flats using the "P" setting on the 40D - camera decides best exposure time etc for itself. If I put the lens and viewfinder caps on after taking my flats without changing settings, the camera will take darks of different exposure times from the flats.
Will this matter, or do the darks have to be a minimum time to work?
Cheers
Doug
Think of your flats as lights. Your darks should be exposed equally.
I've found, with 350Ds, anyway, that taking bias/offset frames has added noise to my images. I don't bother, as the sensor readout noise, should be contained within the darks for the lights, and the darks for the flats.
Think of your flats as lights. Your darks should be exposed equally.
I've found, with 350Ds, anyway, that taking bias/offset frames has added noise to my images. I don't bother, as the sensor readout noise, should be contained within the darks for the lights, and the darks for the flats.
I do it right, but this is just to much for me, come on guys we are just taking piccies, at my age I',m lucky to do that, and I enjoy it, lets just keep doing that. we are delving into much technology and losing the plot, it is not a compition who can out do each other
It's all horses for courses Leon. On most nights for pretty picture stuff with a DSLR, unless the data is looking good coming off the camera I'll slap together some flats and just play processing skills. But if the data is good then it definitely pays to go that extra mile. For more exacting work (photometry - still playing when the skies fine) using say SBIG then I'll go the whole hog.
Paul, I do go that extra mile, I take ICNR, I take flate's with each new session, I process as best I know how, but I do feel that sometimes, that it just gets to critical, and it would turn off new people, like I was once.
Darks must be the same exposure time as your flats Doug. You'll need to identify your flat exposures and apply that to your darks as well.
There is no way my flats can be the same exposure time as my darks. My darks need to be the same as my lights so how do I take a 20 minute flat, you can't. I think you meant to say darks need to be the same as your lights or darks and flats shold have the same F setting. I generally shoot flats to show about 30% on a PS hystogram. ie: 30% exposed.
Just to get it right :-
Light = an exposure of an object. (lenscap off)
Dark = an exposure of nothing. (lenscap on)
Flat = an exposure of a constant uniform light source. (lightbox on)
Bias = A quick dark exposure. (lenscap on)
Yes my images link isn't working, I'm moving it to a cheaper host.
So if your "flat light" is a 1 sec exposure, then you should be taking a "flat dark" of 1 second as well, and subtract them from your flat lights before applying the flats to your lights.
I don't take flat darks at this stage - my flats are usually 1/3s exposure so I didn't think it was worth it. I'll give it a try next time and see if it makes a difference (if I can force the camera to do ICNR on such short exposures).
Leon I know what you're saying, but sometimes it's the small things that make a big difference. I can tell immediately (usually) when an image hasn't had good flats applied. It's almost essential these days.
If subtracting darks from those flats makes the final image even 5% better for some people it will be worth it. Especially if it only adds a few minutes to their workflow.
Thanks Mike. Yep that is what i mean Robin. When I take flats I take them manually with no automatic exposure so my flats can often be 3,4 or even 10 sec+ long. While not critical for DSLR shooting, if I'm doing Photometry with the SBIG then it is pretty much mandatory for some of the programs that are used. If the Bias has been removed from the light by subtracting a dark then the bias (and noise) needs to be removed from the flat to prevent that from being divided into the light.
It's often thought that flats are used to correct for vignetting, dust and other optical faults. This is true, but it's not the only thing they are used for. They're main function, particularly in 'sensitive' work it to equalize out the quantum inefficiencies found in the pixels across the chip. If my chip has a QE of 30%, that is only an average of the pixels. The actual pixels may well vary by a couple of percent. In that situation the flat helps to bring the pixels up the to same level, so to speak. This is why the bias should be removed. The noise in the flats will also effect the predicted QE so it should be removed as well.
Another issue is ISO. There has been a trend recently to use a lower ISO for taking flats because they are 'cleaner'. Changing ISO is basically changing the gain. for optimum flats I believe the ISO shouldn't be changed. If the gain is one then each electron is converted to an ADU so a pixel with 1013 electrons will read 1013 adu. but if the gain is reduced (lower ISO) and the gain is 20 electrons/ adu then that pixel will read only 50 adu, the other 13 being discarded. By dividing this flat into my image I am getting an incorrect adjustment because of the discarded 13 electrons. I hope this is making sense.
Of course if you are really fanatical and if you are using a monochrome camera with colour filters, then you should take flats for each filter separately as the transmission characteristics of the filters may neither be uniform nor the same as each other.
If you use ICNR for your flats you are already subtracting a dark in camera if the exposure is one second or more.
Before I built the fridge I always used to use ICNR for flats and lights. With the camera inside the fridge and the ICNR off for collecting data (lights) it is impractical to switch the ICNR on just for the flats.
I got a very obvious banding pattern in one of my HDR images and it was coming from even seven second exposures at 200 ISO to get the trap in M42! There is no noise in a seven second exposure at 200ISO is a very wrong assumption!
It took me a while to figure out what was going on. I thought I would pass on the info.
Below are a couple of images showing the effect. One is a crop from the top left corner.
In your case Mike it is just a matter of subtracting the bias frame from your very short exposure flats. A bias frame is the shortest exposure your camera can produce. Just take a dark at the same exposure and conditions. ICNR only kicks in at one second exposure or above.
If you use ImagesPlus then this is a simple procedure.
Leon it is not a competition as I would not tell all and sundry how to do it better if it was. It is about sharing info to get better results.
Astrophotography is the most difficult pastime ever invented due to all the variables.
I really enjoy images produced by top end equipment by others. My philosophy is to do the best you can with what you have now. I thought Hubble would stop amateurs producing images. How wrong I was!
I am figuring out ways to overcome light pollution and air pollution and still persevere.
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come on guys we are just taking piccies, 
at my age I',m lucky to do that, and I enjoy it, lets just keep doing that. 
