Take your astrophotography to the next level of quality and accuracy with flat frames — the easy way.
Shooting flats seems difficult, but once you figure it out it’s easy and very reproducible, assuming you’re pedantic about the details. Many people swear by sky flats, but if you’re objective is small enough, or your screen/light panel is large enough, these are easier and more controlled!
When I started astrophotography I was like most people, and only shot light frames, I didn’t want to take the time to shoot darks, bias, and flats. Now I won’t process astronomical data without them. I consider it a wasted imaging session if I’m not able to shoot darks and flats to match (bias are easy and can be shot anytime — more about bias frames in another post). That’s also why, in the interest of time saving, I consider a master dark library one of the most important things you can keep around (but that’s also for a another blog post).
While it’s best to shoot flats immediately after you finish your imaging session, you can buy yourself a little time if you are able to remove the scope from the mount without detaching the camera or moving focus. I’ve also made successful flats this way in the field.
Note: This method works for telescopes and camera lenses, DSLR cameras, CCDs, and one-shot color as well as monochrome.
Why you need to use flat frames
You need to make flat frames for DSO imaging! Here are some reasons that may help you understand why flat frames can add so much quality to your final images, not to mention without them you may not be getting a fully accurate representation of your target.
Flat frames fix vignetting
Vignetting “repair” is, in my opinion, the best reason to use flat frames. Tanja was imaging NGC 6723 using a full-frame sensor DSLR on our 8″ astrograph imaging newtonian telescope, and because of the large sensor size, on just about any scope, the vignetting was INSANE. Check out these before/after shots:
Flat frames fix brightness imperfections
No objective is perfect, it seems. There is always something that gets in the way and causes imperfections and changes in image brightness levels over the imaging plane. Because flats are made with as pure a light source as we can get, it is assumed that anything that shows as a dark patch in the acquired image is an imperfection in the imaging train. It could be a speck of dust, a scratch in a lens or mirror, a bug, a UFO, etc.
It’s simple: if you want to take your final image quality to a higher level, you MUST use all types of calibration frames, and flats are up there as some of the most important. Just do it!
What you need
- A clean white t-shirt or white cloth with little to no visible weave pattern. It must be big enough to fit over the objective end of the scope when stretched.
- A large rubber band or elastic.
- A good laptop LCD screen with a full-screen white image, or a uniformly lit and diffused white light panel that is a little larger than the objective end of the scope or lens. I use a 15″ MacBook Pro Retina, but you don’t need that. Any LCD or white-light panel will work as long as it is uniformly lit.
- If at all possible, do not remove the camera from the telescope. If you must, mark it with tape. It needs to be in the same orientation as the light images.
- Do not move the focus position. Lock it down when you are finished shooting light frames (or even better, before).
- It is very important that everything in the imaging train is exactly as it was when you shot your light frames. Filters, dew shields, correctors, dust, etc. should be in place and not touched or cleaned.
- Don’t be lazy about the details.
How to do it
– Set your camera to MANUAL mode, just like you would with light frames.
– Set the ISO to 400 or lower.
– Start with an exposure duration of 0.5-2.5 seconds.
– You will need to experiment with exposure duration, I needed an exposure length of 13 seconds with a 5nm Ha filter.
– Make sure to shoot separate sets of flats with each filter you use, the imperfections could be different!
For camera lenses
– Make sure the aperture and focus of the lens is exactly as it was when you shot your light frames.
- With the lens cap/telescope cover off, stretch about 2 layers of the cloth/shirt tightly over the primary objective and secure it with the elastic. Ensure there are no folds or creases over the open end.
- For an LCD: create a pure white image to display full screen. I use a new document with a white background in Photoshop and make it full screen. Set the backlight brightness to about half to start with.
- Hold the LCD screen or white light panel up to the objective. If you cannot hold it directly against the objective with no gaps (shown below), drape a dark, light-blocking cloth over over the scope and screen, and/or shoot in a dark room so the only light that enters the scope objective is from the light source screen.
** Important! ** Make sure the objective is COMPLETELY covered by the light source panel. Light leaks from other sources can and will ruin the flats.
- Pick an exposure length in the middle of the suggested range, depending on your camera type (From above: 0.5-2.5 seconds for DSLR, CCD is dependent on the filter) and take a test exposure.
- Now you have to analyze the exposure to make sure the median brightness is correct (~51% or so). If you’re within around +/- 5%, you’re fine [in my opinion, right now]. If you’re shooting DSLR tethered or CCD it’s easy because you won’t need to remove the camera’s media card to access each test exposure.
Read more below about how I measure median brightness for my test exposures.
- If the exposure median brightness is not within +/- 5% or so of around 51%, adjust the light source brightness first to try and get it right. Once you’ve exhausted options for brightness, only then should you adjust the exposure length. Repeat and experiment with the light source brightness and exposure length until you get a median brightness as close as you can to ~51%.
- When you’ve found the sweet spot for your optical train and camera, shoot a lot of them. I usually take at least 30 exposures.
- Done! Now you can calibrate your light frames with these flats (and darks and bias, right!?) to get the most detail and quality out of your astro images.
Measuring Median Brightness
** Important ** Avoid letting the way the flat frame looks influence your opinion. Go by the numbers!
For DSLR, I use the Image Statistics function in PixInsight (shown left).
For CCD, I currently use the per-exposure image statistics information available in Sequence Generator Pro.
Whatever image processing or acquisition software package you use, look at the median and maximum statistics numbers for the test exposure. You’re looking to get the median just a touch over half the maximum, around 51%. That being said, I’ve had good luck at +/- 5% as well.
You want the maximum to be at or near “full-well capacity”, which is as bright as the pixels can get. So, increase brightness until it stops growing, and back up a bit until it’s just below maximum so you don’t overexpose your frames.
Other apps for measuring median brightness
Most, if not all, of the major image acquisition or processing applications should be able to give you image/pixel statistics.
If you want to measure this for free, one way would be to download the free trial version of Nebulosity. The mean and maximum for the currently viewed image is listed under “pixel stats” (left). Mean is not the same as median, but close enough, usually.