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Why no commercial source for "Duncan" Mask?

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#51 stuart keenor

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Posted 29 August 2012 - 04:33 PM

I made one for my little C130 Mak-Cass. It seemed to work just as well as with my C11.

sound intresting may do the 10" mak cass!!what do you guys think? stuy

#52 Steve OK

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Posted 30 August 2012 - 07:24 AM

Since it only takes 20 minutes and costs little or nothing to make, why not? :)

#53 stuart keenor

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Posted 30 August 2012 - 11:52 AM

Since it only takes 20 minutes and costs little or nothing to make, why not? :)

going to have a shot will let you know thanks stuy

#54 frolinmod

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Posted 04 September 2012 - 08:10 AM

Steve, I made one just like yours, but the three arcs did not become lines as focus was moved in. They stayed arcs all the way to focus and I did not get crossed lines at focus. With that in mind, doing it with the mask did not make collimation any easier than doing it without the mask. I'll have to try one more time before making a new mask in the shape as shown by pinbout and nevy.

#55 nevy

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Posted 04 September 2012 - 10:35 AM

I made a new one using Steves diamentions
[image]http://Posted Image[/image]
[image]http://Posted Image[/image]
The silver lines are so I can line them up with the collimation screws,
It did work as its suppose to , I thought my collimation was spot on using a star test ( concentric rings) , with the mask on , the lines crossed in the centre but one was slightly back a little bit so I adjusted it so they were all the same , I then took the mask off and looked at the defocused star , it showed not concentric rings (off to one side slightly ) so I collimated it back to perfect again without the mask , but then the clouds rolled in and had to cover the scope. I don't know why the two methods didn't agree , I'll have a go with it another time and try again . I found the lines to be bright enough but a bit awkward to see as they were very small ( I was using a 7.4 & 6.4mm eyepieces , 378x & 437x mag ) should I be using more power to make the image bigger?

#56 Steve OK

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Posted 04 September 2012 - 12:01 PM

Well, Frolinmod, I don't know what to tell you. How was the seeing when you tried this? My first try with the mask happened to coincide with some (rare) good seeing. In subsequent trials with more average seeing, the pattern has been less distinct. But in all the trials, the arcs do in fact become tiny radial lines just before focus is reached. High power is required to see them. I was using 560x on my C11 the first time.

And to Nevy, I got a little taste of that myself, trying to collimate using my webcam. The two patterns with and without the mask did not agree. I started wondering if having a not-perfectly-centered secondary could cause such a discrepancy. I can't quite get my mind around it. I'll keep messing with the mask, no doubt, but perhaps the tried and true "center the airy disk in the diffraction rings" will win out after all.

Thanks for the feedback, guys!

Steve

#57 stuart keenor

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Posted 04 September 2012 - 03:28 PM

Since it only takes 20 minutes and costs little or nothing to make, why not? :)

going to have a shot will let you know thanks stuy

made one out of cardboard worked very well on the intes micro mak cass ingenious ;)stuy

#58 Steve OK

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Posted 04 September 2012 - 03:43 PM

Good news! Thanks for the report, Stuy.

#59 nevy

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Posted 04 September 2012 - 04:29 PM

I was wundering about the secondary not being centred , sometimes when I've did star tests before , I have noticed that although I get nice concentric rings , now & again I think I'm noticing the tiny dot right in the centre might not quite be in the dead centre. But sometimes it looks like it is , I'm not sure if I'm convincing myself it is centred becaus I bellieve it should be. :confused:

#60 Mary B

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Posted 05 December 2012 - 08:41 PM

Added them to my product line!

#61 barasits

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Posted 05 December 2012 - 09:47 PM

Added them to my product line!


Great! I haven't had the time to make my own Duncan mask, so would you please provide a link to your web site.

Geoff

#62 Bart

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Posted 06 December 2012 - 11:30 PM

Ditto! I'd love to pick one up for a C11.

#63 Mary B

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Posted 07 December 2012 - 03:03 AM

No web site, send me a PM here. I found a web site was costing me more than I was making, this is a very small part time business. I will need dimensions of your scope in order to cut one. Scopes over 10 inches I have to farm out the cutting to someone with a larger laser and it isn't cheap.

#64 A. Viegas

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Posted 08 December 2012 - 04:27 PM

I have a friend who has a new 3D printer. I will see if we can figure out how to print one of these Duncan masks for my C8 and C11... already printed out Bahtinov masks, so should not be that difficult.

Ideally, if anyone has a .svg file with exact dimensions for the Duncan mask, that would make it easier, otherwise I will use the drawings from a few of the posts on this thread. ( I assume there is no "Duncan Mask" generator on the web? http://astrojargon.n...Generator.aspx)

Al

#65 jbalsam

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Posted 12 January 2013 - 08:57 AM

Bump.

I got an email recently from someone asking if I could add Duncan masks to my lineup, so I wanted to see if that would be a useful service to provide to the community. I've read some conflicting posts here about people being able to get results with them, and I've never owned an SCT so I don't know if methods like Greg Nowell's SGCD are easier/better. I don't want to sell something to people that isn't going to work well.

