That's one of the uses my 80mm f/15 Carton excels at too.
Doug

On Sun, Nov 15, 2009 at 8:41 AM, Guido Santacana <foxam2001@...>wrote:

>
>
> Long focus and good quality refractors are a good choice. Using my old
> Royal Astro Optical 76mm f/16 refractor Ihave been able to do splitting of
> doubles that sometimes is surprising. Maksutovs are good at this too.
>
> Guido
>
> --- On Sat, 11/14/09, Scott Holland
<holl8127@...<holl8127%40bellsouth.net>>
> wrote:
>
> From: Scott Holland <holl8127@... <holl8127%40bellsouth.net>>
> Subject: Re: [Refractors] Refractor for Double Stars
> To: Refractors@yahoogroups.com <Refractors%40yahoogroups.com>
> Date: Saturday, November 14, 2009, 10:16 PM
>
>
>
> I would vote for the D&G. Faint close pairs should benefit fron the extra
> inch for resolving power and light grasp, and at f/12, I don't think CA will
> be a problem. Also, I think you'll probably have less out-of-pocket. D&G has
> a fine reputation.
>
> Scott Holland
>
> Lowell, NC 28098
>
> Celestron/Vixen C-102 f/9 (plus a lot of other toys)
>
>
> ----- Original Message -----
>
> From: patconlon
>
> To: Refractors@yahoogro ups.com
>
> Sent: Saturday, November 14, 2009 2:57 PM
>
> Subject: [Refractors] Refractor for Double Stars
>
> Hello everyone recently I have become interested in double stars and
> Refractors, so I was wondering which type of refractor would be best for
> this type of astronomy. I was thinking along the lines of a TEC 140 or a D&G
> 6inch F12. Now I know we are not comparing like with like but would the
> extra inch plus the greater focal length of the D$G compansate for the fact
> it is not an APO. The optical tube would be going on a Losmandy G11 Mount,
> bearing in mind also the D$G is half the price of the TEC.
>
> [Non-text portions of this message have been removed]
>
> [Non-text portions of this message have been removed]
>
>
>



--
Doug
Truckstop Astronomer


[Non-text portions of this message have been removed]

Long focus and good quality refractors are a good choice. Using my old Royal
Astro Optical 76mm f/16 refractor Ihave been able to do splitting of doubles
that sometimes is surprising. Maksutovs are good at this too.

Guido

--- On Sat, 11/14/09, Scott Holland <holl8127@...> wrote:

From: Scott Holland <holl8127@...>
Subject: Re: [Refractors] Refractor for Double Stars
To: Refractors@yahoogroups.com
Date: Saturday, November 14, 2009, 10:16 PM







 









       I would vote for the D&G. Faint close pairs should benefit fron the extra
inch for resolving power and light grasp, and at f/12, I don't think CA will be
a problem. Also, I think you'll probably have less out-of-pocket. D&G has a fine
reputation.



Scott Holland

Lowell, NC 28098



Celestron/Vixen C-102 f/9 (plus a lot of other toys)



----- Original Message -----

   From: patconlon

   To: Refractors@yahoogro ups.com

   Sent: Saturday, November 14, 2009 2:57 PM

   Subject: [Refractors] Refractor for Double Stars



Hello everyone recently I have become interested in double stars and Refractors,
so I was wondering which type of refractor would be best for this type of
astronomy. I was thinking along the lines of a TEC 140 or a D&G 6inch F12. Now I
know we are not comparing like with like but would the extra inch plus the
greater focal length of the D$G compansate for the fact it is not an APO. The
optical tube would be going on a Losmandy G11 Mount, bearing in mind also the
D$G is half the price of the TEC.



[Non-text portions of this message have been removed]

























[Non-text portions of this message have been removed]

Not true. The D&G's hold their original value quite well, and it has the
advantage of costing Much Less than the TEC, while still providing a really
good view.
Doug

On Sun, Nov 15, 2009 at 2:30 AM, Chris Lord <
chrislord@...> wrote:

> No, get the TEC140APO. The Strehl of the achro when focussed in green
> will be about 90% but less than 80% at C & F, whereas the Strehl of the
> TEC140 is ~98% across the visual spectrum. Its not simply a question of
> how much green & folding you have to part with. There's also the matter
> of how well the 'scope holds value. The D&G may cost half as much as
> the TEC, but after a decade the TEC will have a resale value ~ what you
> paid, whereas the D&G resale value will be like that of a used car. In
> the meantime you'll have enjoyed the superb view, instead of wishing
> you'd gone for the real deal.
>
> Chris Lord
>
> On Nov 14, 2009, at 19:57, patconlon wrote:
>
> >  Hello everyone recently I have become interested in double stars and
> > Refractors, so I was wondering which type of refractor would be best
> > for this type of astronomy. I was thinking along the lines of a TEC
> > 140 or a D&G 6inch F12. Now I know we are not comparing like with like
> > but would the extra inch plus the greater focal length of the D$G
> > compansate for the fact it is not an APO. The optical tube would be
> > going on a Losmandy G11 Mount, bearing in mind also the D$G is half
> > the price of the TEC.
> >
> >
>
>
> [Non-text portions of this message have been removed]
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>


--
Doug
Truckstop Astronomer


[Non-text portions of this message have been removed]

Chris Lord wrote:
> http://www.vixenoptics.com/refractors/ax103.html depicts the f/8
> system. The AX103S page has a CA diagram but its unscaled so
> meaningless. Its a 'scope designed for imaging, but f/8 is a tad slow
> for deep sky photography. You'd have to add the dedicated f/5.6 FF. I
> assume the 140 f/5.7 system is similar, i.e. triplet air spaced OG +
> Petzval type field corrector.
With "Petzval like", wouldn't that imply that the four
lenses in two groups would be more like two doublets?
You wouldn't as much need a triplet for an achromat.

No, get the TEC140APO. The Strehl of the achro when focussed in green
will be about 90% but less than 80% at C & F, whereas the Strehl of the
TEC140 is ~98% across the visual spectrum. Its not simply a question of
how much green & folding you have to part with. There's also the matter
of how well the 'scope holds value. The D&G may cost half as much as
the TEC, but after a decade the TEC will have a resale value ~ what you
paid, whereas the D&G resale value will be like that of a used car. In
the meantime you'll have enjoyed the superb view, instead of wishing
you'd gone for the real deal.

Chris Lord

On Nov 14, 2009, at 19:57, patconlon wrote:

>  Hello everyone recently I have become interested in double stars and
> Refractors, so I was wondering which type of refractor would be best
> for this type of astronomy. I was thinking along the lines of a TEC
> 140 or a D&G 6inch F12. Now I know we are not comparing like with like
> but would the extra inch plus the greater focal length of the D$G
> compansate for the fact it is not an APO. The optical tube would be
> going on a Losmandy G11 Mount, bearing in mind also the D$G is half
> the price of the TEC.
>
>


[Non-text portions of this message have been removed]

http://www.vixenoptics.com/refractors/ax103.html depicts the f/8
system. The AX103S page has a CA diagram but its unscaled so
meaningless. Its a 'scope designed for imaging, but f/8 is a tad slow
for deep sky photography. You'd have to add the dedicated f/5.6 FF. I
assume the 140 f/5.7 system is similar, i.e. triplet air spaced OG +
Petzval type field corrector.

Chris Lord

On Nov 14, 2009, at 19:33, Richard F.L.R. Snashall wrote:

> Scott Walker wrote:
>  > Would still expect the NeoAchro to have a lot of chromatic
> aberation. The Petzval design only improves chromatic aberation a
> modest amount. One needs to combine the Petzval design with special
> glasses to achieve the great color correction that is seen in scopes
> like the NP101. I would expect the chromatic aberation to be similar
> to a f8 to f9 achro of the seem aperture.
>  Re: NeoAchro 140
>
>  I wonder what their particular definition of "Petzval like" is...
>


[Non-text portions of this message have been removed]

I agree with you, Scott.
Doug

On Sat, Nov 14, 2009 at 6:16 PM, Scott Holland <holl8127@...>wrote:

>
>
> I would vote for the D&G. Faint close pairs should benefit fron the extra
> inch for resolving power and light grasp, and at f/12, I don't think CA will
> be a problem. Also, I think you'll probably have less out-of-pocket. D&G has
> a fine reputation.
>
> Scott Holland
> Lowell, NC 28098
>
> Celestron/Vixen C-102 f/9 (plus a lot of other toys)
>
> ----- Original Message -----
> From: patconlon
> To: Refractors@yahoogroups.com <Refractors%40yahoogroups.com>
> Sent: Saturday, November 14, 2009 2:57 PM
> Subject: [Refractors] Refractor for Double Stars
>
> Hello everyone recently I have become interested in double stars and
> Refractors, so I was wondering which type of refractor would be best for
> this type of astronomy. I was thinking along the lines of a TEC 140 or a D&G
> 6inch F12. Now I know we are not comparing like with like but would the
> extra inch plus the greater focal length of the D$G compansate for the fact
> it is not an APO. The optical tube would be going on a Losmandy G11 Mount,
> bearing in mind also the D$G is half the price of the TEC.
>
> [Non-text portions of this message have been removed]
>
>
>



--
Doug
Truckstop Astronomer


[Non-text portions of this message have been removed]

I would vote for the D&G. Faint close pairs should benefit fron the extra inch
for resolving power and light grasp, and at f/12, I don't think CA will be a
problem. Also, I think you'll probably have less out-of-pocket. D&G has a fine
reputation.

Scott Holland
Lowell, NC 28098

Celestron/Vixen C-102 f/9 (plus a lot of other toys)


   ----- Original Message -----
   From: patconlon
   To: Refractors@yahoogroups.com
   Sent: Saturday, November 14, 2009 2:57 PM
   Subject: [Refractors] Refractor for Double Stars



   Hello everyone recently I have become interested in double stars and
Refractors, so I was wondering which type of refractor would be best for this
type of astronomy. I was thinking along the lines of a TEC 140 or a D&G 6inch
F12. Now I know we are not comparing like with like but would the extra inch
plus the greater focal length of the D$G compansate for the fact it is not an
APO. The optical tube would be going on a Losmandy G11 Mount, bearing in mind
also the D$G is half the price of the TEC.





[Non-text portions of this message have been removed]

I've owned one of these for almost 2 years.  (140)  Great scope.

Jim

--- In Refractors@yahoogroups.com, "Richard F.L.R. Snashall" <rflrs@...> wrote:
>
> Scott Walker wrote:
> > Would still expect the NeoAchro to have a lot of chromatic aberation. The
Petzval design only improves chromatic aberation a modest amount. One needs to
combine the Petzval design with special glasses to achieve the great color
correction that is seen in scopes like the NP101. I would expect the chromatic
aberation to be similar to a f8 to f9 achro of the seem aperture.
> Re: NeoAchro 140
>
> I wonder what their particular definition of "Petzval like" is...
>

Hello everyone recently I have become interested in double stars and
Refractors, so I was wondering which type of refractor would be best for this
type of astronomy.  I was thinking along the lines of a TEC 140 or a D&G 6inch
F12. Now I know we are not comparing like with like but would the extra inch
plus the greater focal length of the D$G compansate for the fact it is not an
APO. The optical tube would be going on a Losmandy G11 Mount, bearing in mind
also the D$G is half the price of the TEC.

Scott Walker wrote:
> Would still expect the NeoAchro to have a lot of chromatic aberation. The
Petzval design only improves chromatic aberation a modest amount. One needs to
combine the Petzval design with special glasses to achieve the great color
correction that is seen in scopes like the NP101. I would expect the chromatic
aberation to be similar to a f8 to f9 achro of the seem aperture.
Re: NeoAchro 140

I wonder what their particular definition of "Petzval like" is...

Would still expect the NeoAchro to have a lot of chromatic aberation. The
Petzval design only improves chromatic aberation a modest amount. One needs to
combine the Petzval design with special glasses to achieve the great color
correction that is seen in scopes like the NP101. I would expect the chromatic
aberation to be similar to a f8 to f9 achro of the seem aperture.

Scott Walker
   ----- Original Message -----
   From: Chris Lord
   To: Refractors@yahoogroups.com
   Cc: blackpoolastronomy@yahoogroups.com
   Sent: Saturday, November 14, 2009 8:55 AM
   Subject: Re: [Refractors] Re: VIXEN NeoAchro 140 & AX103S



   So what do you reckon to these Vixen designs?

   http://www.vixenoptics.com/refractors/ax103.html
   http://www.vixenoptics.com/refractors/neo140.html





[Non-text portions of this message have been removed]

Jim,

If I said it as I really think it, I'd be in prison!

