Forums › Forums › Farktography General Chat › This week’s contest › 03-09-11 – Stars
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Plamadude30k.
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March 6, 2011 at 10:07 pm #38635
U-Man
Participant…thermal noise… I could google that but I’ll just ask a question instead. Would a long exposure (say 30 minutes) at zero Fahrenheit have less noise that a similar exposure at 70 F?
March 6, 2011 at 10:17 pm #38636ravnostic
Participant…thermal noise… I could google that but I’ll just ask a question instead. Would a long exposure (say 30 minutes) at zero Fahrenheit have less noise that a similar exposure at 70 F?
Considerably, yes. Either, however, would have a good deal of noise.
FWIW, I won’t be doing any stacking, as I have continually delayed my wedge mount purchase, and it’s a must for stackable images. I am toying with my selections, though, and may be going all astral after all (which I’m sure I’ll regret).
I really need to correct that situation.
March 6, 2011 at 10:40 pm #38637ParticipantI suppose there is quite a bit of noise in here – but without a stacked comparison next to it the noise isn’t too bothersome to my eye.
ISO 250, 25 minute exposure, Peleng Fisheye with manual aperture – I forget what I set it at. On a frozen river in Mopsy’s front yard. Appx 10 degrees F. Canon 5D Mk II.
http://www.ullepics.com/d/1920-4/Startrails_4273.jpg
more noticeable noise at 1400 x 1400 – http://www.ullepics.com/d/1925-3/Startrails_4273.jpg
I’ll use one from this series.
March 6, 2011 at 10:47 pm #38638ParticipantOh yeah. As long as the technique is used for astrophotography, I would like to include stacked images for this theme. If anybody has strong feelings against this, speak up now or forever hold your piece.
Still don’t know about the false color thing.
/yes, it was intentional.
March 6, 2011 at 11:09 pm #38639Uranus
Participantheld 😛
/yes, it was intentional.
March 6, 2011 at 11:13 pm #38640ParticipantI suppose there is quite a bit of noise in here – but without a stacked comparison next to it the noise isn’t too bothersome to my eye.
ISO 250, 25 minute exposure, Peleng Fisheye with manual aperture – I forget what I set it at. On a frozen river in Mopsy’s front yard. Appx 10 degrees F. Canon 5D Mk II.
http://www.ullepics.com/d/1920-4/Startrails_4273.jpg
more noticeable noise at 1400 x 1400 – http://www.ullepics.com/d/1925-3/Startrails_4273.jpg
I’ll use one from this series.
There’s not too much noise in that shot, but you can spot it easily in the larger version (hot pixels; show up as ‘stars’ that don’t move). Usually, resizing down takes care of them, and ‘long exposure noise reduction’ does wonders for an initial shot.
And damn–if you’re allowing stacking, I’m doomed. But I’ll roll with it.
March 6, 2011 at 11:34 pm #38641Elsinore
Keymaster…thermal noise… I could google that but I’ll just ask a question instead. Would a long exposure (say 30 minutes) at zero Fahrenheit have less noise that a similar exposure at 70 F?
I’ve done some long exposure stuff (e.g. 10-20 minutes) in freezing temps hoping that would help with the hot pixels (that’s what thermal noise shows up as, in addition to general noisiness). Unfortunately, it didn’t seem to help much, if at all. I think the problem is that the sensor is generating enough heat during the exposure coupled with the fact that the location of the sensor is somewhat insulated from the cold to the point that it just can’t cool it enough during the exposure to really decrease the noise level to any large extent (at least that’s my theory). With the actual astrophotography CCD cameras, they have coolers mounted directly onto the sensors to keep thermal noise low.
Either way, I’ll edit the theme description to allow stacked exposures.
March 7, 2011 at 12:27 am #38642olavf
ParticipantThe AIM-9/R uses liquid nitrogen to keep the IR sensor cool before launch, but that’s probably not good for your DSLR. Especially if you dropped it.
Anyway, I have a general question about photo edits. I have a large amount of string – 33 miles to be exact – and although it’s not cut up into three-inch lengths, there’s a couple of spots where it’s visible in the shot. Is it okay to smudge that? As a side note, what about things like shadows that crept into the picture that bug the hell out of me? Like this one: http://www.follandfamily.com/TF/mypics/110226/7725-karla_red_sm_web.jpg (not for this contest, but it’s the general idea – and I may want to use it down the road)
March 7, 2011 at 1:06 am #38643KeymasterI would see removing things like shadows and strings to be over the line of what’s allowable for Farktography purposes. If retouching is used, it’s meant for things like pimples, hot pixels, a stray hair–in short, things that are just a few pixels here or there. Removing strings or shadows is closer to the removal of tree branches or telephone poles that’s referenced in the rules as being right out.
