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I've been doing a bit of astrophotography and the problem that I almost always have is nailing focus. I'll manually focus on a bright star or the moon (which hopefully isn't an option!) by magnifying on live view, leave the lens on manual focus, and then rotate the camera to the target.

My problems happen two fold:

  • Evenly greatly magnified on the live view screen, there's very little difference between in focus and slightly out of focus of a bring point light source. It's extremely difficult to tell in camera if its right, but painfully obvious once on the computer after the exposure.

  • I think sometimes the focus ring gets bumped while moving the camera to the target. So my focus ends up way off.

What are techniques or methods to help here? I've seen some reference to some sort of diffraction mask that could help, but don't know anything about it. My particular DSLR doesn't tether to a laptop, so computer aided focus is out for me (but it may be worth mentioning in general for a wider audience with similar issues).

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migrated from physics.stackexchange.com Jun 3 '12 at 21:56

This question came from our site for active researchers, academics and students of physics.

7 Answers 7

up vote 16 down vote accepted

A very simple, yet effective method to achieve almost perfect focus is to use a Bahtinov mask. I believe that this is the "mask" that you were referring to. It is a diffraction mask that is placed on the aperture of the telescope, creating three diffraction spikes. When the image is in focus, the three spikes line up perfectly. If it is even slightly out of focus, it is very easy to tell. There are online generators that you can use to make your own, or you can buy more well-made ones from commercial suppliers.

An earlier, and less effective device is the Hartmann mask. It relies on similar principles of diffraction, but it is generally thought that a Bahtinov mask is more precise and easier to use. Besides these, there are other methods of focusing, as you know, but for simple, amateur astrophotography, I'd use a Bahtinov mask.

Just don't forget to take it off before you actually start taking pictures!

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2  
FWIW, I found the Bahtinov beyond my skill levels with an X-Acto knife and soft plastic (it turns out that a shape consisting of long narrow slots turns out to be flimsy -- who knew?). I suggest finding a friend with a cutter or a commercial source... –  Larry OBrien Jun 9 '11 at 16:35
    
+1 for the online generator link and suggestion to try making one yourself. All it took me was a craft knife, an A4 sheet of mounting cardboard and a side helping of patience; now my view is crystal clear. :) –  Jono Dec 15 '12 at 0:01

The best way to nail any focus is to focus on something bright, and the same distance relatively to what you want to photograph.

In terms of Astronomy, that means you need to focus on something that's at infinity, and bright. There are a few common options.

  1. If the moon is up, use it. Your camera will focus perfectly at infinity. Then set it to manual focus, and try not to bump the focus ring.
  2. The next best thing is a very distant light. Use a distant city light if you can, they are bright enough to focus, and far enough away to be infinite.
  3. If none of these works, then you can try a bright star, airplane, etc. If it's a star, you can continue to do #4.
  4. If none of these works, focus manually as best you can, then take a picture at the highest ISO your camera will work at. Zoom in to small locations in the image, and keep trying to get it just right, then resume your ISO levels to their normal values.
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3  
Just don't do what I did once. Pointed my camera at the brightest star in the sky and spent several minutes trying to focus it down to a pinpoint, only to realize that I'd picked Jupiter, and was using a long enough lens that it showed up as a circle in my viewfinder. –  Mark Mar 21 at 7:30

If your camera has live histogram, and if the moon is visible, you may be able to use the histogram to focus. Unless you are so zoomed in that the moon fills the whole field, it should work, at least in theory :-). Since most of the field of view is dark, you should have a peak in your histogram at the left (low intensity). Then, you should have another peak near the high values, corresponding to the moon. You should be able to use the dip between the two peaks to focus. The aim is to get the dip to be as low as possible, or (I think) equivalently, the rise in the second peak to be as steep as possible.

This is because when you have a sharp focus, you have the least amount of pixels receiving the bright light from the moon.

I haven't tried this, so I am not completely sure if it would work, but maybe you can try the method and post your results here. Although, from a quick Internet search, it seems like it should work (for the sun at least!).

Another option is to focus during the daytime, on a distant object, and then make sure you don't bump the focus for the night shots.

If you really wish to be accurate, you would want to take test shots and then analyze the point spread function (PSF) using a computer program. This is the method used by professional astronomers, but then they have a very nice CCD directly connected to a computer. In that case, a software such as Mira will do the focusing automatically for you.

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Use a cheap Cross Screen photographic filter, the sort used time ago to show a four spikes light rays departing from a bright point of light.

I noted that with lens starting from 35 to 200mm delivers outstanding and tack sharp results provided you choose a very bright star and an exposure around a second or more. If you own a "live view" option you'll find it much easier to check and eventually fine tuning the focus in real time using the higher magnification your screen will provide, my EOS 550D works perfect at 10x.

If you don't own that, well nothing is lost but some time, simply take a picture and compare if the rays are tight, sharp and with interference fringes colorful.

Nowadays EOS (but even Nikons, I suppose) have standard software that allows to remotely focus the lens using the keys of the PC. EOS Utilities is a good example, you are allowed to adjust exposure, stop, enabling "live view" routine setting a high magnification in order to watch perfectly the path of the focus while commanding it comfortably without the need to get-up, go to the camera, rotate the focus ring, return to the PC (or camera display) to check the results, return back to adjust focus and so on.

Price of Cross Screen? A few bucks for average sizes (e.g. 52-55-58-62-72mm).

Enjoy and leave Bahtinov mask for larger diameters, higher focal lens telescopes; it works best with the above mating.

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I'm primarily a visual observer, but am just beginning to dabble in astrophotography using a DSLR, a Canon Digital Rebel XT. Like you, I've had major problems with focus but, having done my homework, think I have the solution. I've bought a right-angle magnifying viewer for my camera, which magnifies the image 1.25x and 2.5x. I haven't had a chance to try it yet, though. Be forewarned, these little devices do not come cheap: this cost me $310! They're also scarce: most dealers did not stock them.

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Why would a viewer that magnifies 2.5x help me better than my live view which can do 10x? –  rfusca Jun 3 '11 at 22:07
    
Because the magnification is optical, rather than electronic. Also the right angle prism makes viewing easier. –  Geoff Gaherty Jun 4 '11 at 4:22

As a personal experience I recommend collimating the telescope at regular intervals, if your image is in focus at the center and have a side out of focus more than the other is probably a collimating issue. Having said that, there are multiple options for simplifying the process of focusing.

a) Use a diffraction mask, as the Bathinov (this is what use most of the times). You can even make yours. Bathinov Mask Generator

b) Use a magnifying glass on the camera LCD.

c) Change your focuser for a micrometer style focuser, or electric focuser.

d) Use software to analyze the attained focus, not online, but transferring the images, such as CCD Inspector or similar ones.

e) Use a smaller sensor, to use the most central part of the optics and have a flatter field and have an easier focusing process.

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Keep in mind that while focus is obviously important, vibration of the telescope/mount can also cause blurriness in a long-exposure image. In some cases this may look like defocus.

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