Astrophotography – Flat Frame box

Now that I have the scope in the observatory on a wedge it’s time to start doing a little more astrophotography. And one issue I immediately ran into, especially with the field flattener in place, was the need for decent flat field images. I needed to make a flat box…

When a camera takes a photo, especially through a telescope, not all parts of the image are of equal brightness. And with some scope configurations it can be quite a dramatic difference in intensity from the corners to the edge of the picture. So the idea of a flat field is that you take an image of a uniformly lit surface and then image processing software can adjust your real photo, your ‘light’ image, to the correct even intensity. These flat fames also will capture spots of dust on the camera chip and other imperfections which software can also process out.

Heres a flat with my webcam (lots of dust on the sensor, but it looks clean to the naked eye!):

And one from the DSLR with the field flattner showing a range of brightness across the frame (though at least it looks like the anti-dust mechanism on the 400D is doing its job!):

On a single frame of the Orion Nebula with a camera in a similar (but not quite the same) configuration you can see a similar light pattern:

Since flats are dependent on the telescope configuration, you need to take them with the exact configuration you are using for the light image. So the same focus, similar temperature, same filters, same barlows, same specs of dust on the ccd! etc. etc. In practice this means that nearly every night you want to take astrophotos, you also need to take flats, since most people will not leave the camera attached to the scope permaently. They only take a few minutes so its no big deal in terms of time.

The problem arises in getting that uniformly lit surface. There are traditionally two methods of doing this. Dome flats – which involves pointing the camera at a part of the telescope dome that give uniform brightness. Or Twilight flats – pointing the scope at the twilight sky which for this purpose can be uniform enough. Both of these approaches have potential problems – the light may not be uniform, it may have a gradient, it may not be white. My main problems with them however were that 1. I’m not at the observatory at twilight, and 2. it’s not a dome!

So I decided to build a flat box. Several other amateurs have tried this before with good results, e.g.

My approach was similar. 5 A3 sheets of 5mm foamboard purchased from the local art supply shop, some diffuse plastic used for covering schoolbokks from the stationary shop, and pack of push-on-off led lights from the hardware store.

Here’s the finished product:

The construction is fairly obvious from the image. We have two OTA sized holes at the left so slide this onto the scope. Next to that the first diffuser. At the back we have the lights and beside them the other diffuser. The lights are mounted to the sides rather than the back so that the light gets diffused that bit more. The top and back are held in place with ducktape, the rest of the frame is glued together. The rings and diffusers are just held in place by some spare foamboard glued to the sides so I can remove them if necessary; better diffusers if necessary or a different sized scope.

There were two problems apparent with this:

  1. The light was not white. The led’s has a slight blue hue to them confirmed by the image histogram. I may get actual white light led’s in future, but for now most processing software has options to make the images grey – either normalise a histogram or discard colour data which you can then re-apply colour data to get a normalised histogram.
  2. The box does not fit snugly onto the scope so can bend downwards a little. This causes a slight difference to the field but you need to analyise the images to notice it. Holding the box in place while letting software take the images gets around this.


Leave a Reply

Your email address will not be published. Required fields are marked *