Motion blur

[seismiccwave]

<p>This question is for the astro photographers. Just for fun and a clear night, I pointed a couple of my lenses toward and sky and try to get some pictures. I got Orion. I used my D700, Nikkor 105 mm VR AF-S. f11 and 30 seconds exposure. I locked up the mirror but didn't use the remote. Yes the camera was on a tripod.;-)<br>

My question is if 30 seconds enough time for the earth's rotation to be recorded as a motion blur or do you think it was my clumpsy fingers.<br>

I am not sure you can actually see from the picture I post here because I have to reduce the size.</p><div></div>

[allardk]

<p>Hi Hansen. The short answer is yes, 30 seconds is enough. See my contribution to <a href="../canon-eos-digital-camera-forum/00Qlfh">this post</a> for an example. However, the motion blur is always one-directional, and its amount depends on the part of the sky you are photographing. Your blur is only very slightly one-directional. I guess part of it is due to other reasons. Did you use a self-timer? Are you sure the VR was off for this? VR can also give you some blur on such long exposures.</p>

 

[rainer_t]

<p>-- "My question is if 30 seconds enough time for the earth's rotation to be recorded as a motion blur or do you think it was my clumpsy fingers."</p>

<p>Definately ... with a 100mm lens I would guess you shouldn't expose any longer than 5sec.<br>

Another thing is to correctly set the focus to infinity. (... albeit out of focus starts can also be nice, since they disclose more of the colors of the starts).</p>

[MichaelChang]

<p>Hansen, the calculation is pretty easy: the earth completes a 360 degree rotation about its axis in a 24 hour day, equivalent to 15 degrees per hour, 1/4 degree per minute, meaning you will experience a 1/8 degree shift for your 30 seconds of exposure. How this impact trailing will be dependent on the focal length of your lens; less obvious with a short/wide lens, more pronounced as you move longer. <br /> <br /> I would start with that particular lens wide open, use the highest available ISO and check the focus carefully (your posted example looks slightly off). This should get you a decent picture in a second or two from the dark skies in your post. If not, then gradually extend exposure time just short of the instance when trailing becomes visibly offensive. You can also take multiple exposures, manually rotate each and stack them. <br /> <br /> Remember that many astrophotos are "extracted" from rather messy original captures (see attachment). Factors such as atmospheric disturbances due to the air's thermal gradient, light and air pollution, all contribute to bad "seeing" conditions which, to an extent, can be dealt with in post processing. <br /> <br /> The saving grace is that stars are bright point-sources, so brighter objects will be easy to capture while dimmer objects will be the traditional battle with signal-to-noise ratio as we experience in terrestrial photography.</p><div></div>

[chris_werner]

<p>Actually the calculation is a but more complicated than that as 1 degree at the celestial equator is greater linearly than 1 degree at the celestial pole, so there's a little trigonometry involved. Also, the rotation is actually over a sidereal day, which is both not precisely 24 hours and varies, but for most purposes that doesn't matter.</p>

<p>The length of trails is determined by the following formula.</p>

<p ><img src="http://www.wilmslowastro.com/software/images/formula20.png" alt="Star trail length" /></p>

<blockquote>

<p>where:<br>

F = Focal length of lens scope (trail length in same units as focal length)<br /> E = Exposure length<br /> T = Length of sidereal day in same units as exposure<br /> D = Declination of the star</p>

</blockquote>

<p>Here (http://www.wilmslowastro.com/software/formulae.htm#StarTrails) is an online calculator that will do the math for you.</p>

<p>You got a good capture of Betelguese's color! It's a red supergiant. Rigel (top right - the opposite corner from Betelguese) is a blue supergiant. Nice contrast.</p>

[seismiccwave]

<p>Thanks gentlemen. That got a little overwhelming first thing in the morning. I will try with my remote next time and play with shutter speed versus aperture opening.</p>

[bob_irvine]

<p>This was a 30 second exposure. 200mm f5.6 ISO200</p><div></div>

[robert lee]

<p>Take <a href="http://www.focusmagic.com/examplemotionblur.htm">Focus Magic </a> out for a spin. It has a deconvolution based module for reversing out motion blur. It works very well for simple blurs like those from star trails.</p>

[seismiccwave]

<p>Thanks, so it looks like I was recording a bit of Earth's rotation. After looking into my Photoshop CS-4 I noticed I can also reverse the motion blur by using a smart sharpen command. I can actually dial the angle that the motion is traveling. I am going to give that a try later tonight.</p>

[GerrySiegel]

<p>I guess the bottom line is that everything is moving and stillness is our little illusion. Earth rotates. Circles sun. Solar system moves in galactic plane. Galaxies are moving very fast in all directions. I am getting tired just thinking about it. Last few nights Orion has been pretty sharp, hasn't it. It is splendid to see, and a look into the distant past. We are lucky if we get to see stars with all the sodium lamps and clouds. aloha, gs</p>

[seismiccwave]

<p>Gerry, you comment brought out a lot more philosophical thoughts from me still. I agree about everything is moving and the stillness is our little illusion. We can even expand that thought a little more and call motion an element of time. Our heart is beating, our blood is flowing, our body is generating energy so electrons must be moving. So when stillness becomes reality we die.;-)<br>

Hilo still has much less sodium lamps but a lot more clouds. Fortunately it has been severe clear the last couple of nights and the stars are indeed gorgeous.</p>

[profhlynnjones]

<p>Regardless, if the star circles are not round, the camera moved.</p>

<p>Lynn</p>