Star Photography One on One

J

J-see

Senior Member
Another night with a clear sky but too much moon at the one side and plenty pollution at the other. You not only need to know where to point the lens, that area needs to be clear too.

Not much choice so I did some more dot-shots with the 200mm. Each shot I hopefully learn some more. Live-view rocks for focusing these shots. Now I need to fine-tune the ISO/shutter and maybe pick another lens. I get enough data in the 2 seconds I can shoot the 200 but there isn't much in its FOV.

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I wonder what that bright spot is. I can tell you what the moon is and a couple of star signs but all the rest I distinguish as shiny star and not so shiny star.

I got the Tam out to see if I could get more.

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Not really but even at 600mm I can get the stars nice.
 
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J

Jacknet

Senior Member
This is my first shot with my new 200mm lens. Not much to look at but here it is anyhow.jpg0297.jp.jpg
Orions belt. I got some great views of Orion the other night and wanted to take the dagger nebula but was not in a dark enough area.
 
J

J-see

Senior Member
Still learning while struggling with pollution and limited angles but at least it slowly starts to resemble what's out there.

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J

J-see

Senior Member
wev said:
Download Stellarium -- a free, open source astro program.
Click to expand...

That's a handy program. I did rewind time to the shot taken at my location and unless I'm checking the wrong direction, that should be Jupiter. I wondered if it would be but didn't know how to check. I'm surprised it shows some moons using nothing but a 200mm.
 
J

J-see

Senior Member
Does anyone know how to calculate the exact shutter limit I can use with each lens/sensor?

I read about the rule of 600 at the one place, the rule of 500 at another yet none of them seems to be very correct in my case. I used the 18mm for 20s and even while that's just 360 and within range of both rules, when I check 1:1, I see the stars stretch. The full shot is ok but the moment I want to crop it's a no-go.

Strangely when shooting my 200mm at 2s shutter they're pretty ok even at 1:1.
 
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J

J-see

Senior Member
I didn't know one of the Three Sisters was a double-star. I guess the Tam is at least sharp enough to distinguish that.

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J

J-see

Senior Member
Testing what high ISO does.

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That's the Three Sisters and that bright area to the bottom is the Orion Nebula I think. Or whatever it is called in English. The 200mm is a bit short and the 600's shutter too fast to get enough data.
 
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J

J-see

Senior Member
Just tried some shots to check how the lens does. Too long shutter for this length and too close to the buildings to get depth wide open. Too much moon too. But all in all the stars it grabs is not too shabby at that ISO.

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Bourbon Neat

Bourbon Neat

Senior Member
J-see said:
Does anyone know how to calculate the exact shutter limit I can use with each lens/sensor?

I read about the rule of 600 at the one place, the rule of 500 at another yet none of them seems to be very correct in my case. I used the 18mm for 20s and even while that's just 360 and within range of both rules, when I check 1:1, I see the stars stretch. The full shot is ok but the moment I want to crop it's a no-go.

Strangely when shooting my 200mm at 2s shutter they're pretty ok even at 1:1.
Click to expand...

In respect to the 500 or 600 rule to plan your shutter speed. My curiosity went further as I wasn't satisfied with the results using those rules. I need to know how far the movement of the stars is on the sensor in time. Luckily, like most things, a little arithmetic can get ya there.

This formula will give you more to help in determining your shutter speed. Notice that considering the crop of a sensor is not necessary.

D=T x FD/13750 x is the sign for multiplication

D is the distance of (or size of a pixel)

T is time, what we want to know (How long does it take for the light of the star to move one pixel on the sensor).

FD is focal distance

13750 is what I call hard arithmetic, someone else figured out the movement of the sky as we see it during the rotation of our planet.

To determine D: Take the sensor horizontal dimension and divide by the pixel count across the sensor. Example of the D5500 and D7200: 24.1mm / 6000 pixels = .00392

0.00392=T x 24(24mm lens)/13750

0.00392=T x 0.00174

0.00392/0.00174= T

T= 2.25 seconds the time the light on the sensor moves one pixel.

Using the rule of 500, 500/24=20.83.....20.83/1.5 (crop sensor)=13.88 seconds of exposure. 13.88/2.25=6.16 pixels of travel across the sensor. A 10mm lens will bring that camera up to 5.37 seconds per pixel.

Roughly, with the D5500 and 24mm lens, stars are between 5 and 30 pixels wide at a 13 second exposure. So, knowing that, you can decide how much trailing is acceptable to you.

As you can imagine, high pixel sensors may or may not be the best for night sky. Fat pixels good and slim pixels not as good. With a D4 and a 24mm lens, you get 4.82 seconds of exposure to one pixel of movement (a 12.1mp FX sensor). With that said, we are using the pixel as a measuring unit when each sensor may have a different pixel size.

The D810 and a 24mm lens, will get you 2.80 seconds of exposure to one pixel of movement (a 36.3mp FX sensor).

Note that lens size has an impact too: 10mm very good, 50mm not so good.

Disclaimer......This is just a few cents from a large bottle of pennies, don't take it too serious.
 
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