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Cleaner Stars - How to install an aperture mask on your Newtonian

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#1 pbkoden

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Posted 07 April 2019 - 04:39 PM

Here is a simple inexpensive way to greatly improve the diffraction artifacts around bright stars in your images. Here is the before and after effect both looking into the focuser drawtube, and what the resultant images look like:

 

 

cLa2r1u.jpg

 

 

The ideal primary mirror would have a perfect circular edge profile. But the edges of your primary mirror are not perfect. They diffract light and cause diffraction spikes/rays to extend from the core of bright stars in all directions. In addition, the three mirror clips that overlap the edges of the primary will cause gaps in these spikes where they mask the edges of the primary. The combination is a a bright area of diffraction spikes with three dark lobes in it. The trick to fixing this is to mask the entire edge of the primary mirror, including the mirror clips. This reduces the diffraction effect, and creates a nice even halo around the star core. Here is how to do it.

 

Here is what you will need:

  • Material to cut a masking ring out of and a template to cut to
  • Hobby knife to cut the material
  • Flat black paint or flocking material
  • Double-sided foam tape

The first thing you will need to do is cut out a masking ring. The outside diameter of the ring should be small enough to fit inside your tube easily, but larger than the outside of your mirror. The inside diameter of the ring should as large as possible, while being just small enough to cover the mirror clips when centered over the primary mirror. You can take measurements and then use a free tool like Sketchup or LibreCAD to create a 1:1 print with your inner and out diameters to use as a cutting template. 

 

The mask can be cut from just about any material. I originally hand cut one from black art matting board which worked great for over a year. Eventually it sagged and lost its shape some, so now I am replacing it with a 1/16" thick aluminum water-jet cut ring from an online metal cutting service (metalscut4u.com). The service was very easy to use, their online form includes a ring shape and all you need to do is select it and then type in your ID and OD to get an instant cost on a multitude of materials.

 

 

iMxRPvU.jpg

 

 

If you cut your ring out of something reflective or lightly colored, you will need to paint or flock it to prevent unwanted reflections from entering the light path. In my case I will flock my ring front and back.

 

Once the mask is dark and flat, adhere it to your mirror clips with double sided foam tape, being sure to center it over the primary mirror. In my case, I've removed the flocking from the areas where the tape needs to make contact so I get good adhesion. 

 

 

zAoaTTb.jpg

 

 

L5woKIu.jpg

 

 

And that is all there really is to it. The only downside is that you do lose some mirror area, reducing your effective focal ratio by a practically unnoticeable amount.

 

 

 


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#2 J A VOLK

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Posted 07 April 2019 - 05:08 PM

Worked well - guessing the biggest effect removed was from the mirror clips and possibly the end of the spider arms. I've seen a method where you put little caps on the clips that are like the truncated part of a circle to eliminate spikes. I made a similar mask for a 10" mirror I made that had a great optical figure, but a very narrow turned edge.

Edited by J A VOLK, 07 April 2019 - 05:09 PM.

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#3 Jon Rista

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Posted 07 April 2019 - 06:58 PM

It isn't just cleaner stars. It clearly helps contrast as well, as all that extra scattered light gets strewn all around the stars in a non-uniform manner and that hurts contrast of background details around said stars.


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#4 calypsob

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Posted 08 April 2019 - 06:09 PM

Ha, awesome I have spent the past 2 days trying to decide if I should print a mask or have one cut. Im going to give your suggested link a try, thanks.

#5 Jon Rista

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Posted 08 April 2019 - 06:32 PM

Just curious, is there no other way to hold a newt primary so that the clips don't obscure the path?



#6 calypsob

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Posted 08 April 2019 - 06:43 PM

Just curious, is there no other way to hold a newt primary so that the clips don't obscure the path?

 

They shouldnt be touching the mirror, technically they are just a fail safe to prevent the primary from falling out in transit. Some people will put a shim between the primary clip/mirror and fill the void with silicone and then remove the shims, this prevents astigmatism and lets you ditch the clips, kinda sketchy though imo. One of my primaries has silicone tipped screws that grab the mirror but it still uses clips as a fail safe. They are hard to avoid unless you go with a conical mirror which has no clips or you can get a turned edge beveled mirror where the clips do not cover any reflective surface.


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#7 calypsob

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Posted 09 April 2019 - 07:31 AM

This guy btw can do a CNC wood aperture mask for a lower price than the metal one. https://www.ebay.com...=p2047675.l2559  Just try to limit the obstruction to .125" if possible

 

Im still making up my mind, it would be kind of cool to remove the clips and trace and drill the ring so that it bolts down in place of the clips.



#8 rkayakr

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Posted 09 April 2019 - 08:40 AM

I have a mak-newt that already blocks incoming light around the corrector perimeter. I wonder if a mask at the primary mirror will make any difference?



#9 RaulTheRat

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Posted 13 April 2019 - 10:47 PM

I have a mak-newt that already blocks incoming light around the corrector perimeter. I wonder if a mask at the primary mirror will make any difference?


I've not accurately calculated, but I do plan to mask my own 190mn like this shortly, since my intuition is that off-axis rays will still make it to the mirror edges - on mine the primary is 200mm, and the front aperture 190mm, but even a degree off-axis, over a 60cm tube (and it's actually longer than that) a star one degree off-axis should be able to illuminate part of the mirror 1cm displaced from the aperture (using the 1 in 60 rule) - so I'm pretty sure the mirror clip area and mirror edge are being illuminated.

#10 elmiko

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Posted 14 April 2019 - 11:52 PM

I did the same thing with my Orion imaging Newtonian 8 inch f 3.9. I made mine out of 1/4 foam board . Drew the inner and outer circles using a compass. Then cut them using a Dremel tool. Then mounted it using one of the mirror clip screws on each mirror clip. Really improve my images! Mike


Edited by elmiko, 14 April 2019 - 11:52 PM.

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#11 betelgeuse91

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Posted 15 April 2019 - 09:21 AM

Why can't every imaging Newtonian come with the mask... Everyone wants this!


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#12 ribuck

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Posted 13 April 2020 - 03:26 PM

Lacerta scopes come with a mask pre-installed mask - they take the Skywatcher Scopes - replace the tube and focuser, flock the tube, and install a mask on the primary 

 

They sell the masks on their website for 8" & 10" mirrors



#13 calypsob

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Posted 13 April 2020 - 07:35 PM

I ended up silicone gluing my 6” into place. You determine where to place the blobs, put in spacers, set in the mirror. When the silicone dries remove the spacers. Now you can remove the mirror clips. No need to mask.

#14 ribuck

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Posted 14 April 2020 - 02:10 AM

Hi Wes,

 

I'm afraid i'm not as brave as you.  I'd worry about the glue failing



#15 calypsob

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Posted 14 April 2020 - 06:22 AM

Hi Wes,

I'm afraid i'm not as brave as you. I'd worry about the glue failing

Yea I mean my scope was not very expensive so im not too worried. I do store it upright, but honestly you need a piece of piano wire to remove the silicone. Also note that When its on the mount its always pointing up. I dont think I would glue in something larger than a 6 without an excellent mirror support because you may start to introduce an astigmatism. You need to spend a while reading about the plop methods if you go that route.

Edited by calypsob, 14 April 2020 - 06:24 AM.


#16 ribuck

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Posted 14 April 2020 - 06:53 AM

I think I might just get a pre-made mask that can be removed if needed and try it for myself and see how i get on.

 

Cheers,

Rich.




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