517 replies to this topic

[Dennis Sakva]

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 His web page is http://astrophoto.chat.ru/ but it’s rather outdated.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:

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[Dennis Sakva]

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]

FOCUS!

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[Dennis Sakva]

Extrafocal

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[Dennis Sakva]

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.

[AlanT]

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.

[Dennis Sakva]

Sorry, I didn't add the picture when I was entering the post. It's there now.

[DJMcCrady]

Impressive!

[MtnGoat]

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.

[MtnGoat]

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.

[Dennis Sakva]

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.

[imjeffp]

>(You didn't study German, do you?)

Yeah, speaking of long words!

[Freddy WILLEMS]

Can you post the link directly to the mask ?
Freddy

[Dennis Sakva]

Here is the discussion
http://www.astronomy...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

[astroandy]

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

[Dennis Sakva]

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.

[Gluon]

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.

[Dennis Sakva]

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.

[lukasik]

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

Here is the discussion
http://www.astronomy...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

[Dennis Sakva]

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....0;attach=57433

[Dennis Sakva]

Bob, seems like the link was truncated by cloudynights forum engine. I replaced it. Try again.

[lukasik]

Thank you so much for the links Dennis.

Regards,

Bob

[roestman]

seems very interesting. for everybodys info, you can go to google language tools and translate the page to english or what ever you speak. not allways correct but it can get you through.


http://www.google.co...age_tools?hl=en

[idahoman]

This is interesting. But, the files are for CorelDraw, which I do not have. Does anyone have them in a different format? And, how do you scale them for different focal lengths?

[MtnGoat]

he posted the ratios on page on, Idahoman