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Doc

My new multi-aperture mask

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As I'm off work and completely bored I made an aperture mask, but with a twist as it's three in one with the bonus of a dust cover as well.

First hole is 145mm so my scope becomes a F12.6

Second hole is 110mm so my scope becomes a F16.6

Third hole is 80mm so my scope becomes a F22.8

Then with all holes covered it becomes a dust cover.

I'm going to paper mache it and paint it black.

What do you guys think. Is it going to work? Whats F22 going to look like?

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I really like the look of that, Mick. I'm definately going to have a stab at making one for my 12" Lightbridge.

When you do your brilliant observing reports it will be very interesting to read what sort of difference each hole makes.

Thanks for the idea.

John

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Cheers John. I'm hoping for more contrast on planets. When I'm out next I'll try it on Saturn and see what the view is like.

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wonder what it would be like with a 2x barlow on also, would that take it to f44 ?

why do smaller holes change the f ratios ?

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f = focal length divided by diameter of lens

The different apertures Doc's Doofer2 provides in effect changes the diameter of lens in the equation above...

carl

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this is really interesting. is it this straight forward? It would be great to use this to 'convert' my 12" 1600mm f5.3 dob to an 8" 1600mm f8 dob for planets if there's no adverse effects? does it affect resolution? I think I'd just make a ring to reduce the aperture 'evenly'. fitting a kitchen at the moment so loads of good quality cardboard about me!

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wonder what it would be like with a 2x barlow on also, would that take it to f44 ?

why do smaller holes change the f ratios ?

:headbang: Now thats a thought. As Carl has said it's all due to the focal length. I read somewhere that at F22 you should have APO style views, I doubt it very much but will check it out on Saturn when I get an oppertunity.

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Shane, using a ring will work, although with your secondary in the centre of the tube, you'd have to allow for the diameter of the secondary when calculating the effective new aperture. It is far simpler to had a known size hole, offset so as to be uninterupted, if you wish to "engineer" your new focal ratio.

Doc, that is great, did your 4 year old earn a gold star for it? LOL!

Seriously, a spiffing idea, one which I may very well copy, although I may use a compass for scribing the circles etc. At work, my coffee comes in large plastic containers, and the lids for these are a perfect, positive, fit onto the end of my 114mm newt. To-wit, I am amassing several different plans. Already I have a dust cover (original was missing when I bought it) and a solar filter (I used my aperure mask for this, so need to make another) and now I can busy my self during the quiet periods, by making a variable aperture mask, akin to yourn.

(Idea well and truely cribbed!)

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Doc, that is great, did your 4 year old earn a gold star for it? LOL!

LOL that hurt....

If this idea works I have a few offcuts of 4mm polycarbonate sheets at work I'll remake it from that, get everything cut out nice and neat and then paint it all black.

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Really interested to hear the results from this. Make sure the sub-apertures avoid the spider vanes - then you get unobstructed, CA-free views. I guess it will be apo-like!

I wonder if the new focal ratios mean the scope is less fussy about eyepieces??

Andrew

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Really interested to hear the results from this. Make sure the sub-apertures avoid the spider vanes - then you get unobstructed, CA-free views. I guess it will be apo-like!

I wonder if the new focal ratios mean the scope is less fussy about eyepieces??

Andrew

In theory a F16 scope should have no coma at all.

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this is really interesting. is it this straight forward? It would be great to use this to 'convert' my 12" 1600mm f5.3 dob to an 8" 1600mm f8 dob for planets if there's no adverse effects? does it affect resolution? I think I'd just make a ring to reduce the aperture 'evenly'. fitting a kitchen at the moment so loads of good quality cardboard about me!

You hit the nail on the head. It affects resolution... The smaller the aperture, the lower the resolution will be in theory. The way you gain, is that most 'scopes don't actually work at their maximum theoretical resolution (i.e. the 'diffraction limit'), but are instead limited by things like seeing, optical quality, obscuration diffraction (due to spider vanes/secondary).

As you make the aperture smaller (and the f/ratio longer) you reduce the contribution from seeing/optical quality/obscuration -- but you increase the contribution from diffraction. The trick is to balance the contributions and get the best possible image. In your particular case, you'll almost certainly win on Planets with an off-axis 8" f8 over a 12" f5.3 (probably actually need a 5" hole off-axis and avoiding all the spiders/secondary as Andrew says, to get the best performance).

An off-axis hole will work better than a central 'even' hole, because you will (if you avoid the spider vanes) get rid of the diffraction term from the spiders/secondary. Do make sure the hole itself is nice and round though, or you'll get some interesting looking diffraction rings :)

Nice idea Doc -- like that implementation a lot ;)

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Ok so my knowlege of this stuff is extremely tenuous, but won't you get coma anyway because it's caused by the the light being reflected slightly differently at the edge of the mirror compared to the centre?

I was thinking about this possiblity only last night. But didn't actually give it a try.

I shall watch for the results with great interest - I really want to be proved wrong! Do you get a proportionally smaller field of view?

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Ok so my knowlege of this stuff is extremely tenuous, but won't you get coma anyway because it's caused by the the light being reflected slightly differently at the edge of the mirror compared to the centre?

I was thinking about this possiblity only last night. But didn't actually give it a try.

I shall watch for the results with great interest - I really want to be proved wrong! Do you get a proportionally smaller field of view?

NO according to the maths the FOV stays the same no matter what the aperture.

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Mick,

thanks for clarifying that. The lens cap (or whatever it's called when you don't have a lens) on my 200p appears to have a couple of mini cut outs on it, about 70mm diameter-I'll give one of them a try next time I'm looking at the moon.

cheers

Alan

Edited by oceanheadted
corrected grammar

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If your into solar the reduced aperture also means you don't need to spend a small fortune on a large aperture solar filter too.....just cover one of the holes with solar film and away you go...

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Well I had a chance to use it last night and it did dramatically cut down on the brightness of Mars or Saturn and enabled me to see a little more detail.

The images were darker so I lost some resolution but did manage to see the polar ice cap and I'm sure Sytris Major (I think thts how it's spelt).

Compared to a full 400mm aperture I saw more detail but the image was darker.

Not alot of difference was noted between the 145mm, 110mm and 80mm apertures except for a darker image, the same amount of detail and image size was obtained while using all three apertures.

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At work, my coffee comes in large plastic containers, and the lids for these are a perfect, positive, fit onto the end of my 114mm newt. To-wit, I am amassing several different plans. Already I have a dust cover (original was missing when I bought it) and a solar filter (I used my aperure mask for this, so need to make another) and now I can busy my self during the quiet periods, by making a variable aperture mask, akin to yourn.

(Idea well and truely cribbed!)

Thus far, I have 3 such plastic coffee lids for DIY end cappy thingys for the 114. I wonder if this is why I am having trouble sleeping? ;)

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