eyepiece limits

I know that there are practical upper limits for magnification for a given
scope, but is there also a practical lower limit to magnification? I built a
10" F5 truss tube dob and used a 1.25" focuser on it. My lowest power
eyepiece currently is a 32mm Plossl (40X). So, how low can I go, and would
there be advantages other than FOV to lower power with my setup?
BTW my highest power eyepiece is a 13mm Nagler (97X) and I carry a 2X
Barlow.
Thanks in advance.
Duff Couillard

On Sat, 18 Sep 2004 15:13:42 GMT, "D. Couillard"
wrote:

I know that there are practical upper limits for magnification for a given
scope, but is there also a practical lower limit to magnification? I built a
10" F5 truss tube dob and used a 1.25" focuser on it. My lowest power
eyepiece currently is a 32mm Plossl (40X). So, how low can I go, and would
there be advantages other than FOV to lower power with my setup?
BTW my highest power eyepiece is a 13mm Nagler (97X) and I carry a 2X
Barlow.

The lower limit in a newtonian or other telescope with a central
obstruction is when the size of the shadow of the secondary mirror
begins to approach the size of the entrance pupil of your eye. When
that happens you get an obvious and annoying black spot in your field
of view.

The lower limit in a newtonian or other telescope with a central
obstruction is when the size of the shadow of the secondary mirror
begins to approach the size of the entrance pupil of your eye. When
that happens you get an obvious and annoying black spot in your field
of view.

Exactly right. I hope we don't start getting confused (and confusing) posts
about exit pupil size and wasted light/aperture.

Dennis

As Bill noted, the low power limit occurs when the secondary obstruction
becomes evident. However, he didn't say when that occurs. In my experience,
I find a 10mm exit pupil still acceptable with a 22% obstruction as long as
I'm well dark adapted. YMMV.

Part 2 relates to the advantage of low power - a wide field of view. The
only variable here is the field stop diameter of the eyepiece. Once you hit
27mm in a 1.25" focuser, you're maxed out. A shorter focal length will give
a more appealing view, thus the 24mm Pan, but your 32mm is just fine. A 40mm
would give a less appealing image than you have now.

However, with a 2" focuser, you can get a 46mm field stop and the total
field of view nearly triples. The highest power at this field stop is
typically about a 40mm eyepiece. You will lose a little brightness, unless
you eye can open to 8mm, but you're not likely to see the secondary
obstruction unless your secondary is unusually large.

Have fun,
Frank

"D. Couillard" wrote in message
news:GgY2d.22947$yW6.9263@clgrps12...
I know that there are practical upper limits for magnification for a given
scope, but is there also a practical lower limit to magnification? I built
a
10" F5 truss tube dob and used a 1.25" focuser on it. My lowest power
eyepiece currently is a 32mm Plossl (40X). So, how low can I go, and would
there be advantages other than FOV to lower power with my setup?
BTW my highest power eyepiece is a 13mm Nagler (97X) and I carry a 2X
Barlow.
Thanks in advance.
Duff Couillard

That brings up a related question. If you can make out the central
obstruction in the daytime with, say a 7mm exit pupil, It's still there at
night decreasing contrast right? It's just harder to see. What exit pupil
shouldn't you go above with a 33% CO if one doesn't want contrast decreased
on axis?

--

Thanks,

Chuck

"Frank Bov" wrote in message
...
As Bill noted, the low power limit occurs when the secondary obstruction
becomes evident. However, he didn't say when that occurs. In my
experience,
I find a 10mm exit pupil still acceptable with a 22% obstruction as long
as
I'm well dark adapted. YMMV.

Part 2 relates to the advantage of low power - a wide field of view. The
only variable here is the field stop diameter of the eyepiece. Once you
hit
27mm in a 1.25" focuser, you're maxed out. A shorter focal length will
give
a more appealing view, thus the 24mm Pan, but your 32mm is just fine. A
40mm
would give a less appealing image than you have now.

However, with a 2" focuser, you can get a 46mm field stop and the total
field of view nearly triples. The highest power at this field stop is
typically about a 40mm eyepiece. You will lose a little brightness, unless
you eye can open to 8mm, but you're not likely to see the secondary
obstruction unless your secondary is unusually large.

Have fun,
Frank

"D. Couillard" wrote in message
news:GgY2d.22947$yW6.9263@clgrps12...
I know that there are practical upper limits for magnification for a
given
scope, but is there also a practical lower limit to magnification? I
built
a
10" F5 truss tube dob and used a 1.25" focuser on it. My lowest power
eyepiece currently is a 32mm Plossl (40X). So, how low can I go, and
would
there be advantages other than FOV to lower power with my setup?
BTW my highest power eyepiece is a 13mm Nagler (97X) and I carry a 2X
Barlow.
Thanks in advance.
Duff Couillard

"D. Couillard" wrote in message news:GgY2d.22947$yW6.9263@clgrps12...

