David DRH. 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. 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!!! 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? 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. 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.
This is very important. The flats, darks and bias frames can't be applied accurately unless the images are non-debayered RAW files. It makes more sense now, and I'll try it out when I get the chance! Never thought of doing the flats earlier in the process when the sky is more evenly lit.
Will do that for sure. Approve the Cookies This website uses cookies to improve your user experience. By using this site, you agree to our use of cookies and our Privacy Policy. Register to forums Log in. May 20, 1. LIKES 0.
May 21, 2. May 21, 3. May 21, 4. May 21, 5. May 21, 6. 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.
May 21, 7. David Ransley wrote in post Take a few of them between 10 and 20 is usually enough. May 22, 8. Hi, Don, you say:"Dark Flats are quick and easy to acquire". May 22, 9. May 22, as a reply to DonR's post I've been searching and reading for 3 days now about this and never found an answer to these questions..
May 22, Don thank you! May 23, Jump to forum Not a member yet? Registered members may log in to forums and access all the features: full search, image upload, follow forums, own gear list and ratings, likes, more forums, private messaging, thread follow, notifications, own gallery, all settings, view hosted photos, own reviews, see more and do more Don't be a stranger - register now and start posting! May 21, 2 The Bias frames are very short exposures made with no light reaching the sensor.
May 21, 3 The Help File also comments on the files you are talkng about, but my question: Dark Flat Frames? May 21, 5 Awesome descriptions, thank you! Dark Flats?! What does all this mean?! If you have ever asked someone about how to improve your astrophotography quality, those words were probably peppered in the answer you got. Calibration frames are photos you take in particular ways and under particular conditions, for the precise purpose to improve the quality of your light frames.
To keep it easy, the job of the calibration frames is to calibrate the light frames, i. There are four types of calibration frames: bias , darks , flats, and dark flats. Each type has a specific job to do. Bias frames are used to take care of the readout noise of your sensor , i. In high-end cameras, this type of noise can be extremely low but is never zero, and collecting bias frames is an easy way to remove the readout noise.
Darks frames are used to record the electronic noise your camera produces during the capture of a particular exposure. This noise depends on the length of the exposure and the amplification of the signal, i. Since the longer the exposure, the hotter the sensor becomes, this kind of noise is also called thermal noise. Darks will also take care of stuck pixels in your sensor, i. Hot pixels are much brighter than they should be and are permanently one.
They can be red, green or blue. Cold pixels have lower than normal sensitivity, while dead pixels have zero sensitivity. They appear gray or black. Finally, dark frames also include the readout noise, the one that bias frames are intended to deal with.
Flat frames are used to cure issues in your setup that can alter the uniformity in the brightness of your scene. These can be vignetting from using a filter or a photographic lens wide open, or dirt and dust specks in your optical train. Even for the relatively short flat frames, thermal noise is affecting them, particularly so if the flats are unusually long few seconds.
This allows us to isolate the readout noise from thermal noise and to keep pixel values to their minimum. Because we are recording the readout noise, the temperature at which you took your light frames is not important, and you can build your own bias library and collect the bias frames once a year , for the ISO values you usually use. Instead, you use what is called the gain.
In this case, put the cap on your telescope and take the bias frames with the same gain you used to collect the light frames, setting the camera to its fastest shutter speed. They are the longest calibration frames to take because they have to match the exposure time used to record your light frames : if you shoot your target with 3-minutes long exposures, each dark frame must be 3-minutes long.
They are the trickier calibration frames to take because they are temperature-dependent, as they need to record the thermal noise of your sensor. There are several advantages in using cooled cameras over a classic DSLR, Mirrorless or non-cooled astro camera:. For a start, there is not a simple way to relate the ambient temperature to the sensor temperature. Then, the sensor temperature increases as the imaging session goes on because the sensor warms up by taking one exposure after the other.
Lastly, some cameras record a temperature in the exifs attached to the images, but that is luckily not the real temperature of the sensor. The best way to take darks with non-cooled cameras is to get them directly in the field, at the end of the imaging session.
I usually take my darks while packing up. For these reasons, some say darks are a waste of time if you do not have a non-cooled camera and that sometimes dark frames even degrade the image quality. With this option, mathematical models are used to scale the master dark to match at best the temperature, ISO, and exposure time of your lights.
You should experiment with that as it sounds like a great way to calibrate light frames from uncooled DSLR. This way, one could create once a year a master dark from integrating a couple of hours of darks and scale it when calibrating the images, thus saving time in the field. Flat frames are not used to suppress digital noise in your images, thus they are not temperature-dependent.
0コメント