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12-22-2010, 10:08 AM   #1
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Star Trail 1hr - But what is the purple hue??

I was attempting to shoot star trails for the first time while on holiday in the mountainous Cederberg region last week. The moon was out, so the first evenings attempts were limited to about 10mins. The second night we got up in the wee hours after the moon had set to watch a meteor shower, and I left the camera running for about an hour.

Perhaps my own stupidity, but I turned long exposure noise reduction off to get to see the images immediately. I expected to see some noisy spots, but the purple patches surprised me, and I have no idea what causes them. With the moon out on the first night, I thought they might be light bending in from the moon or via the eyepeice. The second night the moon was down, and I covered the eyepeice with some white insulation tape (all I had, but very opaque).

Any ideas or tips for this purple hue problem, or what else to try to improve image quality? Thanks in advance.

Sigma 10-20 @ 11mm, f/8, ISO 280, time-3200s

Going back in Jan for another 6 days, so plenty time to experiment then.



12-22-2010, 11:18 AM   #2
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I'm guessing that you were shooting south. If so, the patches on the right side are probably the Larger and Smaller Magellanic Clouds, 2 small satellite galaxies to the Milky Way. The rest of the patchy stuff would be part of the Milky Way itself. Lucky you did turn NR off as you would have had to wait an other hour for the camera to process the dark signal. And you would have still be left with your purple hue/patch "problem". Otherwise, pic looks fine to me.
12-22-2010, 11:41 AM   #3
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one HOUR exposure?! some of that is probably Magellanics, the rest is probably noise. good thing nr was off otherwise, well you probably know, another hour wait to see results. maybe try many shorter exposures & stacking next time. google 'startrails' for plenty of good advice.
12-22-2010, 01:44 PM   #4
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It's artefact as a result of stray ambient light creeping its way to the sensor, either tangentially through the front element or through the eyepiece. Next time, frame the shot, shield any street lighting from the lens, focus and then ensure your eyepiece cover is on during the exposure.

There should be no reason why you cannot do a single noise-free multi-hour exposure (with a consistent power source) on your K200D - great CCD sensor.

12-22-2010, 02:05 PM   #5
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It's sensor noise. With long exposure at high ISO almost all DSLR's suffer from the problem.I have had a lot of experience with LE images and all of them exhibit this issue. You can reduce it by using a stopped down aperture and low ISO.

It is not ambient light as some will tell you. I have had this problem on a moonless night in the middle of some of Oregons deepest forests hundreds of miles away from any light source.

You can remove most of it in post and also eliminate it by turning on Slow Shutter Speed NR (item 10 in the Settings menu) This will double the exposure time however because it takes a second dark frame exposure:




Here is the original:



and an extreme example:


Last edited by mtroute; 12-22-2010 at 02:25 PM.
12-22-2010, 02:42 PM   #6
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To prove it's not ambient light pollution just take a 1 hour shot with the cap on
12-22-2010, 03:30 PM   #7
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QuoteOriginally posted by Lowell Goudge Quote
To prove it's not ambient light pollution just take a 1 hour shot with the cap on
In case you were doubting that it was not ambient light, I would like to know how two cameras taking shots in different parts of the world at different times would have the same ambient light patterns...

If you were agreeing that it was NOT ambient light then...You my friend are cool and chicks dig ya!

12-23-2010, 01:19 AM   #8
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QuoteOriginally posted by pixelsaurus Quote
I'm guessing that you were shooting south. If so, the patches on the right side are probably the Larger and Smaller Magellanic Clouds, 2 small satellite galaxies to the Milky Way. The rest of the patchy stuff would be part of the Milky Way itself. Lucky you did turn NR off as you would have had to wait an other hour for the camera to process the dark signal. And you would have still be left with your purple hue/patch "problem". Otherwise, pic looks fine to me.
Indeed shooting south, and the white splodges are the Magellanics. The sky was very dark, so they stood out to the naked eye.

QuoteOriginally posted by rhodopsin Quote
one HOUR exposure?! some of that is probably Magellanics, the rest is probably noise. good thing nr was off otherwise, well you probably know, another hour wait to see results. maybe try many shorter exposures & stacking next time. google 'startrails' for plenty of good advice.
Thanks for Google tip. I was forgetting the obvious limitless info source. Although it seems that the sites are divided on long exposure versus stacking. It seems that stacking is essential if the sky is too bright for long exposure. Will need to play with both. They all seem to advise going fully open and low ISO. Stacking is also advanced as way to reduce LE noise.