I know that there are practical upper limits for magnification for a given
scope, but is there also a practical lower limit to magnification? I built a
10" F5 truss tube dob and used a 1.25" focuser on it. My lowest power
eyepiece currently is a 32mm Plossl (40X). So, how low can I go, and would
there be advantages other than FOV to lower power with my setup?

Yes, you can go lower than that using a 40mm Plossl. However, there is
no benefit to doing so, because you have already achieved the maximum
field of view possible with your scope when using a 1.25-inch eyepiece.
There's a lucid discussion of this in the October issue of Sky & Telescope,
and it has also been hashed over innumerable times on s.a.a. Try a
Google search.

BTW my highest power eyepiece is a 13mm Nagler (97X) and I carry a 2X
Barlow.

Going lower is one thing, but you can *certainly* go higher than that --
much higher! In fact, I would go so far as to say that you are wasting
a large part of the capability of your scope by sticking at such low
powers. Your current max of 194X is fine for deep-sky viewing, but you
should get *much* better images of the planets at 250X or 300X on a
steady night.

- Tony Flanders

William Hamblen wrote in message . ..

The lower limit in a newtonian or other telescope with a central
obstruction is when the size of the shadow of the secondary mirror
begins to approach the size of the entrance pupil of your eye. When
that happens you get an obvious and annoying black spot in your field
of view.

That's certainly right in theory, and it's a real practical problem
for daytime viewing or when looking at the Moon. But it's almost
impossible to go low enough in a normal scope for that to be an
issue for deep-sky observing, which is the only time you're likely
to want a super-wide FOV at night.

The extreme case is an F/4 Newt -- very fast -- with a 25% central
obstruction -- very large -- and a 40mm eyepiece. That still only
gives a 2.5mm dark spot, which is pretty much swallowed up in any
normal person's dark-adapted pupil.

- Tony Flanders

Thanks Tony, I think I was part of that protracted discussion awhile ago.

--

Thanks,

Chuck

"Tony Flanders" wrote in message
...
William Hamblen wrote in message
. ..

The lower limit in a newtonian or other telescope with a central
obstruction is when the size of the shadow of the secondary mirror
begins to approach the size of the entrance pupil of your eye. When
that happens you get an obvious and annoying black spot in your field
of view.

That's certainly right in theory, and it's a real practical problem
for daytime viewing or when looking at the Moon. But it's almost
impossible to go low enough in a normal scope for that to be an
issue for deep-sky observing, which is the only time you're likely
to want a super-wide FOV at night.

The extreme case is an F/4 Newt -- very fast -- with a 25% central
obstruction -- very large -- and a 40mm eyepiece. That still only
gives a 2.5mm dark spot, which is pretty much swallowed up in any
normal person's dark-adapted pupil.

- Tony Flanders

On Sat, 18 Sep 2004 23:44:08 -0600, "Chuck"
wrote:

That brings up a related question. If you can make out the central
obstruction in the daytime with, say a 7mm exit pupil, It's still there at
night decreasing contrast right? It's just harder to see. What exit pupil
shouldn't you go above with a 33% CO if one doesn't want contrast decreased
on axis?

The thing that counts in this case is the entrance pupil of the eye
rather than the exit pupil of the telescope. In daylight your pupil
is 1 or 2 mm across so the shadow of the secondary may fill or nearly
fill the entrance pupil of the eye, so there is a dark spot. At night
the entrance pupil of the eye is 5 - 7 mm so the shadow has less
effect.

"Chuck" wrote in message ...

That brings up a related question. If you can make out the central
obstruction in the daytime with, say a 7mm exit pupil, It's still there at
night decreasing contrast right? It's just harder to see. What exit pupil
shouldn't you go above with a 33% CO if one doesn't want contrast decreased
on axis?

That's a very interesting question; I'd never thought of it that way.
After thinking about it for a while, I've concluded that the answer
probably varies from one individual to another.

If the eye were optically perfect, then it would be easy to compute
the loss of contrast due to the magnified CO. The way I figure it,
if your eye opens to 7mm and the central 2.5mm of the light cylinder
coming out of the eyepiece is obstructed, it is just like a
2.5/7 = 35% CO in a 7mm telescope.

However, hardly anybody can resolve anywhere near the theoretical
limit of a 7mm telescope. If a 7mm eye were optically perfect, we
would be able to split a 0.3' double naked-eye, but in fact very
few people can split even a 2' double and fewer than 50% can split
the Double-Double (4'). My own limit is around 6'.

OK, so why are we so far from perfect? Insofar as it's due to the
granularity of the retina, it won't be affected by CO at all.
Insofar as it's due to imperfections in the eye's lens, CO will
have a particularly deleterious effect, because the CO masks the
part of the eye lens with the best resolution -- the part that's
used for daytime viewing -- leaving only the sloppy outer portion
that's used under low-light conditions.

In practice, I bet that the effect of the CO for low-power images
is negligible for almost everyone up to a CO of 3mm or more. And
if you *really* wanted to see high contrast in small features,
you would increase the magnification, rendering the whole question
irrelevant.

- Tony Flanders