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Dithering.....How Much, How Often?

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#1 neaptide

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Posted 17 September 2019 - 08:38 PM

Sharpcap has a dithering function and I am hoping that someone can help me with the settings. I've been experiencing some " Walking Noise" and I am hoping dithering will help. Problem is I don't know what my settings should be. I'm imaging on my C9.25 @ F/10 with an ASI294. I'm taking 1 minute exposures for now which I may increase to 2 minutes if my guiding holds up. When I dither at the settings in the screenshot my guiding goes from ~.7" to .8" or .9". I don't know how often or how many pixels I should be dithering at.

 

Anyone got any advice?

 

Thanks

John

 

Screenshot (22).jpg


Edited by neaptide, 17 September 2019 - 09:53 PM.


#2 Jon Rista

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Posted 17 September 2019 - 08:58 PM

Dither no less frequently than every 3 frames. The fewer frames you stack, the more often you should be dithering. For longer exposures (over 5 minutes), you should be dithering every frame since the dithering overhead will be smaller. Dither aggressively. A couple arcseconds difference is nothing....you probably are not dithering enough. When dithering occurs, you should see fairly large spikes in your guide graph. The star should move anywhere from a couple arcseconds to 6, 8, 10, 12 arcseconds, in both axes. Dithering in a single axis is less than useless, as it can actually CAUSE correlated (walking, raining) noise!! Always dither both axes.

 

Dithers should not take a long time to settle. Anywhere from 5-10 seconds, maybe slightly longer if it was a large dither. If it is taking you 40 seconds, a minute or longer to settle, then you have something misconfigured. Settling factors are usually in pixels...so if it says something like settle at 0.25, that would be one quarter of a pixel. If your guide RMS is say, 0.7 pixels, then trying to settle at 0.25 pixels is only going to happen by luck...on the off chance...and it will take FOREVER. Settle at no less than 0.7 pixels, maybe slightly above, to ensure that settling occurs efficiently.


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#3 Stelios

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Posted 17 September 2019 - 09:01 PM

Sharpcap has a dithering function and I am hoping that someone can help me with the settings. I've been experiencing some " Walking Noise" and I am hoping dithering will help. Problem is I don't know what my settings should be. I'm image on my C9.25 @ F/10 with an ASI294. I'm taking 1 minute exposures for now which I may increase to 2 minutes if my guiding holds up. When I dither at the settings in the screenshot my guiding goes from ~.7" to .8" or .9". I don't know how often or how many pixels I should be dithering at.

 

Anyone got any advice?

 

Thanks

John

 

attachicon.gif Screenshot (22).jpg

Your guiding may go up, but that's because dithering is figured as part of your guiding statistics--but that part does *not* affect your images. You can have (with strong dither) actual RMS of 0.5" and RMS with dither of 0.8" or even higher (depending on settle time and exposure length). 

 

With exposures of only 1' (presumably you are stacking 100 or more) you can probably dither every two or three frames. With my mono camera I usually dither every three frames for LRGB (1 to 3 min exposures) and every frame for Narrowband (6-min exposures). 

 

To dither every 3 frames, you would want a dither interval of about 180 (Sharpcap will *wait* for the frame to be fully captured before starting the dither). The rest of the settings look OK. You need to monitor how long it actually takes your mount to finish waiting for the dither--if it is over 15" on average, I would lower the amount (currently 1.5pixels), if less than 8" I would raise it. 

 

Note that when imaging at F/10 you need to expose twice as long as you would at F/7 for the same SNR. Is your seeing good enough that you benefit from a 0.40"/px image scale? I doubt it for NY. If not, you may want to consider adding an F/6.3 reducer (a bit over $100 for the Celestron one).


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#4 PirateMike

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Posted 17 September 2019 - 09:18 PM

I usually take long exposures, sometime up to 20 minutes (with a 100mm scope), and I dither before every frame and allow one minute for settling. Even though the settling time I set is long it actually adds up to only 5% of my imaging time. I can shoot 20 frames at 20 minutes each before I loose a frame due to dithering.

 

John has a few comments above about dither settings that I was not aware of. So next imaging session I will try out his suggestions and see how it goes. Thanks John waytogo.gif.

 

Work on your dithering, it is the cats meow if you want smooth and dark black sky.

 

 

Miguel   8-)

 

.


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#5 Alex McConahay

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Posted 17 September 2019 - 09:31 PM

You might want to visit here:

 

 https://www.cloudyni...etween-dithers/

 

Alex


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#6 rockstarbill

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Posted 17 September 2019 - 09:34 PM

I dither every frame. 5 pixels, which most nights is about 3"-10" of dither (depending on which system of course). The mount I use does correct nearly instantly though, so you may want to evaluate what that would mean to you in terms of overhead. 