QuoteOriginally posted by Ash Quote
It's artefact as a result of stray ambient light creeping its way to the sensor, either tangentially through the front element or through the eyepiece. Next time, frame the shot, shield any street lighting from the lens, focus and then ensure your eyepiece cover is on during the exposure.

There should be no reason why you cannot do a single noise-free multi-hour exposure (with a consistent power source) on your K200D - great CCD sensor.
Ash, Unlikely to have been stray light, as Cederberg is very remote. About 100km from nearest town. No light sources (and then only minor lights) within about a mile, and nothing line of sight from photo site. But I need to retrieve the proper eyepiece cover for this work.
12-23-2010, 01:38 AM   #9
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QuoteOriginally posted by mtroute Quote
It's sensor noise. With long exposure at high ISO almost all DSLR's suffer from the problem.I have had a lot of experience with LE images and all of them exhibit this issue. You can reduce it by using a stopped down aperture and low ISO.

It is not ambient light as some will tell you. I have had this problem on a moonless night in the middle of some of Oregons deepest forests hundreds of miles away from any light source.

You can remove most of it in post and also eliminate it by turning on Slow Shutter Speed NR (item 10 in the Settings menu) This will double the exposure time however because it takes a second dark frame exposure:
Thanks mtroute. It seems your LE noise pattern is almost identical ito the purple patches. Must be a camera/sensor thing. I was very fairly convinced it was not a stray light issue (I suspect the Cederberg is nearly as remote as Oregon, just dry and hot). Would be fascinated to see if the camera produces these patches for a pure dark image. Lowell Goudge's suggestion might be worth an experiment for me.

Did the LE noise reduction remove the purple patches for you?

BTW: your pic of the boat and star-trail is really good imho. Great framing.
12-23-2010, 03:37 AM   #10
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The mention of the eye piece cover is interesting I had forgotten about that little piece of kit. I had supplied with a K100d but not my K10d, Kx [now sold ] and K-r . i wonder why Pentax no longer supply them.

cederburg, like the whole Cape Province a beautiful place
12-23-2010, 04:03 AM   #11
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QuoteOriginally posted by mtroute Quote
and an extreme example:
Did you fire a strobe off-camera to the right on this one? Some of the shadows give that appearance.
12-23-2010, 04:23 AM   #12
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I stand corrected.
Interestingly my LE noise patterns differed from scene to scene on my K10D, and diminished with use of the eyepiece cover, which led me to believe that light entering the eyepiece had something to do with the artefact.
12-23-2010, 07:02 AM   #13
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QuoteOriginally posted by Ash Quote
I stand corrected.
Interestingly my LE noise patterns differed from scene to scene on my K10D, and diminished with use of the eyepiece cover, which led me to believe that light entering the eyepiece had something to do with the artefact.
They do supply a co ER for just this use

I have a film camera Ricoh XR2s with an eyepiece shutter
12-23-2010, 08:57 AM   #14
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QuoteOriginally posted by Ash Quote
I stand corrected.
Interestingly my LE noise patterns differed from scene to scene on my K10D, and diminished with use of the eyepiece cover, which led me to believe that light entering the eyepiece had something to do with the artefact.
On the first night I was also thinking it was light entering the eyepiece, because the moon was at about 90deg side on to the camera. But the purple intensity did not change when I put the tape across the eyepiece. I also though it might be from the red light head-lamp I was using intermittently (to find the camera in the dark..). But after the second night with no moon, and minimal interaction with camera, I was fairly sure it was not stray light.

Fascinating how the purple patches are almost identical on mtroute's image. I wonder if this is something caused by proximity of CCD to other electronic components in body. Heat or EM interference...
12-23-2010, 12:57 PM   #15
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QuoteOriginally posted by KevinR Quote
Fascinating how the purple patches are almost identical on mtroute's image. I wonder if this is something caused by proximity of CCD to other electronic components in body. Heat or EM interference...
I've also experienced similar purple coloring in upper corners and one left of center on a K200D. First I thought it was caused by light coming thougth viewfinder, but now I think it might be related to camera...
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