RE: RE: Re: Is this RBI and how does it affect your imaging strategy
one reason i thought i needed to preflash the bias frames is i do use them to scale darks AND i use them to "dark subtract" the flats. fact is, the non preflash bias frames look identical to the pre-flashed bias and same for the flats. (your comment made me examine them more carefully - than just my assumption -- we know what assumptions do)
because of image shift from dithering and data shift from filter changes i have decided for cameras that matter, using the RBI pre-flood can only help. but i do need to get some time to create an image (same object) with and without pre-flash to see if it is actually noticeable in "pretty pictures"
--- In firstname.lastname@example.org, <email@example.com> wrote:
Lee, you're welcome. Preflashing flats does add a lot of time to your work flow. Also, if you are not scaling your darks, and making darks that are the same time exposure as the lights you are calibrating, you don't need to apply bias frames. Adam block covers this in his teaching series on using CCDStack.
- Forgive my ignorance, what is a “flash dark?”Gregory
Here is a corrected and more insightful method:
For a given exp time:
1) take 2 virgin darks. Subtract one from the other and measure STD. virgin_noise = STD*gain/sqrt(2);
2) take 2 flashed darks (flash each dark). Subtract one from the other and measure STD. flashed_noise = STD*gain/sqrt(2);
noise from flashing = sqrt(flashed_noise^2 - virgin_noise^2)
A "flashed dark" is an otherwise normal dark frame taken immediately after flashing.
"Flashing" floods the CCD with infrared light just prior to the exposure, in order to saturates the substrate. The CCD substrate can retain electrons; i.e. the pixel well is not completely emptied by readout. Those retained electrons bleed out during the next exp(s) to produce a "ghost image" from the prior exp. Flashing destroys ghost images by making the entire CCD a uniform ghost.
If the camera does not have a built-in pre-flash device then another method is to avoid super-cold temps (colder CCDs retain more electrons in the substrate) or warm and re-cool the CCD (esp prior to taking darks).
But don't worry about this unless you have a an obvious problem. Usually ghosts are not a problem unless Vega or other very bright star is the field.
--- In firstname.lastname@example.org, <fyrframe@...> wrote:Forgive my ignorance, what is a “flash dark?”Gregory