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#7 neaptide

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Posted 17 September 2019 - 09:44 PM

Thanks for the advice all. I'm dithering every 180 seconds with a max dither step of 20 pixels which is giving me a spike of ~ 3". My settle time is ~8" with the settle threshold at 5 pixels. I was settling almost instantly with this setting at 1.5 pixels but the spike in guiding was really not even there.. It seems weird to work so hard to get my CGX guiding at .6-.7 arc seconds and then purposely mess it up, but I understand that this is the way to a better image. 

 

I am currently experiencing the best seeing conditions I've had in many months so I figured I'd give F/10 a try. I would like to try to capture some planetary nebula tonight. We'll see how it goes.


Edited by neaptide, 17 September 2019 - 09:46 PM.

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#8 lakeorion

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Posted 18 September 2019 - 11:08 AM

Your guiding may go up, but that's because dithering is figured as part of your guiding statistics--but that part does *not* affect your images. You can have (with strong dither) actual RMS of 0.5" and RMS with dither of 0.8" or even higher (depending on settle time and exposure length). 

 

This and suspending guiding during autofocus are making it difficult for me to determine how good my guiding is.  I wish PHD had an option to suspend statistics during dithers and pauses.



#9 bobzeq25

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Posted 18 September 2019 - 11:48 AM

This and suspending guiding during autofocus are making it difficult for me to determine how good my guiding is.  I wish PHD had an option to suspend statistics during dithers and pauses.

Key point.  How "good" your guiding borders on irrelevant.  The only question is, how "good" are your stars?

 

The PhD2 graph is not an end in itself.  It's main value is in diagnosing issues that are messing up your stars.  For that purpose you can omit dithering and autofocus, and do testing.

 

If you like your stars, you're done.  <smile>



#10 Stelios

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Posted 18 September 2019 - 12:36 PM

This and suspending guiding during autofocus are making it difficult for me to determine how good my guiding is.  I wish PHD had an option to suspend statistics during dithers and pauses.

Simple if laborious: Make a copy of the guide log, edit out the sections around auto-focus and dither commands. Run PhDLogViewer, and it will show you the "real" guiding performance. 

 

I did it once, then figured it wasn't really worth the bother knowing the exact details :)


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#11 Stelios

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Posted 18 September 2019 - 12:38 PM

Key point.  How "good" your guiding borders on irrelevant.  The only question is, how "good" are your stars?

 

The PhD2 graph is not an end in itself.  It's main value is in diagnosing issues that are messing up your stars.  For that purpose you can omit dithering and autofocus, and do testing.

 

If you like your stars, you're done.  <smile>

Yes in general, but also not entirely because you may be getting round but bloated stars, and it's hard to evaluate how "bloated" the stars are--is it seeing or guiding? 


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#12 kingjamez

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Posted 18 September 2019 - 01:40 PM

Yes in general, but also not entirely because you may be getting round but bloated stars, and it's hard to evaluate how "bloated" the stars are--is it seeing or guiding? 

Exactly. The measure of good guiding is getting as much resolution as the sky allows. Round stars are a sign of equal error in RA and DEC but are not necessarily a sign of good guiding. 


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#13 Der_Pit

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Posted 18 September 2019 - 06:27 PM

Simple if laborious: Make a copy of the guide log, edit out the sections around auto-focus and dither commands. Run PhDLogViewer, and it will show you the "real" guiding performance. 

 

I did it once, then figured it wasn't really worth the bother knowing the exact details smile.gif

Uhm - why?  phdlogview can/does automatically mark the dither settling, and you can control-mark the areas that you also want to exclude.  RMS statistics, drift (and also frequency analysis) will be done only on the selected parts....

Deselected areas are greyed.  With all of them selected, the reported RMS would have been 0.84/0.34 arc sec instead of the 'real' 0.35/0.23

 

phdlogview.jpg


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#14 Jon Rista

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Posted 18 September 2019 - 06:49 PM

This and suspending guiding during autofocus are making it difficult for me to determine how good my guiding is.  I wish PHD had an option to suspend statistics during dithers and pauses.

The PHD log viewer allows you to select multiple concurrent fragments of the graph, so you can calculate your real RMS and things like that without all the starts and stops and dithers interfering with it all.


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#15 SebaAstro

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Posted 14 March 2021 - 08:22 AM

Uhm - why?  phdlogview can/does automatically mark the dither settling, and you can control-mark the areas that you also want to exclude.  RMS statistics, drift (and also frequency analysis) will be done only on the selected parts....

Deselected areas are greyed.  With all of them selected, the reported RMS would have been 0.84/0.34 arc sec instead of the 'real' 0.35/0.23

 

attachicon.gifphdlogview.jpg

How do you de-select areas that are gray? I tried but i cannot figure it :(



#16 Der_Pit

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Posted 14 March 2021 - 10:29 AM

How do you de-select areas that are gray? I tried but i cannot figure it frown.gif

select     = CTRL+LMB+move

deselect = SHIFT+LMB+move

 

So Shift+click left of the area to de-select, and pull the cursor across while holding the mouse button pressed.

wink.gif


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