Multiple aperture off axis mask with white light film on dob

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BillyBoyBoy
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Multiple aperture off axis mask with white light film on dob

Post by BillyBoyBoy » Thu Jan 03, 2019 5:35 am

Hello and happy 2019!

I have purchased a Sumerian Alkaid 12" f/5 dob for a travel visual scope. For those not familiar, it packs down to a suitcase that is smaller than carry on dimensions.

The plan is to use this scope for night viewing, yes, but also have a white light filter for solar viewing.

I know the limiter is seeing when considering aperture for solar viewing. And I know that more aperture supports better resolution and higher magnification. So it's a balance to be struck. I would think that ideally, to have a means to increase or decrease aperture based on current seeing seems like a way to go.

The apertures allow sunlight through a white light filter, and film is my choice for this plan... Baader film.

Online, but not in person to gauge viability, I have seen off axis aperture masks with multiple apertures. I have seen the approach of three different sizes, where one uses one of the openings in the mask according to seeing and the other openings are covered. I have seen others where you have six openings and none are covered up.

My idea is to have an off axis aperture mask with four equally spaced holes (each hole the same size of either 70mm or 80mm) circling the spider vane center, and each opening placed between spider vanes. I would have four flags that could cover up any number of the openings to allow more or less light in, depending on the seeing. All flags covering all holes during transport, thus protecting the film. The hole size of around 70 or 80mm in diameter allows room for the unused flags to reside between holes.

So, in effect, I would have one, two, three or four openings. So, the aperture opening area of the 70 or 80mm hole x 1, 2, 3 or 4.

Is this a viable idea here? It would be homemade and I already put the design together using CAD. Is the transmission of sunlight through a variably set number of four same-size openings on an off axis mask a viable analogue for a set of four off-axis masks with a single opening whose aperture sizes vary from mask to mask?

As this is a travel scope for air travel, the goal is to figure how versatile this scope can be for visual use.

Any and all insight or input is appreciated and welcome!

Cheers, and sunny skies!

Bill

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marktownley
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Re: Multiple aperture off axis mask with white light film on dob

Post by marktownley » Thu Jan 03, 2019 10:28 am

There's a detailed thread about this somewhere on SC, multiple off axis sub aperture masks are a bad idea. Just go for the one aperture, maybe 80mm, this will be fine for visual observing. Bear in mind the sun is blank in whitelight at the moment and will likely be for a while yet.
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BillyBoyBoy
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Re: Multiple aperture off axis mask with white light film on dob

Post by BillyBoyBoy » Thu Jan 03, 2019 1:29 pm

The longest solar minimum ever is the one you are in the middle of when you discover solar astronomy.

Hey, at least it buys me time to research and save up for the Ha gear.

I get the impression that it might cause diffraction issues, as if the mask part were really wide spider vanes in between the holes. I've looked and looked, but I will look some more. If you or anyone else that reads this thread finds that thread that Mark is referring to, please drop a link! I like to know the why and the why not.

Thank you Mark! I have been intending to reach out to you and pick your brain about your experience on your HaT.

Bill

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Re: Multiple aperture off axis mask with white light film on dob

Post by BillyBoyBoy » Thu Jan 03, 2019 2:50 pm

Another question.

Does the aperture have to be circular? As long as it's one aperture, it can be any shape, like oval, right?

Bill

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Re: Multiple aperture off axis mask with white light film on dob

Post by BillyBoyBoy » Sun Jan 06, 2019 5:12 am

Sorry, that was a dumb question.

Bill

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Re: Multiple aperture off axis mask with white light film on dob

Post by christian viladrich » Sun Jan 06, 2019 10:51 am

As explained by Mark, this is a bad idea. Make it simple and go for only one aperture.

Here is a simulations comparing the effect on the figure of diffraction of full aperture, two and four holes :
Image

http://astrosurf.com/viladrich/astro/in ... action.htm
Christian Viladrich
Co-author of "Astronomie Planétaire"
http://www.astroplanetes.com/
Co-author of "Astronomie Solaire"
http://www.astronomiesolaire.com/

BillyBoyBoy
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Re: Multiple aperture off axis mask with white light film on dob

Post by BillyBoyBoy » Tue Jan 08, 2019 4:39 am

Oh, wow.

Thank you guys.

Bill

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