I’ve been gradually working my way through the many things to learn in astro-imaging. One of the things I encountered in my readings was the concept of using “flat frames” to counteract problems in the camera (dust motes, field vignetting, etc.). It seemed complicated and I wasn’t convinced it was worth the effort.
Well, once I reached the point of actually taking some test exposures, it became clear that it is a necessary technique. Even after a careful cleaning, my camera is showing dust specks and moisture blooms that need to be removed from the images. And, there is a noticeable gradient of brightness from the centre out to the edges that needs to be balanced. So, flat frames it is. I needed a way to photograph a flat, very uniformly-lit neutral surface. While I read of several photographers who have successfully used the twilight sky as a flat frame, most produce their flats using some kind of a “light box” — an artificial target that fits over the telescope and contains a uniformly-lit surface.
I read numerous online articles from others who have built light boxes. The articles that appealed to me the most were
From these, I made a basic plan of what I planned to build.
The idea was to build an oblong box that would fit over the dust shield on my SV80S OTA. At the other end would be a light source and, inside the box, some kind of diffuser surfaces to spread out the light to an even field.
What followed was what I’d call “seat of the pants design” — this was not thought out carefully in advance. Rather, it was a series of little steps that combined what materials and tools I could readily find with experiments to see what seemed to work. I think it worked out fairly well, and I offer this photo-journal of the construction to encourage others to try this simple and useful project.
I had in mind an oblong box, width sufficient to slip over the dew shield on my SV80s, and long enough to cover the shield sufficiently for stability, and with enough interior space for diffusers for the light source.
At the left are two panels with holes large enough for the OTA. The two panels are to provide support — I’d place them far enough apart that they would hold the box without sagging. Then I planned a third panel with a slightly smaller hole; the OTA could “butt up” against this panel to ensure that the box doesn’t slide down too far. At the far end of the box would be a light source of some kind, and between the light source and the OTA, a couple of diffuser panels of some kind. I thought that adjusting the position of the two diffuser panels would allow me to adjust the light to a suitable flat field.
I decided to use 5mm (3/16″) foam board for construction, because it is stiff but very light, and because a local craft store has a ready supply.
I used two 600mm by 914mm (2-by-3-foot) sheets, costing about $6.
The outside diameter of the SV80S dew shield measures 111mm (4-3/8″), so I decided to use
- 178mm (7″) square panels as the basic construction unit. This allows a good amount of room for holes, including diffusers somewhat larger than the OTA diameter.
- 113mm (4-7/16″) holes to pass over the OTA;
- 104mm (4-1/8″) hole in the depth-stop panel — larger than the glass diameter, but smaller than the physical diameter, of the OTA.
A third board has a slightly smaller circle that doesn’t fit over the OTA, and will act as a depth stop.
I found these nice little Sylvania LED lights at Canadian Tire. You push the lens to light the 3 bright white LEDs.
Opening the back shows there is no room at all to sneak a little switch in. (Those are 3 AAA batteries.)
Remember that two of the panels must be wider than the other two, by twice the thickness of the foam board.
OTA view of the illuminated diffuser.
Note: The light looks very blue here, but it isn’t — that’s just poor white balance in these low-light photos taken without flash to see the illumination. It also doesn’t matter – the objective is to have a uniformly-illuminated surface – the colour of the surface doesn’t matter.
Total cost was about $30.
How well does it work? Here’s an image of M13 without and with flats produced by this box.
(And, to be fair, some additional processing of the “with flats” version, since I lost the image that should go here and have to use a processed one).