| |
Main
Date: 13 Sep 2006 18:56:28
From:
Subject: refractor 120mm : f/5 or f/10 ?
|
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-quel-tel-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
|
|
| |
Date: 13 Sep 2006 14:19:06
From: Willie R. Meghar
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
<Thierry > wrote:
>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 could be "reasonably" happy with an f/5 achromat for casual visual
observations of the sun, moon, and stars. For the planets I would
prefer a longer f-ratio for an achromat. Much planetary detail is of
a subtle nature.
>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).
Regardless of which way you go (with an achromat) you will at times
wish you had gone the other way! Image brightness will be the same
with either scope provided the same magnification is used. The
primary restriction of the f/10 would be field of view (and of course
portability). For example, for observing the North America Nebula you
would definitely want something shorter than the f/10.
>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 ?)
If at all possible I would go with an f/6 apochromat.
If an apochromat is not a possibility: If portability is an important
concern, I would get the f/5 achromat and enjoy it in the areas where
it performs well. If a wide field of view isn't important for you,
and portability isn't important for you, and the sun, moon, and
(especially) the planets *are* important to you, then I would suggest
the f/10 (or even f/12) achromat -- assuming the mount you already
have will handle that size scope.
>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 ? ).
The chromatic aberration would be of no concern for visual use on star
fields and deep sky objects. Some would view it as a problem with the
sun and moon; others would not. I wouldn't be *too* concerned with
chromatic aberration when observing the sun and moon. The primary
problem with the chromatic aberration, in my opinion, would be with
the planets.
If an apochromat or a refractor with an ED lens is a possibility for
you, I would suggest an f/6 apochromat or ED.
>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...;-))
I understand your indecision! It took me more than a year to decide
on purchasing my 130mm f/6 apochromat. I've *never* regretted that
decision!
Willie R. Meghar
http://meghar.blogspot.com/
|
| |
Date: 13 Sep 2006 20:56:18
From: Anthony Ayiomamitis
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
Salut Thierry,
If you have any plans to also travel (for example to observe a total
solar eclipse), I suspect you will be much more comfortable with the
shorter f/5 OTA. If image scale is an issue, you can always pick up a
quality 2x barlow to get back to f/10.
Anthony.
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-quel-tel-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
>
>
|
| | |
Date: 14 Sep 2006 01:08:11
From:
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
"Anthony Ayiomamitis" <anthony@perseus.no2spam.gr > wrote in message
news:ee9gp0$424$1@mouse.otenet.gr...
> Salut Thierry,
>
> If you have any plans to also travel (for example to observe a total solar
> eclipse), I suspect you will be much more comfortable with the shorter f/5
> OTA. If image scale is an issue, you can always pick up a quality 2x
> barlow to get back to f/10.
In writting the post I thought about you.too ;-)
At first sight I haven't imagine how such a scope could be "cumbersome" with
its 1.2m...
Cumbersome is not the word, but it is quite long indeed to travel even at
short distances
Otherwise it really looks to what I image to be a refractor à la Galileo, a
lo-ong tube, symbol of the astronomer ;-)
But you are right, it is bit long.
I will check your web again to see what scope you use (I remember more or
less)
My question is then, is a 120 mm f/5 achromat compatible with the
observation of bright objets (e.g. planets or moon). The chromatic
aberration and mainly the bleu-violet light couldn't ruin the image, showing
arounf all objets bright colored halos ?
Who knows ? or who as pictures ?
By the way, could the Chromacorr be useful ? Has someone out there tested it
on a small apochromat 120 mm f/5 ?
and is it always available (some months ago it was no more)
Thierry
>
> Anthony.
>
> 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-quel-tel-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
|
| | |
Date: 13 Sep 2006 14:17:52
From: Tom Royer
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
"Anthony Ayiomamitis" <anthony@perseus.no2spam.gr > wrote in message
news:ee9gp0$424$1@mouse.otenet.gr...
> Salut Thierry,
>
> If you have any plans to also travel (for example to observe a total solar
> eclipse), I suspect you will be much more comfortable with the shorter f/5
> OTA. If image scale is an issue, you can always pick up a quality 2x
> barlow to get back to f/10.
>
> Anthony.
I agree with this advice. A 120mm refractor, particularly f/10,
is not a small telescope. I have a 100mm f/6 mounted on a
GEM that I consider about the max size for "grab and go".
The one drawback to the shorter FL is that the scope may
be more susceptible to false color.
Good luck with it.
>
> 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-quel-tel-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
|
| | | |
Date: 14 Sep 2006 07:28:16
From: William Hamblen
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
On Wed, 13 Sep 2006 14:17:52 -0400, "Tom Royer" <troyer@mitre.org >
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
--
The night is just the shadow of the Earth.
|
| | | | |
Date: 14 Sep 2006 08:18:28
From: Alan Charlesworth
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
In article <quhig2pov286u5hnk3kdbd8bca37guoi6b@4ax.com >,
William Hamblen <wrhamblen@comcast.net > wrote:
> On Wed, 13 Sep 2006 14:17:52 -0400, "Tom Royer" <troyer@mitre.org>
> 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)?
|
| | | | | |
Date: 14 Sep 2006 22:33:01
From: William Hamblen
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
On Thu, 14 Sep 2006 08:18:28 -0700, Alan Charlesworth
<alan.charlesworth@sun.com.invalid > wrote:
>In article <quhig2pov286u5hnk3kdbd8bca37guoi6b@4ax.com>,
> William Hamblen <wrhamblen@comcast.net> wrote:
>
>> On Wed, 13 Sep 2006 14:17:52 -0400, "Tom Royer" <troyer@mitre.org>
>> 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.
|
| |
Date: 14 Sep 2006 07:07:18
From: David Nakamoto
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
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.
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-quel-tel-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
>
>
|
| | |
Date: 14 Sep 2006 22:57:19
From:
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
"David Nakamoto" <david.nakamoto@verizon.net > 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)
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-quel-tel-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
|
| | | |
Date: 16 Sep 2006 06:21:23
From: David Nakamoto
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
Thierry wrote:
> "David Nakamoto" <david.nakamoto@verizon.net> 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-quel-tel-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...;-))
|
| |
Date: 14 Sep 2006 00:38:10
From: Wally
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
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.
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-quel-tel-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
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Date: 14 Sep 2006 23:15:32
From:
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
"Wally" <woodfield@ai5.net > wrote in message
news:4508EAC2.BAE6412A@ai5.net...
> 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-quel-tel-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
>
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Date: 18 Sep 2006 04:53:03
From:
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
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-quel-tel-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.
|
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Date: 19 Sep 2006 19:22:35
From:
Subject: Re: refractor 120mm : f/5 or f/10 ?
|
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
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