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Why no commercial source for "Duncan" Mask?

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#1 jrbarnett

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Posted 01 August 2012 - 10:08 AM

Most of you probably know what a Duncan Mask is. For those that don't, it's a collimation tool for catadioptrics conceived of by UK amateur Duncan Evenden. It consists of an aperture mask with cut-outs opposite of the collimation screws. Here's an example together with basic instructions on its use:

http://stargazerslou...attach_id=65090

My question is this. Given the exorbitant prices fetched for SCT collimation tools like the Hotech unit, why hasn't any commercial producer bothered to manufacture Duncan Masks for commercial SCTs? Heck, Celestron and Meade could *include* them with their scopes. Producing a solidly mounted, precision mask would literally cost pennies.

Orion? Astrozap? Scopestuff? Are you listening? :grin:

- Jim

#2 brianb11213

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Posted 01 August 2012 - 10:40 AM

Heck, Celestron and Meade could *include* them with their scopes. Producing a solidly mounted, precision mask would literally cost pennies.

For a few pennies they could upgrade the power cord connector to a reliable type.

For a few dollars they could stick a dew shield in the retail box (SCTs and Maks are essentially useless without a dewshield) and add forced ventilation fans.

These companies are more interested in what they can pay their shareholders now than in whether or not they have a repeat customer in five years.

#3 nevy

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Posted 01 August 2012 - 11:36 AM

I was thinking of making one of those masks, do they work and if they do does it get collimation spot on ?

#4 bilgebay

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Posted 01 August 2012 - 01:36 PM

I built one for my C11 but didn't like it as the pattern it produced was too dim. Maybe I didn't understood the formula and made a mistake during the design phase. I will produce another one asap.

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#5 gnowellsct

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Posted 01 August 2012 - 03:44 PM

My question is this. Given the exorbitant prices fetched for SCT collimation tools like the Hotech unit, why hasn't any commercial producer bothered to manufacture Duncan Masks for commercial SCTs? Heck, Celestron and Meade could *include* them with their scopes. Producing a solidly mounted, precision mask would literally cost pennies.

Orion? Astrozap? Scopestuff? Are you listening? :grin:

- Jim


Jim, they don't bring out product because they know they can't beat the Sheer Genius Collimation Device.

It's priceless.

http://www.astromart...?article_id=718

Greg N

#6 gnowellsct

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Posted 01 August 2012 - 04:04 PM

There's a close up in this thread.

sheer genius collimation device

#7 bluedandelion

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Posted 02 August 2012 - 12:13 AM

You should have said, "The SGCD is so simple, it cannot be invented. I am not the inventor." :lol:

The uses of a stick are many, but how does one invent one? You found a new use for an old device.

Ajay

#8 nevy

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Posted 02 August 2012 - 07:02 AM

I built one for my C11 but didn't like it as the pattern it produced was too dim. Maybe I didn't understood the formula and made a mistake during the design phase. I will produce another one asap.

I think it should be a round disc with 3 cutouts In the shape of the grey area, it should look like a 11" cats eye hotspot
http://i1172.photobu...19/ccd762f7.jpg

#9 jmiele

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Posted 02 August 2012 - 07:39 AM

Exactly. The grey is what should make up the mask. Light grey that is. :) Also the edge should account for the retaining ring so as not to make the 3 arc's become to slender.

Joe

#10 Mary B

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Posted 02 August 2012 - 03:27 PM

Be easy to laser cut, what is the math behind it?

#11 gnowellsct

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Posted 02 August 2012 - 04:11 PM

You should have said, "The SGCD is so simple, it cannot be invented. I am not the inventor." :lol:

The uses of a stick are many, but how does one invent one? You found a new use for an old device.

Ajay


No no Ajay, I didn't invent "the stick." I invented *the properly placed notch*! ™

Greg N

#12 shiner

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Posted 14 August 2012 - 12:10 PM

I'm going to look into making one of these. The lid of a five gallon plastic paint tub should do the trick....

#13 Starhawk

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Posted 15 August 2012 - 12:30 AM

I've used the shadow of my arm in place of the stick.

It works. I think the reason no one makes one is it's like trivial. Print out a radiation warming symbol and get some scissors.

-Rich

#14 jrbarnett

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Posted 15 August 2012 - 12:21 PM

You mean "trivial" like commercially available flexible dew shades, Bob's Knobs, magnetic "speaker magnet" counterweights, Baader solar film ring filters, "shower cap" style solid tube Dob end caps, collimation caps, etc.?