March 7, 2011 at 1:11 am #38644ParticipantNo worries. I didn’t think so, but the perfectionist in me made me ask =P
March 7, 2011 at 3:49 am #38645chupathingie
ParticipantI suppose there is quite a bit of noise in here – but without a stacked comparison next to it the noise isn’t too bothersome to my eye.
ISO 250, 25 minute exposure, Peleng Fisheye with manual aperture – I forget what I set it at. On a frozen river in Mopsy’s front yard. Appx 10 degrees F. Canon 5D Mk II.
http://www.ullepics.com/d/1920-4/Startrails_4273.jpg
more noticeable noise at 1400 x 1400 – http://www.ullepics.com/d/1925-3/Startrails_4273.jpg
I’ll use one from this series.
There’s not too much noise in that shot, but you can spot it easily in the larger version (hot pixels; show up as ‘stars’ that don’t move). Usually, resizing down takes care of them, and ‘long exposure noise reduction’ does wonders for an initial shot.
And damn–if you’re allowing stacking, I’m doomed. But I’ll roll with it.
Long-exposure noise increases with all of: ambient temperature, ISO, exposure duration. Some of you folks may be unfortunate enough to have a camera that suffers from amp glow, which will show up as a rather prominent gradient in longer (>5 min-ish) exposures. It gets worse until the camera gets as warm as it’s going to get. Only way to get rid of it is let the camera cool down or deal with it in post if it hasn’t saturated (either way you lose data…either by manipulation or lost acquisition time).
U-Man, that looks awesome…I’d imagine the noise is much more noticeable at full-rez, but still…25 minutes is a LONG exposure… I’m gonna have to try that myself and see how the 5DII fares at somewhat warmer temps.
March 7, 2011 at 4:13 am #38646ParticipantI’ve got 40 minute and 37 minute pics at 100 ISO as well. One is with a 5D Mk II and the other is from a 7D. You’ll see at least one of those this week.
March 7, 2011 at 4:19 am #38647ParticipantLooking forward to seeing them… I might enter a couple of older shots I have filed away from way back if I even still have them.
The last time I opened a shutter for any appreciable length of time I used a 300D 😆
I’ve been wanting to find time to play with the low-ISO end of the 5DII just for star trails, but I guess I’ll never get to unless I quit answering the phone when work calls looking for help…
March 7, 2011 at 4:31 am #38648ParticipantI took these during the Christmas holiday this year at Mopsy’s in Michigan’s U.P. I walked out and set up using ‘Bulb’, clicked the remote shutter and then went back inside. It was total guesswork. I went back to pick up my camera “a while” later. 🙂
March 7, 2011 at 5:44 am #38649Plamadude30k
ParticipantLet me briefly explain what I mean by false-color imaging.
With big telescopes, the only way we have of making a full color image is taking separate images with different colored filters (the ones I typically use are Harris B, V, and R filters which have center wavelengths of 433, 546, and 647 nm respectively. I sometimes also use near IR and near UV filters.) This results in 3 black and white images at different wavelengths:
b: https://picasaweb.google.com/lh/photo/Y1T_Tb6enZuc-MAAiMF35BxSdVnS5UXY6FXQUhEu-Uw?feat=directlink
v: https://picasaweb.google.com/lh/photo/FWs6OJHJzFphuAhfcFbPjhxSdVnS5UXY6FXQUhEu-Uw?feat=directlink
r: https://picasaweb.google.com/lh/photo/PoQMN-Hp08YBuNRiMA9ZuhxSdVnS5UXY6FXQUhEu-Uw?feat=directlinkAfter some (relatively minor) noise-reduction techniques, there are a couple of ways I can turn these three images into one color one. The easiest way involves a program called SAOimage DS9, where I can load each of the images in the appropriate color (for example, the r image will be red). I’ve used other methods in the past, but those are way more time intensive-this is probably the simplest. Here’s a result using that method (without applying any noise-reduction):
color: https://picasaweb.google.com/lh/photo/NUUH7qWas58IRqc56RQVFRxSdVnS5UXY6FXQUhEu-Uw?feat=directlink
I know for sure that this is way beyond the usual farktography rules, and wouldn’t be used in any normal contest. If I can’t use any images like this in this particular contest, I’ll just wait for the next astro-themed one.
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