Dithering

[Carole]

I've read a lot of threads about dithering recently, both on here and other forums.  I think I've sort of got the idea what it is, but how to achieve it is another matter.

I gather this is to shift the position of the object being imaged fractionally in your field of view in order to erase/overlap hot/pixels and other blemishes, I guess in much the same way as taking an AVI would do when it is stacked in Registax. 

In a very simplistic way would turning the guiding off for a short while and then re-starting achieve this if there is slight drift without guiding?

Carole

[MarkS]

In a very simplistic way would turning the guiding off for a short while and then re-starting achieve this if there is slight drift without guiding?

Yes - that would work - your understanding is correct.

Before I discovered Nebulosity I used to shoot a few frames; switch off guiding; manually slew the mount by a few random pixels; switch guiding on at the new star position and shoot another few frames.  Very laborious but it did the trick.

Mark

[Carole]

Thanks Mark. 

How often would you say this needs to be done to be effective?

I have sometimes had to turn the guiding off anyway because a cloud came over or something, but it's only been about once an hour on average.

I guess finding some software that would do it for you would make life easier.

Carole

[mickw]

Your guiding turns itself off anyway  

I think nebulosity will do dithering  during capture

Nope it doesn't - but when you link phd to nebulosity you can pause guiding during image download - I assume that would work the same.

I have seen a capture program that does dithering but can't remember it 

[Carole]

Your guiding turns itself off anyway

Actually it does continue guiding now unless there is a cloud or something, but not all the guiding is good.

Still it's much better than it was before.  Just means I waste some of the subs which is annoying.  

Have now got myself a regulated power supply, so I am hoping that might help.  Not had a chance to use it yet.  

Carole

[mickw]

Just checked - Link Nebulosity to phd and you can dither

[Carole]

Just checked - Link Nebulosity to phd and you can dither

Interesting, will bear that in mind.

Carole

[MarkS]

Just checked - Link Nebulosity to phd and you can dither

Interesting, will bear that in mind.

That's exactly how I currently dither!

[Carole]

If I try to link Nebulosity to PHD, does this mean Nebulosity has to be my capture programme? 
I am quite happy with Canon Utility which I currently use.

Carole

[mickw]

Yes

[The Thing]

Hi Carole,
On this thread we have been discussing alternative capture programs that do or will support PHD dithering. Canon Utility is fine but it's much more convenient to be able to see your images immediately not to mention the other useful features these programs offer.

http://forum.orpington-astronomy.org.uk/index.php?topic=7166.msg45439#msg45439

Duncan

[RobertM]

On the subject of dithering, Fay did a lot of that on Saturday but I don't think she used PHD. 

[doug]

This is a new one for me.  As I`m not an imager, I always thought that dithering was to be uncertain as to something, or to be hesitant. I didn`t know it was an astronomical term; is it a new one??   

Doug.   

[Carole]

Hi Duncan, I think that thread was one of the ones I had been referring to.  

it's much more convenient to be able to see your images immediately

I can see my images immediately with Canon Utility, as soon as the 5mins sub (or whatever) has finished they download in a couple fo secs and I can see them straight away.

I think I need to ponder on this.

or to be hesitant

Doug, I think in effect Dithering in imaging does mean a slight hesitancy (or rather pause and wait) in guiding.  Probably where it got its name.  

On the subject of dithering, Fay did a lot of that on Saturday but I don't think she used PHD

Carole

[MarkS]

Doug, I think in effect Dithering in imaging does mean a slight hesitancy (or rather pause and wait) in guiding.  Probably where it got its name.  

A pause and wait in guiding would have no effect if your mount had perfect tracking. Instead, a dither is a deliberate slight movement of the star field across the image frame between subs.

But where does the word come from?

According to: http://en.wikipedia.org/wiki/Dither
"Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as "banding" in images."

I think the term was first used in printing - where you wish to create a wider grey scale or a greater number of colours from a limited palette.  Its usage in astro-photography is slightly different - it is something that happens at the acquisition phase.

In astro-photography, dithering can prevent the "square star" look in images or banding across images.

Mark