Flat frames - using a calibrating technique to find dust

What's that madman doing this time?

A few nights ago I was out in my back yard taking photos of planets. Using a monochrome camera and filters, I got a couple of images. They're not great, but that's another story.

While I was out there, I noticed a problem. Jupiter had a big dark blob in front of it. The blob was about the same size as the planet, and roughly circular. Because Jupiter was a smallish image on the sensor, I was able to move my mount so that the blob didn't interfere with the image, but I suspected I had dust on one of my filters.

Next morning, I looked at the filters and the camera. Try as I might, I wasn't able to see anything.

Dust is a frustrating problem, especially for planetary photographers. With photos that use the whole frame, you can compensate for dust, but for tiny planets you can't do this.

I decided to find it using the camera itself, by getting a "flat frame". This is a photo of what should be a blank featureless field. Because there's no background, dust shows up as round or donut shaped blobs. It also highlights vignetting, where the centre of the frame is brighter than the edges.

To do this I went and got a white tee-shirt and put it over the lens. I stretched it so the weave was consistent - it wasn't tight on one side and loose on the other. 

I went outside and pointed the scope at some clouds to get an unfocused, consistently illuminated field. I exposed the photo so none of my pixels were black, but none were white either.

The image was boring - just a grey box. But when I fiddled round with the levels tool in Photoshop, the vignetting and two dust shadows emerged.

They were on the luminance filter, one above and one below the line of the sensor's long side.

Now that I knew where the dust was, I was able to have a closer look, and I was able remove them using a blower brush.

Once I took a second flat frame, and processed it similarly, you can see that it's more or less clear of dust.

Back to work!