In search of the flat field

I’ve been thinking about the problem of evaluating my flat field lighting and target, and two approaches came to mind. One is to make some flats, rotate the camera 180 degrees, make more flats, and then measure the difference.  I haven’t done that, but it seems like it should work.  The other method is to photograph the target  in a wide field with my dslr and look at color rendering and the light distribution over the field.

Here’s a stretched look at the master flat I had made.  It actually did a pretty good cosmetic job on the NGC80 image.

stretched maser flatIt shows the vignetting produced by the optical train.  The “donuts” are shadows of dust bunnies on the cover glass of the ccd chip; they’re shaped like that because my optical system has a secondary mirror that forms a central obstruction.  When this image is applied to the image of NGC80, all that crap is magically removed.  So flat fielding  removes artifacts of the optical system and ccd, but requires an evenly lit target to work.  Here’s the DSLR picture of the first try at a target:

first flat field target

First, it’s really red.  I’m using a 15 watt tungsten bulb to illuminate the target, which is bouncing off a white reflector before hitting the white target.  It looks like about a 2800K source, that gets even redder bouncing off the plywood interior of the hut.  Color matters because I will do photometry, and try to get very accurate measurements of star or minor planet brightnesses, and the sensitivity of the chip varies a bit with wavelength.  So I’m aiming at a more “daylight” balance on the theory that that’s a pretty average star color.  Being the color of our star.  Let’s see how even it is.

I converted the image to grey scale and used the horizontal box tool in Maxim to see average values across the image.  You can see that the illumination drops off on the right hand side.  It looks to me like if I can level out the profile, I should have a pretty good target.  The light is falling off because the scope is casting a shadow on the target.  It worked ok with my previous setup, but needs a fix with the new mount.  Here’s the way the setup should look.

I put some tough blue, a lighting gel used to convert tungsten to daylight, on the lamp, and fiddled with the reflectors.  The blue really helps the color balance.

The reflector and target are both white foamcore.  I need to come up with an intelligent mount for the cards, right now they are just taped and wired, and aren’t parallel. Also, I find that the sweet spot for the scope is right on the stud.


The illumination still looks pretty blotchy, so back to Maxim for more measurements.

At least the slope is in the opposite direction!  I still have about 10 percent variation across the target.  Next fix is to properly mount the target and reflector, which I think will make it pretty close.

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