|
Peter,
I find I get good flats doing either of these methods:
1. A whitish cloth cover over the end of my telescope. In my case my wife made me a whitish elasticised cover to cover the ends of both my CDK and RHA scopes. I use these when doing flats to diffuse the light and prevent stars showing up.
2. I take flats at dusk and take 6 of each filter and binning I am going to use.
I take flat darks meaning a dark same exposure lengths as the flat. So I try to use the same exposure time for each to keep that simple. 4 seconds is what is working for me for all filters and narrowband. I usually expose to about 22,000 to 29,000 ADU. Too bright and they can overcorrect and the vignetted areas go lighter than the rest of the image. Too dim and the vignetting is not entirely removed.
3. If I don't take dusk flats I take them the next day with my rolloff roof closed and the white cover on and the scope pointing to a neutrally lit part of the interior wall. At my dark site that is the grey underside of colorbond sheeting and at my home observatory that is flat black painted insulation.
4. When creating a master flat I opt with CCDstack to not subtract a bias at that point. I subtract a bias or a flat dark at the time of applying the flat.
I have found a slight mismatch with the bias or flat dark on some scopes can wreck the flat process.
I set my Proline to fast downloads as time is of the essence in falling light at dusk and you have to be fast and organised. I name the file set up the save as box and binning and next filter in advance whilst the current flat is being taken to save time when doing dusk flats.
Both of these work. I also subtract a master bias from my darks and use exact same exposure length and temperature darks usually about 16 of them to form a master.
The CDK has been touchy with flats in the past less so now.
I have never used the flat illumination panels or light boxes. But many do and are happy with them. I think you'd have to make sure they were very even in illumination and quite broadband in their light as CCDs have different sensitivities to different wavelengths.
Greg.
|