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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

 
ptcanon3ti
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Jul 20, 2016 17:43 |  #3241

pdxbenedetti wrote in post #18072857 (external link)
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.


I'm asking this ONLY because I'm just curious...I'm definitely NOT a math head. So, where did you get those figures and calculations, and do you actually use them when you go shoot star shots? I'm NOT trying to be a smart ass here. I really want to know. :)


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pdxbenedetti
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Post has been edited over 1 year ago by pdxbenedetti.
Jul 20, 2016 17:58 |  #3242

ptcanon3ti wrote in post #18072862 (external link)
I'm asking this ONLY because I'm just curious...I'm definitely NOT a math head. So, where did you get those figures and calculations, and do you actually use them when you go shoot star shots? I'm NOT trying to be a smart ass here. I really want to know. :)

Ha, no I don't use them when I do star shots, I use tracking mounts so star trailing isn't something I worry about. Roger Clark has several pages of technical resources for doing calculations like these, about half way down this page is where I stole the information for doing those calculations:

http://www.clarkvision​.com/articles/nightsca​pes/ (external link)

He lists plate scales for various focal lengths and camera sensors here:

http://www.clarkvision​.com/articles/platesca​le/ (external link)


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Davenn
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Jul 20, 2016 19:00 |  #3243

ptcanon3ti wrote in post #18072862 (external link)
I'm asking this ONLY because I'm just curious...I'm definitely NOT a math head. So, where did you get those figures and calculations, and do you actually use them when you go shoot star shots? I'm NOT trying to be a smart ass here. I really want to know. :)


no you don't ... that was a serious mouthful

more simply .....
for a full frame camera 500 / lens focal length = time before trailing eg 500 / 24 = 20.8 sec ... make it 20 sec

for a crop sensor camera 500 / (lens focal length x crop factor) = time before trailing eg 500 / (24 x 1.6 = 38.4) = 13 sec

Dave


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FEChariot
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Post has been edited over 1 year ago by FEChariot.
Jul 20, 2016 20:03 |  #3244

pdxbenedetti wrote in post #18072155 (external link)
Trailing depends ENTIRELY on exposure length, not focal length.

pdxbenedetti wrote in post #18072738 (external link)
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

Holy crap! You completely contradicted yourself. Then you go on and write paragraph after paragraph on the 600 rule further contradicting yourself.

Are you simply one of those people who are incapable of admitting fault?


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pdxbenedetti
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Jul 20, 2016 23:12 |  #3245

FEChariot wrote in post #18072962 (external link)
Holy crap! You completely contradicted yourself. Then you go on and write paragraph after paragraph on the 600 rule further contradicting yourself.

Are you simply one of those people who are incapable of admitting fault?

What? In no way did I contradict myself, I don't understand what's so hard with this, in VERY simple terms: at Xmm focal length on Y camera a star will move Z pixels per second, so the number of seconds will determine the number of pixels that star moves over the exposure and produces a trail, it's not rocket science. Nowhere did I say in my post that star trailing was dependent on focal length, that specific comment said exposure length was dependent on focal length as calculated by various rules used by different people.


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FEChariot
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Jul 21, 2016 06:38 |  #3246

pdxbenedetti wrote in post #18073100 (external link)
What? In no way did I contradict myself,

If you can't read the two statements you wrote (that I quoted above) and see they are complete contradictions, then we need to work on your comprehension skills.

So what is it? Does focal length affect how long you can expose for without seeing star trails or not? You have said both yes and no now.

I don't want I dissertation answer I just want to know which of your statements was made in error. Both yes and no cant be the answer.


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TCampbell
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Jul 21, 2016 10:24 as a reply to FEChariot's post |  #3247

It turns out some photographers will tell you to use the "600 rule" and some will tell you to use the "500 rule". They are really the same rule... just a different baseline. The fact that there are two different baselines should suggest that the amount of star trailing is subjective.

If you plan to blow up the image to a very large size and mount on the wall, you might see elongated stars if you use the "600 rule" ... so maybe you go with the "500 rule" to be a bit more conservative. If, on the other hand, you plan to share the image on the web, it won't be large enough to notice star trailing if you follow the "600 rule" and you get to collect a bit more light.

You can calculate the angular dimensions of your camera frame based on the sensor size and lens focal length (this site has a calculator that can make that easy: http://www.tawbaware.c​om/maxlyons/calc.htm (external link) ). If you convert the degrees into arc-seconds (multiply by 3600 to convert degrees to arc-seconds) and then divide that by the number of pixels in your cameras physical dimensions (sensor resolution on the horizontal and vertical axes) you'll know how many arc-seconds per pixel you have when you use that particular lens.

