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azrabella

Recommedations/Suggestions please

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I'm thinking of doing some visual (white light) solar astronomy with either an ST80 or a 150mm Dob. Thing is, would it be better to use a Baader Solar filter across the full aperture of either or use the offset 50mm aperture built into the caps of these scopes. Is there any advantage/disadvantage either way apart from the cost involved?

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A filter(Baader solar screen) fitted to the ST 80 would probably give you as good if not better results to a 50mm aperture with the Dobbie.

 

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There's been many debates about what those little caps are for haha.

There's a persistent theory that they're to aid with solar observing or for stepping down aperture's to cut through bad seeing but it's never been conclusively proved. It appears even skywatcher don't know! 

But yeah, as above, you'd want to use full aperture, solar film is sold in such quantities that there wouldn't be a cost saving either way.

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Posted (edited)

Using the ST80 you'll have the same aperture in a smaller overall package if the dobsonian option is reduced aperture only.

Using the dobsonian you can get full aperture solar film if you want but that would cost more.

Regarding the ability to reduce the aperture of the ST80, doing this will also increase the focal ratio which will reduce aberrations. In effect the view will be dimmer and the maximum detail resolvable will be less but the view you do get will be cleaner/sharper. When I use my ST80 l often reduce the aperture for bright targets.

Edited by Paz

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1 hour ago, Paz said:

Regarding the ability to reduce the aperture of the ST80, doing this will also increase the focal ratio which will reduce aberrations. 

I've often wondered about that...

It wont really increase the focal ratio will it? I'm aware of the fact that focal ratio is a product of focal length divided by aperture. But similarly it is also true that the focal length is also a fixed number; determined purely by the objective lens. You can't turn an f5 mirror or lens into an f10 mirror or lens just by covering up a bit of the aperture? Its still an f5 lens surely? I mean if that were really true then the exit pupils would all change too.

Or maybe I'm wrong (that happens a lot too 🤔haha)

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2 hours ago, Mr niall said:

I've often wondered about that...

It wont really increase the focal ratio will it? I'm aware of the fact that focal ratio is a product of focal length divided by aperture. But similarly it is also true that the focal length is also a fixed number; determined purely by the objective lens. You can't turn an f5 mirror or lens into an f10 mirror or lens just by covering up a bit of the aperture? Its still an f5 lens surely? I mean if that were really true then the exit pupils would all change too.

Or maybe I'm wrong (that happens a lot too 🤔haha)

The objective lense is not physically changed itself but the effective focal ratio that you are using does change and the exit pupil will reduce.

If you have a theoretically perfect scope then there would be no benefit as there would be no aberrations to reduce, but in an ST80 spherical abberation and chromatic aberration would reduce.

Stopping down might also dodge the worst of tube currents, and makes the image easier on your eyes and your eyepieces as they are not tested so much either.

In the end the proof is in the pudding - it's easier than changing an eyepiece to test full and stopped down aperture back to back.

A bright star like Vega is a good test, and the bright planets are also.

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There used to be an argument about the merits (otherwise) of larger
apertures versus (solar... daytime) SEEING etc. In the *distant* past, it
was said (by "Sir Patrick"?) that a 4" scope was "best" for white light.
Or rather a 5" refractor "collected too much light & HEAT". But then,
today's *observing techniques* have changed quite a bit as well... 😉

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