Taking Flats.

I finally managed to arrive at an easy way to do this and I thought I'd share it.

Basically I used a sheet of whiteboard (clean). I bounced a light off the rear wall of the observatory so that the whiteboard was evenly illuminated. Then it was simply a matter of pointing the scope in its imaging configuration at the white board and taking shots until the CCD pixels were half saturated (average values around 32000).

I then took a stack of these shots. It certainly makes a huge difference getting rid of the dustmotes.

Now that I have an easy way of doing it I'll be taking them every shot!

Having not dabled into your level of astrophotography (yet!), I ask out of curiosity - do you do this procedure to ensure your optics are as clean as possible Dave? Or can it also be used to determine the average longest exposre value of your CCD, and thus when hot pixels could show up? And, what is 'taking flats'? :oops:

Taking Flat Fields: Flat fields are used to correct for noise in the optical path of the telescope system. The use of a flat field is essential for a good image. Keep these ideas in mind when creating a good flat field.
It must have even illumination.
It must be taken with the same optical setup as the image you are going to apply it to (don't change the focus).
It should be about one-third to one-half the saturation values for the camera.
Multiple exposures (8 to 12) should be taken and averaged using the median filter.
Take separate dark frames to use with the flats.
Take flats for each color filter for color imaging.
Check the pixel value across the image. The image should have variations of only 5-10 units.
There are many ways to achieve a flat field, and I've outlined a few below.

Dome Flat: Close the dome and with the lights on take an image of the painted white image of the shutter. Move the telescope a small amount to different parts of the shutter for averaging out irregularities. This method is good if you need to change the focus of the optical system during the night.


Sky Flat: Work this one in twilight before the sky gets too bright. You will need to change your exposure to compensate for changes in twilight brightness. You need a similar average value for each exposure. Be careful about getting faint stars in your images as well. Move the telescope slightly so they average out.


T-Shirt Flat: This can be done in twilight or the daytime. There is a white cloth, and a rubber band in the observatory box. Place the cloth over the front of the telescope pulling it tight. Secure it with the rubber band and place the dew shield around the telescope. Point the telescope toward the ground and take your exposures.