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When 20" is not enough .....

John

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cotterless45

cotterless45

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Yep, that would look nice in my garden. Don't know what my wife would say though. Probably I wouldn't be worrying about cloud cover !

Agree about the ladder. I get vertigo, can't even stand on chair without falling off.

Nick.

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ollypenrice

ollypenrice

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Great stuff, but in these huge apertures I'd want to go below F4 myself. It isn't just the ladder businesss, it's the fact that your FL is getting ever longer, excluding more and more of the showpiece objects and chasing the nudges as well. F6 seems a strange F ratio to choose in my view.

Olly

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quaoar

quaoar

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Great stuff, but in these huge apertures I'd want to go below F4 myself. It isn't just the ladder businesss, it's the fact that your FL is getting ever longer, excluding more and more of the showpiece objects and chasing the nudges as well. F6 seems a strange F ratio to choose in my view.

Olly

Great stuff, but in these huge apertures I'd want to go below F4 myself. It isn't just the ladder businesss, it's the fact that your FL is getting ever longer, excluding more and more of the showpiece objects and chasing the nudges as well. F6 seems a strange F ratio to choose in my view.

Olly

My Webster 28" is F4.2. I can honestly say I have no issue with observing or climbing the ladder. You soon get used to it and it's no big deal anymore. A 28" F4.2 is actually shorter than the Obsession 25" F5. My most favoured eyepieces are the 17mm and 10mm Ethos with Paracorr II and occasionally lower TV Ethos or Naglers if the seeing permits. If I were to buy it again ( and I'm considering doing just that :) I would probably go a 32" F 3.65.

I'm just wondering what objects I'm missing out on?.post-18787-0-00328100-1405198009_thumb.j

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swamp thing

swamp thing

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Great stuff, but in these huge apertures I'd want to go below F4 myself. It isn't just the ladder businesss, it's the fact that your FL is getting ever longer, excluding more and more of the showpiece objects and chasing the nudges as well. F6 seems a strange F ratio to choose in my view.

Olly

It seems somewhat strange that as the owner of a large Dob you're kinda missing the whole point of large aperture. You don't buy a large Dob for wide field views Olly. They don't do them, you buy large aperture for the increased image scale. That's what aperture gives you. If you want wide fields use a small scope.

Remember the lowest usable power for each aperture remains the same whether f/3 or f/6 so focal length is immaterial. It's merely the eyepiece you choose to get you there that is different.

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ollypenrice

ollypenrice

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It seems somewhat strange that as the owner of a large Dob you're kinda missing the whole point of large aperture. You don't buy a large Dob for wide field views Olly. They don't do them, you buy large aperture for the increased image scale. That's what aperture gives you. If you want wide fields use a small scope.

Remember the lowest usable power for each aperture remains the same whether f/3 or f/6 so focal length is immaterial. It's merely the eyepiece you choose to get you there that is different.

I don't think I'm missing the point at all. Astronomical telescopes have two purposes, to make small things look big and to make faint things look bright. Once you are up to an aperture giving 300x you are about done on the 'small things made big' side. Sure, it is more complicated because if the small thing is also faint you will need 'faint things made bright' to come into play or magnification will just make it disappear.

I don't use my 20 inch to get more image scale. Absolutely the opposite. I usually run it at the lowest power I can, a 26 Nagler giving 78.8 times. I sometimes use a 13 Ethos giving 158x. I don't use it for higher powers than that. In other words I primarily use it, as do our guests, for the second purpose of making faint things bright. Now if a large aperture scope condemns itself to high powers by virtue of its focal length it no longer suits my prurposes (though it might suit yours and that's fine.) Like any sane person I would like more than 20 inches of aperture  :grin:  but I absolutely do not want to 'spend' that aperture on increased image scale. I want to spend it on more brightness at the image scale I already have. And I don't want to be condemned to more frequent nudging, either, especially when giving tours to beginners. Mine is not a minority opinion if the large number of ultra fast light buckets being made is anything to go by.

Olly

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swamp thing

swamp thing

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I don't think I'm missing the point at all. Astronomical telescopes have two purposes, to make small things look big and to make faint things look bright. Once you are up to an aperture giving 300x you are about done on the 'small things made big' side. Sure, it is more complicated because if the small thing is also faint you will need 'faint things made bright' to come into play or magnification will just make it disappear.

I don't use my 20 inch to get more image scale. Absolutely the opposite. I usually run it at the lowest power I can, a 26 Nagler giving 78.8 times. I sometimes use a 13 Ethos giving 158x. I don't use it for higher powers than that. In other words I primarily use it, as do our guests, for the second purpose of making faint things bright. Now if a large aperture scope condemns itself to high powers by virtue of its focal length it no longer suits my prurposes (though it might suit yours and that's fine.) Like any sane person I would like more than 20 inches of aperture  :grin:  but I absolutely do not want to 'spend' that aperture on increased image scale. I want to spend it on more brightness at the image scale I already have. And I don't want to be condemned to more frequent nudging, either, especially when giving tours to beginners. Mine is not a minority opinion if the large number of ultra fast light buckets being made is anything to go by.

Olly

But you are using your 20" for image scale you just said so in how you're observing Olly :)

I'm not talking about high power. You are confusing my point.

You are using the increased image scale your 20" gives you even at minimum power. You are using the advantage of aperture to keep the objects at max brightness with increased image scale. That is exactly what aperture is for.

You cannot increase surface brightness of extended objects. That's impossible, all you can do is make them bigger for the same brightness, that is what aperture does.

A 30" scope will never be able to match a 20" for field size it's simply not possible regardless of focal length as the 30" scope has higher minimum power. If you are looking for a 30" scope for field size your search will be never ending.

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