Dark substraction issue with narrowband filters (OSC camera)

A place to report problems and bugs in SharpCap
Forum rules


If you have a problem or question, please check the FAQ to see if it already has an answer : https://www.sharpcap.co.uk/sharpcap-faqs

Please also read about Troubleshooting USB Issues before posting.

*** Please do not post license keys - please report any problems with licensing to 'admin' by private message ***

Please include the following details in any bug report:

* Version of SharpCap
* Camera and other hardware being user
* Operating system version
* Contents of the SharpCap log after the problem has occurred.
[If SharpCap crashes, please send the bug report when prompted instead of including the log]
Post Reply
Steph
Posts: 2
Joined: Sun Jan 19, 2020 4:38 pm

Dark substraction issue with narrowband filters (OSC camera)

#1

Post by Steph »

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
bug_sc.jpg
bug_sc.jpg (350.69 KiB) Viewed 1489 times
Regards.
User avatar
admin
Site Admin
Posts: 13330
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Dark substraction issue with narrowband filters (OSC camera)

#2

Post by admin »

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
Steph
Posts: 2
Joined: Sun Jan 19, 2020 4:38 pm

Re: Dark substraction issue with narrowband filters (OSC camera)

#3

Post by Steph »

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.
User avatar
admin
Site Admin
Posts: 13330
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Dark substraction issue with narrowband filters (OSC camera)

#4

Post by admin »

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
joeastro
Posts: 10
Joined: Thu Feb 13, 2020 7:09 am

Re: Dark substraction issue with narrowband filters (OSC camera)

#5

Post by joeastro »

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
Post Reply