Taking flats

[iceman]

So from what I understand, flats (using a DSLR) should be taken using AV mode at ISO100, so that the camera works out the best exposure to evenly fill the histogram.

Problem I had is, how do I use ImagesPlus to automate the capture process? When I set the camera in AV mode, I don't seem to be able to use ImagesPlus any more because the "Capture" and "Bulb Capture" tabs both want an exposure time, and won't fire off any images without it.

How do you guys do it? Do you press the shutter manually? Is there something i'm missing?

[[1ponders]]

I stick to the manual setting Mike and just adjust the exposure until the histogram shows to about 1/3 - 1/2 across the range after a 2 - 10 sec exposure. I don't adjust the ISO at all, I leave it the same as my lights.

[iceman]

Thanks Paul. Do you take dark flats to subtract from the flats, given the higher ISO and therefore more noise??

[[1ponders]]

Not for DSLR shots normally, but I have done it. If you try it you will have to dark subtract you flats before using them as the automated process won't handle that.

[h0ughy]

no thats what your darks are for and the bias shots, as for the 2 second bit, I change mine to P on the camera and leave the ISO at what I shot at?

[[1ponders]]

Flat darks are a bit different h0ughy. Sometimes it pays to take them if you really want to get fussy about reducing any callibration induced noise.

[h0ughy]

fair enough, but having a cooled camera I don't really suffer from much noise:whistle:

[[1ponders]]

Maybe we should do a test when you come up for Astrofest to see just how much. ;)

[Lee]

Why is there an exposure time (ie 2-10s) requirement??? Won't any exposure that gives the right histogram do the trick??

[Lee]

How does this sound.....
This is with a DSI, can't see how it would be different to any other CCD though....
I have wound down the gain, exposure to 200ms - I get a nice smooth frame, intensity all over about 21000. I can't see the dust unless I stretch the histogram.

Is it the difference between the areas that matters in a flat? Ie the difference in intensity between the corners and the centre, or the dust spots and the rest???

[Dennis]

Here’s how I think of flats and light frames. When calibrating your images, Flats are “divided” into the light frame rather than being subtracted like we do in a dark frame.

Say you have a dust spot in the galaxy (light) frame, and it reduces the intensity of light on the pixels in that small region by say, 50%.

When you take the flat field, the intensity in the dust spot region will be 50% of that in another region that has no dust, or vignetting. That is, a clean non-vignetting region can be said to have an intensity level of unity or 1.

So, when you divide the flat into the light frame, dividing by ½ will “double” the intensity to compensate for the 50% loss caused by the dust spot, whereas dividing by 1 will not make any difference where there is no dust or vignetting.

Likewise, if a flat field region is say x2 as bright due to the effects of a localised hot spot, then dividing the light by the flat (÷2) will bring the intensity back down to unity.

Cheers

Dennis