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How do larger apertures work with the moon?


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Hi all, I'm slightly confused about getting greater resolution on the moon.

If you have a larger aperture, you can get greater resolution right? But the moon is very bright, so people often put the end-cap back on the newtonian and open the "little cap" on it to reduce the brightness. This must reduce the resolution I suppose?

So what's the deal here - is this why you have a lunar filter, to cut down on the light hitting your eye but letting the scope collect more photons?

Many thanks!

Rob

Edited by RobH2020
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Well yes a greater aperture is better resolution, but that is on paper and theoretical. Another consideration needs to be that there is likely no lens or mirror on earth that performs to the theory exactly. And the portions that are acting furthest from the theoretical ideal are the edge areas and the bigger the mirror or lens, the more "edge" there is and  the worse they perform and so the worse the image.

It is as ever a 3 way choice: Size, Cost, Quality. Pick the 2 that you want and the third is beyond your control.

Scope design also has an impact.

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Thanks for the reply! 

Basically I'm just wondering if the new 8" dobsonian I have will provide a more detailed view of the moon than my old 6"!

If I put the end cap on and open the little cap to reduce brightness, it'll be the same... so am wondering if lunar filter / sunglasses would give me a better view!

Thanks,

Rob

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3 minutes ago, RobH2020 said:

if the new 8" dobsonian I have will provide a more detailed view of the moon than my old 6"!

Going from a 6 inch aperture to 8 inches will reduce the Dawes limit (i.e. increase the resolving power). At least, in theory! The value changes from 0.77 arc-sec to 0.58. Both of those will be smaller than the resolving power limit imposed by atmospheric seeing.

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If you reduce the aperture of the scope by using the small opening in the end cap, your "new" aperture is that of the hole so around 52mm if I recall correctly. So then you will have the resolution and light grasp of a 52mm scope, ie: significantly less than that of a 200mm scope.

 

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its all about image scale

get a little mirror and see your face reflection it

then get i big woman mirror then you will be able to see the hairs growing out of your massive face 😝

same applies to the scope

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I have no explanation as to why some people find the moon too bright. It's only reflected sunlight like you get in daytime.

Full brightness also gives you more contrast than if you were to use a filter. Black is still black, but the whites are much cleaner. Crisp and clean, that's how I like my moon :wink2: 

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I also never understood why people would want to dim the brightness of lunar views, until I used a 35mm Aero ED in a 12" dob... man that was bright! Couldn't look at it for more than a couple of seconds without looking away. When you increase the mag the image naturally gets dimmer, so if it's too bright just use a shorter FL eyepiece! If you do really enjoy low mag views and they are a bit bright, I'd go for a lunar filter rather than using the little cap.

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I don't filter or stop the scope down to observe the moon, even with my 12 inch dobsonian. I do tend to use high magnifications though.

I carry a moon filter with me for outreach sessions though, in case anyone feels uncomfortable observing the moon without one.

 

 

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4 hours ago, RobH2020 said:

Thanks for the reply! 

Basically I'm just wondering if the new 8" dobsonian I have will provide a more detailed view of the moon than my old 6"!

If I put the end cap on and open the little cap to reduce brightness, it'll be the same... so am wondering if lunar filter / sunglasses would give me a better view!

Thanks,

Rob

I have been using my 8" dob on the Moon quite a bit recently, in fact just about to go out, as soon as it clears the chimney stack. Typically I may start at 66X and work my way up, settling usually at 171 - 240X. Never actually occurred to me to use a filter, particularly if its a dedicated Lunar session, your pupil quickly adapts and as mentioned. mid to high power creates more tone anyway.  Also often, the Moon may naturally become slightly more reduced in brightness due to thin cloud passing over. In as far as detail is concerned the 8" dob will enhance many extraordinary features.

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Having just bought my 200P Dob I attached my DSLR a Canon 60D with iso100 at 250sec I managed this shot of the moon Friday night, no filters just the cameras speed control to control the light, for my first photo with this setup I am quite pleased, it can only get better as I learn more, Can't it?

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