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08-03-2016, 03:41 AM - 1 Like   #1
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U3 Astrophoto setting uses AA filter simulator

For the first time in a Pentax DSLR, Pentax engineers have pre-programmed the User mode settings on the K-70. U1 defaults to "clear tone", U2 defaults to "advanced HDR", and U3 defaults to "astrophoto".

Here's my question for discussion:

In the U3 Astrophoto preset, the AA filter simulator is set to Type 1 (weak). Is moire even a concern when shooting stars? Could they have done this to reduce noise or false color?


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08-03-2016, 04:52 AM - 1 Like   #2
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The anti-moire setting ensures all the stars have the correct color. Without AA, if a high-resolution lens images a star as a point of light and that point of light falls only on a green pixel, the result would be a green star. The AA filter simulator ensures that each star's light falls on enough different Bayer filter pixels to get a more accurate measurement of color.
08-03-2016, 04:56 AM   #3
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Interesting. So... an alternative approach might be to use no AA filter simulation but engage pixel-shift instead, yes?
08-03-2016, 05:02 AM   #4
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QuoteOriginally posted by BigMackCam Quote
Interesting. So... an alternative approach might be to use no AA filter simulation but engage pixel-shift instead, yes?
Doubt it, because of motion.

QuoteOriginally posted by photoptimist Quote
The anti-moire setting ensures all the stars have the correct color. Without AA, if a high-resolution lens images a star as a point of light and that point of light falls only on a green pixel, the result would be a green star. The AA filter simulator ensures that each star's light falls on enough different Bayer filter pixels to get a more accurate measurement of color.
Interesting point. I would have thought that the motion in the sky would be sufficient, but this makes sense. There are inevitably going to be stars so small that they only land on a single pixel.


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08-03-2016, 05:05 AM   #5
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QuoteOriginally posted by Adam Quote
Doubt it, because of motion.
Ah, of course. We're talking long-ish exposures, aren't we? I'll shut up now
08-03-2016, 07:56 AM   #6
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There is an approach called "dithering", that is used to both randomize and reduce noise - increasing the effectiveness of stacking. This AA movement is very similar. I have always wondered with the ability to move the sensor, why Pentax has not applied it previously for this purpose. For dithering you want a movement of about 12 to 15 pixels in order to pickup a different noise pattern. Coupling the combination of dithering and pixel shift would be excellent, especially for better color. Then combining this with Astrotracking would be tremendous. If the shift amount and direction were included within the EXIF so that stacking would be exact, that would be excellent (especially if Pentax collaborated with a software stacking utility).
08-03-2016, 09:58 AM   #7
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QuoteOriginally posted by Adam Quote
Doubt it, because of motion.



Interesting point. I would have thought that the motion in the sky would be sufficient, but this makes sense. There are inevitably going to be stars so small that they only land on a single pixel.
Assuming one is using an exposure time that causes star trailing, the motion can help except if the star trails are at certain angles relative to the sensor. If the motion would make trails that happen to be either vertical or horizontal, then about half the stars would trail across only red and green sensitive pixels (creating yellow stars) and half would trail across blue and green sensitive pixels (creating cyan stars). If the motion would make trails that are at a 45 angle, then some stars would trail across only green sensitive pixels (creating green stars) and some would trail across red and blue sensitive pixels (creating magenta stars). Longer arcing or circular star trails would cycle through the various colors.

BigMackCam's suggestion of using pixelshift is a good idea if as long as the total pixel shift image creation time of (4*shutter_time + 3/4 sec) is much shorter than the max exposure time that avoids trailing (about 500/focal_length) or if the camera is on a tracking mount.
08-03-2016, 01:56 PM   #8
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QuoteOriginally posted by photoptimist Quote
Assuming one is using an exposure time that causes star trailing, the motion can help except if the star trails are at certain angles relative to the sensor. If the motion would make trails that happen to be either vertical or horizontal, then about half the stars would trail across only red and green sensitive pixels (creating yellow stars) and half would trail across blue and green sensitive pixels (creating cyan stars). If the motion would make trails that are at a 45 angle, then some stars would trail across only green sensitive pixels (creating green stars) and some would trail across red and blue sensitive pixels (creating magenta stars). Longer arcing or circular star trails would cycle through the various colors.

