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FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial 
Thread started 20 Oct 2011 (Thursday) 11:46
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Milkyway nightscapes

 
davidmtml
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Jul 15, 2016 09:41 |  #3226

I was worried I wasn't going to be able to get much last night due to the moon being so bright. Pretty pleased with how this turned out, but might need some tweaking to the processing as I was working on it at 2:00 in the morning! This is a 14 image stitch with the Samyang 14mm f2.8.

IMAGE: https://c1.staticflickr.com/9/8761/28290525096_8771104911_b.jpg
IMAGE LINK: https://flic.kr/p/K6Wk​u5  (external link) Morony Dam at Midnight (external link) by David Rabenberg (external link), on Flickr



  
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Jul 15, 2016 10:10 |  #3227

NCHANT wrote in post #18067428 (external link)
Doesn't seem to have the colours of a meteor?

This is true, it could be that they've clipped the highlights and lost the color information, all the stars are white in the image which makes me think that could be what happened.


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Jul 19, 2016 18:45 |  #3228

After checking up on this tread a few weeks ago, I captured this from my buddy's back deck in VT. I'm happy with the results! Funny thing is there's more light pollution in AK where I live then here in VT.

IMAGE: https://dl.dropboxusercontent.com/u/16139001/2016/20160711-_O1C1990.jpg

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Jul 19, 2016 18:57 |  #3229

NCHANT wrote in post #18067428Doesn't seem to have the colours of a meteor?

pdxbenedetti wrote in post #18067688 (external link)
This is true, it could be that they've clipped the highlights and lost the color information, all the stars are white in the image which makes me think that could be what happened.


yup agree, good call, pdxbenedetti


in that image you are not seeing the full trail of the meteor as the shutter closed BEFORE the meteor trail faded
therefore you are still seeing the fireball at the head of the trail, rather than a full brightening and fading trail :-)

Dave


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potinam
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Jul 19, 2016 20:23 |  #3230

Nice pictures. When I took picture with 30s I can see star trail, I just wonder if it possible to take astronomy with 2.8 lens.




  
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Jul 19, 2016 20:37 |  #3231

potinam wrote in post #18071975 (external link)
Nice pictures. When I took picture with 30s I can see star trail, I just wonder if it possible to take astronomy with 2.8 lens.

There's some useful information here...

https://photography-on-the.net …showthread.php?​p=18066056


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Jul 19, 2016 20:46 as a reply to  @ potinam's post |  #3232

If you have a wider focal length lens, you'll get less trailing.


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Jul 19, 2016 23:38 |  #3233

archer1960 wrote in post #18072002 (external link)
If you have a wider focal length lens, you'll get less trailing.

No, not true. Trailing depends ENTIRELY on exposure length, not focal length. Focal length will dictate the length of exposure you can take before you get star trailing, but focal length combined with aperture will dictate the amount of LIGHT you collect with any given exposure. That is the single most important thing with night photography, you have to balance field of view with the ability to collect as much light as possible in conditions with minimal light. Too wide of a lens will usually mean not fast enough of an aperture (ultra wide angle lenses normally have apertures from f2-f2.8), even with a 30 second exposure at f2.8 with an ultra wide angle lens you will collect half or even less as much light as using a 24mm f1.4 lens taking a 10-15 second exposure.


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Jul 19, 2016 23:48 |  #3234

pdxbenedetti wrote in post #18072155 (external link)
No, not true. Trailing depends ENTIRELY on exposure length, not focal length.

You should probably correct that statement. Then why have a 600 or 500 or 100 or what ever rule you want to use that includes focal length in the rule?


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Jul 20, 2016 05:31 |  #3235

pdxbenedetti wrote in post #18072155 (external link)
No, not true. Trailing depends ENTIRELY on exposure length, not focal length. Focal length will dictate the length of exposure you can take before you get star trailing, but focal length combined with aperture will dictate the amount of LIGHT you collect with any given exposure. That is the single most important thing with night photography, you have to balance field of view with the ability to collect as much light as possible in conditions with minimal light. Too wide of a lens will usually mean not fast enough of an aperture (ultra wide angle lenses normally have apertures from f2-f2.8), even with a 30 second exposure at f2.8 with an ultra wide angle lens you will collect half or even less as much light as using a 24mm f1.4 lens taking a 10-15 second exposure.

