Am I dithering enough?

[smr]

Hi all,

This is a quick levels and bit of curve adjust on some data from last night. Just wondering if I am dithering enough. I can see that its working by virtue of being zoomed in on APT between images and seeing the pixels move around slightly. I have APT dithering at default settings. 

Or is this just general light pollution noise ?

 

 

Here it is at 50 percent crop...

What is this mottling colour a result of ? ie. type of noise? Or is it more so because as you can see I've stretched the data excessively? Will more signal improve it? I hope to get a lot more signal but on this but I didn't want to attain said data sub optimally if there was something I could do via software, as hardware wise I am limited with no cooling on my DSLR.

 

Edited August 26, 2019 by smr

[wimvb]

A rule of thumb is to dither 12 - 15 pixels when using a cmos camera. Since your camera isn't cooled, you'll need a lot of exposures to reduce any noise. Especially if you're imaging from a light polluted site. Colour mottle can be reduced in post processing, but at the cost of colour loss in the faint nebulosity. More data is always the best recipy for noise reduction.

[symmetal]

Here's a video by Tony Hallas describing  the colour mottlle effect when using DSLRs and agressive dithering is the best way to combat it. As Wim says at least 12 pixels is recommended.

Alan

[pete_l]

6 hours ago, symmetal said:

Here's a video by Tony Hallas describing  the colour mottlle effect when using DSLRs and agressive dithering is the best way to combat it. As Wim says at least 12 pixels is recommended.

In the video, Hallas suggests "a couple" of star diameters dithering between shots. (See 8:30 into the video)

He also says (for DSLRs) you don't have to shoot flats or darks - just let the software: Adobe Raw plugin for PS, do the work for you. That's the sort of advice I like 😎

Edited August 31, 2019 by pete_l

[michael8554]

13 hours ago, pete_l said:

He also says (for DSLRs) you don't have to shoot flats or darks

The plugin does some good stuff but it won't remove any dust bunnies in your imaging train - that's the other use of Flats.

Michael

[wimvb]

22 hours ago, pete_l said:

He also says (for DSLRs) you don't have to shoot flats or darks -

For dslrs, flats may be the most important of the calibration frames. You will also need bias frames to calibrate the lights and flats. There is no general recipe here that always works. You will have to experiment.

[alan potts]

16 minutes ago, wimvb said:

For dslrs, flats may be the most important of the calibration frames. You will also need bias frames to calibrate the lights and flats. There is no general recipe here that always works. You will have to experiment.

Is that was Bias are for Win, I just put them in, never have known why, when you know nothing it pays not to question too much.

Alan

[wimvb]

1 hour ago, alan potts said:

Is that was Bias are for Win, I just put them in, never have known why, when you know nothing it pays not to question too much.

Alan

Every pixel has a small offset (bias) in the electric signal when it's read. Moreover, this offset can vary between pixels, creating a so called read pattern. The signal coming from every pixel in a sensor consists of photons + dark current + offset. The first two of these are time dependent, offset isn't. For short exposures, such as flats, the second (dark current) is much lower than offset, and can often be ignored. But if you don't subtract the offset from your flats, your images won't get calibrated properly. This usually shows itself as a circular pattern similar to vignetting.

For completeness; if you let the stacking software scale dark frames (eg taken at a different exposure time than the lights), these will also need to be bias subtracted. Otherwise the scaling won't work properly.

In a perfect calibration work flow, you would provide lights, darks, flats, dark flats, and bias frames. Shortcuts are: leaving out dark flats if the flat exposure time is short, or leaving out bias frames if you don't scale darks.

In my workflow with a cmos camera that has amp glow, I use lights, darks, flats, and dark flats, but no bias, as I don't scale my darks.

Edited September 1, 2019 by wimvb