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Field Rotation on Alt/Az Mounts

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#1 HeyJP


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Posted 12 August 2013 - 07:13 PM

A mini-lesson on something I "re-learned" while shooting photos Saturday night: Field rotation with an Alt/Az mount.

I have been playing with astrophotos on my CPC1100 which is on a Alt/Az fork mount. With an Alt/Az mounting, you are limited in length of photos because of "field rotation": as your alt/az mounted scope tracks your target object, the object stays centered, but the field of view is rotating about the center point of observation.

The general rule of thumb is that you can get away with 30s exposures on alt/az before the rotation becomes noticeable. However, the actual time you can "get away with" varies depending on your location on earth (lat/long) and the location of the object in the local sky (alt/az) at the time you start your photo. If you want to keep the field rotation to less than 0.1 degrees, the max exposure time varies from a couple of seconds to more than 3 minutes… if you map it out, you can see that 30s, actually is a good rule of thumb: for a large part of the sky, but not all.

I found a good article on this topic in Michael Covington's Book "Digital SLR Astrophotography". ( Kindle Version: http://www.amazon.co...y-ebook/dp/B... ) Also, if you Google "astrophoto field rotation" you'll find quite a few websites and articles covering this topic and the equations).

The equation takes into account:
- Maximum allowed rotation (degrees)
- Your location on earth (Lat/Long) (Only the latitude matters)
- Object's location in the sky (Alt/Az)

The latter is NOT the RA/Dec of the object, but rather the actual alt/az spot between horizons at the time you start your image. If you point your finger to any spot in the sky… ANY object at that spot will have the same rotation rate.

The equation is:

--- MaxTime (secs) = MaxError (deg) / rotation rate (deg/sec)

If we allow 0.1deg of error then the full equation is:

--- MaxTime = (0.1 * cos(Alt)) / (0.004167*cos(Lat)*cos(Az))

When I took a 30s image of the Crescent Nebula at midnight Saturday night... there was (as you can see below) LOTS of field rotation. I programmed the rotation equation into my HP Calculator App (an HP41C that runs on iPhone!). When I popped in the Lat/Long of the Crescent (which was close to Zenith), the "allowed time" came out to 4.5 seconds. Clearly, my 30 second image was 6.6 times too long which means I should rightly get 2/3 degree of rotation which is obvious in the photo.

I shoulda run my calculator BEFORE rather than AFTER. ;-)

Here's some general guidance about the "sensitive areas of the sky" for alt/az photography:
- You can't get much time on objects in the north or south
- You cant' get much time on objects at high altitude (towards the zenith)
- You CAN get good exposure time on objects low in the east or west

Unfortunately, the ideal position for shooting is when an object is high in the sky (less atmosphere to shoot through). But, physics doesn't allow you to shoot long way up there!

If you use an equatorial mount or wedge on an alt/az mount, properly aligned, you can go all night (well, for a while at least) since rotation is accounted for and not an issue. The above only applies to alt/az mounts such as typical for an SCT or Dob.

Learning (and re-learning) every day,

Jim in Boulder

Attached Thumbnails

  • 6021555-Crescent-30s 001.jpeg

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