hyperfocal?

[sally_tomato]

first i need to say--i love this section of the forum! you guys are amazing!

i have a question. why is focusing on celestial objects so difficult? i have read in many threads that "focusing was difficult" or whatnot. doesn't the hyperfocal distance come into play? i feel like i am missing some fundamental truth.

ok, thanks in advance!

[Bernoulli]

We're not actually focusing the camera in the sense of turning the barrel of a lens, we're focusing telescopes that have cameras attached. The reason that's "complicated" let's say, is that the entire stack shakes a little when you try and do very fine corrections in focus. You have to move the knob, let the apparatus stop shaking (takes a couple of seconds) and check your new focus. I, at least, am not able to smoothly change the focus while viewing the results in real time.

I understand there are electric focusers that'll get around this, but I just have a knob on a telescope to turn. I can get a great focus every time because I've done it so much I know the tricks for my own setup. But it's the most complicated part of my shoot.

[Nighthound]

Hyperfocal is achieving focus on objects near to infinity. In astrophotography we are focusing at infinity.

As Bernoulli mentioned focusing a telescope rig at high focal length is complicated by vibration from simply touching the focuser adjustment knob. Electric focusers are very effective for hands-free adjustment and there are other devices out there to help with focus(see below).

Achieving critical focus at infinity takes some practice but isn't all that difficult on most nights. I say "most" because atmospheric conditions can make critical focus tricky and when conditions are poor, not much worth the effort.

At times deep sky objects are void of many bright stars to focus on so it's necessary to use nearby stars and then move the telescope back to the object to be imaged.

I shoot at f/4ish with my telescopes and have no problem with focus using the Stiletto(300 lpi) focusing aid.
http://www.stellar-international.com/index1.html

The latest popular mask for focusing a telescope:
http://www.focus-mask.com/

And a very inexpensive method is using fishing leader(black nylon works well) criss-crossed at the front of a refractor or camera lens will produce cross hairs on bright stars. When the cross hairs are showing single merged lines then you are right on or very close to critical focus. By checking the tiniest stars on the LCD (laptop is better for enlargement)you can tell whether you've nailed it or not.

[Bernoulli]

And a very inexpensive method is using fishing leader(black nylon works well) criss-crossed at the front of a refractor or camera lens will produce cross hairs on bright stars. When the cross hairs are showing single merged lines then you are right on or very close to critical focus. By checking the tiniest stars on the LCD (laptop is better for enlargement)you can tell whether you've nailed it or not.

Nighthound - Can you do this with a star in Live View? I'll try tonight but I have to have a camera attached instead of a down-loadable CCD and I might not be able to see stars that way.

[Nighthound]

Should be no problem at all. The spikes are visible when looking through an eyepiece as well. On a refractor, SCT or lens the line can be attached with masking tape and then removed if you don't want spikes in your images. I like the black nylon if you keep it on for imaging, the spikes are less prismatic in color.

[ssracer]

The other thing that makes focusing a scope so difficult is that if you are like me and don't have a motorized mount, then as you increase your magnification the object that you are trying to focus of stays in frame for shorter periods of time.

Even with prime focus on my scope (900mm) the moon isn't in frame for all that long before I have to adjust.

[Adrena1in]

Just wanted to thank you for the link to the Mask website Steve...I've been thinking of making one (or three ) of those, but hadn't found a template yet.

[Nighthound]

You're very welcome buddy. If you go to the link above there is a downloadable template, see the menu at left.

Sean, I know what you are dealing with, I shot for the first year or so with my LXD 55 AR-5, which is not equipped with Autostar for tracking. It's a lot of extra work for sure.

[ssracer]

Yeah, right now I am just using a Bushnell that I got for Christmas. Going to be a while before I can afford to upgrade. Works pretty good, but the fact that it is a long, thin tube gives me about f/11 which means that unless I crank up the iso some shots just need too much exposure time to get without ending up with a streak...lol