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Old 19-02-2019, 04:27 PM
Startrek (Martin)
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To Dither or not to Dither ?

Iím set up for dithering but havenít tried as yet
Just wanted to know how many people Dither and are the image results from dithering a lot better than dark frame calibration for both DSLR and CCD
Also if your guiding is not great ( say 1.50 to 2.0 arc sec error ) is it worth dithering as the mount may take a along time to recover in between frames which would make it a very long nights imaging ?
Would you both dithering with short exposures say 30 sec to 60 sec ?
Thanks in advance
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Old 19-02-2019, 04:49 PM
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Camelopardalis (Dunk)
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Martin, dithering is especially useful for a DSLR with unregulated cooling. Cooled cameras can be calibrated with darks.

If you're guiding isn't great, then work on the mechanical issue(s) in the first instance. All dithering does is issue commands to the mount to move a small amount in a given direction, in the same way that the guiding software does. Improve the mechanical issue(s) and it's win-win
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Old 19-02-2019, 08:24 PM
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As an aside, another gem of wisdom (or not) is to let the sky (The Earth's rotation) dither for you, by stacking (short*) exposures from a fixed mount/tripod, with a generous gap between exposures, of between 2 to 3 times the exposure length. Somewhat off-topic, but useful if you're on holidays without all your gear/EQ-mount.

*Short as in sufficiently "short" exposures such as not to cause obvious star trailing, as defined by using typical 600 exposure rule, except that I prefer to use 300.

Best
JA
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Old 19-02-2019, 08:33 PM
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Lack of dithering in the last shots I did with a 350D (The first ones where I could run to more than 30 seconds, so a toasty warm sensor) left measles in the resulting shots where darks subtracted out the hot pixels. They were hot as in max values so after dark subtraction there was no data left. COmbine that with guiding so the target was more or less stationary in frame and it really looked spotty.



I noticed less issues with the ASI294 but there was still an improvement by dithering, it was though a lot more subtle.
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Old 20-02-2019, 08:10 AM
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I dither. The only problem is if you have mount issues especially with the DEC axis and backlash. You might find you have a long settling time after dithering - so takes longer to collect lights. But if you are already guiding and using DEC axis also then you are probably okay?

I have a reasonable amount of backlash on my mount. So I only dither every 3 - 5 subs. Which is fine as I use a cmos camera - loads of short subs.
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Old 20-02-2019, 01:24 PM
Startrek (Martin)
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Thanks all for the great advice and information 👍
Iíve made a set of notes on my phone regarding dithering with BYEOS and PHD2 so I can refer to them if I need assistance during imaging with dithering as follows -

General Dithering Settings in BYEOS
Set PHD2 to Spiral Dither
Adjust below to suit

Dither Aggressiveness
Low 1 to 2
Med 3 to 4
High 5

Settle Dither at
0.25 default
0.30 for guiding below 1 arc sec
0.35 for guiding 1.0 to 1.5 arc sec
0.40 for guiding 1.5 to 2.0 arc sec

Calm down period
Default 25sec
Maximum 2 minutes
Depends on guiding, seeing conditions, mount tracking error , exposure time etc....
***Higher Aggressiveness results in longer calm down period


Dither ( Frames)
30 sec exposures every frame (1x)
60 sec exposures every frame (1x)
120 sec exposures every 2nd frame (2x)
150 sec exposures every 2nd frame (2x)
240 sec exposures every 2nd or 3rd frame (2x or 3x)
300 sec exposures every 2nd or 3rd frame (2x or 3x)

Cheers
Martin
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Old 20-02-2019, 01:46 PM
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I would actually tend to go the other way in one respect. Shooting with a DSLR you are likely to have hot pixels and the longer the exposure the hotter they are going to be, to the point that maxed out pixel values are likely. If you subtract them out with darks you are going to be drilling holes in your real signal. Better IMO to keep moving them around frame by frame to allow for the holes to be averaged out by stacking after the dark subtraction.
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Old 20-02-2019, 02:14 PM
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Stonius (Markus)
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Quote:
Originally Posted by Startrek View Post
Thanks all for the great advice and information 👍
Iíve made a set of notes on my phone regarding dithering with BYEOS and PHD2 so I can refer to them if I need assistance during imaging with dithering as follows -

General Dithering Settings in BYEOS
Set PHD2 to Spiral Dither
Adjust below to suit

Dither Aggressiveness
Low 1 to 2
Med 3 to 4
High 5

Settle Dither at
0.25 default
0.30 for guiding below 1 arc sec
0.35 for guiding 1.0 to 1.5 arc sec
0.40 for guiding 1.5 to 2.0 arc sec

Calm down period
Default 25sec
Maximum 2 minutes
Depends on guiding, seeing conditions, mount tracking error , exposure time etc....
***Higher Aggressiveness results in longer calm down period


Dither ( Frames)
30 sec exposures every frame (1x)
60 sec exposures every frame (1x)
120 sec exposures every 2nd frame (2x)
150 sec exposures every 2nd frame (2x)
240 sec exposures every 2nd or 3rd frame (2x or 3x)
300 sec exposures every 2nd or 3rd frame (2x or 3x)

Cheers
Martin
I thought dither was pretty much set and forget. Are these tweaks only applicable to DSLR's or to Cooled CMOS as well? What is the rationale behind them? I dither because it helps remove fixed pattern noise, but I also use darks, flats and flat darks too (every little helps, I guess).

