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Old 13-03-2015, 05:34 PM
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Bassnut (Fred)
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DSLR noise reduction no darks no flats!

I tripped on this randomly.
https://www.youtube.com/watch?v=PZoCJBLAYEs
Its a talk by Tony Hallas on how he produces noise free DSLR images. Its right out of left field, im not sure what to make of it. I havent heard of this method before.

He doesnt bother with darks or flats.
He sees DSLR noise as "colour Mottle", constant large diameter random colour dots 5-10 pixels in size that are the same in each exposure. Ive seen this effect as everyone has, but I didnt know it was constant!.

He dithers each sub by at least 5-10 pixels (during capture), aligns the subs, combines them, and bingo, no noise!, no other noise reduction used!.

Have I missed something?.
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Old 13-03-2015, 10:34 PM
Redshift13 (Rohan)
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Might give it a try - if it works it could be a handy processing method.
Wonder how this method would work for nightscapes if you had, say, some trees, a lake or a rock formation in the foreground...
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Old 13-03-2015, 10:53 PM
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Shiraz (Ray)
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that's what Mike S has been saying for years - no calibration, just dither. Dither de-correlates any fixed pattern noise and then it is subject to reduction by stacking. Without dither, fixed pattern noise is not reduced by stacking.
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Old 13-03-2015, 11:21 PM
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AstroJason (Jason)
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Cheers for sharing Fred, very informative. Looking forward to giving this a try. I wonder how much benefit there would be to cooling the camera in addition to using his dithering technique?

Rohan, you could definitely use this technique with nightscapes. You would take all your frames of the sky and landscape foreground. If you're using a static tripod, just pick one of those frames to mask the landscape back in over the top of the stacked image within Photoshop so the landscape looks sharp.
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Old 14-03-2015, 11:07 AM
Redshift13 (Rohan)
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After doing some more reading on the subject this morning, it seems to be the case that to achieve dithering, your camera needs to be attached to a telescope mount, with the dithering controlled by software. My nightscape setup is much more simplistic - camera on a tripod.
So I'm guessing that this makes 'manual' dithering much more difficult, if not impossible?
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Old 14-03-2015, 12:41 PM
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Yes, I think it would make it more difficult to slightly pan the camera on just a simple tripod. Although, if you can keep track of where and how far you have panned I think this technique it would still be possible. Worth a try anyway.
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Old 14-03-2015, 01:01 PM
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Camelopardalis (Dunk)
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With a manual non-tracking mount surely the rotation of the Earth is dithering for us automatically?
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Old 14-03-2015, 01:05 PM
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Very good point Dunk!
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Old 15-03-2015, 10:36 PM
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strongmanmike (Michael)
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What Ray said

Actually, I just last night dithered..?..without dithering.

Like most, without guiding I get slow drift due to polar missalignment and tube/mount flexure etc so because I was doing lots of very short subs I didn't bother getting the software to move the scope between subs ala my normal dithering (this would have added too much time as I was trying to image through breaks in the clouds ) I just let the dozens of quick subs come down and over time the slow drift created enough shift between subs (with out elongating stars) so that when I median combined - bingo noise gone

Got THIS image which proves you can image when it is cloudy

Mike
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Old 15-03-2015, 11:18 PM
DJT (David)
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Great share, Fred. I have a 60d and a Teegul Patrol I havnt used yet so will have a play with this approach over the next couple of weeks.
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Old 16-03-2015, 03:52 AM
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It would be interesting if you could use a Pentax or similar camera that moves its sensor (normally for Opitcal image stabilistation) and program it to automatically dither. The amount would be exactly known, so in theory, if the manufacturer wanted to, this could be implemented in the camera itself.

With the nightscape photography, perhaps mounting the camera on a micrometer stage or macro set might work?
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Old 16-03-2015, 07:16 AM
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an interesting read - thanks for posting the link.
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Old 16-03-2015, 12:05 PM
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rustigsmed (Russell)
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scroll down to Ivo's comments.

http://www.cloudynights.com/topic/48...-using-a-dslr/

Cheers

Rusty
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  #14  
Old 16-03-2015, 12:22 PM
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Bassnut (Fred)
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Thanks Russell, Ivos post in cloudynights is very interesting, WOW !!!!.

edit: its worth reading the comments after Ivos post too.

I thought something wasnt right, it all seemed so out of left field.
Aint astrophotography and what ppl do with it interesting!.

Last edited by Bassnut; 16-03-2015 at 12:46 PM.
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Old 16-03-2015, 01:04 PM
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multiweb (Marc)
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Yes, Ivo knows his stuff and is on the money. A lot of quackery in that video. Pretty misleading actually. There's no freebie in data acquisition and calibration.

Last edited by multiweb; 16-03-2015 at 01:41 PM.
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Old 16-03-2015, 02:49 PM
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Quote:
Originally Posted by rustigsmed View Post
scroll down to Ivo's comments.

http://www.cloudynights.com/topic/48...-using-a-dslr/

Cheers

Rusty
I find myself agreeing with Ivo.
Geoff

Last edited by Geoff45; 16-03-2015 at 04:02 PM.
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  #17  
Old 16-03-2015, 04:56 PM
Poita (Peter)
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Still, a good reminder of the advantages of dithering.

What would be great is if someone had some really nice data from a DSLR and ran it through the traditional method and the Hallas method to show just what is lost doing it the Hallas way.
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Old 16-03-2015, 09:06 PM
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Shiraz (Ray)
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Agree, lets not throw the baby out with the bathwater. Looks like some of the processes that Hallas used are not very sensible for high resolution imaging, but the basic idea of using dither instead of calibration to remove fixed pattern noise is very powerful. It would be great if someone has a set of dithered DSLR subs that they could stack using either full calibration or dither+stack without cal to see what happens - hope this is not too far from your original post Fred?

For comparison, I just did a test to see how effective the two stacking methods are on 30 subs of dithered mono, temp controlled data. With dither stacking I did a bad pixel removal followed by a simple average stack and no calibration at all. With standard stacking I used an average stack with bias (100 bias subs), darks at the same temperature (70 darks) and flats (100 flats). The full calibration stack image showed evidence of hot pixel effects (no outlier rejection was used) so a bad pixel map was also used to remove the odd warm pixels from the images after the initial calibration.

the results: Dither really can do the job just as well as a full calibration - left image is dither only, right image is full calibration. There is a slight brightness difference where I did not get the offsets exactly equivalent, but that's it. The SNRs from the two processes were effectively identical as measured by the sky background variance.
Attached Thumbnails
Click for full-size image (stacking.jpg)
91.7 KB26 views

Last edited by Shiraz; 17-03-2015 at 11:20 AM. Reason: better pic
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  #19  
Old 17-03-2015, 07:04 AM
Poita (Peter)
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Great stuff Ray, perhaps we should start a new thread inviting people to upload their data and compare results.
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  #20  
Old 17-03-2015, 09:36 AM
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multiweb (Marc)
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Quote:
Originally Posted by Shiraz View Post
Looks like some of the processes that Hallas used are not very sensible for high resolution imaging, but the basic idea of using dither instead of calibration to remove fixed pattern noise is very powerful.
Hi Ray, no doubt dithering is a proven solution and a very powerful tool to remove noise and to some extend dust motes and other static patterns.
All the processes highlighted in the video aren't bad, but it is a bit rich to talk about noise reduction with non linear data. We might as well stack JPEG files and call it a day hey?
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