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Date: 11 Aug 2006 18:36:06
From: dickknox
Subject: What would I see if...
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Imagine 3 telescope set ups
1. a f/10 150mm parabloic with a good plossl 30mm eyepiece
2. a f/5 150mm parabolic with a 2x barlow and the same 30mm eyepiece
3. a f/5 150mm parabolic with a good plossl 15mm eyepiece
Will the experience be the same?
It seems that they all have the same resolving power - about 2 seconds
of arc - ie about 4% of the diameter of jupiter. It seems they all have
the same exit pupil - about 3 mm
Will I get the same experience from all of them? If not, how will they
differ. Can expect them to have the same coma, astigmatism, etc,
assuming that the mirror is properly ground.
The only obvious difference that I see immediately is that the diagonal
mirror will block a larger percent of the light on the f/5 - and thus
maybe cause more aberration?
Can someone explain to me - either from a real world observing
experience point of view or a theoretuical point of view - how will
they differ?
New at this and trying to understand better. Any help would be
appreciated.
Dick
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Date: 12 Aug 2006 00:50:38
From: John Carruthers
Subject: Re: What would I see if...
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dickknox wrote:
> Imagine 3 telescope set ups
> 1. a f/10 150mm parabloic with a good plossl 30mm eyepiece
> 2. a f/5 150mm parabolic with a 2x barlow and the same 30mm eyepiece
> 3. a f/5 150mm parabolic with a good plossl 15mm eyepiece
>
> Will the experience be the same?
>
> It seems that they all have the same resolving power - about 2 seconds
> of arc - ie about 4% of the diameter of jupiter. It seems they all have
> the same exit pupil - about 3 mm
>
> Will I get the same experience from all of them? If not, how will they
> differ. Can expect them to have the same coma, astigmatism, etc,
> assuming that the mirror is properly ground.
>
> The only obvious difference that I see immediately is that the diagonal
> mirror will block a larger percent of the light on the f/5 - and thus
> maybe cause more aberration?
>
> Can someone explain to me - either from a real world observing
> experience point of view or a theoretuical point of view - how will
> they differ?
>
> New at this and trying to understand better. Any help would be
> appreciated.
>
> Dick
Resolution is a function of aperture, they are all 150mm so all
theoretically resolve to 0.76".
The less glass you use the better but seeing counts a lot and quality
counts more ;-)
jc
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Date: 13 Aug 2006 09:12:58
From: Stephen Paul
Subject: Re: What would I see if...
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dickknox wrote:
> Imagine 3 telescope set ups
> 1. a f/10 150mm parabloic with a good plossl 30mm eyepiece
> 2. a f/5 150mm parabolic with a 2x barlow and the same 30mm eyepiece
> 3. a f/5 150mm parabolic with a good plossl 15mm eyepiece
>
> Can someone explain to me - either from a real world observing
> experience point of view or a theoretuical point of view - how will
> they differ?
>
The most basic difference will be the maximum field of view favoring the
750mm focal length of the F5. A simple 1.25" 32mm Plossl will give you 2
degrees of field.
With the F10, you'd need a 2" focuser to achieve the maximum field of
1.7 degrees.
Given that a 6" scope isn't exactly a "light bucket", you might be
better served by going with the "rich field" nature of the F5. The
primary reason to go with the F10, is interest in the moon and planets,
both of which provide plenty of (reflected) light.
The F10 will achieve high powers with inexpensive, "simple design" long
focal length eyepieces which have better eye relief than their shorter
focal length brethren. Shorter focal length eyepieces are necessary for
same powers in the F5 (half the focal length to be exact).
Given excellent quality optics, and the budget to support them, you
could get the F5 and a couple of short focal length, high end eyepieces
with long eye relief to achieve high power.
Of course the most popular and readily available 6" reflectors are F8.
The F8 with a 2" focuser would seem the ideal compromise, allowing for
the same 2 degree field you'd get with the F5 and a 1.25" focuser.
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Date: 14 Aug 2006 05:53:07
From:
Subject: Re: What would I see if...
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dickknox wrote:
> Imagine 3 telescope set ups
> 1. a f/10 150mm parabloic with a good plossl 30mm eyepiece
> 2. a f/5 150mm parabolic with a 2x barlow and the same 30mm eyepiece
> 3. a f/5 150mm parabolic with a good plossl 15mm eyepiece
>
> Will the experience be the same?
Pretty nearly -- from a strictly optical point of view. Obviously, a
150mm f/10 Newt is a fairly large scope, so from the point of view
of mechanics and ergonomics, there's a very big difference.
An f/5 mirror exhibits more coma than an f/10 mirror, so in theory
you'd see more coma with alternatives 2 and 3. But in practice,
the coma would be pretty modest. Detectable by some people
but probably not by most. Likewise, a Plossl begins to exhibit
some astigmatism at f/5, but most people would barely notice it.
If you changed the question to from f/10 versus f/5 to f/8 versus f/4,
then you'd have a whole nother story -- coma and astigmatism are
major issues at f/4. Conversely, if you changed the question to
f/12 versus f/6, hardly anyone could tell the difference.
In theory, extra glass (i.e. the Barlow) can't help the image and can
only hurt. But in practice, top-end Barlows are so good that you'd
get less variation with or without the Barlow than between one
good Plossl and another. Which is itself pretty small.
For me, the biggest difference would be that the 15mm Plossl in
alternative 3 has fairly short eye relief. So I would distinctly prefer
the alternatives that use a 30mm Plossl on that ground alone.
- Tony Flanders
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Date: 14 Aug 2006 08:50:46
From: Willie R. Meghar
Subject: Re: What would I see if...
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tony_flanders@yahoo.com wrote:
>dickknox wrote:
>
>> Imagine 3 telescope set ups
>> 1. a f/10 150mm parabloic with a good plossl 30mm eyepiece
>> 2. a f/5 150mm parabolic with a 2x barlow and the same 30mm eyepiece
>> 3. a f/5 150mm parabolic with a good plossl 15mm eyepiece
>>
>> Will the experience be the same?
>
>Pretty nearly -- from a strictly optical point of view. Obviously, a
>150mm f/10 Newt is a fairly large scope, so from the point of view
>of mechanics and ergonomics, there's a very big difference.
>
>An f/5 mirror exhibits more coma than an f/10 mirror, so in theory
>you'd see more coma with alternatives 2 and 3. But in practice,
>the coma would be pretty modest. Detectable by some people
>but probably not by most. Likewise, a Plossl begins to exhibit
>some astigmatism at f/5, but most people would barely notice it.
>
>If you changed the question to from f/10 versus f/5 to f/8 versus f/4,
>then you'd have a whole nother story -- coma and astigmatism are
>major issues at f/4. Conversely, if you changed the question to
>f/12 versus f/6, hardly anyone could tell the difference.
>
>In theory, extra glass (i.e. the Barlow) can't help the image and can
>only hurt. But in practice, top-end Barlows are so good that you'd
>get less variation with or without the Barlow than between one
>good Plossl and another. Which is itself pretty small.
>
>For me, the biggest difference would be that the 15mm Plossl in
>alternative 3 has fairly short eye relief. So I would distinctly prefer
>the alternatives that use a 30mm Plossl on that ground alone.
It may be worth adding that a 30mm Plossl will result in a vignetted
field when used with at least some (such as the ones I use) 1.25 inch
Barlows. A vignetted field means at best that the field edge is
darkened (not fully illuminated) and at worst, in addition to the
darkened edge, the field of view will be reduced.
Willie R. Meghar
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