10 replies to this topic

[robjme]

I am very interested in increasing the focal ratio of my AT130 EDX by stopping down the aperture using an aperture mask. There are many informative threads on CN about their benefits. I have never had one or used one, so I have questions about making one or more for my scope.

 

First, I did a search online and found focusing masks and off axis masks, but none made for refractors.  I admit that I have been observing for decades and never tried one because I didn't know about them or think about them. Now I wonder if an aperture mask is superior to a Barlow lens for increasing the focal ratio. They have zero glass, less weight, less expensive, and should work fine with my binoviewer. Why aren't these things more popular?

 

Incidentally, my 4.5" reflector from 1970 came with a cover and a central 2" lens cap for viewing the sun. The scope also came with a solar filter that threaded onto the .965 barrels. It's scary to think that I used that filter a few times to look at sunspots.

 

My plan is to make a 5" objective cover like the one that came with the scope and cut a 4" diameter hole in the center. On top of this, I would add a second cover with a 3" opening. And maybe add a third cover with a 2" opening. I would like to try all of these- it will be like swapping scopes without the effort.

 

So, how do I make this thing? Metal is good and would have a sharp edge but is difficult for me to work. Wood is easy to work with but doesn't have a sharp edge. That leaves plastic. I would want this to be light weight and easy to use. I would also want to be sure that it doesn't mar the finish of my new scope. And I definitely want to keep this away from my precious objective lens so it will have to mount over the dew shield. I have a wood shop and woodworking skills, but no 3D printer. I plan on making some paper masks sometime soon and experiment with them before making the final product.

 

Does the mask perform better when it is close to the objective, or doesn't that matter? What material do you think is best? Should I consider making different masks with different openings rather than this all-in-one idea of mine? Should I use felt to hold it in place like the original objective cover or something else? How important is the sharp edge of the opening? I will probably cut 1/16" plastic sheet using a jig saw, so the edge will not be too sharp.

 

I would be very interested in seeing any homemade masks and learning from your experience.

 

TY, Rob

Edited by robjme, 16 March 2025 - 11:50 AM.

[tturtle]

If it were me I would give wood a try. You can buy balsa sheets 3 or 4 inches wide and glue them edge to edge to get the width you need and then there are various ways to cut the circles.  As for the edge some simple sanding can make the edges as sharp as you want. You can line the outer edge with felt to get a snug fit inside the dewshield. If you want to ensure that it stays flat you can glue some ribs to one face.

 

That’s all of course assuming you put the aperture in front of the objective. What might be more interesting is putting it near the focuser with the ability to slide it up closer to the objective. You could literally do this while you are viewing a planet or the moon to see how the image changes, presumably getting dimmer yet sharper.  

Edited by tturtle, 16 March 2025 - 12:03 PM.

[Astrojensen]

Why on Earth people keep overthinking aperture stops like there's no tomorrow, never ceases to amaze me. Just make them from thin cardboard and be done with it. 

 

 

Now I wonder if an aperture mask is superior to a Barlow lens for increasing the focal ratio. They have zero glass, less weight, less expensive, and should work fine with my binoviewer. Why aren't these things more popular?

For one thing, an aperture mask reduces the resolution of the scope down to the aperture of the mask, obviously. Secondly, it also doesn't change the available back focus, unlike a Barlow, so it won't help with reaching focus when using a binoviewer... 

 

Using aperture masks are usually for experimenting with the resolving power of different, small apertures, as well as masking off turned-down edge in the objective. 

 

 

Clear skies!

Thomas, Denmark

[robjme]

One of my other hobbies is model airplanes. So I have a lot of balsa laying around. Maybe a good idea for a preliminary mock-up.

I forgot to mention that I would also like this thing to look pretty on my scope.

[Sketcher]

 

 

Plastic ("For Sale" signs etc. at your local hardware store for raw material), posterboard, and cardboard can be suitable materials.  For larger (than 6-inches or so) apertures 1/8-inch hardboard can be used.

 

The masks can be fun and educational to experiment with, but in my experiences, unless your telescope has a poorly designed objective, you'll most likely prefer sticking with full aperture for all of your observations.

 

Filters (in my experience) tend to be better for dimming overly bright objects as well as for taming chromatic aberrations -- without reducing resolution.  But not everyone has the same preferences . . .

 

* * * * * * * * * * * * *

I used a 1-inch aperture mask to stop down my ST-80 for my series of 1-inch aperture observations:

 

 

 

 

etc.

 

But that wasn't to "improve" performance of the telescope.  Instead, that was to show what a modest, little 1-inch aperture was capable of.

 

I also once used a series of aperture masks to determine the smallest aperture at which Cassini's Division could be detected at, but that resulted in a self-proclaimed "expert" denying that my observation was even possible.  (The telescope I had stopped down for that observation was an Astro-Physics oiled triplet).

 

Anyway, I can certainly recommend doing your own experimenting with the use of aperture masks.  After all, they're dirt cheap to make and very easy to use.

[geovermont]

Cardboard. It's free.

[doveofthesea]

I used several old CDs to make aperture masks for my AR102 solar scope project. My solar results were noticeably improved at 64.28mm (it was the biggest hole saw I had) bringing me from f/5.88 to f/9.3. Adding a 2X barlow got me to f/18. The backyard results impressed me such I just completed a 3D project for a solar film, aperture mask (65mm and 75mm) and 75mm ERF holder. Haven't had a chance to use it yet since I am paying the cloud tax....

 

 

 

   

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[izar187]

For bigger newts just use a pizza box.

[AlamoBob]

For bigger newts just use a pizza box.

When you do this on a Newt, you're not changing the size of your secondary, so it is a larger and larger percentage of your total optical path - doesn't that make for some very odd results?

[Tangerman]

When you do this on a Newt, you're not changing the size of your secondary, so it is a larger and larger percentage of your total optical path - doesn't that make for some very odd results?

Aperture masks for Newts aren't usually centered. You cut a hole between the spider vanes. Gets rid of diffraction spikes, no more secondary obstruction, greatly reduced resolution and brightness.

Edited by Tangerman, 20 March 2025 - 01:58 PM.

[izar187]

When you do this on a Newt, you're not changing the size of your secondary, so it is a larger and larger percentage of your total optical path - doesn't that make for some very odd results?

If you make a significantly down sized on-axis centered mask,

then yes you do end up with a disproportionally large secondary

obstruction on your now smaller newtonian reflector.

 

But it will still out resolve the even smaller yet aperture of the

off axis masked newt, where all secondary obstruction is gone.

 

On-axis masking often yields good detail, but somewhat muted.

Off-axis masking yields aesthetically contrasty, but less detailed.

IMHO+E

 

An alternative method C, for some folks is an apodizing mask.

Which would more correctly be a full aperture apodizing filter.