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Dither Settings - SharpCap and General

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#1 ESzczesniak

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Posted 05 August 2020 - 09:45 AM

Two weekends ago I got all software/settings are working for me to start dithering with SharpCap and PHD2.  However, it seems the default settings leave a lot of wasted time in capturing, and may be a bit aggressive based on what I've been reading.  

 

I am imaging with a WO Z81/0.8x flattener and an ASI294MC-P that gives me an image scale of 2.14"/pixel.  

 

The two main settings I'm wondering about are dither size and settle time.  The default dither size is 20 pixels, but browsing the forums I see a lot of people using 4-8 pixels with similar setups.  And the default settle time is 60 seconds, but I'm seeing a lot of mention of 15-20 seconds.  Could anyone provide some feedback regarding these settings?  I'd love to not throw away 60 seconds every 3-4 frames, or worse, an entire 120 second exposure.

 

I know some people are dithering every frame, but it seems that every 3-4 frames may be a reasonable starting place?

 

And can anyone explain what the settle parameter/pixels are (I don't have SharpCap up and can't remember the exact name)?  I'm understanding this somewhere on the order as once motion is <x pixels per frame, the image is "settled".  But what is the readout showing when is says 8 pixels, then 4.4, then 4.2?  Is this motion?  Or how many pixels it moved from the starting position?

 

Thank you for any input on this.  I am trying to be understanding this is in part going to be trying different settings, but I'd love to get in the ballpark without throwing out precious limited imaging time.  

 

 



#2 Gipht

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Posted 05 August 2020 - 09:57 AM

I dither about every 3rd frame and allow only 5 seconds to settle. 98% of the time, that is enough settle time, but occasionally I have to reject a blurred frame. My dither size for your setup would be  around 10 pixels.  These are all numbers based on a fairly new use of dithering.  Other people may have better advice.



#3 ESzczesniak

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Posted 05 August 2020 - 06:25 PM

Thank you!  Sounds like I should at least drastically reduce the settle time.  Even if I still leave it at 15-20 seconds, it saves a huge amount of time.



#4 calypsob

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Posted 06 August 2020 - 11:02 AM

Is settle time determined purely via trial/error?

#5 Stelios

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Posted 06 August 2020 - 07:36 PM

It depends on your settle threshold, which I hear no mention of here.

 

Settle threshold is the amount of deviation between actual and target star position (in the *guider*) in pixels that will be considered "settled." Multiply by the guider image scale to see what deviation that would be in arcsecs.

 

Settle time is how long you want the guider to *remain* in that range before you consider your guiding "settled."

 

I have a feeling that you are all setting the settle threshold WAY too high, so the thing that triggers imaging is the minimum settle time. This is BAD. Your guider may very well NOT have settled by then. 

 

If your target is around 1" RMS guiding, you would want no more than around 2.5" deviations--probably about  0.4 to 0.6 pixels for a guidescope. You would want this maintained for at least 3 exposures and more likely 4, so if you have, say, an Orion Starshoot Autoguider, and use 2" exposures, you'd want 0.6 pixels for 8". The "minimum" should not matter, the "maximum" should be for those cases where seeing is awful and you can't quite stay under 2.5" deviations for long enough. 

 

Do not confuse the amount of pixels to dither (which is set in PhD2 with choices of dither size) and settle threshold. Entirely different things. 



#6 ESzczesniak

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Posted 08 August 2020 - 10:53 AM

Stelios, thank you for the reply. The settle threshold was what I was referring to, but didn’t have the name in my head and didn’t have SharpCap to open and check. That all makes much more sense all put in context now.


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