refractor 120mm : f/5 or f/10 ?

"Wally" wrote in message
...
well f7 or f8 would be ideal, if the achromat is well corrected. I made an
f8
in 120mm dia once and I wish I had never given it away.

I agree. and we can check this to manufacturers.
The Meade achromats are f/8, f/9, AP apos are f/8 or higher, only TV sells a
f/6.3 but it a apo as well..
I am even not sure that using ED lens on a 120 mm f/5 achromat we can
prevent chromatiism.
For a so short focal we need a minimum Fluorite lenses.
So I think that I have to find anotehr 120 mm close to f/7-f/8 indeed.

Thierry



Thierry wrote:

Hi,

I would like to buy a small achromat (not sure to take an apo) OTA of 120
mm
in dia. but I can't put my finger on the right focal length to select :
600, 900 or 1200 mm. Say to make simple, 600 or 1000 mm, f/5 or f/10 ?
I know quite well optics and its problems, what change in the FOV,
"speed",
etc
see at http://www.astrosurf.org/lombry/menu...el-acheter.htm (i
translate ;-) how to select a scope)

This new scope will be mainly used for visual ovservations (moon, bright
planets, sun, starry fields...), and casually.
I have the mount so I only need the OTA.

I wonder if in purchasing a f/5 I will not regret the f/10 with its
greater
magnification on planets and if I buy the longer one, if it will not be
too
dark visually (I owned various scope but usually just in the middle,
.f/7-8,
no luck).

But what is your feeling ?
If you own a 100-120 mm refractor f:/5 or f/10,
- if you had to make a new purchase, if you own a f/5 'd you buy the same
one in f/10, and conversely
- if you own a f/10, 'd you rather buy the f/5 ?
and why 'd you change ? (magnification, brightness, problem of focusing
with
some accessories, other ?)

Of course in changing from f/10 to f5, and essentially with a refractor,
they are much chances that you improve a lot the chromatic aberration on
a
achromat. The prevent this I could also buy the OTA with ED lens (go with
itn, but what f/ratio ? ).

All your opinions will be appreciate to make my choice.
(it will be easier and cheaper than buying both scopes and test them a
year
long...;-))

Thierry

---
Auteur de "Un siècle de Physique, 1- La Physique Quantique", AEGEUS,
2005
http://www.astrosurf.org/lombry

On Thu, 14 Sep 2006 08:18:28 -0700, Alan Charlesworth
wrote:

In article ,
William Hamblen wrote:

On Wed, 13 Sep 2006 14:17:52 -0400, "Tom Royer"
wrote:

The one drawback to the shorter FL is that the scope may
be more susceptible to false color.

There's no "may" about it. A 120 mm aperture f/5 telescope achromat
with common glass will have a lot of false color. You normally want
the focal ratio on an achromat to be approximately the aperture in
centimeters, or about f/12 for a 120 mm telescope.

Bud

Has anybody tried the new Orion 120 ED f/7.5 (price $2000)?

Uncommon glass, or so they claim.

--
The night is just the shadow of the Earth.

Thierry wrote:
"David Nakamoto" wrote in message
news:Gc7Og.52096$Qb2.863@trnddc08...
Hi Thierry,

I actually got a 120mm f/5 refractor, and then got a 127mm f/12 Mak
because I couldn't get enough magnification for planets and the moon, with
eyepieces or a web camera. Even on deep sky objects, the slight contrast
improvement going to the longer focal length is probably worth it.

Yes. It is indeed what I am afraid of.
the max useful magnoification is about 300x; thus using a.... 2 mm eyepiece
!
Now, there is the psosibility to add it a quality barlow (I know that some
also take into account a chromatic correction, they are not simple "apo"
barlow)

But you still have to deal with the chromatic aberration from the
telescope itself and its short f/ratio. I think if the observer's main
interest is Sun, Moon, and planets, then go with the longer focal length.

--- Dave


Thierry

Just my humble opinion.

Clear and Steady Nights !
--- Dave


Thierry wrote:
Hi,

I would like to buy a small achromat (not sure to take an apo) OTA of 120
mm in dia. but I can't put my finger on the right focal length to select
: 600, 900 or 1200 mm. Say to make simple, 600 or 1000 mm, f/5 or f/10 ?
I know quite well optics and its problems, what change in the FOV,
"speed", etc
see at http://www.astrosurf.org/lombry/menu...el-acheter.htm (i
translate ;-) how to select a scope)

This new scope will be mainly used for visual ovservations (moon, bright
planets, sun, starry fields...), and casually.
I have the mount so I only need the OTA.

