Flat Frames are a special type of calibration frame that are used to correct vignetting, gradients and dust in your astrophotography. In essence, a flat frame is a evenly illuminated photo of a blank light source. I, myself, left it the longest time before I began taking flat frames and when I did, it changed my entire workflow and made editing a lot easier. If I put them off the longest time, then I bet you might be putting it off as well. In this post I’ll explain to you how to take flat frames.
Fantastic Flat Frames
Once you learn the art of flat frames – as well as a hack to make life easier – you’ll quickly be able to take consistent and effective flat frames. I’ll be showing you how to take flat frames with both DSLR cameras as well as dedicated astronomy cameras. Dedicated astronomy cameras are a bit more complicated, it isn’t as intuitive as using a DSLR. I’ll begin with DSLRs as I suspect if you’re reading how to shoot flats, you’re towards the beginning of your journey and may not have yet moved on towards a dedicated astronomy camera.
I’m going to begin also with explaining what one piece of equipment you’ll need is. A white t-shirt and an elastic band. Now, some people dispute using a white t-shirt when shooting flat frames because “you didn’t use a t-shirt when taking your light frames”. I dispute this. A white t-shirt helps diffuse the light falling onto the sensor and makes life easier, contributing towards a more consistent flat frame. You also want a nice flatly illuminated sky. An overcast day, or a nice clear day away from the sun is also required or ideal. An artificial light source can be used also.
Stretch the white shirt over the aperture of the telescope. Make sure it’s nice and taut with no ripples, and remove any debris, hair or dirt on the shirt. Secure it in place with the elastic band.
Really Important Aspects Of Flat Frames!
When shooting flats there are two, extremely critical aspects to know about. That if not adhered to, will remove the ability to take meaningful flats.
Keep Focused!
Keeping your instrument in focus is critical. That is, do not move the focus!. It’s important. Changing focus can sometimes change apparent focal length of the telescope, and it means that sometimes dust may come in or out of focus. This is important for both DSLR camera lenses and telescopes.
If the focus is moved between your light frames and flat frames then you need to either return to that focus point or you’ve missed the chance to shoot good flats. It can be unforgiving.
With a telescope you can make a mark on the focus tube, or just use the focus lock on the tube. That way it can be kept in the same place from lights to flats.
Use STICKY TAPE!
One hint/tip/hack I can give you, especially when using DSLR camera lenses, is to use sellotape or a sticky tape of some description. I would find focus first, and then (whilst still in live view) I would then stick a bit of sellotape on the focus ring.
A Word From The Wounded
In the past I’ve been bitten when taking my flats using my camera lens. Particularly the Canon 50mm f1.8 prime lens. It’s because of the electronic focuser. What happened was the lens would pull itself back in when the camera’s turned off. As you can see in the image above, the lens moves slightly. So when it retracted, it would change the focus. As mentioned above, that means I couldn’t take good flats.
If your camera has this sort of electronic focus ring, then be sure to go into your camera settings and disable “Camera Lens Retract On Power Off”. On Canons this is found in the Custom Functions menu. But all cameras are different, so be sure to consult your manual.
Keep The Rotation The Same
The other aspect to know is the camera rotation. Make sure that stays the same. Dust calibration is achieved by somewhat lightening a photo in a specific place. If the dust was in one part of the image, but you rotated it around, then the image will be brightened elsewhere.
One thing I would do in order to keep camera rotation similar is use a couple bits of masking tape between the camera’s flattener (if using) and the focus tube. And just use a pen mark in order to have a visual indication that rotation is the same.
How To Take DSLR Flat Frames
Right, with all that out the way it’s time to take some flat frames! As mentioned, I’m beginning with a DSLR. It couldn’t be easier. We’re going to use a mode on the camera called Aperture Priority Mode’. This is a semi-automatic mode which lets the camera work out the shutter speed required. All you need to do is input ISO and your aperture.
Using a camera lens means you need to set the lens aperture and the ISO. If you shot your lights at f/3.5 and ISO 800, then when you come to your flat frames, you’d again set your camera to ISO 800 and f/3.5. Lens aperture can change focus after all, as well as vignetting.
From there, you could now begin shooting your flat frames. However I’m going to just start live-view to demonstrate the light meter.
Flat Frames, Histogram And Light Meter
On a DSLR you’ll see the light meter. It’s underneath the aperture and mode on the camera. It goes from -3 up to +3. In this example we’re going to say -3 is pure black. 0% let’s say. +3 will be pure white (100%). For flat frames we want 33-50% luminosity. This is where the light meter will really come in to help. The arrow will point to how luminous the image is.
