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debmanna
member
Reged: 01/11/07
Posts: 66
Loc: Cypress, CA
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I just found a new way of focusing my DSLR at prime focus. I have never read about this technique anywhere.
The idea is that when we focus our camera using the telescope we basically focus at infinity.
That means if we can focus correctly at infinity everything should remain at focus whatever direction we aim our camera.
To do this I did the following.
1. I used my DSLR at prime focus and aimed at Moon. Since it is very bright I had no problem viewing it through view finder of my camera and achieving a sharp focus.
2. I took a pencil and mark the focuser draw tube position on the tube (see the picture). It means to achive focus every time I have to extend the focusert tube to that point where the line just starts coming out of the focuser tube housing.
I used this technique today. First I pointed my refractor at M81 (I could see it as a very faint smudge). Then I placed my DSLR at prime focus and using the focus knob extended the tube to such extent that the pencil mark just aligns with the outer portion of the focuser i.e. I extended the focuser precisely that amount so that the DSLR will achieve prime focus. I didn't bother to look through the view finder.
Then I took a 30 second exposure. After downloading the picture to my computer I found I am right! The picture was decently focused. My theory is as precisely I can figure out that focusing mark, the more accurate focus I will achieve.
This has certainly openend new possibilities for me. I do not need to worry about focusing any more. It is so easy. I will try this method on some other DSOs. But I am hopeful this will work.
I am attaching the picture of the pencil mark I did on my focuser draw tube. Next I will post a picture of M81 and M82 I took using this technique. The picture is only a 30 sec exposure so there is not much detail but the focus is more or less okay i.e. I now need to concentrate on increasing the exposure time.
Please let me know your comments/ advice/ suggesstion on this.
Deb
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Debangshu
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Equipment:-
Orion Astroview 100mm EQ
Nikon D50
Nikon N80
Nikkor 75-300mm Zoom
Nikkor 50mm Prime
Galileo 20x60 Binocular
Orion 1.25" Sky Glow Filter
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debmanna
member
Reged: 01/11/07
Posts: 66
Loc: Cypress, CA
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here's the frame of M81 and M82 at prime focus. I used the same technique described above to achieve the focus. I think I got a decent focus.
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Debangshu
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Equipment:-
Orion Astroview 100mm EQ
Nikon D50
Nikon N80
Nikkor 75-300mm Zoom
Nikkor 50mm Prime
Galileo 20x60 Binocular
Orion 1.25" Sky Glow Filter
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s58y
Post Laureate
Reged: 12/12/04
Posts: 4850
Loc: Eastern NY
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If you have a rack and pinion focuser, another possibility is to put a mark on the focus wheel (as well as the drawtube), for even greater accuracy.
Of course, different temperatures sometimes cause the point of best focus to shift a little. Non-parfocal filters may also shift the focus point.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, SV66 guidescope
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http://www.pbase.com/s58y
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jgraham
Postmaster
Reged: 12/02/04
Posts: 5370
Loc: Dayton, Ohio
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Nest! I do the same thing to get my CCD cameras in rough focus, then set the final focus using a mask. If your focusser is a tad loose it helps to always set the focus while moving the focusser in the same direction; always in or always out.
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-John
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Orion StarBlast, BinoViewers, Coronado PST
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Tasco 60mm Refractors
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David Rosenthal
Pooh-Bah
Reged: 06/23/06
Posts: 1201
Loc: Midland Park, NJ
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Deb,
That's the same trick I use to achieve rough focus when I use my ZS66SD APO. The drawtube on that scope has etched white numbered lines. I made a chart which shows my starting point for all different configurations of accessories, visual and photographic. It works quite well. Nice discovery !
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David Rosenthal | Midland Park, NJ
My Image Gallery
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5000 26mm, Hyperion 17mm, TV 11mm
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werper
member
Reged: 05/14/05
Posts: 80
Loc: S.E. Wisconsin
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I simply use a parfocal eyepiece to get a good rough focus, then aim at a bright star and fine tune the focus live on-screen with my DSI. After that, everything I point at is in perfect focus.
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Thomas Bardenwerper
Celestron C8-N on a Roboscoped Vixen Polaris GEM - the BorgScope
Celestron C4.5
Lomo 60 MAK
Meade DSI-C
Meade 11x80 Binoculars plus a few other binos
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Raven911
Pooh-Bah
Reged: 03/12/05
Posts: 1441
Loc: New Mexico
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I use a short bit of tape on the side of my focus tube and make index marks for each filter and different camera type. I then focus it by taking short images and slowly adjusting the focus. Works great. I also use the parfocal eyepiece routine for my Mak-Cass.
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http://smg.photobucket.com/albums/v287/Raven911/Astrophotos/
http://www.eye-of-the-raven.blogspot.com/
Celestron SC6
IM M66 Mak-Newt
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SBIG ST7/Canon 300D/DSI Pro Mono/Starshoot DSCI
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Dragon Man
sage
Reged: 03/07/06
Posts: 229
Loc: Snake Valley, Australia
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Quote:
Of course, different temperatures sometimes cause the point of best focus to shift a little.
I totally agree. Although only minor, focus will shift as temps drop (or rise).
I also use lines I have marked on the drawtube of my Crayford Focuser. It gives me almost correct focus every time, then I use the Orion Accufocus for the fine focussing. As the temp drops I just keep it in focus with the Accufocus.
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IMAGE EDITING AND COMMENT APPROVED
Ken - 12" Dob on a Restored EQ | 8" EQ Reflector | EQ6 & synscan | ED 80 | Modded 840 Toucam Pro II | 20x80 Triplet Binoculars |
Snake Valley Astronomical Association and my own site Australian Astronomy
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Mike Nordquist
super member
Reged: 03/08/07
Posts: 110
Loc: Stevensville, MT
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Since I've just started to mess with prime focus, and have not acheived much focus success, I'm certainly going to try this and the other suggestions in this thread.
Thanks all!!
Mike
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Vixen ED80sf
Vixen PortaMount
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