APO refractor apeture

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.

But on nights of superb seeing, how far can you take a superb 7" APO, like a TMB or an AP? Can you view Saturn at 500x? 600x? When does decreasing brightness become a problem, and when does additional detail fail to appear?

Thanks
JB

"Jim Beam" wrote in message
...

when does additional detail fail to appear?

I think 0.8mm exit pupil was on the table, which holds to the 31x per inch
rule. Beyond that, you are just making the image larger, which has the
double edged sword effect of also making the image dimmer (as you
suggested). So the questions are, and this is subjective, at what exit pupil
are images too dim for you, and at what exit pupil do your floaters (if any)
become a serious bother? These values will hold in any scope. You then
simply scale up the aperture, to increase image scale.

Stephen Paul

I think 0.8mm exit pupil was on the table, which holds to the 31x per inch
rule. Beyond that, you are just making the image larger, which has the
double edged sword effect of also making the image dimmer (as you
suggested). So the questions are, and this is subjective, at what exit pupil
are images too dim for you, and at what exit pupil do your floaters (if any)
become a serious bother? These values will hold in any scope. You then
simply scale up the aperture, to increase image scale.

Stephen Paul


Hi Stephen, thanks for your response. I don't understand. You mention
0.8mm as if it has almost the authority of a speed limit. Is there a reason
why 0.8mm would be the point where you go from seeing more detail to
just making things bigger? Does it have to do with 0.8mm making the
Airy disc visible to the eye?

JB

"Jim Beam" wrote in message
...

I think 0.8mm exit pupil was on the table, which holds to the 31x per inch
rule. Beyond that, you are just making the image larger, which has the
double edged sword effect of also making the image dimmer (as you
suggested). So the questions are, and this is subjective, at what exit
pupil
are images too dim for you, and at what exit pupil do your floaters (if
any)
become a serious bother? These values will hold in any scope. You then
simply scale up the aperture, to increase image scale.

Stephen Paul


Hi Stephen, thanks for your response. I don't understand. You mention
0.8mm as if it has almost the authority of a speed limit. Is there a
reason
why 0.8mm would be the point where you go from seeing more detail to
just making things bigger? Does it have to do with 0.8mm making the
Airy disc visible to the eye?

I'm not ignoring this post by the way. I just don't have an answer that I
like.

Stephen Paul wrote:
Hi Stephen, thanks for your response. I don't understand. You mention
0.8mm as if it has almost the authority of a speed limit. Is there a
reason why 0.8mm would be the point where you go from seeing more detail
to just making things bigger? Does it have to do with 0.8mm making the
Airy disc visible to the eye?

I'm not ignoring this post by the way. I just don't have an answer that I
like.

At an exit pupil of 0.8 mm, the Airy disc has an apparent (that is,
magnified) size of about 2.3 arcminutes (for light at 550 nm). Where
that figure comes from is out of the scope of this post, but that is
around the point where the disc becomes more obviously a disc and not
a point of light.

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

Jim Beam wrote in message ...
I think 0.8mm exit pupil was on the table, which holds to the 31x per inch
rule. Beyond that, you are just making the image larger, which has the
double edged sword effect of also making the image dimmer (as you
suggested). So the questions are, and this is subjective, at what exit pupil
are images too dim for you, and at what exit pupil do your floaters (if any)
become a serious bother? These values will hold in any scope. You then
simply scale up the aperture, to increase image scale.

Stephen Paul


Hi Stephen, thanks for your response. I don't understand. You mention
0.8mm as if it has almost the authority of a speed limit. Is there a reason
why 0.8mm would be the point where you go from seeing more detail to
just making things bigger? Does it have to do with 0.8mm making the
Airy disc visible to the eye?

In fact it might not be the case. It depends on the visual acuity of
the observer. Most observers, expecially so if no longer young, are
comfortable with seeing details when they cover about 3 arcminutes but
they can still do see them, albeit less comfortably, when the apparent
size is 1 arcminute if the relative contrast is very high. Returing to
the hypothetical 7" this means anything between 94x to 282x for
details at the instrumental limit (in green light, where the peak
visual acuity is) of 0.64". The pupil exit size therefore varies from
1.8mm to just 0.6mm, or from 13x to 40x. I would consider 75x as the
normal upper limit in very good seeing for any quality scope,
regardless whether it's an APO or whatever else. With my 6" mak 282x
is my normal planetary magnification, regardless of seeing.