Chris Lord

On Nov 14, 2009, at 16:15, jimcurrymaine wrote:

> Chris:
>  When I clicked on your link this is the what came up: "Error 403 -
> Forbidden
>  You tried to access a document for which you don't have privileges"
>
>  I was able to log on by eliminating everything after the .org. Great
> website, great essays. Next time don't hold back, though. Tell us what
> you really think :>)
>
>  Jim


[Non-text portions of this message have been removed]

So what do you reckon to these Vixen designs?

http://www.vixenoptics.com/refractors/ax103.html
http://www.vixenoptics.com/refractors/neo140.html

Chris:
When I clicked on your link this is the what came up: "Error 403 - Forbidden
You tried to access a document for which you don't have privileges"

I was able to log on by eliminating everything after the .org.  Great website,
great essays.  Next time don't hold back, though.  Tell us what you really think
:>)

Jim

--- In Refractors@yahoogroups.com, Chris Lord <chrislord@...> wrote:
>
> It has no bearing on Strehl. Scratch-Dig ratio is a surface quality
> measurement used by opticians. Scratch numbers are defined by the width
> in units of 0.1 microns; dig numbers are defined by dig diameters in
> units of 10 microns.
>
> Commercial optics have a 60-40 Scratch-Dig ratio, MilSpec optics 20-10
> or 10-5.
>
> The reason it has no bearing on Strehl ratio is because cosmetic
> defects on a lens surface scatter light and lower contrast on extended
> objects. Strehl is a measure of how much light is present in the
> central part of the Airy disc of a point source compared to the
> theoretical maximum. Strehl is linked to PSF, not cosmetic defects of
> the optical surface.
>
> See p54 of my Eyepiece Monograph on the publications page of my website.
> <http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/
> BRAYOBS%20PUBLICATIONS.html>
>
> Chris Lord
>
> On Nov 13, 2009, at 19:02, jimcurrymaine wrote:
>
> > I'm seeing this spec. show up on lens websites. How does this
> > specification relate to Strehl ratio?
> >
> >
>
>
> [Non-text portions of this message have been removed]
>

It has no bearing on Strehl. Scratch-Dig ratio is a surface quality
measurement used by opticians. Scratch numbers are defined by the width
in units of 0.1 microns; dig numbers are defined by dig diameters in
units of 10 microns.

Commercial optics have a 60-40 Scratch-Dig ratio, MilSpec optics 20-10
or 10-5.

The reason it has no bearing on Strehl ratio is because cosmetic
defects on a lens surface scatter light and lower contrast on extended
objects. Strehl is a measure of how much light is present in the
central part of the Airy disc of a point source compared to the
theoretical maximum. Strehl is linked to PSF, not cosmetic defects of
the optical surface.

See p54 of my Eyepiece Monograph on the publications page of my website.
<http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/
BRAYOBS%20PUBLICATIONS.html>

Chris Lord

On Nov 13, 2009, at 19:02, jimcurrymaine wrote:

> I'm seeing this spec. show up on lens websites. How does this
> specification relate to Strehl ratio?
>
>


[Non-text portions of this message have been removed]

I'm seeing this spec. show up on lens websites.  How does this specification
relate to Strehl ratio?

I don't think these will do much for the inherent false color due to the achro
nature of the doublet lens.  Careful collimation of the front cell will work
best for this.  This device is only for atmospheric induced color (which I have
never noticed).
 
Or... am I wrong and does this device actually help cut down on CA?
 
It also seems to add a fair amount of glass to the optical path - what effect
does this have on light transmission and the brightness and contrast of the
target?
 
Would be good to have more info from an actual user.
 
Warp
 


--- On Fri, 11/13/09, Frank <mega75mark@...> wrote:


From: Frank <mega75mark@...>
Subject: [Refractors] Re: Meade LXD 75 AR6 Refractor
To: Refractors@yahoogroups.com
Date: Friday, November 13, 2009, 8:43 AM


 



I have an atmospheric dispersion prism and they do work. With Jupiter at a
rather low declination this year the prism removes the red and blue fringe
around the planet and gives it a clean edge. Adirondack sells them, but they are
pricey.

Frank K.