Markus
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Old 20-02-2019, 03:14 PM
Startrek (Martin)
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These are setting parameters in BYEOS for DSLRs only
ďDither framesĒ at the bottom I added as a guide only and could be totally incorrect
So for longer exposures the read and shot noise is greater therefore you would Dither every frame using your DSLR ? Thatís going to limit capturing quite a number of DSOs as sometimes youíre u start your capture at 1.00am or 2.00am with possibly 50 to 60 subs and the sun will be rising before you finish if your Dither recovery time isnít reasonable ?
Next night might be cloudy ?
What Iím suggesting is that there is a small sacrifice to pay in dithering, itís just press the button and everythingís ok , there are a few things to consider like everything in AP
Cheers
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Old 20-02-2019, 07:03 PM
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rcheshire (Rowland)
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Dithering will improve image quality, particularly with older DSLRs. The imperfections of equipment that limit imaging time because guiding becomes limiting are very likely those that make dithering necessary in the first place.


The argument against dithering is often confused by reference to dark noise, hot pixels and so on. Dithering does a whole lot more. The argument seems to originate from a desire to avoid dark frames, where dithering should only cater for the imperfections inherent in the image reduction process.


From experience, dithering will not necessarily remove or hide hot pixels, unless they are first reduced by image calibration. Image flatness is the beauty of dithering. But dithering is not always explained well. The focus around dark noise has relevance but misses the point.


Edit: same target same equipment - dither no dither Canon 1000D circa 2012.
Attached Thumbnails
Click for full-size image (dither-no-dither.jpg)
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Old 20-02-2019, 07:14 PM
Startrek (Martin)
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Rowand
Understand the point you put across well explained
Just need some clear nights now to Dither my images

Thanks to all who replied and provided good advice
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Old 20-02-2019, 07:30 PM
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Part of the discussion seems to center around the time lost while dithering through a night of imaging. I thought about that initially and discounted it. I found very quickly that effective dithering paid a better dividend in finished image quality than the extra subs I might have got without dithering.
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Old 20-02-2019, 08:00 PM
Startrek (Martin)
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Paul
Didnít even think about the fact that you could possibly save time by reducing the number of subs in your capture plan if those subs are of a much better usable quality than those that are not dithered
Good point !
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Old 20-02-2019, 09:12 PM
gb44 (Glenn)
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Dithering takes no extra time in my case.

I use a 10sec interval between exposures with dslr camera control in ImagesPlus. Dithering with PHd2 takes place at the end of an exposure automatically. Guiding and the next exposure start simultaneously.

Even manually, one can watch the PHd2 exposures and move the mount a random few pixels. Then restart guiding and watch the graph till it settles and then start an exposure. The settling of the mount must be an issue - you can assess the settling timespan and use that as the interval between exposures.

Then in post-processing try Drizzle.

GlennB
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Old 21-02-2019, 09:38 AM
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Watching the dither process between APT and PHD2 I reckon on a reasonably still night it takes around 30-40 seconds. Only a handful of seconds to actually make the dither, the rest of the time is the time taken for guiding to settle down again (Which APT monitors via the distance to the guide star meeting a threshold in pixels) and then additional wait time configured in APT for settling.


Like everything in AP, I found that trying to scrimp on the settle time meant time saved but worse results, but I am imaging at approx 1500mm so YMMV as they say.
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Old 21-02-2019, 01:15 PM
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I found I had a decent amount of settling time after each dither, so I got around it by doing captures like LRGB-dither, or LRGBLRGB-dither, or LLRRGGBB-dither, etc. The latter being a compromise and working on my mount not being perfect anyway.
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Old 22-02-2019, 07:34 AM
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I reckon LRGB-dither would be the trick. One sub per filter then dither so the next of each will have fixed pattern noise and hot pixels in a different place on the target.
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Old 22-02-2019, 08:03 PM
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I use the Sky X for imaging many times. Sometimes I revert to the tried and tested reliable CCDSoft.

Occasionally the Sky X does not go to plan. Like getting 3 x blue, 3 x green yet only 1 x red as the dithering settling process must have taken up extra time and the timer went off to prevent the mount hitting the pier before I finished my imaging run! (plus of course the next night is cloudy!).

But dithering is one reason I use Sky X. It does make a difference. You'll see less artifacts in your colour images and lower noise in general. Its worth it.

Greg.
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Old 22-02-2019, 09:48 PM
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Quote:
Originally Posted by The_bluester View Post
I reckon LRGB-dither would be the trick. One sub per filter then dither so the next of each will have fixed pattern noise and hot pixels in a different place on the target.
It depends on how many subs you are going to take. All you need is to not have the unwanted pixels becoming statistically significant such that they avoid rejection during stacking.

Also, you are assuming that your tracking and guiding are pixel perfect in the first place...more likely, there’s some “natural” dithering going on throughout a set of subs. This will depend, to some extent, on the resolution of your guiding and main cameras. It’s possible to guide well, but still shift a few pixels over a period of time in the main imaging camera.
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Old 08-03-2019, 02:25 AM
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Astrofriend (Lars)
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Hi,
I take my astro photos with dithering. My problem has also been that it take some time for the auto guiding to stabilize after every dithering, about 15 to 20 seconds, earlier much more.

I try to not do to big dithering moves, about 10 to 20 pixels on the main camera (color camera), then it work.

My main camera is a Canon 6D which has low static pattern. I only dither to take away the hot pixels.

Here I have write it down how I use the dithering:
http://www.astrofriend.eu/astronomy/...dithering.html

/Lars
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