Dithering

[P Holdsworth]

Hi all, it says no such thing as a silly question.......well this could be the one !

I'm going to try dithering on my next target. I've found the settings in ASI air so good to go but was wondering if you just dither the light frames or should you dither the dark frames also ?

Thanks Paul

[TerryMcK]

Just the light frames Paul

[P Holdsworth]

Thank you Terry

[AstronomyUkraine]

Posted by: @p-holdsworth

I'm going to try dithering on my next target. I've found the settings in ASI air so good to go but was wondering if you just dither the light frames or should you dither the dark frames also ?

Depending on the length of your exposures, you should not need to dither every frame. My preferences are dithering every frame for 5 minute exposures, dithering every 2 or 3 frames for exposures of 30s upwards, or you will spend half the night waiting for the mount to settle.

[P Holdsworth]

@astronomyukraine

Thanks for the advice.....I tend to do mainly 3 min exposures so will try dithering every 2. Hopefully it helps with dslr noise. 

[MarkAR]

Never thought about dithering darks, when you think about it, it won't do anything!

I dither every frame, only takes a second with Ekos.

[Gina]

I've never actually trued dithering.  I know many people use it,

[DaveS]

Dither your Lights but don't dither your calibration frames. TBH I'm not sure what difference it would make since you're not recording a star background.

[Carastro]

I stopped dithering some years ago when I got a CCD camera as the capture Software I use = Artemis - doesn't support dithering and I love Artemis too much to change. 

So I tend to do a sort of manual dither, in that either I do a meridian Flip, or a multi night image, or otherwise stop the imaging part way through and let it drift, or move it a few pixels once in a while.

It seems to work for me.

Carole 

[Padraic M]

Dithering darks would be a total waste of time when you think about it - you can take your camera off the scope entirely and cover it with a lens cap to take your darks. It can't see a thing anyway!

Very interested to hear other people's advice on dithering lights. I'm dithering (in all the meanings of the word!) but haven't really thought about the consequences. 5-min subs mostly so I should probably continue to dither.

[P Holdsworth]

@markar

Hi Mark, yep when I thought it through it wasn't the brightest question to ask but thanks for your help. I'm going to stick with the ASI air for the time being as I'm fairly comfortable with it now. I've also used APT which seems quite good and again I'm okay with that software and it also has dither settings. If the dither takes too long with the asi air I'll have a look at EKOS. 

[P Holdsworth]

@carastro

Thanks Carole, I'm going to stick with the ASI air and APT for the time being. Both support dithering so will give it a go with the software I've got used to so far. There seems to be more capture software out there than I realised so may well have a look at more advanced software in the future. Thanks again for your help

[P Holdsworth]

@gina

Hi Gina, I think given I'm using a dslr  it's worth a try. Many have suggested it so hopefully it works on my set up. Fingers crossed !

[don4l]

I ticked the "dither" box in my CCDCiel for the first time ever just last week after seeing the dreaded "walking noise" in an evening's imaging.

I may have seen this noise once before over the last three years.

I've been wondering why it suddenly became an issue for just this one image and I think that there are two reasons.

The first reason is that I had three hours on the same target without any problems at all.  So the mount just kept tracking in exactly the same position.

The second reason is that I was capturing OIII data on a weak target, so the signal to noise ratio was very low to start with

I suppose that I am (in a long winded way) saying that you shouldn't worry too much unless you can see that you have the issue.

[Astroarg]

I dither, and if you use PHD2 don't panic at the spikes you suddenly get when it sets to dither.  As I use NINA to capture, I have it dithering in PHD2 every other image for 5m subs (It was set to every 6, but someone told me that was too long between dithers) and now I'm playing with 10m subs I'm dithering every sub.  Probably lose 5m in every hour of imaging just for dithers this way.

 

When you stack, you'll need to crop a small amount to remove the artefacts left by dithering. 

[PeterBolt]

Has anyone tried the "spiral" dither pattern in PHD2? I read that it might help mounts with high Dec backlash?

[Astroarg]

@petedevon yes, I used that when I had issues with my CEM25P, now I've tightened the belt I've gone back to Random.

I don't really know what difference there is in terms of final image

[PeterBolt]

@Astroarg thanks for the reply, I think I will give it a try next time out

[Stub Mandrel]

Posted by: @don4l

I may have seen this noise once before over the last three years.

I'm pretty sure I only see 'walking noise' when thin, uneven cloud has been drifting across the field of view.

[TerryMcK]

I dither with the defaults in PHD2, which I believe from memory is 2 pixels, or when using the inbuilt guiding is EKOS which I also have set to 2 pixels. It ensures that with cameras, especially CMOS, any hot/cold pixels, fixed pattern noise or cosmic rays are minimised.

When the frames are viewed in Blink it is quite mesmerising to see the frames wandering about randomly. So when integrated during stacking you only lose a few pixels from each edge.