I wonder if in purchasing a f/5 I will not regret the f/10 with its
greater magnification on planets and if I buy the longer one, if it will
not be too dark visually (I owned various scope but usually just in the
middle, .f/7-8, no luck).

But what is your feeling ?
If you own a 100-120 mm refractor f:/5 or f/10,
- if you had to make a new purchase, if you own a f/5 'd you buy the same
one in f/10, and conversely
- if you own a f/10, 'd you rather buy the f/5 ?
and why 'd you change ? (magnification, brightness, problem of focusing
with some accessories, other ?)

Of course in changing from f/10 to f5, and essentially with a refractor,
they are much chances that you improve a lot the chromatic aberration on
a achromat. The prevent this I could also buy the OTA with ED lens (go
with itn, but what f/ratio ? ).

All your opinions will be appreciate to make my choice.
(it will be easier and cheaper than buying both scopes and test them a
year long...;-))

Thierry wrote:
Hi,

I would like to buy a small achromat (not sure to take an apo) OTA of 120 mm
in dia. but I can't put my finger on the right focal length to select :
600, 900 or 1200 mm. Say to make simple, 600 or 1000 mm, f/5 or f/10 ?
I know quite well optics and its problems, what change in the FOV, "speed",
etc
see at http://www.astrosurf.org/lombry/menu...el-acheter.htm (i
translate ;-) how to select a scope)

This new scope will be mainly used for visual ovservations (moon, bright
planets, sun, starry fields...), and casually.
I have the mount so I only need the OTA.

I wonder if in purchasing a f/5 I will not regret the f/10 with its greater
magnification on planets and if I buy the longer one, if it will not be too
dark visually (I owned various scope but usually just in the middle, .f/7-8,
no luck).

But what is your feeling ?
If you own a 100-120 mm refractor f:/5 or f/10,
- if you had to make a new purchase, if you own a f/5 'd you buy the same
one in f/10, and conversely
- if you own a f/10, 'd you rather buy the f/5 ?
and why 'd you change ? (magnification, brightness, problem of focusing with
some accessories, other ?)

Of course in changing from f/10 to f5, and essentially with a refractor,
they are much chances that you improve a lot the chromatic aberration on a
achromat. The prevent this I could also buy the OTA with ED lens (go with
itn, but what f/ratio ? ).

All your opinions will be appreciate to make my choice.
(it will be easier and cheaper than buying both scopes and test them a year
long...;-))


If you are serious about either deep-sky and planetary observing, you
need light-gathering power, so please consider a 200mm or larger
Newtonian with quality optics. If you also need a very wide field of
view, consider getting an -additional- telescope, with a short focal
length, instead of trying to make your main scope serve all purposes.

Achromatic refractors, except in short focal ratios and small sizes,
are largely obsolete. Newtonians are cheaper, more powerful, and
sometimes more portable. SCTs and Maks are less cumbersome and not much
more expensive. Apochromats avoid many of the problems of the other
designs (at a price) but are limited in aperture.

Thierry wrote:

I would like to buy a small achromat (not sure to take an apo) OTA of 120 mm
in dia. but I can't put my finger on the right focal length to select :
600, 900 or 1200 mm. Say to make simple, 600 or 1000 mm, f/5 or f/10 ?
This new scope will be mainly used for visual ovservations (moon, bright
planets, sun, starry fields...), and casually.
I have the mount so I only need the OTA.

Well, I own a 100mm f/6 achromat, which presumably has only 2/3 as
much chromatic aberration as a 120mm f/5 achromat. It's a superb
deep-sky scope, but I would *not* recommend it to somebody who
wants to spend much time viewing the planets. The problem isn't so
much the colored fringes -- I can put up with those -- as the loss of
contrast. If you put it side-by-side with a 100m f/6 apochromat, the
difference is shocking. And when you're viewing Jupiter or Mars, you
really need all the contrast you can get.

It's not half bad on the Moon, though the violet fringes are *truly*
garish.
Still, the Moon has something to show at every magnification, and the
100mm f/6 beats my high-quality 70mm f/6.9 achromat hands down
on such a high-contrast subject.

On the other hand, a 120mm f/10 doesn't appeal to me at all -- just
too darned big. I'd much prefer a 125mm Mak or a 150mm f/6 Newt.
But obviously, other people feel differently.

- Tony Flanders