APO refractor apeture

Jim Beam wrote in message ...

I read different numbers for how much you can milk out of a
specific aperture. I am considering a 7" APO. Some people say
30x per inch, thats it for detail. Well, thats only 210x which
seems conservative for a 7" APO. Many people say 50x per inch,
and thats 350x for a 7" APO, sounding better.

To my mind, this is utterly the wrong way of looking at the problem.
What magnification you choose to use depends on your eyes, your
tastes, and the target you're viewing. It is *not* a useful measure
of how much detail you can see through a telescope -- certainly not
when comparing among different observers.

For instance, I customarily view Jupiter at 120X in my 70mm refractor,
about 40X per inch. That works out well for me, and I have an eyepiece
that I like that delivers that magnification with that scope. Other
people might prefer 80X and yet others might prefer to use 150X. Yet
all of us would probably be seeing a similar amount of detail.

In any case, I certainly don't consider 40X per inch too high for that
little refractor; in fact, I have used twice that for viewing the Moon,
or for splitting tight doubles. 40X per inch is also a tad on the low
side (for me) when viewing Saturn's rings or details on Mars.

However, the inherent limitations of 70mm aperture are already *quite*
visible at 120X. On nights of mediocre seeing, I also customarily use
120X for viewing Jupiter in my 177mm reflector, so I end up using the
same magnification on both scopes despite the fact that one has 2.5x
the aperture of the other. But that doesn't mean that I see the same
amount of detail in both scopes. On the contrary! Except on truly
terrible nights, I always see more detail at 120X in the 177mm scope
than at 120X in the 70mm scope, despite the fact that this magnification
is still nowhere near the small scope's "limit" -- if such a concept
is meaningful.

- Tony Flanders

I don't see how diffraction affects one and not the other, Rat.

Andrea T.

On a star, the diffraction is thrown outside of the star image. On an extended
image diffraction smears the image itself.
rat
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For instance, I customarily view Jupiter at 120X in my 70mm refractor,
about 40X per inch. That works out well for me, and I have an eyepiece
that I like that delivers that magnification with that scope. Other
people might prefer 80X and yet others might prefer to use 150X. Yet
all of us would probably be seeing a similar amount of detail.

In any case, I certainly don't consider 40X per inch too high for that
little refractor; in fact, I have used twice that for viewing the Moon,
or for splitting tight doubles. 40X per inch is also a tad on the low
side (for me) when viewing Saturn's rings or details on Mars.

I agree. When I had my Ranger it did fine up to 120x on Jupiter and Saturn.
Actually I have a hard time seeing much on those subjects at any lower mag than
that.

For Mars, I was able to use 160x to best effect with teh 70mm aperture.

As the aperture increases (the 4.1" Traveler for instance), things start to
slow down a bit, and for it about 140x is ideal for Jupiter to my own eyes.



rat
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Ratboy99 wrote:
On a star, the diffraction is thrown outside of the star image. On an
extended image diffraction smears the image itself.

Unless the binary is very tight, I don't see how diffraction will throw
light outside the secondary star--even mostly outside the secondary star.

I suspect the main reason why higher powers seem tolerable on binary
stars (and lunar observing) is because the input contrast is so high.
Even when you are low on the MTF, the output contrast is still high
enough for your eye to pick up on the detail.

In contrast, on DSOs, the contrast is low, so that being low on the
MTF means low output contrast as well. If the object is magnified too
much, the variations in brightness may be too difficult for the eye to
pick out.

Another aspect of binary observing that might be relevant is that it is
a relatively well-circumscribed observation. Primary color, primary
brightness, secondary color, secondary brightness, separation, and
position angle. That's about it. Whereas other sorts of observation
are much more varied, and benefit much more from greater contrast.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Unless the binary is very tight, I don't see how diffraction will throw
light outside the secondary star--even mostly outside the secondary star.

I was thinking about it and I realize my first answer was rather silly
actually. If I understand it correctly, the Airy disk is ALL diffraction
pattern. After all a star is a point source, so strictly speaking its image is
completely obliterated by diffraction. We have just come to accept that this is
what a star looks like in a telescope, and we aren't particularly looking for
low contrast details on stars, are we?

The effects of diffraction on an extended image, while not as catastrophic as
those on a star image, still blur lines where light and dark meet, smearing
detail. For this reason, I am more "bothered" by diffraction on extended
objects than on point sources, even though it is obviously the star that gets
the real short end of the stick when it comes to diffraction.


