Aperture mask for Newtonians

[Maddie]

Hi everyone

I’m looking to buy an aperture mask for a Newtonian telescope. Some companies might do customized rings with a specified inner and outer diameter? Any recommendations??

Thank you!

Maddie

[Tropo-Bob (Bob)]

I made some by cutting up some cardboard to suit the tube diameter and then cutting a hole in it that was small enough to accept light that was not interfered with by the diagonal spokes.

I found that I could split doubles that previously just looked like a blurry mess.

[bojan]

I used black folder cover plastic sheet and cut hole with this tool.

[Maddie]

Thank you. I’ll have a look and see what I can do.

Maddie

[astro744]

What is the diameter of your Newtonian primary mirror and secondary mirrorr (minor axis of latter)?

Why do you want an aperture mask?

You mention inner and outer diameter. This implies an on axis mask perhaps placed directly over the mirror to block the effects of an edge defect in the primary, e.g. turned down edge.

You would not use an on axis mask at the front of the tube as it would reduce contrast due to the secondary diameter now being a greater percentage of primary diameter. (Contrast is a function of diameter not area). The effect can be minimal to extreme depending on the obstruction.

An off-axis mask is typically used and is limited by the physical dimensions of components used. Typically a 10" telescope with 2" secondary can accommodate 4" off-axis diameter portal that clears the secondary vanes as well as secondary mirror providing 4" of clear aperture.

Again you have to ask why do you want a mask. If the primary mirror is of good quality then when seeing permits, full aperture even with the central obstruction of the secondary will outperform especially in terms of resolution and light gathering a masked telescope. The benefits to double star observing are real in that the diffraction spikes no longer 'hide' secondary stars from view if they happen to have a position angle causing them to be behind a spike. However resolution is still less than at full aperture.

[AaronH]

Originally Posted by astro744

Why do you want an aperture mask?

I've heard of plenty of people imaging with Newtonians adding mask to the very edge of the primary to block out the mirror clips, and to also reduce the impact of any other intrusions into the scope (bolts, etc.). This masks off a few millimetres at the edge of the primary, so should have little impact on performance.

The theory is that it can reduce the minor diffraction artifacts that can surround brighter stars in images, and produce a crisper star shape.

[Maddie]

Originally Posted by astro744

What is the diameter of your Newtonian primary mirror and secondary mirrorr (minor axis of latter)?

Why do you want an aperture mask?

The aperture mask is intended for the 203mm primary mirror, to mask the mirror holding clips.

Thank you for the tips, I’ll keep them in mind.

[Camelopardalis (Dunk)]

The mirror clips shouldn’t be in focus when astronomical objects are.

Consider whether you really want to add something to your scope that isn’t necessary

[Maddie]

Originally Posted by Camelopardalis

The mirror clips shouldn’t be in focus when astronomical objects are.

Consider whether you really want to add something to your scope that isn’t necessary

I am hoping to get cleaner looking stars. In the before and after image below, it appears that the aperture mask contributes to the image. Thoughts?

https://www.cloudynights.com/topic/6...our-newtonian/

[Camelopardalis (Dunk)]

Ah interesting…I’d seen that effect on a couple of my images but doesn’t really bother me. Good to know what causes it though

Looks like it should be easy enough to make one out of black foam or plastic, and a circular cutter as bojan suggested.

The CN thread also talked about affixing the mirror to the clips with silicone…I’m not convinced that would be without its issues though. With my f/4 newt any mirror slop in the primary cell wrecks collimation.

[appiice (Ed)]

Hello Maddie

I know I'm a bit late, but, I can and am cutting masks for telescopes.
I have cut Bahtinov, Tribahtinoc and Duncan masks for members ( and others ) on IIS - I am using either 1.5 or 2mm black plastic sheet.
If you have a sketch or a drawing I can quote it for you, generally delivery ( sending from here in Bathurst ) is about 24 -36 hours.

If interested PM or email me appinsw@outlook.com

Ed