You can also see the vignette on the live view. This is one of the main things Flat Frames will help calibrate out.
Now, just rack off a load of flats. I would use about 15-20 for DSLR images. We want the histogram to show a peak in the middle. Or 33-50%. If you see a histogram like this, then you’re on the right track for fantastic flat frames. As I said, DSLR flats can be really simple.
Flat Frames And Dedicated Astronomy Cameras
With a dedicated astronomy camera the same principle applies. We want to keep focus the same, camera rotation the same and 33-50% of the histogram. The trick is finding out what exposure to take in order to get that 33-50% luminosity for the flats. With no Aperture Priority Mode to choose with dedicated cameras, we need to use some technical information and a bit of maths.
In this example I’m going to be using the brilliant software Astrophotography Tool. With APT there’s a mode literally for this called the ‘CCD Flats Aid’. Within CCD Flats Aid, it asks for the ‘Target ADU’.
You might be wondering “How do I figure that out?!”. That’s where the camera’s technical sheet comes in. We’re going to use it to find the camera’s bit-depth – or Analogue to Digital Charge (ADC), and from there we do maths.
Some Examples
- The brilliant ASI 183mc Pro Camera is 12-bit ADC
- The Excellent ASI 071mc Pro Camera is 14-bit ADC
- The substantial ASI 6200mc Pro is 16-bit ADC
- The CCD Starlight Express 674 is 16-bit ADC
Flat Frame Maths
Now I did warn you that there’s going to be a bit of mathematics. But it’s okay. It’s not that bad. The calculation we’re using is
2^ADC/2
So what we basically do is raise 2 by the bit depth as a power, and then divide it by 2 in order to get 50% illumination.
- 12 bit = 4096 ADU
- 14 bit = 16384 ADU
- 16 bit = 65536 ADU
Then divide that by 2.
- 12 bit = 2048 Target ADU
- 14 bit = 8192 Target ADU
- 16 bit = 32768 Target ADU
From there, enter the target ADU in and hit RUN. APT will then sort it out all the stress for you. It’s a lot easier than it sounds. Then let it make a plan for you, and shoot them frames. For a dedicated astronomy camera and flat frames, I use about 50 flats, and 50 dark flats.
What are Dark Flat Frames?
Dark Flat Frames
Dark Flats are very similar to dark frames. With your light frames, darks are used to calibrate them and remove noise. A similar concept applies for dark flat frames. Taking dark flats are also extremely easy. In fact, easier. All the headache has been done for you!
DSLR Dark Flat Frames
With a DSLR, make a note of what Aperture Priority mode said. If it ended up taking an image that was 1/1000, f/3.5, ISO 800. Then you’d go to Manual mode, and input them exact same settings. Do not get too caught up with it. You may notice that some of your flat frames are 1/1000, some may be 1/800, others 1/1200. Just pick whatever value is in the middle. During stacking these values are averaged out anyway.
Put your settings in, and then put your lens cap on. Focus is no longer an issue too much really, but avoid changing anything if you can. Just put the cap on, or a hat over the lens, and fire off the same amount of dark flats as flats.
Dedicated Dark Flat Frames
The same, again, applies with a dedicated astronomy camera. In APT for example you can easily just go from the Flat Frame plan, add a new Dark Flat plan, and APT will copy the settings. If it doesn’t then just input the same settings. Put the lens cap on, and fire off another 50 or however many standard flat frames.
And there you have it!
Summary
So again, don’t be intimidated by flat frames. I know they’re a bit awkward at first but you can get used to taking them fast. As mentioned earlier, you can add something like an LED light panel as your light source in order to be able to take flats then and there. Especially if you have to pack up at the end of the night. I use an A4 artist’s trace panel from Amazon these days and it takes all the stress out. Don’t bother with the t-shirt if you’re placing a LED panel on the end of your scope. It won’t be required.
- Keep Focus The Same
- Keep Rotation The Same
- Aperture Priority Mode Is Your Friend
- Dedicated Astro Flats Are Not Hard
- Take Dark Flats – The Hard Part Has Already Been Done
- Take A Healthy Amount – They Don’t Take Long To Shoot
- Don’t Get Caught Up On Exact Exposures – They Get Averaged Out
- Take Flat Frames!
Thanks very much for reading everyone. I hope this guide has been useful for you. If you’d rather watch how to do it, here’s my video from my YouTube channel.