[Aavso-photometry] Taking flats

[arne]

gianlucaros at gmail.com wrote:
> I wonder whether taking flats from a white screen pc is suitable to
> take flats. I have a 17" TFT portable pc and I display a white pdf for
> taking flats. I measured the brightness of the screen with the
> exposure meter of a film camera and it seems even. Does anyone use
> such a method for taking flats? I put the screen very close to the
> telescope mouth.What if the screen does not cover the telescope mouth
> precisely?

It needs to cover the telescope mouth, no gaps.  To check
for evenness, take a series of flats with the screen in one
orientation, then rotate it 90 degrees and take another set.
Ratio the two sets.

The TFT screens typically use a cold cathode fluorescent lamp for
illumination, which has spikes throughout its spectral
response.  It is "ok" for V-band calibration, since this
is near the peak of the CCD response and there is little
difference between pixel responses across the V bandpass;
almost any illumination source at V will give acceptable results
for most programs.  You should use a different light source
if you are using other filters, such as B or Ic, where there
are differences between pixel responses and a smooth wavelength
source is needed.  If you use a LCD display, keep it perpendicular
to the telescope tube and probably consider some sort of diffuser
since these screens tend to be glossy.

The important thing to remember on all illumination sources is
that they tend to look good to the human eye, but the CCD has
a much larger wavelength range, and the light sources tend to
be much poorer over that range.  So if it looks white to the eye,
it is probably ok at V, but don't trust it elsewhere without tests.
Arne