How to Take Flat Frames with a DSLR
Beginners often skip flat frames in their astrophotography workflow because they’re not as easy to shoot as darks or bias frames. This was certainly true for me when I was starting out, but learning how to take flat frames with a DSLR does not need to be an overly complicated process.
A little practice is all it takes to shoot effective flat frames. In this guide I will show you how to do it step-by-step using a DSLR camera attached to a telescope, but the same process can be followed for a CMOS camera.
The most common way to take flat frames is to use the white t-shirt method. Stretch a white t-shirt over the objective of your telescope and secure it with an elastic band or similar.
You’re looking to achieve an even level of light across the aperture of your telescope, by pointing it at the sky early morning or early evening, you should have a good level of light entering your scope but not so much that it washes the image out.
You should take your flat frames using the same ISO setting that was used for your imaging session.
Set your camera to Aperture Priority (Av on a Canon DSLR). This will select the right exposure time for your flat frames automatically. As a guide, I generally find at ISO800 1/180th second is optimal, don’t worry if yours varies slightly. If your exposure is significantly different than this then you may need to choose a brighter/darker spot.
Important – Your flats must be taken with the telescope in the same focus that your light frames were taken.
I have found that 20-30 flat frames is perfectly adequate to create a good master flat file. Of course you could opt to take more than this, which won’t have any negative effects other than running up the shutter count on your camera! Any less than 20 and you may not have captured enough data for a really useful master file.
Good flat frames will account for the uneven illumination across your telescope’s field of view, and also remove any dust spots that were on your camera sensor. The result gives a cleaner, more evenly illuminated image, meaning your image processing will be easier, especially when it comes to stretching, as your image will have even illumination rather than having washed out areas.
This will improve your astrophotography like you wouldn’t believe!
Follow my calibration frames tutorial below to see how to take flat frames with a DSLR
Taking Flats at Night
Taking Flat frames at night can be achieved with an external white light source. This has the added benefit of being able to dismantle your astrophotography gear with the knowledge that you won’t need to spend the following morning taking calibration frames, allowing you to get that pot of coffee on and go straight into your image processing workflow!
You could purchase a flat field panel, which will allow you to control the brightness. You will still need to use a white t-shirt but taking Flats this way means you will have consistent results as you’re in control of the light source. This is especially helpful when you live in the UK like me, which seems to just be various shades of grey no matter the time of year!
Another light source you could use is a laptop or tablet screen. Download a light source app that will allow you to have a white light across the screen and adjust the brightness until you achieve a good Flat frame. The benefit of this is that you won’t need to buy additional equipment like you would if you went down the light panel route.