There are loads of things that are utterly trivial to make yourself, yet they sell well when made available commercially. I would also like to see "stop down" dust caps for refractors with threaded concentric rings allowing reduction in aperture in 5mm increments (and corresponding extension of focal ratio).

Regards,

Jim

#15 Steve OK

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Posted 16 August 2012 - 05:52 AM

I made one yesterday (20 minutes with an X-acto knife and a piece of photo matte board), and got to try it last night. I am very excited by how well it worked! I did have decent seeing, and my scope was within 2°F of ambient temp when I pulled it out of the shed. I used a 5mm EP on my C11 for 560x, and even then the pattern of three lines was pretty small. I could easily see the effect on the pattern of tiny movements of the collimation screws. I think I achieved the best collimation I've ever had for my C11. I don't think I'd be overreaching to say that the Duncan mask is to collimating an SCT what a Bahtinov mask is to focusing my camera, at least for me. It gives the same kind of feedback anyway. I'm sure that the purists out there that are blessed with consistently good seeing can get perfect collimation without a mask, but the feedback it gives is a huge help to me. I leaves me wondering...why did I just now find out about this? And what other wonderful tricks are hidden out there?

Steve

#16 Pinbout

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Posted 16 August 2012 - 04:46 PM

Be easy to laser cut, what is the math behind it?


if you send me one for 8in I'll tell you the math... :grin:

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#17 elwaine

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Posted 16 August 2012 - 06:41 PM

Good point Jim. A mask like that will certainly sell.

So which one of you guys are going to commercialize this?

#18 Bart

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Posted 16 August 2012 - 07:21 PM

I'd buy one for a C11, hint, hint, hint.....

#19 Bart

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Posted 16 August 2012 - 07:32 PM

http://stargazerslou...attach_id=65090

With the above link you come across this sentence: "When they all meet collimation is achieved, when they cross evenly you are in perfect focus." I'm confused by it's meaning and hoping someone can clarify.

Would the sentence above be more accurate if it was written this way? "When the lines start to meet, collimation is almost achieved. When they all cross evenly, like the last picture, that is when you have achieved perfect collimation."

Thanks
Bart

#20 barasits

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Posted 16 August 2012 - 11:37 PM

Thanks for the report, Steve. You've convinced me to put this at the top of my project list.

Geoff

#21 OzAndrewJ

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Posted 17 August 2012 - 12:19 AM

Gday Greg

Jim, they don't bring out product because they know they can't beat the Sheer Genius Collimation Device.

It's priceless.



Unless you have bicycle spokes :grin:
About 5c each.
This allows you to adjust the collimation easily whilst looking through the EP and no risk of hitting the corrector with your fingers or allen keys etc.

Andrew

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#22 Steve OK

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Posted 17 August 2012 - 07:37 AM

Bart asks "Would the sentence above be more accurate if it was written this way? "When the lines start to meet, collimation is almost achieved. When they all cross evenly, like the last picture, that is when you have achieved perfect collimation."

No. As you approach focus, whether collimated or not, the three little spikes (produced by diffraction through the three open arcs) converge. If you have ever used a Hartmann mask it behaves the same way. If you are collimated, the three little spikes all arrive at the center at the same time. Conveniently, there is a tiny dot at the point where they converge, presumably the Airy disk of the star. If you are not quite collimated the spikes do not arrive at the same time. In my case, two of the three spikes touched the dot together while the third spike hadn't quite made it. Luckily it was the spike at the 12:00 (or 6:00) position so I knew which knob (Bob's) to turn. I gave the "top" knob maybe 1/8th of a turn. Checking the spikes again the errant spike now reached the dot before the other two, so I knew I had gone too far. I moved the top knob back a little at a time, checking the spikes' behavior, until they appeared to arrive at the dot in unison. By "checking the spikes' behavior" I mean making small movements of the focus knob and watching the spikes move radially towards or away from the little central dot.

#23 Bart

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Posted 17 August 2012 - 08:06 AM

I guess I'm getting confused with the idea of collimation and focusing being the same thing.

So as represented with the third picture, second from right, when all the lines converge at the beginning of the lines, collimation has been achieved. Then when you focus, the lines move on top of each other as suggested in pic number four, first from right. Is that correct?

Thanks
Bart

#24 HeyJP

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Posted 17 August 2012 - 08:42 AM

Now you have it, Bart. When you collimate,, you intentionally de-focus your scope, then adjust the collimation screws until an endpoint of the three lines just touch in the center. You can then re-focus your scope which causes the three lines to overlap and cross their centers.

Jim

#25 Bart

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Posted 17 August 2012 - 09:14 AM

Thanks.






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