BigMackCam's suggestion of using pixelshift is a good idea if as long as the total pixel shift image creation time of (4*shutter_time + 3/4 sec) is much shorter than the max exposure time that avoids trailing (about 500/focal_length) or if the camera is on a tracking mount.
ie a 24mm lens exposed in pixel shift mode for 20 seconds at f2.8 and 3200 iso?
with stacking and a dark frame, I bet that would turn out pretty good... I'm not an astro photo guru by any means, but maybe there's a use for pixel shift I could actually utilize.
and after reading some of those articles, I may stick to sleeping in the hours of darkness,,,, eeegad


Last edited by nomadkng; 08-03-2016 at 02:21 PM.
08-03-2016, 07:56 PM   #9
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During a longish astro exposure, just where would the sensor movement take place for the virtual AA filter effect? Does the sensor divert slowly, during the exposure or during astrotracing?
Also, if during astrotracing, why not try pixel shifting? The stars are remaining stationary in the frame anyway.

Last edited by shiner; 08-03-2016 at 08:05 PM.
08-04-2016, 01:24 AM   #10
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QuoteOriginally posted by shiner Quote
During a longish astro exposure, just where would the sensor movement take place for the virtual AA filter effect? Does the sensor divert slowly, during the exposure or during astrotracing?
Also, if during astrotracing, why not try pixel shifting? The stars are remaining stationary in the frame anyway.
Pixel shift is unavailable in bulb mode.

As for how the AA filter simulator works, it just constantly shifts the sensor around to get light to spill over to adjacent pixels, mimicking the effect of a hardware AA filter. So this would happen throughout the exposure.

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08-04-2016, 02:24 AM   #11
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QuoteOriginally posted by Adam Quote
Pixel shift is unavailable in bulb mode.

As for how the AA filter simulator works, it just constantly shifts the sensor around to get light to spill over to adjacent pixels, mimicking the effect of a hardware AA filter. So this would happen throughout the exposure.
What about on a camera-timed 30 sec exposure (on astrotracer)?
08-04-2016, 03:05 AM   #12
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QuoteOriginally posted by shiner Quote
What about on a camera-timed 30 sec exposure (on astrotracer)?
It still counts as bulb mode, so the pixel shift setting gets greyed out. If you go to M mode with a 30-second exposure, you won't have the astrotracer, on the other hand.

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08-04-2016, 05:14 AM   #13
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QuoteOriginally posted by photoptimist Quote
The anti-moire setting ensures all the stars have the correct color. Without AA, if a high-resolution lens images a star as a point of light and that point of light falls only on a green pixel, the result would be a green star. The AA filter simulator ensures that each star's light falls on enough different Bayer filter pixels to get a more accurate measurement of color.

Thanks for the explanation. I didn't think of this but it make sense to me why I had green stars in first shots.
08-04-2016, 06:36 AM   #14
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QuoteOriginally posted by gaweidert Quote
Thanks for the explanation. I didn't think of this but it make sense to me why I had green stars in first shots.
You're welcome.

As a related aside: the Kepler space telescope is actually designed to operate slightly out-of-focus so that each star covers multiple pixels. If they did not do this, then the measured light from the stars would vary if the focused pinpoint of star light fell on the in-sensitive parts of the detector between the photosites.
08-05-2016, 12:20 AM   #15
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QuoteOriginally posted by Adam Quote
It still counts as bulb mode, so the pixel shift setting gets greyed out. If you go to M mode with a 30-second exposure, you won't have the astrotracer, on the other hand.
Thanks, Adam!
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