How To: Best Way to Make Flats
What when why?
Flat frames are evenly lit solid grey images designed to show anomalies in your image train; sensor, filters, and lenses. These anomalies can be dust, smudges or vignetting. In a perfect world when shooting a solid uniform source your image would show that same solid uniform light across the entire frame. Flat frames show the deviation your current equipment has from perfect.
Flat frames must be taken after shooting and before making any changes to your equipment other than; cover the telescope with a diffuser, point it at a light source, and change the exposure speed such that the histogram is 1/3 to 1/2 from the left side. Temperature doesn't matter so you can do these inside or during the day, just don't change anything other than speed from when you were shooting.
The current recommendation for shooting flat frames is to cover your telescope with a t-shirt (for a diffuser) held with a rubber band and point it at your iPad. The problem is that I don't own an iPad; just kidding, that is never a problem. The problem is that we're suppose to be shooting a solid uniform source and none of that produces it.
Hold a t-shirt up to the light and look through it. You see the pattern. Did you know t-shirt are made with various types of fabric. The pattern can be different type and size and more or less visible with different fabrics.
Wrapping a t-shirt around a cylinder and holding it in place with a rubber band? Try it, repeatedly, and see if you get reproducible results. The edges are going to bunch up and there will be wrinkles throughout.
The evenly illuminated light source. Ok, the iPad actually does work good for this, if you have one lying around, and it's not too old. Most people use the sun but clouds tend to make that uneven and unreliable. Some people use laptop screens, don't do that, those are never evenly illuminated.
I saw a recent video from Peter Zelinka where he had a piece of a t-shirt held in a ring. Brilliant. We just need something more uniform than a t-shirt.
Embroidery Rings. This is the way to hold a stretched piece of fabric smoothly in place for use as a circular diffuser cover. A set of them is only $7.
Brushed Microfiber. You know those cloths you use to clean glass and LCD panels with. I went on Amazon looking for top quality fine linens and found pillow cases with large amounts of the fabric I needed for less than $20. Use a couple layers at 90 degrees to each other to offset whatever pattern there still may be.
EL Panel. Expensive but perfect. What I use.
LED Photography Light. Moderately priced. Popular choice with good results. Probably a better value than the EL Panel.
Tracing Light Panel. Cheap. People use them. But you know with cheap comes lower quality control so you take your chances with this choice.