#66 m2k

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Posted 14 January 2013 - 07:16 AM

Mary B. (Echo Astronomy and Electronics) in a previous post stated that these masks can be laser cut up to 10" in diameter...
I'm wondering if a larger diameter disc can be used to "stop-down" a larger diameter scope to 10" and still work? I'm thinking here of my C-14. :confused:
-Mike

#67 Hikescdnrckys

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Posted 19 June 2015 - 04:52 PM

For those of you with 9.25 SCTs I made one today using fibreboard and a Martha Stewart circle cutter and protractor. The dimensions are 9 3/4" for the overall diameter, 8 1/2" for the next diameter then 6 1/2" for the next diameter (that leaves about a 5/8" so the mask lies on the retainer ring). Finally the inner circle which fits over the secondary collimation screw cap is 3 5/8".



#68 Dunc

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Posted 02 October 2015 - 01:10 PM

Hi Guys, 

A little intro. I'm Duncan Evenden and you are discussing a mask diagram in a PDF I put on-line a couple of years back. Thanks to my mate Robert for first posting it on one of the major astro forums... I am amazed there is so much interest and glad it has been useful. I have only just joined Cloudy Nights so I could post this after a friend told me about this thread. 

 

I got my first scope, which is still my scope, on 2 Jan 2006 and immediately ran into collimation issues as one of the screws fell out onto the grass first time out! This is a Meade LX90 8" UHTC, now deforked and tuned with Vacuum Grease and a Lakeside Focuser. Then I discovered Bobs Knobs as I found I kept losing the allen key (hex wrench to some of you). Then I found out the difficulties of collimation. A long web search led me to this idea.

 

My first mask was made of a Wickes (DIY chain) repair plaster lid which is white plastic (very good for flat fields as well if no holes:) ). The slots were quite narrow. I later experimented with wider slots, this was the time I produced the PDF. Trouble is narrower slots work better as the diffraction that changes the curved slot into the three sets of three (if you look closely) lines produces a sharper image, but the trade off is you need a brighter star.

 

My current mask uses the same size lid but dark blue (from another Wickes product). It has thinner slots and the dark colour stops light transmission so improves contrast. BTW the hoop is the dew heater which usually sits on the retaining ring.

 

Originally I used a 5mm Baader EP (400x). Now I use a Canon 1000D (modded) and AstroPhotoTool,  using 10x LiveView to view the image. The star must be in the centre of the FoV/chip as this scope has terrible optics at the edges. With this camera I have to use a really bright star, Vega is ideal as I can see it in the Live View ;{. Otherwise I use single short exposures and the APT Magnifier which takes a while. Those with nice expensive CCDs should have no problem. It still takes me a couple of goes to get the collimation spot on. I would recommend that if you change the imaging train, re-collimate. I do a bit of planetary imaging as well as deep sky and it's critical to be accurately collimated.

 

As to mask design, experiment to find what's best for your scope, a friend of mine used cardboard pizza bases for ages for this. Maybe the dimensions I use can be scaled up or maybe they should be kept the same. I suspect that the slots should be the same width and distance from the edge but different lengths, but I am not a mathematician, I make and try and remake (more fun). The slots must respond to the adjustment knobs so the slots can't be too long or one knob turn will affect more than one slot/difraction pattern. Then make the final one, my current mask is now 4 years old and should last as long as I have the scope.

 

Hope that helps,

 

Duncan

Orpington Astronomical Society, UK.

 

DuncanCollimationMask.jpg


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#69 Scott in NC

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Posted 02 October 2015 - 01:30 PM

Welcome to Cloudy Nights, Duncan!  Thanks for stopping by to share your expertise with us. :)



#70 Steve OK

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Posted 02 October 2015 - 03:08 PM

Yes, welcome!  And thanks for your contribution to our beloved hobby!

 

Steve



#71 Ptkacik

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Posted 02 October 2015 - 05:11 PM

One note is that the line lengths may be related to the radial depth of the slot so if the mask is not cut accurately, then the lines might not line up correctly.

That is, a laser cut mask perfectly mounting on the secondary might be necessary. A scissored cut pizza box might lead to a wrong answer.

I'm not sure about this but it seems that this could solve the mismatched error described earlier.

Clear skies,
Peter

#72 Dunc

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Posted 03 October 2015 - 03:26 AM

Thanks for the warm welcome. Yes, one thing I've found with this hobby is when you make things accuracy is good!



#73 gunfighter48

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Posted 12 October 2015 - 10:54 PM

Hi Dunc,

 

I've been using your mask for about 4 years and it works great. Gets me in the ballpark and a little tweaking on a bright star get it perfect. I like the Duncan Mask because I can use it during daylight/twilight so I can see what I'm doing!! Thanks for your invention!!!!!!!!!!!!!


Edited by gunfighter48, 12 October 2015 - 10:56 PM.



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