Alt-az tracking and widefield imaging

[jambouk]

Using a skywatcher alt-az supatrak GOTO mount (like this http://www.firstlightoptics.com/skywatcher-mounts/skywatcher-supatrak-mount-and-tripod.html), how long would I be able to get exposures for just using a DSLR and a 70-300mm lens before field rotation appeared?

Does anyone have any experience of this?

Thanks

James

[jambouk]

I just found this interesting website, which doesn't tell me the answer, per se, but is clearly very helpful:

http://calgary.rasc.ca/field_rotation.htm

James

[glappkaeft]

The moment you open the shutter you start to get field rotation. How much field rotation to tolerate is a personal preference and is also dependent on how good the focus is (the size of the star matters) and the resolution of the final image. Say that you decide that 0.1 degrees of field rotation is tolerable. On a image with a width of 5000 pixels this will stretch the stars around 4 pixels at the short edge of the image (tan(0.1)*2500 and a little more at the corners.

Then you can use the formula on the linked page to calculate how long your exposures can be. With a limit of 0.1 degrees you'll get something like 5 seconds near zenit, a few minutes low in the west and east and around half a minute to the south and north (assuming latitude 40N).

[tingting44]

thats right field rotation is a personal preference, i actually like it lol....just a tad of field rotation in my shot here  (got 2 Perseid's meteor's in this one if you can spot them )

timelapse data blended into star trail by tingting44, on Flickr

[glappkaeft]

Actually that's regular rotation, not field rotation. You only get field rotation with a tracking mount (Alt-Az, poorly aligned Eq mount or guidescope) and only around the center of the image. Still, very nice image...

[tingting44]

Actually that's regular rotation, not field rotation. You only get field rotation with a tracking mount (Alt-Az, poorly aligned Eq mount or guidescope) and only around the center of the image. Still, very nice image...

ah i get ya! well you learn something new everyday! thanks bud thanks for the nice comment tho