Flat frames are a bit finicky in that they are to be taken at the same focus position and camera orientation as the Image Frames that were taken. This means that in the Field, if you are going to shoot a second target, you should take your Flats right after shooting the 1st target, as you’re likely to change the camera orientation or even change the camera between the 1st and 2nd objects. You don’t really have the option to wait for dawn and twilight to shoot your flats unless you’re very good at indexing the camera and focus positions.
Also, at a star party you’ll upset a lot of folks if you light up your area with a white light source, so a Light Box or similar that fits neatly over your scope is what’s needed. I’ve built a couple that I use regularly. One is flat(ish) and uses an EL panel as the light source, the other is a box and uses LEDs. I’ve had good success with both. I’ll describe both and include pictures of the construction. Neither of these are perfect or works of art, but they do work for me. They were inexpensive and can be built with simple tools and common materials, although some scrounging around may be involved to find some lightly tinted paper or plastic.
The first is an Electroluminescent (EL) Panel sandwiched between pieces of foam board. EL panels along with the required inverter can be purchased online from places like Glowhut.com (just Google EL panel). Color may be an issue with these panels. Although the piece I purchased was supposed to be white (pink when turned off), it was in fact a blueish green color and substantially red deficient. As I mention in my post about taking flats, it’s best if the color is pretty well balanced for a couple reasons: 1) makes for equal flat exposure times if shooting filters and 2) a deficiency in the color balance can upset the results if using an OSC camera. So I had to find a way to correct the red deficiency in my panel.
I this case I tried various tints of paper and plastic sheet to lay over the EL panel, and after several tries, finally found a plastic sheet that gave me a decent color balance. Hobby stores are a good source for materials. You also might find some scrape plastic from a sign making business. Milk White plastic is great material if you can find it.
So for my case I simply built a frame using white foam board to hold the EL panel and covering plastic sheet. Then I used half a 4” plastic pipe coupling, which just happened to nicely fit over the dew shield of my 80mm scope, attached it to a another scrap of foam board and clipped to the EL panel frame. Works like a charm. Here’s couple pictures:
The next box is one I built for my 8” SCT. Again, this box is built from white foam board glued together with a hot glue gun. The light source is bright white LEDs which again a little red deficient, so some red LEDs (standard intensity) were added. Getting the light balanced right takes some experimentation. You can trim the resistors in the LED circuit to tweak the color balance. Note there are several ways to wire it. You can find several examples of how to hook up LEDs online so I won’t repeat it here. I chose a circuit and values so I can use up to a bit over 12 volts since I always have a battery for my rig, but it will light up with as low as 5 volts.
The difficulty in the design of the box was diffusing the light evenly in the box to give a uniform brightness to the scope. You can see in the pictures the approach taken. The lights are placed in little shield tubes with an angled frosty shield to diffuse the light against the back of the box. The back of the box has odd angled white reflective surfaces to bounce the light around and some some white stuffing to avoid any dark corners.
A piece of particle board cut to fit the OD of the scope and some white plastic & paper to finish off the front. Pretty simple and works well.
Here are the pictures:
I hope this gives some ideas on how to construct your own “Flat Box”.
Comments, suggestions, or corrections welcome.