What does Field Rotation means??

Jingguo · Post by **Jingguo** » Mon Apr 16, 2007 11:53 pm

hi while i was reading S&T, i came across the word Field Rotation. what does it actually means?? Does Field Rotation = Drifting?? how it affect astro imaging?? i heard some software actually auto correct field rotation. isit true?? thanks

aquillae · Post by **aquillae** » Tue Apr 17, 2007 1:17 am

hello jingguo, field rotation is caused by imperfect polar alignment, you'll see the circular movement of stars near the edge of the fov.

example:

i took this m51 image, a long time ago. I did a decent job manual guiding it on a star nearby the galaxy. But due to bad polar alignment, you'll see the field rotation.
http://www.geocities.com/goteng/pic/200 ... ril3rd.jpg

another example from recent spaceweather p.o.d:
http://spaceweather.com/swpod2007/15apr07/Strauch1.jpg
the author took several still images of Venus+Pleiades, and since he aligned the images on Pleiades, we can notice the field rotation.

i hope it helps hehe , kindda having hard time to explain it.

oh btw, yup there are some software out there to help fix field rotation.

Post by **weixing** » Tue Apr 17, 2007 1:28 am

Hi,

i came across the word Field Rotation. what does it actually means??

This basically mean that the stars at the center of the image is stationary while the stars not at the center will show as star trails around the center of the image in a long exposure image. This usually is cause by using a GOTO telescope (or doing guiding) to take long exposure image without or poor polar alignment. Since the GOTO telescope (or guiding) able to compensate the error of polar alignment to follow the star at the center accurately, but as time goes by, the star not at the center will start to rotate.

Does Field Rotation = Drifting??

It's different. Drifting will cause all stars to have star trails, but field rotation will cause star not at the center to have star trails.

how it affect astro imaging??

The stars at the center of the image is stationary while the stars not at the center will show as star trails around the center of the image in a long exposure image.

i heard some software actually auto correct field rotation. isit true??

As long as the exposure is short enough that star trail didn't appear, software will be able correct the error during alignment.

Have a nice day.

yanyewkay · Post by **yanyewkay** » Tue Apr 17, 2007 10:32 am

wiki time? :p

chrisyeo · Post by **chrisyeo** » Tue Apr 17, 2007 12:16 pm

Imagine that you are looking at the Southern Cross. Early in the evening, the top is pointing to the left. Later on at night, the top is pointing towards the right. This is caused by the Earth's rotation.

Imagine that you are using an alt-az GOTO telescope to track Acrux. At the beginning, the stars will be oriented this way, but after several hours, it become oriented another way. The GOTO telescope in alt-az mode does not compensate for rotation of the field-of-view in the eyepiece about the object being tracked. In long exposure photographs, this results in smearing of a improperly or non-polar aligned telescope. Some people use a field-derotator to physically rotate the camera to compensate for this effect in alt-az goto scopes.

chrisyeo · Post by **chrisyeo** » Tue Apr 17, 2007 12:17 pm

yanyewkay wrote:wiki time? :p

Of course! Go right ahead and add it .. !

raychan · Post by **raychan** » Wed May 30, 2007 8:05 pm

chrisyeo wrote:Imagine that you are looking at the Southern Cross. Early in the evening, the top is pointing to the left. Later on at night, the top is pointing towards the right. This is caused by the Earth's rotation.

Imagine that you are using an alt-az GOTO telescope to track Acrux. At the beginning, the stars will be oriented this way, but after several hours, it become oriented another way. The GOTO telescope in alt-az mode does not compensate for rotation of the field-of-view in the eyepiece about the object being tracked. In long exposure photographs, this results in smearing of a improperly or non-polar aligned telescope. Some people use a field-derotator to physically rotate the camera to compensate for this effect in alt-az goto scopes.

Wah....really cheem !!!!

Think I'll stick to photography and computing :oops:
At least it has less "alien" words for me to understand......
Must really wiki it !!!

Post by **VinSnr** » Thu May 31, 2007 12:57 am

Wah....really cheem !!!!

Think I'll stick to photography and computing :oops:
At least it has less "alien" words for me to understand......
Must really wiki it !!!

Need not worry too much la. Nowadays if you use DSLR, the exposure is very short. It also depends which part of the sky you shoot.

There are software available that can correct this. If not, most of the "trails" are around the edges....just crop away those area lor.

But if you are a perfectionist, then just go do your drift alignment.