The Earth's rate of rotation is 15.04 arc-seconds per second (that's our "angular" rate of rotation). But this is the rate of speed that a star will seemingly travel (due to diurnal motion) IF (and only if) that star is located directly above the Earth's equator (a star located at "declination 0º" -- declination are sky coordinate values by the map to Earth's latitude values. A star that has a declination component of it's coordinate as 0º is directly over Earth's equator and a star with a declination component of +90º is located directly over Earth's north pole and a star with a declination component of -90º is located directly over Earth's south pole.) BTW, I've sort of ignored that the since the field of view is wide, the stars in one part of your image are moving faster than the stars at the in the opposite end of the image -- so in reality you'd also account for that.

It turns out that if the star is located at declination 90º then that star will not appear to move at all. Polaris is "close" ... but not quite at +90º (it's about 2/3rds of a degree off the pole but we call it the pole star because it's within 1º of true North.)

Anyway... by the math, since the rate of star movement is slowed as you get nearer to the pole, you can express that movement mathematically by multiplying the 15 arc-second per second rate of rotation by the COSINE of the declination of the object you are imaging. So assuming you are imaging an object at, say, declination +60º it would be: 15 x cosine(60) = 7.5 (the cosine of 60º happens to be .5). So that means the rate of movement of that star (at declination +60º) is only 7.5 arc-seconds per second. Knowing the number of arc-seconds per pixel on your camera sensor (given he focal length of your lens) you can determine how many seconds you can image before 1 pixel of movement has occurred. You'll never notice 1 pixel's worth of movement (and I don't care how good your eyes are) ... but what about 2 pixels... 3 pixels? What about 5 pixels?

So back to Eric's point... the real question is, how much trailing can occur before you will even notice it? This probably depends a bit upon the image size you plan to use for displaying the image.

You can do the long version of the math and be precise. But if you don't want to do the long version, we can suggest that if you follow either the 600 rule or the 500 rule then "most people" would probably think you have no star trailing in your image (a subjective judgement, but you'd find it's probably the majority subjective judgement.)

But mathematically, if we imported your image into PixInsight, sampled a dozen or so stars, and ran them through the point spread function (PSF), I bet it would detect that your stars aren't really "round"). Of course, you can also have non-round stars just from sagittal or meridonal axial blurring depending on the quality of your lens and it's ability to handle the focal ratio; even if you were on a tracking mount with perfect tracking (so technically no star trailing). Astrophotographers are always trying to look for ways to make sure their stars are "round". It's a never-ending quest. (Vibration, flexure, alignment precision, periodic error, collimation, optical imperfections... the list of reasons goes on and on.)




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pdxbenedetti
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Jul 21, 2016 10:50 |  #3248

FEChariot wrote in post #18073294 (external link)
If you can't read the two statements you wrote (that I quoted above) and see they are complete contradictions, then we need to work on your comprehension skills.

So what is it? Does focal length affect how long you can expose for without seeing star trails or not? You have said both yes and no now.

I don't want I dissertation answer I just want to know which of your statements was made in error. Both yes and no cant be the answer.

Maybe you need to try reading the whole comment instead of picking and choosing small portions of it which suit your arguments. I said EXACTLY:

"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."

NOWHERE in this entire statement is a contradiction. Nowhere. Wider focal lengths mean you can take longer exposures before star trailing becomes apparent. Exposure length dictates star trailing. So in VERY SIMPLE $%*(ing terms: Exposure length (which dictates how much star trailing ACTUALLY OCCURS) is calculated in various forms using various equations based upon what people can accept in terms of star trailing, that exposure length they calculate will then produce an image with X amount of star trailing. Exposure length ENTIRELY dictates how much star trailing occurs in an image.


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FEChariot
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Jul 21, 2016 17:32 |  #3249

pdxbenedetti wrote in post #18073495 (external link)
Maybe you need to try reading the whole comment instead of picking and choosing small portions of it which suit your arguments. I said EXACTLY:

I am reading the entire comment. I am simply quoting the parts in question for two reasons. One is to keep you focused on the issue and not allow you to go on a dissertation tangent and getting off the point. Second is because you can't make an erroneous sentence and expect to maintain your credibility. You can't say one thing and then contradict it in the next sentence. That is not how the English language works. Your case is exceptionally special because you are so good with your photography: aspiring astro photogs see your work and then read what you write as gospel due to your talents behind the camera and then you confuse the crap out of them when you contradict yourself.

Lets start with the original problem:

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

This is statement is 100% inaccurate and you not only know it, you have written other statements saying the opposite. Star trailing is due to both focal length and exposure length. if you shoot a portion of the sky with both a 300mm lens and a 16mm lens for a given exposure length, the 300mm image will see light across more pixels on the sensor for a given exposure length. Even if you are tracking with 99% accuracy, the 1% tracking error will show up in the 300mm image first. TCambel above wrote a great explanation of why above.

