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Patrick
Postmaster
Reged: 05/16/03
Posts: 6789
Loc: Franklin, Ohio
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Quote:
the cheapest solution to easy camera control for older Canons is a Photix TC 80N3 knock-off. Lots of people here have used them and reported they work.
I'm very happy with my TC80N3 for exposure control. It's pretty versatile and can be used in a variety of situations, including astronomy and terrestrial applications.
For instance, I used it to capture multiple exposures when doing wide field constellation imaging with nothing more than my camera sitting on a photo tripod. See Here for the results.
Patrick
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Discovery 10" f/6 Split Tube Dob
Celestron C6 SCT
Denk Binoviewers
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Oberwerk 9x60 Binocular
Celestron Regal 8x42 Binocular
Canon 30D DSLR
Mini EQ1
My Astronomy Pages
Edited by Patrick (09/06/08 07:34 AM)
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RichardShell
Vendor Stellar Technologies International
Reged: 02/21/08
Posts: 9
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Actually, diffraction focusing is not as straight-forward as some would think. Similar to using a ronchi screen, resolution is dependent on the interference pattern generated. At low f/ratios, two kite sticks in a cross pattern is not going to do it. The number of 'squares' in the diffraction pattern created should be directly proportional to the f/ratio being focused at. For many years, 35mm photographers were amazed at how sharg a focus could be obtained by focusing through a window curtain. Indeed, algorithms have been developed to show the number of spikes needed at certain f/ratios.
And I would take exception to your assertion of it being faster unless you have ten fingers on each hand. It takes 10 seconds to attach the Stiletto, another 20 seocnds to focus, and another 10 second to stick the camera back on. I would challenge anyone to beat 40 seconds using either Live Preview or diffraction focusing using a computer. Besides, with the Stiletto you don't need the laptop at all and don't have to get your knees dirty. 
Clear Skiee!
Richard
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Gus_Smedstad
professor emeritus
Reged: 07/02/06
Posts: 546
Loc: Boston 42° 16' N 71° 08' W
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Quote:
At low f/ratios, two kite sticks in a cross pattern is not going to do it.
Funny, because I've focused successfully several times now at f/4.9 with essentially that. The diffraction mask I've been using is a Hartman with 3 toothpicks. The main issue with focusing with this method is that there's sometimes ambiguity as to where the 6 diffraction spikes meet, since the blob of the star blocks the intersection.
Maybe you're mistaking the focusing method? The approach I'm talking about depends on seeing the intersection of diffraction spikes, not the sort of pattern you see with the Ronchi screen.
Richard, I know the Stiletto is your baby, but you're exaggerating on the times. Of course part of my problem is that my particular stiletto is PITA to take off the T-ring, but even discounting that, if you're not rushing things it does take longer than 10 seconds to take it out of the case and attach it, and longer than 10 seconds to take it off and put it away properly. Heck, I can't change camera lenses that fast - I timed myself once, and it's more like a minimum of 30 seconds to take one off, put it away, and mount a second one.
Focusing time might be less than 20 seconds, though. It's fast if you discount the time to attach and remove.
And Live View really is fast. I haven't timed it, but I would not be surprised if I've focused in under 40 seconds using Live View. It's only 10-15 seconds tops once you're close enough to actually see the star. The big problem is getting it close enough so you can see at least a diffuse ring, since Live View is significantly less sensitive than the view through the Stiletto if you're viewing a severely out of focus star.
It's pretty silly arguing about which is faster, because the differences are fairly trivial compared to the time required for most other methods.
- Gus
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Gus Smedstad
Orion Atlas 8 EQ-G
Orion 80mm f/11.4 guidescope
Orion Starshoot Autoguider
Canon 40D
Philips SPC900NC
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RichardShell
Vendor Stellar Technologies International
Reged: 02/21/08
Posts: 9
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Hi Gus -
What? Me biased! )
Actually, years ago I was thinking about developing diffraction masks to complement the ronchi screen approach. I suggest you try different screen sizes available to see the differences in resolution using this method. Aluminum screens are available from several manufactuers with different mesh sizes. What you will find is that at lower f/ratios, you will notice an big improvment in focusing as the size of the mesh is increased (e.g. more squares), while at higher f/ratios the mesh needs to be coarser. I even have a proprietary software program which indicates the proper mesh interference pattern given the f/ratio.
Having said the above, any method is better than using your eye. The big failing of the Hartmann mask is that you still need a pretty bright star, and have pretty good eyesight. With the Stiletto, you just need to tell the difference between light and dark. Hartmann masks, diffraction focusing, and the use of ronchi screens are all very similar techniques that require resolving a pattern and not a pinpoint. Resolution-wise, the ronchi screen is at the top of the list, diffraction focusing with mesh is second, and the Hartmann mask third. And yes, cutting more matching holes in the Hartmann mask will increase resolution, so it does not hurt to experiment.
And do send that Stiletto mount back for a replacement when you have a chance.
Clear Skies!
Richard
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Gus_Smedstad
professor emeritus
Reged: 07/02/06
Posts: 546
Loc: Boston 42° 16' N 71° 08' W
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Since I have lousy weather today, I was prepping the Stiletto for replacement and I started fiddling with it. I think I'm satisfied I've found a method of depressing the latch with my thumb which doesn't stick, the way it does when I use a finger. Basically, I'm pushing straight back instead of down and back, which is natural when pressing the lever.
When I push straight back, the detent retracts fully. When I press it in the more intuitive way, the detent rubs against the side of the channel and stops short of full depression.
So I think I'm OK.
- Gus
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Gus Smedstad
Orion Atlas 8 EQ-G
Orion 80mm f/11.4 guidescope
Orion Starshoot Autoguider
Canon 40D
Philips SPC900NC
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