But its not about the light. Its about trailing. And if you use a 50mm lens at 2.8 and 30s to shoot the milkyway, or you use a 16mm lens at 2.8 the wide angle shot will have a lot less starttrailing. I partly agree thats it about exposure time too, but not how you stated entirely. Its a mix of FL AND exposure time.

I dont know why some people must always correct another person when it was clear what he meant. Especially when the correction is not totally correct either.


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Jul 20, 2016 15:09 |  #3236

FEChariot wrote in post #18072163 (external link)
You should probably correct that statement. Then why have a 600 or 500 or 100 or what ever rule you want to use that includes focal length in the rule?

Again, the rules that people use to determine exposure length are dependent on what they feel are reasonable amounts of star trailing and are dependent on focal length. Wider focal lengths do not mean more or less star trailing, wider focal lengths mean you can take longer exposures before that significant star trailing becomes apparent. Exposure length dictates star trailing, but exposure length is not the only factor for collecting light and collecting light is the most important thing for producing astrophotography images.

davidfarina wrote in post #18072303 (external link)
But its not about the light. Its about trailing. And if you use a 50mm lens at 2.8 and 30s to shoot the milkyway, or you use a 16mm lens at 2.8 the wide angle shot will have a lot less starttrailing. I partly agree thats it about exposure time too, but not how you stated entirely. Its a mix of FL AND exposure time.

I dont know why some people must always correct another person when it was clear what he meant. Especially when the correction is not totally correct either.

It's not about the light? The whole point of photography is to capture light, stars produce light, the whole point of doing astrophotography is to take pictures of stars (and other night sky subjects), ergo the whole point of astrophotography is to capture the light produced by stars/other night sky objects. The amount of acceptable star trailing will vary from person to person, some use a "rule of 600", others a "rule of 500", others a "rule of 200", in any case the focal length is used to determine exposure length and exposure length will determine the amount of star trailing. But like I said above, exposure length is just one factor for collecting light, light which is absolutely vital for producing an image. If you collect too little light you won't see anything in your image, if you expose for too long to capture more light you will get star trailing.

Efficiently capturing light will yield better Signal to Noise ratios, that's why you want a lens that is fast and is reasonably wide angle (like the much heralded 24mm f1.4), those lenses will capture significantly more light (or worst case, equivalent levels) with shorter exposure times. Think about it this way, if a 24mm f1.4 lens captures ten times as much light with an exposure compared to an 14mm f2.8 lens you could use a 3 second exposure with the 24mm lens to capture an equivalent amount of light as a 30 second exposure with the 14mm. A 3 second exposure with the 24mm will have significantly LESS star trailing than a 30 second exposure with the 14mm lens, using the rule of 500 the maximum exposure with 14mm is ~35 seconds, max with the 24mm is ~20 seconds. 30 out of 35 seconds is almost 86% of the maximum exposure time, 3 out of 20 seconds is only 15% of the maximum exposure time. You could theoretically capture 10 separate exposures with the 24mm at 3 seconds each in the time it takes to capture one 30 second exposure with the 14mm, you could either stitch those images to produce a wider field of view or stack them to improve SNR and a much better overall final image.


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Jul 20, 2016 15:16 |  #3237

pdxbenedetti wrote in post #18072738 (external link)
Again, the rules that people use to determine exposure length are dependent on what they feel are reasonable amounts of star trailing and are dependent on focal length. Wider focal lengths do not mean more or less star trailing, wider focal lengths mean you can take longer exposures before that significant star trailing becomes apparent. Exposure length dictates star trailing, but exposure length is not the only factor for collecting light and collecting light is the most important thing for producing astrophotography images.

It's not about the light? The whole point of photography is to capture light, stars produce light, the whole point of doing astrophotography is to take pictures of stars (and other night sky subjects), ergo the whole point of astrophotography is to capture the light produced by stars/other night sky objects. The amount of acceptable star trailing will vary from person to person, some use a "rule of 600", others a "rule of 500", others a "rule of 200", in any case the focal length is used to determine exposure length and exposure length will determine the amount of star trailing. But like I said above, exposure length is just one factor for collecting light, light which is absolutely vital for producing an image. If you collect too little light you won't see anything in your image, if you expose for too long to capture more light you will get star trailing.

