Dumbe Question, but really dont understand

[jkdnys]

So, I know that a wedge or EQ mount will allow you to angle your scope to the appropriate angle to you can take long exposure pictures, etc...and not see star trailing.  But I don't get why then a Goto Alt-Z mount like the nexstar can compensate for the same angle needed.  Because the scope is on a fork or arm, can't it just be rotated and angled and achieve the same effect? 

Total newbie question, I'm sure but just wondering

Thanks

[jnb]

With an altaz mount the image would rotate as it tracks.

It's a little difficult to explain without a diagram but image you are photographing a rocket flying overhead. As it appears on the eastern horizon you see the point of the rocket uppermost as you look at it, as it flies overhead and then disappears below your horizon you would see the point of the rocket down as it disappears below the western horizon. Photograph that with an altaz mount and, while it would track, in so doing it would cause the image to spin. A photograph of an astronomical object would do the same. Now imagine putting your altaz mount at the north pole. At this point the problem goes away because as you look at it all astronomical objects move parallel to the horizon, if you move your scope parallel to the horizon you can follow it and it no longer seems to rotate. An equatorial mount tilts your scope so that one axis is oriented as it would be were it at the north pole.

[ronin]

Never easy as you tend not to see the full track during a night.

Imaging 3 stars bit like Orions belt, that rise in the East.

Colour the first Red, the second Green, the third Blue.

The Red comes up, followed by Green then the Blue.

The Red one is at the top, Blue at the bottom.

During the night they "move" from East to South.

When South they are now all in a horizontal line, no Up/Down but Left/Right.

The Red being the one on the Right, Blue at the Left.

Wait a bit longer and by dawn they are on the Western Horizon and going down.

The Red one is the first to hit the horizon, above it is the Green then highest the Blue.

They started out Red at top and end up Red at bottom.

It is this rotation that an Alt/Az scope cannot compensate for.

This rotation is not overly apparent to the eye, but it means that on a sensor the image of the 3 stars has rotated, and so blurred.

At this time Orion, Hyades and Pleiades are an example that is easy to see and grasp the idea. If you have a couple of hours to watch them move around the sky (not across). Even when you what is happening and why it takes a bit to explain easily.

[bingevader]

Two very good answers.

This might help in showing you what you are trying to overcome (was posted in the Astro-Lounge recently)!

Imaging 3 stars bit like Orions belt, that rise in the East.

I wonder if anyone has tried imaging this on an Alt/Az mount aligned to the central star?

Might make a pretty, swirly, catherine wheely kind of effect!

[JED-E3]

So, I know that a wedge or EQ mount will allow you to angle your scope to the appropriate angle to you can take long exposure pictures, etc...and not see star trailing.  But I don't get why then a Goto Alt-Z mount like the nexstar can compensate for the same angle needed.  Because the scope is on a fork or arm, can't it just be rotated and angled and achieve the same effect? 

Total newbie question, I'm sure but just wondering

Thanks

You can mount the scope on a wedge like so.

This allows you to image without field rotation.

[bingevader]

Also a very good answer!

[hazegood]

You can mount the scope on a wedge like so.

This allows you to image without field rotation.

Only is you can put the mount into EQ  mode instead of AZ mode though surely, otherwise it is still tracking in both axis and will be all over the place

[rowan46]

Only is you can put the mount into EQ  mode instead of AZ mode though surely, otherwise it is still tracking in both axis and will be all over the place

That is correct not all goto alt/as mounts have this function but the se mount does

[jnb]

Only is you can put the mount into EQ  mode instead of AZ mode though surely, otherwise it is still tracking in both axis and will be all over the place

Or leave it in altaz mode and tell it you are at the north pole

[Dave Lloyd]

"Making Every Photon Count" explains field rotation very well. A must for any budding A.P er.

[jkdnys]

Thanks to everyone, I think I get it now. Especially after seeing the video.  Very helpful and I see either a wedge for my nexstar or an advanced VX in my future,  So I'm sure I'll have more post on how to properly do a Polar alignment, etc...

Thanks Again!

[ronin]

It is one of those things that makes sense while you arte being told it and it is being explained.

Then 30 minutes later you start wondering all over again that it doesn't make sense.

One part of understanding it is that it doesn't work for a single star - being a point you can spin those round all you want. The object in view has to be of some size. Some stupid thought has just gone through my mind saying "What about the Ring Nebula?"

So thinking of aiming at Betelguese and just imaging that single star (why?) sort of gets you back to asking why does it not work. Whereas aiming at Orions belt you can imagine the rotation that occurs.

[RT65CB-SWL]

Here is another view of 'field rotation' on something a little closer to home by another SGL'er...

http://stargazerslounge.com/topic/236069-jupiter-moon-dance/ that was posted just over a week ago.