Now lets talk about the contradictions is the same paragraph from the post above:

pdxbenedetti wrote in post #18073495 (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.

Here you say that star trailing is dependent on focal length. this is not what you said above. You specifically went out of your way to say trailing is not dependent on focal length in the quote above.


pdxbenedetti wrote in post #18073495 (external link)
Wider focal lengths do not mean more or less star trailing,

Hold on now you are saying that wider focal lengths do not mean more trailing which is the exact opposite of saying:

pdxbenedetti wrote in post #18073495 (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.




pdxbenedetti wrote in post #18073495 (external link)
wider focal lengths mean you can take longer exposures before that significant star trailing becomes apparent.

Now you are back to saying you can take longer exposures with a wider lens which means focal length is again a dependent factor in star trailing. The amazing thing is that here you contradicted yourself in the same sentence. This blows my mind that you wont recognize this.

pdxbenedetti wrote in post #18073495 (external link)
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."

This is you going on a tangent. Nobody is saying otherwise.

pdxbenedetti wrote in post #18073495 (external link)
NOWHERE in this entire statement is a contradiction.

Umm actually... yes there is.

pdxbenedetti wrote in post #18073495 (external link)
Wider focal lengths mean you can take longer exposures before star trailing becomes apparent.

Wait a minute, so what you are saying here is the exact opposite of what you said here: (Star trailing is dependent on focal length too)

pdxbenedetti wrote in post #18073495 (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.


pdxbenedetti wrote in post #18073495 (external link)
Exposure length dictates star trailing.

This is only half of the truth. Focal length also plays into it. Define the 500 rule as you know it and feel free to substitute any other number you want. I am guessing you would word it something like this:

500 Divided By the Focal Length of Your Lens = The Longest Exposure (in Seconds) Before Stars Start to “Trail”

If focal length didn't play into it, why is it in the equation?

pdxbenedetti wrote in post #18073495 (external link)
So in VERY SIMPLE $%*(ing terms: Exposure length (which dictates how much star trailing ACTUALLY OCCURS) is calculated in various forms using various equations based upon what people can accept in terms of star trailing, that exposure length they calculate will then produce an image with X amount of star trailing.

See the 500 rule bolded underlined in red above. See how there is a focal length in the equation? You are missing half the equation when you made the above comment.


pdxbenedetti wrote in post #18073495 (external link)
Exposure length ENTIRELY dictates how much star trailing occurs in an image.

No both focal length and exposure length dictate together how much star trailing occurs in an image.


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pdxbenedetti
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Jul 21, 2016 19:15 |  #3250

I'm done. You know what, I don't really give a flying %*)&, continue to interpret things however the *%(& you want to interpret them. You can believe whatever you want to believe.


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Maureen ­ Souza
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Post has been edited over 1 year ago by Maureen Souza.
Jul 21, 2016 19:16 |  #3251

Folks, it would be appreciated if you took this conversation into the talk thread & left this for photo sharing. Thanks.


Life is hard...but I just take it one photograph at a time.

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kukulkan
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Post has been edited over 1 year ago by kukulkan.
Jul 21, 2016 23:39 |  #3252

Just take it outside and trow some ...words:grin:

Anyway to much talking and writing science, just go out shoot and practice and more practice and more practice.
Can someone share some pictures !!




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Maureen ­ Souza
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Jul 22, 2016 01:31 as a reply to post 18074124 |  #3253

Time to be done or I will close this thread. This place is for photo sharing, no arguments or lengthy discussions allowed!!!!!!!!


Life is hard...but I just take it one photograph at a time.

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davidfarina
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Jul 22, 2016 04:32 |  #3254

Time for nchant or the other fantastic contributors to add some nice pics to get over all these discussions :)


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sandwedge
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Post has been edited over 1 year ago by sandwedge.
Jul 22, 2016 07:15 |  #3255

I'll get the ball rolling again. I posted one from Yellowstone about a week ago. Here are a couple more. As I stated earlier, I don't do a lot of Milky Way stuff, but it was fun to play around with it.


1. Taken on Dunraven Pass. There weren't many options for the foreground.

IMAGE: https://photos.smugmug.com/Yellowstone-2016/i-G4WZMFL/0/L/ys%2014-135-L.jpg
[IMAGE'S LINK: https://photos.smugmug​.com ...s%2014-135-L.jpg&lb=1&s=A] (external link) on Smugmug

2. Old Faithful. The Moon was up that evening. Old Faithful was light painted by someone on the other side - I was planning on using just moonlight, but that isn't always an option with so many people around.

IMAGE: https://photos.smugmug.com/Yellowstone-2016/i-4Gg9G9L/0/L/ys%2023-342-L.jpg
[IMAGE'S LINK: https://photos.smugmug​.com ...s%2023-342-L.jpg&lb=1&s=A] (external link) on Smugmug

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