Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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The best focusing device EVER invented!
Let me first say that this device was invented by a respected Russian astrophotographer Pavel Bahtinov. His web page is http://astrophoto.chat.ru/ but it’s rather outdated. I believe he didn’t update it since 2002.
Most of you have probably heard about (or used) Hartmann mask (http://cometman.com/Mask.html). It’s a whole aperture mask with 2-3 holes (triangular or round) that is supposed to facilitate focusing. Frankly, I always found these mask close to useless, as when 3 stars come close to each other it’s almost impossible to tell if you are before focal point, after or right in it.
Pavel’s invention looks like so:
Edited by Dennis Sakva (07/27/08 03:41 PM)
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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When you put this mask over scope’s entrance it introduces very special diffraction spikes much like those produced by Newtonian spider vanes. How they are useful for focusing? One picture worth thousand of words. See yourself. Pay special attention to central spike.
Intrafocal
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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FOCUS!
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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Extrafocal
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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The difference in focuser position between these three pictures is very small, a few tenth of a mm. The method is incredibly sensitive. And much faster than FHWM, or anything else I know (yeah, autofocusing also). It also works for visual! You don’t need to guess if you focused well (when air is unstable) just put the mask and everything is absolutely clear. You have to try to believe. It works for different scope sizes and focal ratios. The general rule to holes’ width is f/s=150-200, where f – is focal length, s is hole step size. So for telescope with 1500mm focal length the step size is 1500/150=10mm (5mm hole, 5mm for uncut space). If holes become too small the formula becomes 3f/s=150-200. eg for 800mm scope you need to cut 2400/s=200 s=2400/200=12mm (6mm cut +6mm uncut).
The angled cuts are placed at 40 degrees to each other.
I cut mine at laser engravement services for $30.
Edited by Dennis Sakva (07/27/08 03:42 PM)
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AlanT
sage
Reged: 08/20/07
Posts: 484
Loc: 122º36' W, 47º37' N, WA USA
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Looks really useful.
What's the mask look like? I poked around the website, but didn't find it.
al
Ahhhh.... now I see. Thanks.
--------------------
al
Meade 80mm APO
Celestron 6" SCT
CGE, GM-8, & CG-5 GT
ST-2000xcm, DSI Pro II, DSI Pro
( www.alberts-astro.com/astro )
Edited by AlanT (07/27/08 03:50 PM)
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Dennis Sakva
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Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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Sorry, I didn't add the picture when I was entering the post. It's there now.
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DJMcCrady
Carpal Tunnel
Reged: 06/27/03
Posts: 1692
Loc: Seattle, WA
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Impressive!
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++don;
http://starryvistas.net
Tak Sky 90, Tak EM200, STL-4020M
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MtnGoat
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Reged: 02/18/07
Posts: 1212
Loc: Columbia Gorge, WA
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Nice! I've got to make one of these. I'm hoping keeping the ratios about the same will do the trick.
I've been experimenting with different shapes myself, and wondering about other ways of using the diffraction properties and intra/extra focus effects to best advantage, that looks like a very intelligent way to do so.
My fav mask now consist of two slits at right angles to each other.
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"Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use." Mark Twain
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MtnGoat
Pooh-Bah
Reged: 02/18/07
Posts: 1212
Loc: Columbia Gorge, WA
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Been trying to find the page on his site with the mask details...can't read Russian. Boy, Russian uses a lot of long words! Can't read them of course, but I am struck by how many characters the average word appears to have.
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"Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use." Mark Twain
Edited by MtnGoat (07/28/08 01:11 PM)
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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Pavel didn't post any details on mask on his web-page. He posted it on www.astronomy.ru/forum But that one uses even more looong words  (You didn't study German, do you?)
If you have any questions on mask - please ask. I'll be glad to help.
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imjeffp
Aluminum Falcon
Reged: 09/30/03
Posts: 4901
Loc: Cedar Park, Texas
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>(You didn't study German, do you?)
