Long Exposures with Alt/Az Mount

[Peacefield]

I have a Celestron NexStar 11 with an alt/az mount. I've purchased the appropriate T ring and want to try my hand at astro photography.

So my questions are:

- how long an exposure can I get away with using an alt/az mount?

- Is there an area of sky that is more friendly to longer tracking with this type of mount than other areas?

Thanks.

[Sorarse]

Not sure what length of exposure you'll be able to get away with, as I am not familiar with your scope.

The problem you will get with long exposures and an alt/az mount is field rotation. I would suggest that the place to point your scope to keep this to the minimum would be towards the meridian.

I stand to be corrected by any of the wiser sages on this forum though.

ETA Having read Steve's excellent explanation below, it would appear I got it totally wrong about the meridian, so just ignore me, I'm an idiot! ;)

[SteveInNZ]

If you like math the formula is

time in seconds = Allowable rotation in degrees * cos(Altitude) / (0.004167 * cos(latitude) * cos(Azimuth))

Here are some guideline numbers for 0.1 degree rotation.
Directly overhead - 5 seconds
At the pole - 25 seconds
Meridian at horizon - 30 seconds
East or West horizon - 230 seconds

Info is from Covingtons Digital SLR Astrophotography.

Steve.

[Peacefield]

That's interesting, thanks to both of you.

Anecdotally (which I'm much better at than math), I was guessing nearer the east or west horizon. I'm assuming that an alt/az mount behaves most like an eq down low; it's up overhead that the barrel of the scope rotates so that the eye piece always is in the up position whether facing east or west. And north should be the worst as the barrel does not rotate with the "spin" of the north sky. Unfortunately, I don't have a clear shot at the sky until about 25deg up, but I'll take what I can get.

Thanks!

[MintMark]

Thinking about it, I guess field rotation is not exaggerated by focal length the way star trails would be, so as long as you can track the position of the object then your long focal length won't be a disadvantage.

Maybe it'll be OK!