I suspect the main reason why higher powers seem tolerable on binary
stars (and lunar observing) is because the input contrast is so high.
Even when you are low on the MTF, the output contrast is still high
enough for your eye to pick up on the detail.


Again, not much detail there. And the other thing is I have a much easier time
looking around floaters in my eye on objects that subtend less than a half arc
sec in total.

In contrast, on DSOs, the contrast is low, so that being low on the
MTF means low output contrast as well. If the object is magnified too
much, the variations in brightness may be too difficult for the eye to
pick out.

DSO's... I wasn't thinking about them. I was thinking mainly about Jupiter.

Another aspect of binary observing that might be relevant is that it is
a relatively well-circumscribed observation. Primary color, primary
brightness, secondary color, secondary brightness, separation, and
position angle. That's about it. Whereas other sorts of observation
are much more varied, and benefit much more from greater contrast.

Yup.

rat
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As expected . . . The larger the telescope,- the greater it's resolving power
"Dawes' Limit". . .

SO weather your talking about 10X or 1000X The larger telescope will RESOLVE
more detail than a smaller telescope will at the same magnification!

So if you got a Killer 70mm capable of 100X per inch, with the most perfect
seeing the finest detail you could detect would be 1.4 arc seconds across.
While a 177mm working at 50X per inch would produce the SAME image scale, but
you could detect detail as small as .66 arc seconds across

- Roger Hart - ?:^)

XXXXXXXXXXXXXXXXXXXXXXX

snip Except on truly terrible nights, I always see more detail at 120X in
the 177mm scope
than at 120X in the 70mm scope, despite the fact that this magnification is
still nowhere near the small scope's "limit" -- if such a concept
is meaningful.

- Tony Flanders

You guys use some whimpy powers!
I dont even get warmed up unless i'm using over 450x. I like 500 to 850x the
best, but only if seeing is super...

Chas P.

And the lower magnification might even show low contrast details on Jupiter
better than the higher magnification does..

When we were in the Keys 2 years ago, 350x was ideal on Jupiter with my 10"
Mak-Cass, but seeing detail on Gannymede was impossible until we reached powers
of 600x to 900x. At 900x, subtle detail stood out during moments of excellent
seeing on Gannymede, but most surface detail on Jupiter itself was much less
pleasing.

There is no one power that works for every object. That's why Uncle Al makes a
gazillion eyepieces, and you all ought to buy one of each (two if y'all have a
binoviewer - er, a Baader bino if'n ya want t' see the sharpest detail).

Rolando
aka Uncarollo

When we were in the Keys 2 years ago, 350x was ideal on Jupiter with my 10"
Mak-Cass,

35x per inch in great seeing? Interesting. I always leave out the seeing
aspect. Seeing here is often such that using over 200x in any scope results in
poor images. So while I am happy to contribute my two cents, I must alos at
this point make it clear that I make no claim to have figured it all out...yet.

but seeing detail on Gannymede was impossible until we reached
powers
of 600x to 900x. At 900x, subtle detail stood out during moments of excellent
seeing on Gannymede, but most surface detail on Jupiter itself was much less
pleasing.

I would love to see detail on Ganymede this season.


There is no one power that works for every object. That's why Uncle Al makes
a
gazillion eyepieces, and you all ought to buy one of each (two if y'all have
a
binoviewer - er, a Baader bino if'n ya want t' see the sharpest detail).

Rolando
aka Uncarollo

Preaching to the choir uncle Rolando. I shudder to think how many eyepieces I'm
up to now.
rat
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Subject: APO refractor apeture
From: (Chris1011)
Date: 9/23/2004 7:20 PM Eastern Daylight Time
Message-id:


When we were in the Keys 2 years ago, 350x was ideal on Jupiter with my 10"
Mak-Cass, but seeing detail on Gannymede was impossible until we reached
powers
of 600x to 900x. At 900x, subtle detail stood out during moments of excellent
seeing on Gannymede, but most surface detail on Jupiter itself was much less
pleasing.

There is no one power that works for every object. That's why Uncle Al makes
a
gazillion eyepieces, and you all ought to buy one of each (two if y'all have
a
binoviewer - er, a Baader bino if'n ya want t' see the sharpest detail).

Rolando
aka Uncarollo
****************************
I was using powers from 850 to 1100x to see details on Gannymede with a 15"
Obsession - OMI Torus mirror last year!

But only nites of super seeing would allow such high powers...

Chas P.