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it's supposed to be f/9 or so, where the image deterioration from spherical aberration becomes smaller than the scope's resolution limit

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This is true of a 4.5 inch, but as the mirrors get bigger the focal ratio at which a spherical mirror is defraction limited goes up. If I recall, in a 6 inch you have to be at about f/10.5 and in a 8 inch you have to be at about f/13.

That's why you don't normally see spherical mirror used in newts much bigger than 4.5, the focal length required to get close to defraction limited gets really long with bigger mirrors and makes for a scope that you need a ladder to get to the eye piece.

For instance, an 8 inch newt with a spherical mirror would have to have a tube about 100 inchs long (8 1/2 feet) to be defraction limited.

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This is true of a 4.5 inch, but as the mirrors get bigger the focal ratio at which a spherical mirror is defraction limited goes up. If I recall, in a 6 inch you have to be at about f/10.5 and in a 8 inch you have to be at about f/13.

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force a fast scope to become a slow scope by stopping it down.

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So, do I have a roughly stopped down f15 (912/60)?

(The original spherical mirror has been replaced with a e-scopes/coulter 1/8wave.)

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Sounds more or less right. The thing is that by covering the edges of the mirror, you effectively "convert" it to a smaller mirror at the same focal length. You can basically get whatever slower focal ratio you want this way, along with any benefits, like reduced spherical or chromatic aberration, for example, or a scope that will work well on simple eyepieces. The draw-back, of course, is that you lose aperture, and so your theoretical resolving power is less. Unless the mask is offset, you also have the same central obstruction that you usually have, which -- relatively speaking -- becomes much larger after a point.

Chris

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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."

"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson

"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan

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Sounds more or less right. The thing is that by covering the edges of the mirror, you effectively "convert" it to a smaller mirror at the same focal length. You can basically get whatever slower focal ratio you want this way, along with any benefits, like reduced spherical or chromatic aberration, for example, or a scope that will work well on simple eyepieces. The draw-back, of course, is that you lose aperture, and so your theoretical resolving power is less. Unless the mask is offset, you also have the same central obstruction that you usually have, which -- relatively speaking -- becomes much larger after a point.

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Chris, if I understand what you're saying is that a 8 inch newt with a 1200mm FL could also serve as a 6 inch with a 1200mm FL if a mask is made at the front of the tube? Would this make it as easy to collimate the 8 inch with a mask as a 6 inch. I don't know if I understand what you meant, but this doesn't seem right.



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Jimmy

"Rarely Have So Many Understood So Little About So Much" - Palle Yourgrau

"...since that time, I have not complained about the weather one single time. I’m glad there is weather." – Alan Bean, Apollo 12

What do you mean by “Saving the Earth”? The Earth is not in danger! Don’t worry about the planet it will be here long after we are extinct...

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Chris, sounds like those who have a Tasco Luminova ota have a "secret weapon" for the spherical mirror. I guess for lunar/planetary, doubles, whatever.....

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Chris, if I understand what you're saying is that a 8 inch newt with a 1200mm FL could also serve as a 6 inch with a 1200mm FL if a mask is made at the front of the tube? Would this make it as easy to collimate the 8 inch with a mask as a 6 inch. I don't know if I understand what you meant, but this doesn't seem right.

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Well, yes and no; think of it like this:

By masking off the 8" to 6", converting your 8" f/6 to 6" f/8, you probably do increase the tolerance for misalignment compared to what the 8" would show at full aperture, yes. You can probably reduce a 12" dob to 6" and not see coma even without a paracorr.

It will only work, though, as long as it's behaving as a 6" mirror. In the case of your 8" example, you can't put the mask on, collimate it, and take it back off expecting not to see coma because you collimated it with an aperture mask on. The thing that makes the 6" less picky is the focal ratio. An f/6 mirror (the full 8") is somewhat more picky about being perfectly in line than an f/8 mirror (the stopped down version); you can take advantage of this and get the view of a perfectly enough collimated 6" from your not-quite-there 8", but you can't use it to get the collimation any closer to perfect so that it will work right when you un-stop the mirror.

Chris

Edit: What I'm saying here is that in absolute terms, your perfectly collimated 6" optical system and your not-perfect-enough 8" optical system are the same distance from ideal. It's only that at the slower focal ratio, you have more room for error before you start seeing it in the image.

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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."

"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson

"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan

Edited by Protheus (08/19/08 06:40 PM)

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Well, yes and no; think of it like this:

By masking off the 8" to 6", converting your 8" f/6 to 6" f/8, you probably do increase the tolerance for misalignment compared to what the 8" would show at full aperture, yes. You can probably reduce a 12" dob to 6" and not see coma even without a paracorr.

It will only work, though, as long as it's behaving as a 6" mirror. In the case of your 8" example, you can't put the mask on, collimate it, and take it back off expecting not to see coma because you collimated it with an aperture mask on. The thing that makes the 6" less picky is the focal ratio. An f/6 mirror (the full 8") is somewhat more picky about being perfectly in line than an f/8 mirror (the stopped down version); you can take advantage of this and get the view of a perfectly enough collimated 6" from your not-quite-there 8", but you can't use it to get the collimation any closer to perfect so that it will work right when you un-stop the mirror.

Chris

Edit: What I'm saying here is that in absolute terms, your perfectly collimated 6" optical system and your not-perfect-enough 8" optical system are the same distance from ideal. It's only that at the slower focal ratio, you have more room for error before you start seeing it in the image.

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Chris, I got ya. I didn't expect a mask on the 8 inch to help collimation that's of course logical. But now that you've brought this up (the mask thing) I understand that in a 8 inch with 1200mm FL, you kinf of have a 6 inch with the same FL if you put a mask on since its the outer edge of the bigger mirror (the 7 and 8th inches) that induce the abberations.

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I'm afraid I don't fully understand it myself, but that's the way I think it works, yep. At least in terms of spherical aberrations, I can say with some certainty that this is the case. I really haven't looked at coma well enough yet, and can only assume (so far) that it will be similar...

Chris

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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."

"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson

"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan

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But who wants to lose aperture anyway?

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Yeah, well not me!

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Jimmy

"Rarely Have So Many Understood So Little About So Much" - Palle Yourgrau

"...since that time, I have not complained about the weather one single time. I’m glad there is weather." – Alan Bean, Apollo 12

What do you mean by “Saving the Earth”? The Earth is not in danger! Don’t worry about the planet it will be here long after we are extinct...

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But who wants to lose aperture anyway?

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Yeah, well not me!

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Me neither; I think I mentioned the problem of the larger relative secondary obstruction a while back... Still, once in a while, it's good to know that this kind of thing can be done.

Chris

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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."

"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson

"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan

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So my father had the chance to try is scope on Jupiter last night with the 25mm plossl. He said that although the planet was small (only 36X), it looked pretty clean. He would like to try higher power. Just as a guide line, can we expect this scope to work at 100X or so on planets? Any of you have tried this before?

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