# 05 Jun, 2018 10:02
I got again problems by integrating my last session of widefield pictures.
i dont know why, I do the same like usual,
but skip the calibration.
why ? I don’t know how to make good flats with my lens. The T-shirt method doesn’t work , my lens is the Samyang 14mm f2,8.
camera is a Canon eos 1000da.
on Skywatcher Star adventurer.
my result is confusing to me.
after Integration I don’t see any better image than what my subframe was before. I’ll try to add a Dropbox link, there you see a screenshot of my integration and a debayered sub.
i also can add my lights , only 30, if someone likes to help me with that ?
i would appreciate if someone would have a look at it.
thank you in advance.
# 05 Jun, 2018 10:08
I hope it works , if not I will send it personally
# 05 Jun, 2018 20:57
So what I should tell is what I all tried.|
first I tried the pre processing steps out of lightvortexastronomy , without the calibration steps , only added my bias, nonfarks and flats.
I debayered them , rggb pattern. and Then registered them with star alignment, I used 2d spline . Checked first with detected stars if it looks okay , and it did.
then I added all my debayered images and increased noise scales to 2 and hot pixel removal to 2 as well. Noise reduction yp
then the registered files were integrated , I used the first picture as reference, just because it was my fist try , and the others were getting a little bit lighter by the moon shine .
As for algorithm I tried the winzorised, linear fit clipping , and sigma clipping .
All three almost the same bad result.
alsoni tried out the batch preprocessing , and integrating like I explained before.
still the same bad.
so what do you think ?
Am i missing a step? Or something important ?
I hope someone can help.
# 06 Jun, 2018 14:56
"Then registered them with star alignment, I used 2d spline . Checked first with detected stars if it looks okay , and it did.|
then I added all my debayered images"
Are you saying you added your debayered files back to your debayered registered files for integration? My process is - RAW capture-Calibrate with Masters and create a set of .xisf files - Debayer the calibrated .xisf files - finally star align the debayered .xisf files. I just take the defaults in star align and have not had any issues for any of the objects I have shot.
# 06 Jun, 2018 17:17
|Bias frames without flat frames are useless… bias basically is removing noise from flat frames … stock them just as light frames and dark frames and see your results…. … if you wanna use more calibration frames, flat are best done, like 30 min after sunset with your white t-short on your lens, look for histogram , must be around 1/3 distance from your left if you shot them at 1.5 -2 sec long …camera focus best if set exactly like light frames ….good luck|
# 06 Jun, 2018 18:34
Hey David , maybe it sounded wrong , I meant I put all my debayered images into the target frame section.|
not doubel or so.
and I would calibrate them.
darks is no problem. I only have to make some, flats it difficult , I’m not sure how , I basically know how , but with that lense , it’s difficult to put a T-shirt in front of it .
Thank you Greg for your help though.
but even without calibrating , it should be more than the quality of a sub frame.
did you check my screenshot in the drop box link ?
# 06 Jun, 2018 21:20
just made a quick & dirty attempt on your lights: (using PI)
Result: It is the missing flats, that cause your problems. All the information is in the integrated image, but it is hidden below a very strong gradient. At least the light train generated part of that gradient could (and should) be eliminated by flats. I fear, that there might be a very strong gradient of light pollution too.
Well, as first step i just did an ABE and - voila - all the information is here. Further processing would start with excessive DBE to eliminate all gradient in the picture, then the usual stretching and so on. I am sure, with some effort, you will be able to create a great picture out of these lights.
Here is a link to what I have achieved: https://astrob.in/350235/0/
HTH, CS Fritz.
PS: Of course I will delete the images once you have seen them
# 06 Jun, 2018 21:38
thank you for checking on them,
really ‽ that’s the problem ? Hmm well I didn’t really make the effort to go to a dark sky , so I just set out my star adventurer while looking at Jupiter and Saturn.
so the picture taken was over our little town and well, maybe also the rising moon did his job to it.
do you know how to get good flats with this type of lens ? It’s the 14mm Samyang 2,8
maybe you see why i feel like I struggle with it a little .
thank you so much so far !
PS: you can now erase the picture
# 06 Jun, 2018 22:37
the best and easiest way to get flats with that lens is "sky flats". As long as you do not change anything in your light train (e. g. focus) you can make them the next day: Set your Canon to "AV" point to a evenly lit (=clear) part of the sky and make your flats. Before processing them, make sure that there are no clouds in the pictures.
That is the way I make all my flats when traveling with our sailboat. There is no room for much AP equipment on the boat so all I have is a star adventurer, a camera and 3 lenses.
Please feel free to ask any further questions.
PS: I have removed your pictures from my staging area
# 31 Dec, 2018 18:17
Flats correct uneven field illumination, which can be cause by a) irregularities in your equipment (e.g. a dust mote on the sensor or on the lens) b) vignetting, the inescapable optical fact that the center of the sensor receives more light than the corners.
For wide field photographs with a DSLR I have found that (b) is much more of an issue. You can't really salvage a very dark splodge caused by a dust mote using flats.DSLRs might only pick up dust when you change lenses and you are better off just cleaning it. At least this is my personal experience.
[For telescopes or huge lenses things are probably different. It is probably impossible to have a large Newtonian open and pointing at the sky for a couple of hours without at least some dust or moisture making it into the mirror and it is not only necessary but also easier to correct: a dust mote on a large mirror probably translates to a tiny area of the image becoming slightly darker, while a dust mote on a tiny sensor makes hundreds of pixels almost black. ]
To accurately correct vignetting, you need an evenly illuminated field for your flats. You want your original subject to be as close to a constant value everywhere as possible. That way you know that were your flat has recorded 80% of the maximum value, it really is due to the equipment imperfection that you want to correct. Otherwise you are not really correcting, you are only dividing an unevenly illuminated field by another.
Personally I find it quite difficult to obtain a flat field without gradients using the sky method, especially with a wide angle lens. The sky is always brighter on the side that has the sun. Again I think with a telescope it doesn't matter much because the field of view is tiny, but for anything smaller than 50mm or so the gradient is very much noticeable and could easily exceed the amount of vignetting that you are trying to correct.
Previously I would just maximize a white window on my monitor (e.g. a notepad) and hold the camera perpendicularly, moving it randomly across the monitor in between shots. Take 15-20 shots like that, average, behold a perfectly good flat for a wide angle lens.
Then I got a lightbox, a $5 or $10 white box with USB leds all around and a hole to stick your lens through. Same principle really, just more convenient.
Lately I am not using flats at all and use profiled lens corrections instead but that's another story.
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