Hello, as someone getting into astrophotography using a DSLR, lens, and tracker, I have a possibly stupid question lol.
When taking flats. Is there a certain brightness to target, for the light source specifically, if I was using a tablet with a white screen? I know I have to adjust my camera shutter speed to hit a certain spot on the histogram, but does the light source matter?
Thanks.
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The light source can matter. Many people use light panels that are originally meant for drawing on, like tracing things. The problem I've had in the past trying to use the one I have, is that if I try to dim it too much I can see lines or like a flicker from the frequency of the light. You may have to cover the tablet with a thing white t-shirt or something to be able to get your exposure how you want.
As for your histogram, I always aim for 1/4-1/3 from the left. You don't want anything clipped to black, and you really don't want it clipped to white.
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I am thinking that as long as your exposure is long enough to negate any adverse effect a tablet screen (flickering) may have, you should be good. I use a commercial flat panel light that is adjustable in color and intensity, and have never had an issue with 2 - 3" rendering a Flat somewhere just before or at the center of the histogram. Anywhere my panel color is (between 3K and 5K) has never caused a problem in calibration.
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you don't have to worry about that; it just needs to be a moderately bright light source
but,
as others have mentioned, your tablet probably has a strobing pattern coming from the display; you might be able to see this with a slow-mo camera
that can affect your flats in a negative way
I recommend, for someone who doesn't have dedicated flat panels, to just put a white T-shirt on the scope, remove ripples in it, in front of the objective lens, using rubber bands maybe, or paracord (or if you want to iron it beforehand also), and point at the sun/daylight sky through a window or outside, or even at an evenly illuminated wall inside your house, like, one that the sunlight is hitting; these will give you the best budget flats
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Oscar:
you don't have to worry about that; it just needs to be a moderately bright light source
but,
as others have mentioned, your tablet probably has a strobing pattern coming from the display; you might be able to see this with a slow-mo camera
that can affect your flats in a negative way
I recommend, for someone who doesn't have dedicated flat panels, to just put a white T-shirt on the scope, remove ripples in it, in front of the objective lens, using rubber bands maybe, or paracord (or if you want to iron it beforehand also), and point at the sun/daylight sky through a window or outside, or even at an evenly illuminated wall inside your house, like, one that the sunlight is hitting; these will give you the best budget flats So my tablet has a high refresh rate screen. That should lower the flicking the camera will see right? And when I say high I mean 120htz
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Oscar:
you don't have to worry about that; it just needs to be a moderately bright light source
but,
as others have mentioned, your tablet probably has a strobing pattern coming from the display; you might be able to see this with a slow-mo camera
that can affect your flats in a negative way
I recommend, for someone who doesn't have dedicated flat panels, to just put a white T-shirt on the scope, remove ripples in it, in front of the objective lens, using rubber bands maybe, or paracord (or if you want to iron it beforehand also), and point at the sun/daylight sky through a window or outside, or even at an evenly illuminated wall inside your house, like, one that the sunlight is hitting; these will give you the best budget flats
So my tablet has a high refresh rate screen. That should lower the flicking the camera will see right? And when I say high I mean 120htz
I honestly am not sure I never really learned about flat panels or tablet flats because the sun served my purpose I just know that any strobing is bad if you want me to guess, I think you would see double the strobe lines of 60 hertz, but it's just a guess
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You'll have much less to worry about and fuss with if you just use the sky or a white wall. The t-shirt really isn't even necessary unless the FOV is super wide.. Just make sure everything is the same as when you took your images. Same filters, focus and rotation. Expose 1/3 to 1/2 from the left side of the histogram, that's it.
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Hello, as someone getting into astrophotography using a DSLR, lens, and tracker, I have a possibly stupid question lol.
When taking flats. Is there a certain brightness to target, for the light source specifically, if I was using a tablet with a white screen? I know I have to adjust my camera shutter speed to hit a certain spot on the histogram, but does the light source matter?
Thanks. Using an LED panel is where I ended up after many other attempts - so you are starting in a perfect spot, especially as you have a controlled setup you can replicate any time and place! (white T-Shirts also work well, but tuning it to fit for different filters always needed some tweaking) Regarding exposure time/light intensity I would look out for a. make sure you exposure time much longer than refresh rate of LED screen - and not too short, otherwise you wil get moiree effect artefacts b. aim for histogram peak to be between 30-50% of your total range c. take flats every single time for each filter - any shortcut of usings last nights flats might work but often does not Arny
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I used to shoot sky flats, just before an imaging session without a white tee shirt.
then I discovered that sometimes bright stars were visible and ditched that method.
now I shoot sky flats looking east, about 45 minutes or an hour after the sun has set in the west, and I have added the white tee shirt.
I Target the Flats brightness at 30K ADU, as expert suggest that is the correct target value, and if I time things right and the sky is not too bright
my flats exposure time will often be between 2 and 3 seconds. I use EKOS to get the target brightness right.
I usually shoot about 30 Flats and they get stacked to create a Master Flat.
this Sky Flats method seems to work well, so I have no intention of buying a Flats Panel
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I have had that happen too, even when shooting narrowband flats well after sunrise. It really surprised me the first time I saw it. Certainly something to be aware of.
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I use a white cleaning cloth stretched over my lens with a rubber band and a flashlight when I use just my camera and lens setup for my flats. And I aim my telescope at either a bright light or tilt it straight up and cover it with a white shirt and rubber band also using the flashlight. What I mean by using the flashlight is I make sure my lens or scope is perfectly upright when I cover it then I turn on the flashlight and place it upside down on top of my setup so the light shines down into the shirt. I also always have my camera set to AV mode which helps with the histogram.
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The flickering is indeed the biggest problem I see with drawing panels. But my solution is to increase the number of flats I take. Unless you hit the exact frequency of the flicker with your exposure time, the dark and bright stripes are always at different positions (btw, I never saw the flickering while using my tablet screen, though). I build a DIY adjustable panel and I am trying to keep the exposure time of the flats as long as possible to prevent the flickering effect. This works well with NB filters. For LRGB (especially L) it doesn't work all the time. But there you can lower the ISO speed of your camera if needed. It won't have a huge impact on the flats, if the flat sensor gain doesn't match the light frame's gain.
However, it is just a matter of the statistics. Taking more flats will even out the strobing effect on the master flat. I typically use 30 flats to get an even master frame. Sometimes I go even higher to 50. I never had issues this way. There are so many things to me that are more important and have much more impact on the final image quality than the possible left over pattern in the master flat using this method.
I know, that not all of you may agree with me, but this is what I experienced.
CS
Christian
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There are two potential issues with using a tablet. 1. flickering/banding as mentioned above. Normally you can remedy this by playing with brightness and exposure time. Ultimately you're only just looking for a histogram at 1/3 - 1/2 of the max. Another way to deal with this is to rotate the panel halfway the run. In that way, most non-conformities from your panel will even out during stacking. 2. color. Tablet screens are not white. Most have a lot more blue in it than red and green. In itself not a problem, but when you dial in the exposure (brightness/shutterspeed), do not only look at the luminance histogram, but also the RGB histogram. You don't want to expose lets say at 40% of max, and then discover that your blue channel is 70% of max or so. I've done quite some digging into the use of light panels, including tablets, and have written a number of blogs about it. If you're interested, feel free to check this out. The second blog is probably most applicable to your question.
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