Andrea T.

Subject: APO refractor apeture
From: Jim Beam
Date: 9/22/2004 5:17 AM Eastern Daylight Time
Message-id:


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.

But on nights of superb seeing, how far can you take a superb 7" APO, like a
TMB or an AP? Can you view Saturn at 500x? 600x? When does decreasing
brightness become a problem, and when does additional detail fail to appear?

Thanks
JB
*************************************
With the excellent made APO's like AP, TAK ,TMB, you can use up to 100x per
inch in super seeing! The image brightness starts to really fall off at such
powers, but for the moon and planets you can still see detail...

Chas P.

On Wed, 22 Sep 2004 04:17:49 -0500, Jim Beam wrote:


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.

But on nights of superb seeing, how far can you take a superb 7" APO, like a TMB or an AP? Can you view Saturn at 500x? 600x? When does decreasing brightness become a problem, and when does additional detail fail to appear?

Thanks
JB

I bought a TMB115 earlier this year, and in initial tests used
magnifications of up to 335x on Jupiter and Saturn, though the image was
starting to get a bit dim (75x per inch). Generally I preferred to use
somewhat lower powers-say around 50 per inch, but bear in mind the scope
was not mounted equatorially at the time.
I would agree with others that the high end apos will take around 100x per
inch when conditions allow, which with something as big as a 7" is not
likely to be all that often unless you have an extremely good site.
Phil

I read different numbers for how much you can milk out of a specific
aperture. I am considering a 7" APO.

I own a few apos ranging from 3" to 8". I find that for low contrast detail ON
JUPITER that 30x per inch shows just about all an aperture has to show.

This is due to a combination of diffraction effects from the aperture, as well
as defects in the eye.

When it comes to double stars I am basically able to double this figure to get
the max resolution from an aperture due to stars being point sources, and not
as affected by either diffraction in the aperture or defects in the eye.

I think the Moon, being a higher contrast subject, lies somewhere between the
two vis a vis maximum magnification for maximum detail.

210x on Jupiter is enough magnification to proffer excellent images on Jupiter
given top notch optics. You see, the problem isn't so much how small the
details are as how poorly they stand out from their background (low contrast).

The more magnification you use, the more contrast is reduced.

So ideally you want to use the lowest power on Jupiter that will show the
detail in order to maintain the highest possible contrast.

Terence Dickinson in his "Back Yard Astronomer's Guide" recommends 25-35x per
inch for Jupiter.

I will be real happy if I am able to get max performance at 35x per inch this
year on Jupiter (using a binoviewer).

With my 6" APO I find that 180x (30 x per inch) shows just as much fine detail
as any other magnification ON JUPITER from my location and the views at that
magnification are quite satisfactory given good seeing.

A 7" APO at 210x in good seeing is not going to be something to sneeze at.

And then again you might be able to easily push it to 250x on Jupiter. I myself
will know more 9and be able to say more) about it in six months or so.

When it comes to Saturn, low contrast markings on the globe will be easier to
observe at lower powers. However, the rings are quite high in contrast so I
think it is easy to go well beyond the 30x per inch guidline to very good
effect when observing them.

I was alway impressed as to how good they looked in my 8" SCT. I finally
realized it was because they were of such high contrast. I could never get
quite the same effect using that scope to observe the fine detail on Jupiter
regardless of magnification.

My 2 cents,
rat
~( );

email: remove 'et' from .com(et) in above email address

t (Ratboy99) wrote in message ...
I read different numbers for how much you can milk out of a specific
aperture. I am considering a 7" APO.

I own a few apos ranging from 3" to 8". I find that for low contrast detail ON
JUPITER that 30x per inch shows just about all an aperture has to show.

This is due to a combination of diffraction effects from the aperture, as well
as defects in the eye.

When it comes to double stars I am basically able to double this figure to get
the max resolution from an aperture due to stars being point sources, and not
as affected by either diffraction in the aperture or defects in the eye.

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

Andrea T.