Efficiently capturing light will yield better Signal to Noise ratios, that's why you want a lens that is fast and is reasonably wide angle (like the much heralded 24mm f1.4), those lenses will capture significantly more light (or worst case, equivalent levels) with shorter exposure times. Think about it this way, if a 24mm f1.4 lens captures ten times as much light with an exposure compared to an 14mm f2.8 lens you could use a 3 second exposure with the 24mm lens to capture an equivalent amount of light as a 30 second exposure with the 14mm. A 3 second exposure with the 24mm will have significantly LESS star trailing than a 30 second exposure with the 14mm lens, using the rule of 500 the maximum exposure with 14mm is ~35 seconds, max with the 24mm is ~20 seconds. 30 out of 35 seconds is almost 86% of the maximum exposure time, 3 out of 20 seconds is only 15% of the maximum exposure time. You could theoretically capture 10 separate exposures with the 24mm at 3 seconds each in the time it takes to capture one 30 second exposure with the 14mm, you could either stitch those images to produce a wider field of view or stack them to improve SNR and a much better overall final image.

Of course photography is all about light. I was reffering to the topic of star trails...

Gosh, some people too anal over here


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Jul 20, 2016 15:48 |  #3238

davidfarina wrote in post #18072747 (external link)
Of course photography is all about light. I was reffering to the topic of star trails...

Gosh, some people too anal over here

I'm just trying to use reason to justify why you'd use a certain lens or exposure time and what the aspects are that one should focus on for producing high quality astrophotography images, not being anal. Can you get good astro images with a ultra wide angle lens with a moderately fast aperture? Sure, but if all you are focusing on is exposure length then you are missing out on some other key technical aspects which could help improve image quality AND maybe even save you time.


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Jul 20, 2016 16:35 |  #3239

pdxbenedetti wrote in post #18072776 (external link)
I'm just trying to use reason to justify why you'd use a certain lens or exposure time and what the aspects are that one should focus on for producing high quality astrophotography images, not being anal. Can you get good astro images with a ultra wide angle lens with a moderately fast aperture? Sure, but if all you are focusing on is exposure length then you are missing out on some other key technical aspects which could help improve image quality AND maybe even save you time.

You said when it comes to star trails it depends only on exposure time which is not true. That was my main point. Of course it depends on the exposure time, but not entirely. The Focal Length also adds to that equation. The amount of star trails of 15s at 24mm will be not less than lets say a 20s exposure at 20mm. It is less time to cause startrails, but it is also zoomed in a bit more due to the focal length. That was my only point, to disprove your sentence saying star trails depend entirely of exposure time, not on FL.


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Jul 20, 2016 17:25 |  #3240

davidfarina wrote in post #18072818 (external link)
You said when it comes to star trails it depends only on exposure time which is not true. That was my main point. Of course it depends on the exposure time, but not entirely. The Focal Length also adds to that equation. The amount of star trails of 15s at 24mm will be not less than lets say a 20s exposure at 20mm. It is less time to cause startrails, but it is also zoomed in a bit more due to the focal length. That was my only point, to disprove your sentence saying star trails depend entirely of exposure time, not on FL.

Actually yes, the star trails will be less in a 15s exposure at 24mm vs a 20s exposure on a 20mm assuming the same camera is used. Let's say you take an exposure with a Canon t5i, the Canon t5i has a 22.3 x 14.9 mm sensor and image size is 5,184 x 3,456 pixels, so the pixel pitch is 22.3 / 5184 = 0.0043 mm. The plate scale = 206265 * 0.0043 / 20 = 44.35 arc-seconds with the 20mm lens and plate scale = 206265 (number of arc-seconds in one radian) * 0.0043 / 24 = 36.96 arc-seconds 24mm lens. So a star will move 1 pixel using the 20mm lens every 44.35 / 15 (this is the rate in arc-seconds per time second stars move in the sky based on earth's rotation) = 2.96 seconds. With the 24mm lens a star will move 1 pixel every 2.46 seconds. So in a 20 second exposure with the 20mm lens a star will move 6.77 pixels (20 / 2.96) and in a 15 second exposure with the 24mm lens a star will move 6.09 pixels (15 / 2.46). Is that enough to be noticeable in an image? I don't know, depends on cropping and what-not, but the star trail will be longer in a 20 second exposure with the 20mm lens vs a 15 second exposure with a 24mm lens.

The point is that focal length is a factor used to determine exposure time, the actual exposure time will determine the amount of star trailing after taking into account focal length and sensor information.


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