12" Newtonian Sub Aperture Mask

[russ.will]

As Moonshane keeps banging on about sub aperture masks, particularly with respect to high mag planetary viewing, I thought I'd give it a go.

However, as no worth challenge is worth undertaking without complicating it further, I've decided to go multi-aperture, with a rotating sub mask, utilizing some spare 4mm marine ply I have lying around. A Sketchup or two is worth a thousand of my words, so:

As Sketchuped, there are 75mm dia (F20), 100mm (F15) and 112.5mm (F13) apertures. A quarter circle cutout of 112.5mm diametre for a maximum unimpeded light gathering is included for experimental purposes. All are sized to avoid the spider vanes and secondary by a few millimetres and also sit within the 305mm dia of the primary. The rotating mask will obviously allow me to switch apertures easily. The intention is to spray paint everything matt black and to possibly flock the rear of at least the aperture mask.

The questions are:

What size apertures do people think are worth including? I've included that lot, because once the router is up and running, I might as well and I have an idea for adapting the 75mm aperture to take a white light solar filter.

The quarter cut out will actually have rounded corners to avoid diffraction and indeed all apertures will have their edges rounded over for the same reason, but is the quarter cut out actually worth including?

Is there anything else I should have thought of?

As ever, the experience, thoughts, advice and if justified, derision of all would be most welcome.

Russell

PS. My gallery has some images of the work undertaken so far. Once done, I'll do a buld thread in the DIY section.

[Peter Drew]

I think you will find it a well worthwhile project with plenty of interesting experiment to repay the effort. When my 30" F4 Newtonian was in use I had a 3 aperture mask offering unobstructed apertures of 12" F10, 10" F12 and 8" F15. I placed the mask close to the primary to reduce field vignetting.

[russ.will]

Interesting. So in your view, avoiding field vignetting is preferable to the sort of mask I'm proposing, which seeks to avoid the central obstruction of the secondary and spider?

I can see that it would allow apertures in the F6 to F10 range, that my current idea doesn't.

Russell

[Moonshane]

cheeky sod! seriously though it's such a cheap and easy mod, I'd definitely recommend it and anyone doing it will be shocked at the results I'd wager.

Peter is much more knowledgeable than I an I have done this only for solid tube scopes currently where a primary end mask is impractical. for a mahoosive one (30"!!!!) truss dob, this makes sense I think.

For mine, I made a multiple mask too and have a 170mm f11 (the largest circle I can fit between the vanes), a 130mm f14 and a 100mm f18. These last two were arbitrarily chosen to allow comparison with two common(ish) sizes of refractor.

My understanding is that you really need to retain a round window as otherwise weird things happen to the star shapes and other aspects of the view (I am edging toward the cliff of ignorance now though). maybe try it if you have some spare material but I suspect that the quarter aperture idea will be less effective than the 112.5mm round opening.

What I can say is the following:

in my scope, the 170mm f11 unobstructed view of planets, doubles and moon compares with nothing else.

the view of some star clusters (e.g. the ET / Owl Cluster) are enhanced due to the darker background (smaller exit pupil)

it's the cheapest and most effective mod I have ever done. check that your adjusters for the secondary and your secondary housing are below the level of the vanes or you may need a cut-out so it sits well on the vanes. be careful using it for solar of course - i.e. make sure you cover all the outside edges to prevent light coming through directly.

[russ.will]

Cheers for that. Clearance to the secondary is fine. I've made it so it fits the tube like a large lens cap. Also, the quarter cut-out has been made with round corners. I figured that if corners cause diffraction and hence, odd star shapes, it might be best to have no corners. If it doesn't work, at least it makes a good handle!

Peter's suggestion had got me thinking - I have some spare plywood, so I shall knock up some larger masks to sit on top of the lower OTA between the truss tubes. I'll do an F8 and F10 and possibly one the same size as the 112mm, to see which is preferable. The one issue I could see with that is the comparatively large obstruction of the 75mm secondary, which my mask sidesteps as an issue.

I'll post some more piccies later as I did all the routing last night.

Russell