Flat Field Light Box Notes

One of the drawbacks to taking ccd images from under a Mag 3.5 sky, like I have at my home in Houston, TX, is that the sky glow demands that you have to do flat fields to kill the dust doughnuts, vignetting in the corners,, etc., even when shooting deep sky objects. Under a dark sky, flat fields do not seem to be nearly as critical for deep sky objects. When shooting color images, each filtered image needs its OWN flat since the optical path is changed with the extra glass in the path. That adds up to a lot of flats! In addition, the flats need to be taken wish LDC OFF, and will also need to have their own master dark frame with LDC off. The problem I had was trying to get sky flats, or evenly illuminated screens when I would not be able to be set up before dusk, or did not stay up till dawn, and also having to do this in the presence of a lot of stray light making remote screens not evenly illuminated . To this end, I decided to make a simple light box that I could attach to the front of the scope and do flats at my leisure.

The concept is simple: use low level indirect lighting to evenly illuminate a frosted screen. Block out all sources of "stray" light and build it as lightweight and inexpensive as possible.

I used foam board for the rings, baffles, and back end, and posterboard for the walls. Frosted drafting mylar provided the screen, a 7.5 watt bulb for dim illumination (with an adjustable dimmer switch to cut it down even further), and then thin plastic over the outside to protect things from dew. A hot glue gun was used to assemble all the parts.

This shows the "Parts" ready to

be assembled. The light baffle has

already been mounted to the rear

panel with its PVC standoffs

The two following photos show the construction. The opening end is deep enough to not allow any of the screen to be seen directly into the focuser by off axis light (my secondary cage is very short and I normally use an extension during imaging to keep stray light from entering the focuser). Temporary "longerons" hold the rings and back plate in position while the outer skin is "rolled" onto them, using the hot glue gun to attach the skins. The light socket is from an old lighting kit for a ceiling fan. The light bulb side of the baffle is covered with aluminum tape to both increase its reflectivity and prevent light "hot spots" from being seen thru the frosted screen. All light to the screen must bounce around the edges of the baffle to get to the screen. The baffle is mounted with three short sections of PVC tubing (hot glue used here too).

The two "rings" and rear panel

are held in place with temporary

longerons as the "skin" is rolled

and hot glued to them

Almost done with attaching the "skin"

The configuration of the lightbox when in use is quite simple. It clips over the front of the secondary cage with two aluminum "hooks" as shown in the photo below that grab the front ring of the 14" scope’s secondary cage for attaching it to that scope. I simply aim my 5" Maksutov into the open end of the light box for flats with that instrument, and do the same when using various 35mm camera lenses (which really seem to benefit from flat fielding). Note the mylar screen is NOT at the edge of the lightbox, it is recessed a bit so any stray illumination from the screen cannot make it into the focuser on the 14" scope, and it allows me to insert the objective of the smaller instruments into the front of the lightbox to avoid stray light for them.

The configuration looks like this when mounted on the front of the secondary cage of the 14" scope. Note the shiny outer covering of thin plastic to keep the dew from damaging the white poster board that forms the outer cover of the lightbox.

The 7.5 watt white light bulb that is used to provide illumination is far too bright, so a dimmer switch is used. I initially had used a 7.5 watt red bulb, which worked fine for monochrome images and red filtered images (it was still too bright forthem however and needed to be partially taped up to help dim it), but for taking flats for blue or green images the exposures were far too long (ESPECIALLY for blue or cyan). So, I went to a white bulb, and added the dimmer switch to be able to easily adjust exposure durations. The socket for the bulb is a light fixture from a ceiling fan:

To attach the light fixture, I used two long drywall screws that go thru the holes in the light fixture. Since the foam board is not very strong, I hot glued a couple of thin pieces of wood to the foam board for the screws to bite into:

The proof is in the images. I took a series of images with the light box at different rotations and also slightly offset the lightbox in case there was a hot spot in the center that would look the same for the different rotations Remember, for a cookbook camera you need to have LDC turned OFF for flat fields, and the darks that are used with them also have to have LDC off. I then "subtracted" different images from each other after having dark subtracted each of them. The result would show any differences in the light pattern that would make the flat field non-uniform. The results were excellent. The patterns that showed up varied only by 2-5 PV between images that were at a PV of about 2000. (The patterns were VERY difficult to see without first stretching the resulting images to be able to see where to measure!!)

Total cost was about $12 for the materials (plus the dimmer switch), and it took about 3 hours to layout, cut, and glue the lightbox together. I store it in a cardboard box with the frosted screen aimed down (to keep dust/contamination off the screen).

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