SharpCap Forums

Hi,
I've dark substraction issue with my narrowband filter IDAS NB1 (same issue with IDAS lps d2, stc dual narrowband or optlolong l-enhanced)

I've tried differents settings and master dark modified...no way.

Not really a bug, because without filter, dark substraction work perfectly.

Here an example Regards.

Hi,

those trails look like hot (or warm) pixels in the dark not quite matching those in the image frames and therefore leaving darker results which train as the mount drifts slightly.

The most common cause of this is sensor temperature being different between the darks and light frames (although other parameter changes can have the same effect - offset, gain,etc).

Dithering can help reduce this as the dithering movement spreads the problem signal around in a less noticeable way.

Robin

Hi Robin,

Using filters changes the light frames hot pixel values and cannot be correctly substract by the dark frame.

I've modified my masterdark by selecting pixel value ranges and reaffect manually new values. That's better with my 300 newtonian, not perfect, but totaly impossible with my apo.

Unfortunately I can't do dithrering with Sharpcap, because I use EQMOD pulseguiding mode.

Thank you for your reply, and thank for your great software.

Hi,

When you are using dark subtraction in SharpCap, hot pixels are actually dealt with in a moderately sophisticated way. SharpCap will look at the statistics of the dark frame and calculate a hot pixel level that is (I think) six standard deviations above the mean of the dark frame pixel value. Any pixels above this value are flagged as hot. When a pixel is flagged as hot, SharpCap will take a value from a neighbouring pixel (of the same colour for colour cameras) rather than use the potentially incorrect information from the hot pixel.

Under normal circumstances, the approach above should deal well with hot pixels and with variations between the dark and light frames, but maybe it isn't your case.

Cheers, Robin

hi
open the master flat dark.
the black leves or noisey white levels might be too low in the master.fits

i have a problem with image frame size.

the cmos chip is say 5000x3000 pixels bin 1x1
so if you take a dark then switch to 2x2 or other bin
the dark will be rejected when you try to use it.

is the exposure length in the master dark .fits
the same or very close to the image expsoure...

try taking the same imnage expsure length for a new mastredark.fits
with the correctt binning and the correct cmos/ccd chip frame size??

good luck
joe