Obtaining focus

[ollyb]

Having looked through a few celestial photos on this site, I often see comments made about focus. I would have thought focus, or getting the object in question within the DoF, would be relatively easy - just focus at the hyperfocal distance or beyond, and everything else further away is within the DoF so should be in focus.

I take it it's not that simple?

Cheers!

[VIGER]

Somehow easy. I focus using a Right Angle Finder and / or Stiletto focuser for Canon cameras. Many methods can be use ...

http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM

Sometimes it looks like an out of focus picture but are simply and effect of the atmosphere,vibration or poor tracking.

Voila. You know everthing I know.

[Sorarse]

Focussing at the hyperfocal distance maximises what will appear as acceptably sharp in your image. The key word there is acceptably. Sometimes this is not enough for astrophotography, especially when other factors, such as air turbulence, earth rotation and vibration are all conspiring to make your image softer than is ideal.

For those reasons I will always aspire to get my focus spot on, and I am sure many other photographers are of a similar mind.

[SteveInNZ]

Your idea is fine for wide angle shots, although getting a closer hyperfocal distance requires you to stop the lens down which raises other problems in the low light world.

I just calculated the hyperfocal distance for my telescope (8" f/10 SCT) and it's about 18km. I'm not sure that finding something more than 18km away to focus on is really any easier than a star on a black background.

It's also going to be through the maximum atmosphere thickness so the target is going to be jumping around from air currents, etc while a star is a point source through the minimum atmosphere.

Steve.

[Nighthound]

Once you've focused to infinity you've done your job. Achieving critical focus for astrophotography is about micro managing that process. When you get up in focal length (1500mm and up) it's important to be able to zoom in close on an exposure and evaluate the tiniest stars in the field(laptops work great for this). If they're tight(tiny orbs), then you've achieved critical focus and the entire field of view will be in focus. Seeing those tiny stars can be difficult on the camera LCD and as mentioned the atmosphere can further complicate focus. There are nights of poor seeing conditions when even the best focus tools and techniques won't produce crisp images, that's just the nature of the beast. I use the Stellar Technologies Stiletto and find it to be 99% accurate with my refractors but had lower success rates when used with a Schmidt-Cassegrain.

As a side note, during a night where temperatures fluctuate quite a bit it's necessary to refocus, especially with very fast focal ratio telescopes. As the metal in the telescope focuser is effected by the falling temperature the focus will shift a bit. Like I mentioned it does take some micro managing.

[ollyb]

Wow, thanks for the help everyone! Definitely seems to be more to it than simple point and shoot, but I guess that makes the whole rocess more satisfying!