focus aids for astrophotography

[troypiggo]

I'm sure most astrophotogs would be aware of the Bahtinov Mask (http://astrojargon.net/MaskGen.aspx) and how simple, wonderful and powerful it is to aid focusing. If you're spending hours collecting data, no point it being out of focus.

Just came across this thread on Cloudy Nights (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/Imaging/Number/2961332/page/0/view/collapsed/sb/5/o/o/fpart/all) about a free application that helps you focus with a Bahtinov mask even quicker and more accurately. Homepage of the software author is here with links to download (http://www.njnoordhoek.com/?p=325). Thought I'd share.

[A.S.I.G.N. Observatory]

That's BRILLIANT Troy! Thanks for that.

I have a plastic Bahtinov mask at home. Haven't really had much success with it. Might have to give it another try.

I see Bintel is now selling them.

Baz.

[troypiggo]

Really? I'll have to check out the Bintel site. I made my own for my ED80. Downloaded one of the free templates and cut out the cover of one of those black plastic A4 binders. Tried it out the other night just by temporarily duct taping it in place and worked great. Bought some of those needlepoint ply rings from a craft shop, and with some duct tape and a bit of elbow grease I'll have a permanent setup very soon. :)

As for your success, I found it easy to see on screen when imaging with DSI II CCD camera because it's a live view type thing. My DSLR, though, is the older 30D and doesn't have live view. You have to take a test shot, preview it and actually zoom in to see the diffraction spikes. With the 30D I focus by eye, rolling the focuser back and forth until I reckon the stars are about as sharp as I can see, then just check with the preview zoomed in. The few times I've done it I got the focus pretty spot on by eye first time.

Next time I'm going to set up EOS Utility to auto download images to netbook so I don't have to touch the camera to minimise the likelihood of bumping scope and mucking up focus/tracking/guiding.

[jblaschke]

Yeah, the Bahtinov mask is great. I found that site last fall, when I was struggling with focusing my XTi on my scope. That pentamirror viewfinder is just so *tiny* and Hartmann masks worked better in theory than in practice. I was dubious about the potential of a Bahtinov mask, but it was that or a stiletto focuser (and I didn't have the cash for a stiletto) so I gave it a shot. Wow. I was, and still am, very impressed by the Bahtinov.

[A.S.I.G.N. Observatory]

I printed one out on clear projector paper and used it on Jupiter for practice.

As you can see, image 1 is slightly out of focus as the central spike is high.

Image 2 is focused as the spike is centred.

Funny thing is, it works in theory, but when I take the mask off and take an exposure, the stars are not sharp. I think I will stick to the numbers in MaxIm instead. Perfect focus every time. Takes a bit longer, but at least it's right.

Baz.

[Catanonia]

Just made one this evening, and blow me down with a Midget, it works....

Exactly as you see in the photos :)

Loving it.

[Bill Boehme]

I received my Canon Angle Finder C yesterday so I have only experimented with it a bit for terrestrial focusing, but I believe that the magnification will be a great aid in focusing manually. It has two settings for 1.25X and 2.5X. I started making a Bahtinov mask, but cutting it out is a tedious task so I have not finished it yet.

[troypiggo]

I printed one out on clear projector paper and used it on Jupiter for practice.

As you can see, image 1 is slightly out of focus as the central spike is high.

Image 2 is focused as the spike is centred.

Funny thing is, it works in theory, but when I take the mask off and take an exposure, the stars are not sharp. I think I will stick to the numbers in MaxIm instead. Perfect focus every time. Takes a bit longer, but at least it's right.

Baz.

G'day Baz, I know this has been answered on IIS (http://www.iceinspace.com.au/forum/index.php), but thought I'd copy the solution here too for others searching the archives.

The reason you had trouble is that the Bahtinov Mask is designed to work on point source light objects like stars. The light from Jupiter is more of a disc. It's ok for rough focus only, but fine focus needs to be done on stars.

[Nighthound]

Bill, you'll find the Anglefinder to work well with focus on the moon. It isn't quite as effective on deep-sky objects like galaxies or nebula due to the additional glass in the light path which consequently dims them considerably.

Troy, excellent point. The Stiletto focuser as well will not work on planets or the moon. Simply not a condensed or intense enough point of light.