how can I get dark , flat and bias frames ???

Darawan · Jun 16, 2014

how I can get dark , flat , and bias frames ???? please explain it very well

??????????????????????????
these is second time but still i cant understand , i will take a shot of milky way , so i wanna bring out much possible details of milky way

for that reason i wanna sub frames , my camera is cropped sensor

canon 60D

.

thank you

tirediron · Jun 16, 2014

I'm sorry but I'm not sure what you're asking. What do you mean by 'bias' frames, and could you post a link to an example image so that we can better understand?

sscarmack · Jun 16, 2014

kcs_8e39d17e.png

Darawan · Jun 16, 2014

Subs & Frames

TCampbell · Jun 16, 2014

Darawan said:
how I can get dark , flat , and bias frames ???? please explain it very well ??????????????????????????
these is second time but still i cant understand , i will take a shot of milky way , so i wanna bring out much possible details of milky way for that reason i wanna sub frames , my camera is cropped sensor canon 60D .

thank you

A "dark" frame is a frame taken at the same ISO setting, shutter duration, and physical camera sensor temperature ... but with the lens cap on. This allows the camera to accumulate noise, but no image data. It aids the image integration software in being able to determine what the signal to noise ratio is and reduce noise levels in the image.

A "flat" frame is a frame taken of a plain illuminated surface which (and this is the critical part) MUST BE EVENLY LIT (I cannot stress that part enough. If you cannot guarantee perfection, then don't even bother taking it because it'll be do more harm than good.) We've had people attempt to use light bulbs to illuminate a sheet of cardboard, but that won't work. The inverse square law saw the part of the paper which is closer to the light bulb will be brighter than the part which is slightly farther away. For small-ish scopes, I've seen people set their iPad to a white screen, hold it in front of the telescope and take a photo of it (and that actually does work extremely well). There's also a white t-shirt method in which you stretch a white (new and clean) t-shirt over the front of the telescope, wrap a bungee cord around to hold it tight (like the head of a drum) and point the telescope (during the daytime) to the part of the sky which is farthest from the sun. (e.g. if it's afternoon and the sun is in the west, then point the scope to the east). The point of the flat frame is that while your normal images may look evenly illuminated, all the "data" tends to be in the dark end of the histogram. You end up "stretching" the data to bring out details. As you "stretch" the data, you end up "stretching" the background illumination. Suddenly the center of the image (which seemed to be the same brightness as the center) is very obviously brighter. The "flat" image can be used to allow the computer to detect the subtle vignetting which is present, and subtract it so that you end up with evenly illuminated background sky.

A "bias" frame is a zero-time exposure. High end imaging CCDs actually have a feature to generate a "bias" frame, but DSLRs do not. To create one with a DSLR, just set the camera a fast shutter speed (e.g. 1/1000ths sec is fast enough) but leave the lens cap on. Ideally the camera should be well-rested before taking these (so that the sensor isn't hot from continued use). The sensor needs electricity to operate. The charge which is applied for no other reason than to power it up ends up being part of the data when the sensor data is read-out from the chip. So the goal of the "bias" frame is to find out how much data is present simply to make the chip work at all.

With this... you can think of the data on the chip in three parts... (1) the bias ... the power level on the chip simply to make it work at all; (2) the "noise" present due to ISO, time duration, and temperature; (3) actual useful signal information (the image you want to take).

Of course what you really want is #3 and you want to figure out how to get rid of #1 and #2. By taking darks, bias, and flats, you can improve the image integration software's ability to do that.