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# Dithering time wasted?

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13 hours ago, michael8554 said:

If you want to dither 12 pixels on your imaging camera, you need to work out how many pixels PHD needs  to move the guidecam.

Imaging pixel scale = say 2 arcsecs/pixel, so 12 pixels is 12 x 2 = 24 arcsecs

Guide pixel scale = say 8 arcsecs/pixel

So to move the guidecam 24 arcsecs, the guidecam needs to move 24/8 = 3 pixels

The PHD dither range is:

Dither Level     |       Pixels

---------------           ---------

1       -                +/-  0.5  x  Dither Scale

2       -                +/-  1.0  x  Dither Scale

3       -                +/-  2.0  x  Dither Scale

4       -                +/-  3.0  x  Dither Scale

5       -                +/-  5.0  x  Dither Scale

So to dither the guidecam 3 pixels you could select 2 in Dither Level, and 3 in the Scale box.

Or 4 in Dither Level, and 1 in the Scale box.

I think I've got that right........

Michael

Edit:  If your dithering is taking 2 minutes to recover it's possible you're dithering too much.

Mine usually takes 45 secs max to recover.

Yes I agree it's taking too long.

So by your calculations my imaging camera is 1.18 arc/pixel

so 12 x 1.18 = 14.16

guide cam is 5.95 arc/pixel

So 14.16 / 5.95 =2.3798

Last part you lost me where is the dither level? I found the scale but not the dither level. I'm using APT as well if that helps.

Thanks for the help appreciated!

Gerry

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5 hours ago, Gerry Casa Christiana said:

Ok here is mine as it is now. What do you think?

Looks good to me. How is the backlash in the mount? DEC will normally overshoot and take the longest to settle. PHD2 next. Here's mine down in a turbulent Sagittarius. If we agree on this, can you post a guide log? HTH.

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I just recently started dithering after many years of imaging, and can say without question that my images are far cleaner than before. I now rarely  need to use noise reduction and can even get away without dark frames. I may have gotten lucky with my dither settings, software (APT/PHD) and backlash free mount (M Uno), but my time between dithered  sub-exposures is never more tha 10-15 seconds. It was far longer and inconsistent before I switched from SGP to APT for what it's worth.

Derek

Edited by schmeah
Another thought
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12 hours ago, Gerry Casa Christiana said:

Last part you lost me where is the dither level? I found the scale but not the dither level.

Tools/ Manual Guide/ Dither and Scale are at the bottom.

Michael

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7 hours ago, michael8554 said:

Tools/ Manual Guide/ Dither and Scale are at the bottom.

Michael

Thanks Michael

As soon as moon disappears I'll give it a go.

Gerry

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On 2017-08-04 at 18:24, ollypenrice said:

You're aiming to remove hot pixels, Steve. Tony Hallas is also trying to remove what he calls 'colour mottle,' the medium-scale colour irregularity seen in so many DSLR background skies. They tend to be very 'colour busy' rather than neutral grey. So the large scale dither benefit doesn't apply to CCD cameras, but to DSLRs.

Olly

Would it apply to cooled CMOS cameras? Same sensor as a DSLR but mono and cooled...

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12 minutes ago, MartinFransson said:

Would it apply to cooled CMOS cameras? Same sensor as a DSLR but mono and cooled...

If it's an osc, then very likely. These sensors are also used in some of the newest dslrs. Cooling decreases noise, but doesn't remove it. The need for dithering will be less, so it may depend on how picky you are.

For mono cooled cmos, it's not that evident, since the same pixels are used for all colours.

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31 minutes ago, wimvb said:

If it's an osc, then very likely. These sensors are also used in some of the newest dslrs. Cooling decreases noise, but doesn't remove it. The need for dithering will be less, so it may depend on how picky you are.

For mono cooled cmos, it's not that evident, since the same pixels are used for all colours.

I was thinking mono, since that is what I will be using soon (yay!)

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Then I guess, you'll soon find out.

Btw, dithering is always a good thing to do. But maybe cooled mono cmos can be dithered just like ccd.

These cameras are often used differently than ccd, and old truths may have to be 'adjusted'. (I won't go as far as saying "don't apply".)

As always with AP: experiment. And report back, of course.

Good luck

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If I am not mistaken dithering is not just for the removal of hot pixel as it should also remove any banding or dust artifacts.  I was also under the impression that it will also remove any satellite or aircraft streaks but could be totally wrong about this.

I would say that with a dedicated cooled cmos or cdd camera then this really is not a massive issue as you can do your darks whenever as you can set your temp unlike a dslr or uncooled camera. But I guess it will still remove dust artifacts and such.

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A link to Tony Hallas view as referred to by Olly

Dave

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2 hours ago, spillage said:

IIRC, it's the algorithm which stacks the light frames as a result of  the dither which removes the satellites. HTH.

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2 hours ago, spillage said:

The sigma clipping algorithm (and neighbours) statistically removes outliers and other extreme values, and averages the remainder, but only on each aligned pixel over all your frames. So satellite / aircraft streaks will be eliminated because they're only on a small subset of your frames. Conversely, hot pixels, optical vignetting and dust motes are on all of your frames.

