Ad Astra Observatory

I finally knuckled under to the pressure of the dust bunnies and decided to shoot “flat field” frames with my astrophotos. A “flat” is a photo of an evenly illuminated surface. Since there’s no “subject” data in the frame, what the flat shows is any darkening aberrations in the optics, like vignetting (darkening at the corners), dust (all those lovely dark donuts), etc.

The problem is, how do you produce an evenly-illuminated surface? This can be a pain in the neck, so most people, myself included, skip flat calibration in their photography. But my photography is starting to get to a point where I think I could get better results if I started with cleaner data. So, in search of flats I went.

There are many methods to produce flats (shooting the sky at twilight, shooting the inside of the observatory dome, creating a lightbox), but the concept is the same: find a way to back-light some kind of diffuser.

So here’s my version. I made it oversized (the box is 22” square by 8” deep, with a 16” aperture) so that I can mount it on the wall of the observatory and in theory use it to do flats for any of my telescopes, including Trixie (the 13”).

The project was a fun build; it was equal parts construction, papercraft, and electronics. Cost about $15 at OfficeMax and RadioShack. The resulting box is very light; I imagine it weighs less than 2 pounds.

I used 6 bright “white” LEDs for the light source (and I added a dimmer switch, ooo…), which look yellow when I shot a photo in a darkened room, but look very very blue when shot through the telescope. I’ll have to work on the blueness; either I’m going to end up adding red and green LEDs to the mix, or I’m going to figure out how to deal with the blue mathematically (it’s all about the grayscale (: ).

The photos are fairly self-explanatory, except for the blueness and for the very uneven lighting in the telescopic image. From my extensive study of lunar eclipses, I think that the upper LH corner is dark
because of actual vignetting in the optics, but that the upper RH corner is dark because the lightbox was not centered over the telescope. It was a quick test, after all.

Anyway, now shooting lights should be as dead-simple as shooting darks and bias frames — the lightbox is (or will be) set on the wall of the observatory such that when the scope is parked, it points at the

lightbox, and voila! Easy flats.

I’m not surprised that it took this long to “get around to” building a lightbox. But I am definitely ready for those dust bunnies to disappear…

Keep looking, um, up?
Jimbo

PS: in case you’re really interested, the way that image calibration works is that you do this:

calibrated image = (original – dark) / flat

The dark (an exposure the same length as the original, but with the lens cap on) is heat-based data that increases linearly over time, so subtracting it is easy.
The flat (vignetting and dust bunnies) needs to be divided out; think of it as “pixel without a dust bunny has stronger response, so you’re averaging the response of all the pixels”.
Once you’ve calibrated the image with flats and darks, all that should be left in the image is what was actually in the sky.