Dither

[brrttpaul]

Just wondering if say I was using a DSLR on my scope , (using Backyard nikon) it has a dither button, if i had PHD is that all I need to dither along with BYN? or does it need a guide cam?

[PeterCPC]

You still need to guide cam for long exposure imaging but BYN with PHD will dither. That's what I do with BYE.

Peter

[brrttpaul]

it wouldnt be for long exposures (max 3mins) probably only one min . its just to get rid of the mottle effect  really . I will go into guiding later on . thanks for the quick reply

[Davey-T]

You can dither manually, just need to move about two star diameters with a DSLR.

Dave

[brrttpaul]

Im hoping its a case of connect BYN run  PHD connect camera? then in BYN just press the dither button. never used phd before so fingers crossed

[brrttpaul]

in phd it asks what camera im using but it dosnt have nikon in the drop down list

[Davey-T]

Just now, brrttpaul said:

in phd it asks what camera im using but it dosnt have nikon in the drop down list

It means what guide camera.

Dave

[brrttpaul]

ok maybe i misread the first reply, do i need a guide cam if its yes then itll have to go on the back burner

[Davey-T]

Don't see how it will work in PHD without a guide camera , don't know if APT can dither using the downloaded images, as I said it's not a problem just to do it manually.

Dave

[brrttpaul]

how do you do it manually?do you just press the slew button a star width?

[brrttpaul]

Ok maybe a stupid question here (if you dont know you dont know),  I have decided to get a 2nd hand synguider it seems to be the best option to get into guiding, i realise this is a do it all bit of kit for guiding but if I wanted to dither at a later date I would still have to get another camera wouldnt I?  ( im just thinking that the camera wouldnt be in the dropdown menu in PHD)

[PeterCPC]

Why do you think that's the best option for guiding?

Peter

[wuthton]

I can see the attraction if you want an ultra portable rig with no laptop involved beyond that I'd pass personally. 

[Physicist13]

the main thing is to know why you are dithering! Its a noise reduction technique but at the expense of resolution.

P

 

[wuthton]

16 minutes ago, Physicist13 said:

the main thing is to know why you are dithering! Its a noise reduction technique but at the expense of resolution.

P

 

You've got me intrigued, how would dithering effect resolution? 

[cfpendock]

30 minutes ago, wuthton said:

You've got me intrigued, how would dithering effect resolution? 

When you stack many dithered subs you can drizzle and this can significantly boost the resolution of the final image.  The technique was invented for the Hubble Space Telescope. This article explains it: http://www.astrosurf.com/buil/us/spe9/lrgb22.htm

Chris

[wuthton]

Just now, cfpendock said:

When you stack many dithered subs you can drizzle and this can significantly boost the resolution of the final image.  The technique was invented for the Hubble Space Telescope. This article explains it: http://www.astrosurf.com/buil/us/spe9/lrgb22.htm

Chris

Yes, but how would dithered subs lose resolution? I can't see how it would have any effect at all.

[PeterCPC]

There is another thread here

Peter

[Physicist13]

apologies!!! my mind had read "binning" for some reason!!  In fact dithering can improve resolution in undersampled images although it can be tricky to implement as often sub-pixel dithering is needed. The more common several pixel dithering is primarily a means of averaging out artifacts (eg hot pixels ).  Drizzle is a form of dithering. 

I need more coffee!!

 

[wuthton]

2 hours ago, Physicist13 said:

apologies!!! my mind had read "binning" for some reason!!  In fact dithering can improve resolution in undersampled images although it can be tricky to implement as often sub-pixel dithering is needed. The more common several pixel dithering is primarily a means of averaging out artifacts (eg hot pixels ).  Drizzle is a form of dithering. 

I need more coffee!!

 

I think you still need some more coffee , I think you've got capture and stacking techniques a little intertwined.

Dither - is purely a method of capture where you just move the telescope a little bit between each sub so the signal areas occupy a different pixel space on the chip. This makes the noise reduction of the standard stacking process much more effective.

Drizzle - is a method of stacking where each pixel of the sub frame is "drizzled" over four or nine pixels to create an image of greater resolution at the cost introducing a little noise, it's most effective when the subs have been dithered. Drizzle is particularly useful for large pixel CCDs and particularly those with a small chip like my Atik 314, it turns my 1 mega pixel subs into a 4 or 9 mega pixel final image. Drizzle is not generally used with DSLRs as who needs a 64 mega pixel image that carries more noise.

[brrttpaul]

Thx for the many replies, the reason I wanted to dither was to lose the noise mainly ( as I understand it a red pixel goes on top of a blue pixel and cancels the red out type of thing) but to dither I would need a guide scope and another camera ( someone did say do it manually but if you dont know how to do that then again you lost. So in looking at cameras (again you need to know what you looking for really which I dont) I stumbled upon the synguider and wandered into that. As it happens there is a 2nd hand one for sale which I have been in contact with the guy and I can have it for £140. It was only later thinking of it I thought hang on how will I dither if its an all in one.  Ideally heres what I am hoping to do. Set my telescope on top of my pier mount, point at object, take photos, if I can manage longer subs without it being washed out great, otherwise revert back to 1 min subs but lots of them (also invest in a LP filter). If I was to scrap the synguider, what sort of guidescope/camera would I need, would I need to be using the EQMOD with it (at the moment i just get no joy with eqmod). I have been looking at a few clubs in my area and thinking of joining just to pick their brains and put me on the right path, as usual finding the time is the problem

[Davey-T]

Manual dithering is pretty simple, just set the slew speed on the hand controller to something that moves the mount a detectable amount then go north east south west and repeat, letting it settle before taking an image.

Compare downloaded subs and try to move about two normal star widths.

Worth a try as it costs nothing

No need for EQmod for guiding , just PHD and a guide camera.

Dave

[brrttpaul]

just occured to me that i could use the CCd that I got couldnt I ? its a starlight osc , just to see how things would run and i could alw3ays buy a proper guide cam at a later date.  what sort of guide scope would be suitable with an ED80

[wuthton]

13 minutes ago, brrttpaul said:

just occured to me that i could use the CCd that I got couldnt I ? its a starlight osc , just to see how things would run and i could alw3ays buy a proper guide cam at a later date.  what sort of guide scope would be suitable with an ED80

Yes you could but, but, but.... A cooled CCD!! it would be a crime against astrophotography. 

What exactly have you got against the CCD? For everyone I know the results you yield with a cooled CCD are much superior to those from a DSLR (FOV excluded). 

[brrttpaul]

Ihavnt got nothing against CCD I suppose frustration more than anything with the lack of clear skies etc, you learn something then by the time the next clear sky comes along you forgot it all, its got to be fun above all. With my DSLR I can see on the laptop exactly what is in front of me instantly, also alignment is a lot lot easier, with the ccd its l all squares so have to click on a few things , im sure i will get into it the more clear skies i get but at the moment just want to take a picture if that makes sense