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Dithering: How long does it take?

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#1 MountainAir

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Posted 10 August 2020 - 03:49 PM

How long does it take your software to dither between frames?

 

I recently picked up a RASA 11, and I'm still getting familiar with it (I've only had it imaging one night so far).  I expected the shorter images, the larger disk space requirements, and the massive stacking-with-drizzle disk requirements.  One thing I didn't fully think about was the amount of time it takes to dither.  In the recent past, the dithering duration just wasn't very noticeable between 300-600 second exposures.  Now, I'm finding that I can capture 30-second images and then spend nearly a minute dithering (sometimes it's as low as 15 seconds, but even that adds 50% to my capture time).

 

I assume part of this is the DEC backlash I have (CGX mount), but I thought I had that pretty well-tuned.  Another contributor is the added weight stress that the RASA puts on my CGX (I'm contemplating sending it in to Celestron, or getting it HyperTuned).  I am using the internal guider in KStars 3.4.3.

 

How about you?  How long does your rig take to dither after a frame?


Edited by MountainAir, 10 August 2020 - 03:50 PM.


#2 RJF-Astro

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Posted 10 August 2020 - 04:21 PM

What I do with short subs is dither an x number of frames. So my lum frames are 30 seconds and I dither every 4th sub. RGB is 150 seconds and I dither every sub. It takes around 30-45 seconds to stabilize. You can experiment with these settings, but dithering every sub is not as important compared to 10 minute narrowband.

#3 Der_Pit

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Posted 10 August 2020 - 04:52 PM

Also KStars here, but guiding via PHD2.  There it helped a lot to have 'fast recenter' activated.  Dither takes between 10 and 40s.  I don't dither every frame, but try to reach something like 50 different dither positions.  So if I take 200 frames, I dither every 4th.  That usually gives a 'healthy' relation between imaging time and dither time 



#4 MikeCMP

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Posted 10 August 2020 - 05:08 PM

Also, 

See if there is a spiral dither option like in PHD2, will help avoid dec backlash. You can probably also dither just in RA as long as there is no fixed pattern noise.

 

Mike



#5 Stelios

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Posted 10 August 2020 - 05:18 PM

Dithering takes me between 15 and 25 seconds. As others mentioned, with short frames I will dither every 3-5 frames, with 4' and longer I dither every frame. 

 

You can't skip dithering. But if it takes a minute, you can probably improve on your settle time and settle under settings. 



#6 AtmosFearIC

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Posted 10 August 2020 - 06:07 PM

I’m probably the exception rather than the rule but it’s one of the reasons I love my ASA mount.

I had my 8” F/3 out over night and about to start packing up, dithering times are at 2-3 seconds which is quite useful when doing 30s Lum exposures.

On my 250mm F/10 they stretch out to 6-7s but a part of that is imaging at 0.4”/pixel and fully loaded it’d approaching the theoretical max capacity of the DDM60.

Used to use an EQ6 and that was usually around 15-30s depending on how clean the grease/worms were on that part of the cycle.

#7 MountainAir

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Posted 10 August 2020 - 06:32 PM

Interesting!  Such a wide range of dither durations.  Throughout that first night I did try dithering every 2, then every 3 frames.  I will check out the other settings in the KStars internal guider.  

 

Thanks everyone!


Edited by MountainAir, 11 August 2020 - 12:21 AM.


#8 Alex McConahay

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Posted 10 August 2020 - 06:41 PM

Might it be better to ask how many guide frames/mount movements it takes before settling? You have people here claiming it takes them less time to settle than I use for a single guide exposure. And, I do not see how one can determine if the dither is complete before that first exposure is finished.

 

Alex



#9 kathyastro

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Posted 10 August 2020 - 07:23 PM

The actual dither takes almost no time at all.  All the delay is in the settling time. 

 

So check your settling time parameters.  If they are too strict, it can take a very long time to be officially counted as "settled".  If they are too loose, of course, the mount may be considered as settled when it isn't.  You will have to fiddle with the parameters to get the right settings for your mount.

 

With my setup, I have set it to be within 0.2 pixels for 0 seconds.  I originally had it at about 3 seconds, but it was taking way too long.  0.2 pixels is a pretty small error.  If it gets that close, it means that the corrections are of an appropriate size.  (It could get there accidentally, but that is rare.) 

 

I sometimes have it take a long time to get to within 0.2 pixels of the target position, especially if the seeing is poor.  But I have never had it go the other way and hit 0.2 pixels when it wasn't ready to guide.  With good seeing, my dithers take 10 s



#10 OldManSky

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Posted 10 August 2020 - 09:06 PM

My setup settles in about 3 seconds.  I have a 10-second timeout limit, as far as I can tell it's never hit it.



#11 AZ Maverick

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Posted 10 August 2020 - 11:13 PM

I dither RA and DEC random extreme with my ES80 and according to my PHD2 log files it takes anywhere from a few seconds up to maybe 20 seconds to dither and settle.


Edited by AZ Maverick, 10 August 2020 - 11:21 PM.


#12 PhotonHunter1

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Posted 11 August 2020 - 07:15 PM

With my CGE Mount it could take up to 8 minutes - severe backlash issues with my old faithful mount. With my iEQ45 it takes 15-25 seconds (similar to Stelios and others).



#13 Ken Sturrock

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Posted 11 August 2020 - 10:55 PM

There are a variety of ways of dithering: If you're unguided, a dither can be a simple mount jog and the settle time will depend on the mount and payload. In other cases, the software will move the guide star location on the chip and then guide to that new location, the speed is limited by guide rate and settings as well as settle. In my case, I break off guiding, jog the mount and resume guiding at the new location, which can take a while. Because I almost always shoot through filters, I only dither between sets of filters (i.e. LRGB <dither> LRGB <dither> LRGB), which helps compensate for the longer overhead time involved.



#14 mewmartigan

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Posted 12 August 2020 - 10:22 AM

When I shoot 3 minute subs I dither every 3rd frame. When I shoot 4 minute subs, I dither every other.

 

My Z103 and ASI2600MC Pro on the CEM40 takes about 13 seconds to finish a 5 pixel dither.

Same equipment on my previous CEM25 took about 45 seconds.



#15 MountainAir

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Posted 12 August 2020 - 06:09 PM

My Z103 and ASI2600MC Pro on the CEM40 takes about 13 seconds to finish a 5 pixel dither.

Same equipment on my previous CEM25 took about 45 seconds.

A bit off topic:  I was pleasantly surprised by my CEM40, too.  Light, lots of features, good price, performs well, and it's just so... easy!  I didn't even think of the CEM40 when starting this post because I don't really notice the dithers -- they are that fast and reliable.

 

On topic:  I'm wondering if there was something wonky with my CGX mount during first-light for the RASA.  Maybe I didn't balance it well, or maybe a cable was brushing between the RA housing and the mount body that I didn't see or something.  I had it out last night and, while I didn't do any dithering, it was performing better all around.  In fact, I was rather shocked at how well it was auto-guiding.  I want to try it again on a real imaging run to be sure it's all working well.




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