Hot pixels are very small by definition, and if you use precise mount + guiding they will land on the same image pixel always; If you applied darks/biases in these conditions, you would get a dark pixel in the image. Small-scale dithering allows hot pixels to not land on the same image pixel, and then be eliminated by clipping.

Dust motes and optical vignetting are usually taken care of by flats. Though dithering might improve the stacked image by spreading the statistical error introduced by the flat correction, it would require large scale dithering and I doubt it would be visible on most images.

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7 hours ago, alacant said:

IIRC, it's the algorithm which stacks the light frames as a result of  the dither which removes the satellites. HTH.

Assuming the sats only affect one sub, dither doesn't add any benefit. Sigma clip will remove them entirely because they don't appear in any of the other subs.

Olly

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Imagine if we could stack threads. Sigma kappa would remove all off-topic comments (like this one) and the arguments on each side of a discussion would become clearer and more focused

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If you are not dithering with a DSLR then you are doing it wrong, you cant get decent darks with a DSLR no matter how much you think you can you cant unless you have it modified with regulated cooling. Dithering and Kapper Sigma Clipping will do almost as good a job and since trying it I have never gone back. But then again I cant think of many targets that I would do sub 5min subs on so its not a big impact for me.... Doing it every 2 mins would potentially lose you quite a bit of imaging time. I dont just use the default setting either I dither by around 20 pixels and that takes a little longer.

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2 hours ago, Adam J said:

If you are not dithering with a DSLR then you are doing it wrong

No one does it wrongly, just differently.

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10 minutes ago, Stub Mandrel said:

No one does it wrongly, just differently.

Possibly just degrees of wrongness, however, ill amend my statement to say "you will not get optimal results without dithering for long exposures where temperature matched dark frames are not available".  I am very confident of that statement (having performed a comparison) and will stand by it.

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25 minutes ago, Adam J said:

Possibly just degrees of wrongness, however, ill amend my statement to say "you will not get optimal results without dithering for long exposures where temperature matched dark frames are not available".  I am very confident of that statement (having performed a comparison) and will stand by it.

Assuming the algorithms are doing their job properly I'm sure you're right.

Olly

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3 hours ago, Adam J said:

every 2 mins would potentially lose you quite a bit of imaging time

Hi. Dither with less frequency? Every third frame perhaps?

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13 minutes ago, alacant said:

Hi. Dither with less frequency? Every third frame perhaps?

I would guess that it would depend on the number of frames you collect in total. Kappa sigma clipping works by calculating if a pixel falls outside of something like two SD from the mean value and if it does it rejects it, I would have to look at the actual maths used in DSS to be 100% sure though.

So in theory you could still reject a pixel that is present in multiple images if the total number of subs is still large as a fraction of the number of un-dithered subs in each group. As a rule you need about 12 subs for kappa sigma clipping to start to work (allows the standard deviation / mean to be accurately calculated), so I would take a stab at you needing somewhere between 36 and 72 subs for it to work with a dither every third frame.

So the answer is yes but you will need a longer total integration.

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4 hours ago, Adam J said:

If you are not dithering with a DSLR then you are doing it wrong, you cant get decent darks with a DSLR no matter how much you think you can you cant unless you have it modified with regulated cooling. Dithering and Kapper Sigma Clipping will do almost as good a job and since trying it I have never gone back. But then again I cant think of many targets that I would do sub 5min subs on so its not a big impact for me.... Doing it every 2 mins would potentially lose you quite a bit of imaging time. I dont just use the default setting either I dither by around 20 pixels and that takes a little longer.

20 pixels! By the time my backlash has finished I'll be photographing the moon instead of aiming at my deep sky object.

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6 hours ago, Gerry Casa Christiana said:

20 pixels! By the time my backlash has finished I'll be photographing the moon instead of aiming at my deep sky object.

DSLR tend to have some macro features to the noise that only show up in longer exposures and so you end up with a mottled effect if you dont do a large dither in my opinion.

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10 hours ago, Adam J said:

DSLR tend to have some macro features to the noise that only show up in longer exposures and so you end up with a mottled effect if you dont do a large dither in my opinion.

Brilliant I'll try that. Can you help a dummy and tell me how I know Ive got 20 pixels. In APT it's a pulse but I'm still not sure how to calculate the correct pixel. I'm winging it!  I'm fairly new to guiding

Thanks

Gerry

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52 minutes ago, Gerry Casa Christiana said:

Brilliant I'll try that. Can you help a dummy and tell me how I know Ive got 20 pixels. In APT it's a pulse but I'm still not sure how to calculate the correct pixel. I'm winging it!  I'm fairly new to guiding

Thanks

Gerry

If you let the mount slew (at 1 x sidereal rate) for as many seconds as your pixel scale, you'll have dithered 15 pixels, in RA anyway. In DEC it's theoretically more complicated, but the same strategy works.

If your pixel scale is 1"/pixel, then obviously, slewing for 1 second will move the view 15" (sidereal rate), which is also 15 pixels. At 2"/pixel, slewing for 2 seconds will move the view 30", which is 30/2 = 15 pixels.

If you slew at 0.5 x sidereal rate, you have to double the time, etc.

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