Dither and/or darks?

[rubecula]

Are dithering and using darks counterproductive?  So far all my CCD imaging has been based on 600s image with a small dither every alternate frame and calibrated with Darks, Flats and Bias frames.  I've just been getting some 1200s darks and wondered if you dither then the small movement means that the noise in the light won't match up with the noise in the dark.  In which case is it worth using dither and darks?  I'm imaging at a focal lenght of 1400mm.  Recent calibration has been done in APP.

If you only did one which would be the best?

Thanks

[tooth_dr]

Hi Robin

The only thing that moves when you dither is the position of where the signal ie your target DSO, hits the chip.

Doing both dithering and darks isn’t counterproductive.  There is no need to choose one or the other since taking darks with your 383L can be done any time and used later.

Some folks don’t use darks but with your camera, I would suggest continuing to use darks.

[DaveS]

Dithering and Darks are both useful, dithering plus Sigma stacking will get rid of most satellite trails. I'm not familiar with the 383, as my CCD is a 694 sensor which is quieter I use Bias instead of Darks, but I think with the 383 you will need Darks as well as Bias.

[michael8554]

 

7 hours ago, rubecula said:

wondered if you dither then the small movement means that the noise in the light won't match up with the noise in the dark.

Probably, which is why you either Dither, which smears the noise pattern, or use Darks.

Michael

[tooth_dr]

8 hours ago, michael8554 said:

 

Probably, which is why you either Dither, which smears the noise pattern, or use Darks.

Michael

I don’t understand how noise in the light frame moves relative to the dark frame?  The sensor is exactly the same for both. 

Dirhering moves around the light signal on the sensor between exposures but the noise signal stays in the same place. Software recognises this and removes these outliers.

My understanding - use darks to remove noise from subs prior to stacking and then the dithering, via sigma stacking, removes other/remaining noise.

[Peakedge]

11 hours ago, tooth_dr said:

I don’t understand how noise in the light frame moves relative to the dark frame?  The sensor is exactly the same for both. 

Dirhering moves around the light signal on the sensor between exposures but the noise signal stays in the same place. Software recognises this and removes these outliers.

My understanding - use darks to remove noise from subs prior to stacking and then the dithering, via sigma stacking, removes other/remaining noise.

Agreed. Pretty sure that darks are applied to each individual frame in APP rather than on the final stacked image meaning that both are valuable in the overall image. Going through the APP work flow, Calibration is the step before any stacking is carried out allowing you to check effectiveness of the calibration frames; darks, flats etc, before Normalisation and Integration. You can select this at the top of the graphic display for each individual frame in APP.

Hope this helps.

David

[rubecula]

Thanks guys, I'll continue to dither and I'm also glad I didn't waste a day capturing 1200s darks.

[tooth_dr]

32 minutes ago, rubecula said:

I'm also glad I didn't waste a day capturing 1200s darks

HI Robin

I'd say that you would be better with darks though, if you plan on taking 20 minute lights.  I run my darks in an evening when it's cloudy, it doesnt take any more than a few minutes to set up the imaging run, and obviously no time to process.  Flats are a PITA, but darks, they are easy

 

[rubecula]

Thanks Adam,  based on the above I intend to use both dithering and darks.