Yeah, speaking of long words!
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Freddy WILLEMS
Carpal Tunnel
Reged: 11/13/05
Posts: 2535
Loc: Hawaii, Honolulu
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Can you post the link directly to the mask ?
Freddy
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Freddy
Meade 14" LX200 GPS UHTC GPS on permanent pier
Celestron C 14" Peltier cooled for planetary imaging.
Meade 10" LX200 & TITAN 50:1 mount Gemini 'go to'
Meade 127 mm f/9 APO & TITAN 50:1 mount Gemini 'go to'
W/O 102 mm f/7 APO doublet
Orion 80 mm f/7 ED
DFK 21AU04.AS
ToUcam 840 II pro
Canon 10D Unmoddified
Canon 40D Hutech moddified
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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Here is the discussion
http://www.astronomy.ru/forum/index.php/topic,10421.msg244210.html#msg244210
The thread is long and there are several different masks. The one I showed in the first post seems to work the best.
But a few people speak Russian, so If you have questions, do not hesitate to ask. If my descriprion of how to make the mask for specific scope is not clear, please tell, I'll try to rephrase it
Edited by Dennis Sakva (07/29/08 09:40 AM)
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astroandy
Carpel Tunnel
Reged: 06/07/06
Posts: 169
Loc: Hong Kong
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Hi Dennies,
Thank you for the useful information.
I note that there's a line running across the aperture, but it is not exactly centered. How much off center should it be? Does it have any effect if I cut it dead center; and most importantly, what software did you use to draw the layout accurately?
CS,
Andy
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FSQ106ED
Zenith Star 70mm APO
Meade sc8
LS60THa-DS FT
EM200 Temma II Jr
Pier Tech 2
ATIK16-HR
ATIK-EFW
Astrodon 9nm Ha, SII; Baader 8.5nm OIII; Meade LRGB, LPS-P2 1.25"
QHY5
DMK21
DBK21
ToUCam Pro II mod.
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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The line is supposed to be centered. But in my case it has no effect on final image. I didn't draw the mask, the guy at laser cutting services used their software. I believe any vector graphics pacage (like Open Office Draw, AutoCAD, etc.) will work.
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Gluon
member
Reged: 02/16/06
Posts: 43
Loc: Austria
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Dennis,
looks really great! Thanks for telling us!
What's the angle of the oblique bars in the left two quarters of the mask?
-martin.
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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You have to try it to belive how helpful it is. Checking focus in the middle of the exposure takes 1 minute
The angle between the bars is 40 degrees.
The mask is quite forgiving to bar width, angles, etc.
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lukasik
Pooh-Bah
Reged: 10/11/05
Posts: 1048
Loc: Lawrenceville, Georgia
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Hi Dennis,
Thanks for posting this info. I'm having difficulty following the link here - I get to the main forum page, but can't seem to figure it out from there since I don't know the language. Do you (or anyone else) possibly have a link to a line drawing I could scale and play with?
Thanks for any help.
Best regards,
Bob
Quote:
Here is the discussion
http://www.astronomy.ru/forum/index.php/topic,10421.msg244210.html#msg244210
The thread is long and there are several different masks. The one I showed in the first post seems to work the best.
But a few people speak Russian, so If you have questions, do not hesitate to ask. If my descriprion of how to make the mask for specific scope is not clear, please tell, I'll try to rephrase it
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1303
Loc: Kiev, Ukraine
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The problem is that you can't easily scale them. Masks are designed to work at a spacific focal length so you can't easily scale mask for 80mm/600mm telescope to 120mm/600mm scope. There were several files I'll post links to them.
Here is the link to 200/F8 telescope by Pavel
http://www.astronomy.ru/forum/index.php?action=dlattach;topic=10421.0;attach=57433
Edited by Dennis Sakva (07/29/08 09:45 AM)
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