Is dithering on DEC-only useful?

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Menno555
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Is dithering on DEC-only useful?

#1

Post by Menno555 »

I finally had a bit of clear sky (a very little bit :cry: ) but long enough to play with the Deep Sky Sequence Planner for the first time.
And I finally managed for the first time to dither without guiding :D Works great! It's indeed only using the Guiding tab and setting the amount of exposures.

But (there is always a but) I now discovered that I can only dither on the DEC of my mount. It has high precision optical encoders on the RA and the mount has 2 options: either filter RA guiding so the encoders correct any tracking errors or Allow RA Guiding so an external guider can be used.
This last settings has to be enabled to get the dither pulses from SharpCap. But that's not an option because I only use tracking so I need those encoders to be active, especially for the longer exposures.
So I can only dither on DEC and I am wondering if that is useful? I did see the movement but it looked like it was repeating step with equal movement each time in the DEC. Or are these dither distances random and I just didn't notice it?

Menno
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Re: Is dithering on DEC-only useful?

#2

Post by admin »

Hi Menno,

glad you like the sequence planner tool!

My guess is that DEC only dithering will be much less effective than dithering in both axes - let me explain why I think that...

The main purpose of dithering is to deal with persistent hot/cold pixel values that can either lead to single pixel problems in the stacked image (if your guiding/tracking is really good) or walking noise as the pixel drifts in most cases. Dithering in two directions spreads out each anomalous pixel to contribute slightly to a brightening (or dimming) of the image over a large number of random pixels in that vicinity of the image. The effect is still there, but much less strongly, and what is present does not seem as obvious to the human eye, since we spot single odd pixels or lines very easily.

If you dither only in DEC then the effect of the bad pixel will be spread out in a line on the image, which I think will be noticeable (just like walking noise is). Mind you, that only applies if your RA guiding is pixel perfect - if there is a slow drift in RA as well then the DEC dithering may actually work reasonably well.

It seems a bit of a shame that your mount cannot pause the encoder operation when a pulse guide command is running and resume after. If it really cannot dither in both directions while encoders are active, surely other people would have run into the problem and complained about it.

cheers,

Robin
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Re: Is dithering on DEC-only useful?

#3

Post by Menno555 »

Thanks for the explanation Robin.
I did compare both settings in the mount and with RA Filtered there was DEC only dither and with Allow RA Guiding there was RA/DEC dither
I think no one complained about it since dithering is typically done with guiding and not during tracking?
But if I understand correctly, the dithering signal is different then a normal slew signal? Cause very tiny slewing commands are no problem.

Menno
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Re: Is dithering on DEC-only useful?

#4

Post by admin »

Hi,

the normal dithering procedure (using PHD for instance) is to adjust the position using 'pulse guiding' commands - these are the same commands that are send when guiding to offset the speed of the mount slightly. SharpCap uses the same commands if you choose the 'dither only guiding' option. It seems very odd that they are not supported!

cheers,

Robin
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Re: Is dithering on DEC-only useful?

#5

Post by Menno555 »

Well, it is supported but only when the encoders are "disabled".
I did mail Ioptron if there is a way around this. Will wait and see for their reply.

Menno
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Re: Is dithering on DEC-only useful?

#6

Post by Menno555 »

Well they are fast this time :)
Reaction is that all this indeed is on purpose to prevent to compete with the high precision encoder correction on RA.
Also some advise which either I don't understand or it's described weird: " .... try dither at a much lower rate, may be once a 2 or 3 seconds."
Any ideas on what they mean with that?

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Re: Is dithering on DEC-only useful?

#7

Post by admin »

Well, it is possible to configure the pulse guiding rate in ASCOM drivers - usually it defaults to about 0.5x sidereal rate, but maybe if it is set much lower then the mount allows the motion? (This is just a guess....)

There is no way currently to set the guide rate in the SharpCap UI, but it can be done via scripting to test - see viewtopic.php?f=5&t=426&p=25926&hilit=g ... ion#p25926

Once it has been set, it should stick at least until you close SharpCap.

cheers,

Robin
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Re: Is dithering on DEC-only useful?

#8

Post by Chuckwagon »

Menno555 wrote: Mon Nov 22, 2021 5:05 pm Well they are fast this time :)
Reaction is that all this indeed is on purpose to prevent to compete with the high precision encoder correction on RA.
Also some advise which either I don't understand or it's described weird: " .... try dither at a much lower rate, may be once a 2 or 3 seconds."
Any ideas on what they mean with that?

Menno
I think they may be referring to the EC mount's tendency to become overwhelmed if guide pulses come it too frequently. The encoders are never turned off, even if you have the setting for Allow RA Guiding turned on. When that setting is off, the mount just ignores guide commands on the RA. That allows people to use guiding, but in effect only guides DEC. But it seems as though the mount doesn't actually apply the pulse commands as they are received, but instead seems to do them in a buffered manner. Almost as if they don't really bypass the encoder but instead try to insert the guide pulses into the encoder stream. When many commands are issued in quick succession, it's almost as if they can't all be handled and end up being delayed, which gets the whole thing upset. However, if you limit the frequency of the pulse input, the mount seems to be better able to handle it. I ended up not guiding my CEM25EC because of the issue. But there are folks who guide their EC mounts, so I think if the commands are spaced out enough it can work.

Robin would have to verify how the dither command is sent and how many pulse commands comprise it. But, I would suspect that if you have Allow RA Guiding turned on, I would think the infrequency of dither commands would cause it no problem, and you could dither in both axes. Dithering commands shouldn't be as frequent as every few seconds, so you wouldn't be having the issue people doing fast guiding see. And if Sharpcap issues the dither as a single pulse command, I would think the mount would handle it.

I suppose the best way to be sure would be to try it and see what happens. :)

Good luck.
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Re: Is dithering on DEC-only useful?

#9

Post by admin »

Hi,

interesting info :) I can confirm that the dithering movements sent direct from SharpCap should just be a single pulse for each axis (I can't remember offhand if they are both sent at once, or one after the other, but it shouldn't matter). Hopefully the mount will deal with that as they are likely to be minutes apart.

Dithering from within PHD2 is different - I think PHD offsets the target location of the guide star and then allows normal guiding pulses to be sent to bring it back over the new location. That could lead to a lot of pulses in quick succession.

cheers,

Robin
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Re: Is dithering on DEC-only useful?

#10

Post by Menno555 »

Thanks both for new insights.
Will try some things for sure. One of them is setting the mount to Sidereal tracking and see what happens. Maybe my User Defined tracking is also interfering because that's a fixed value for the tracking?
Btw the Dithering settings in the General SharpCap Settings. The Max Dither value is standard on 20 pixels. But since I work with FL 2032mm, setting that to for example 10 pixels might help? (my maybe faulty logic says less pixels = less time :) )

Menno
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