--- In Refractors@yahoogro ups.com, "Rich" <astronut1001@ ...> wrote:
>
> Anothher thing which can cause or contribute to this is atmospheric dispersion
when viewing below about 45 to 60 degrees. In fact low angle wedge prisms have
been offered for eyepiece installation to correct this for fanatical planetary
viewers. I have seen them mentioned but have never seen one.
>
> Rich Wood
>
>
> --- In Refractors@yahoogro ups.com, "bonview@" <bonview@> wrote:
> >
> > I did a collimation on this scope but when I look at the moon I have blue
light to the sun side and yellow to the dark side. Are the lens out with respect
to each other? Can I tweak this myself? Can this be tweak with this scope? I
take it that this scope has two air spaced lens.
> > gary saunders
> >
>








[Non-text portions of this message have been removed]

I have an atmospheric dispersion prism and they do work. With Jupiter at a
rather low declination this year the prism removes the red and blue fringe
around the planet and gives it a clean edge. Adirondack sells them, but they are
pricey.

Frank K.

--- In Refractors@yahoogroups.com, "Rich" <astronut1001@...> wrote:
>
> Anothher thing which can cause or contribute to this is atmospheric dispersion
when viewing below about 45 to 60 degrees.  In fact low angle wedge prisms have
been offered for eyepiece installation to correct this for fanatical planetary
viewers.  I have seen them mentioned but have never seen one.
>
> Rich Wood
>
>
> --- In Refractors@yahoogroups.com, "bonview@" <bonview@> wrote:
> >
> >  I did a collimation on this scope but when I look at the moon I have blue
light to the sun side and yellow to the dark side. Are the lens out with respect
to each other? Can I tweak this myself? Can this be tweak with this scope? I
take it that this scope has two air spaced lens.
> >    gary saunders
> >
>

Anothher thing which can cause or contribute to this is atmospheric dispersion
when viewing below about 45 to 60 degrees.  In fact low angle wedge prisms have
been offered for eyepiece installation to correct this for fanatical planetary
viewers.  I have seen them mentioned but have never seen one.

Rich Wood


--- In Refractors@yahoogroups.com, "bonview@..." <bonview@...> wrote:
>
>  I did a collimation on this scope but when I look at the moon I have blue
light to the sun side and yellow to the dark side. Are the lens out with respect
to each other? Can I tweak this myself? Can this be tweak with this scope? I
take it that this scope has two air spaced lens.
>    gary saunders
>

It's not quite the position of the eye but looking through the eyepiece
at an angle.  If the color is in the objective high power makes it
worse.  If the color is in your eye position low power makes it worse.
Also watch out for atmospheric dispersion.

Bud

bonview@... wrote:
>  Thanks, I did have my eye to the glass - from edge to edge. I replace the
Mead focuser with a ScopeStuff focuser plus I was using  the new 14.5mm Orion
edge-on planetry eyepiece. I was like a kid in the candy store. This scope is so
much better than it was for such a small investment. Next moon I will try it
with a higher power eyepiece.
>  Thanks
>  gary saunders
>
>
>
>
>
>
> i
>
> --- On Tue, 11/10/09, William Hamblen <wrhamblen@...> wrote:
>
>
> From: William Hamblen <wrhamblen@...>
> Subject: Re: [Refractors] Meade LXD 75 AR6 Refractor
> To: Refractors@yahoogroups.com
> Date: Tuesday, November 10, 2009, 9:42 PM
>
>
>
>
>
>
> bonview@verizon. net wrote:
>
>> I did a collimation on this scope but when I look at the moon I have blue
light to the sun side and yellow to the dark side. Are the lens out with respect
to each other? Can I tweak this myself? Can this be tweak with this scope? I
take it that this scope has two air spaced lens.
>>
> Even with a reflector your eyepiece will give you blue on one side of
> the Moon and yellow on the other when your eye is not in the center.
> This is very noticeable with a low power eyepiece.
>
> Bud
>
>
>
>
>
>
>
> [Non-text portions of this message have been removed]
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>
>

Hi!

A link to a Rohr-test of the Stellarvue scope was posted on CN:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3440111/page/0/\
view/collapsed/sb/5/o/all/fpart/1

Direct link to test:
http://www.astro-foren.de/showthread.php?t=11109

Any comments by the experts?

What does the thing with the rotation of the flattener suggest?

Best Regards,
Søren

 Thanks, I did have my eye to the glass - from edge to edge. I replace the Mead
focuser with a ScopeStuff focuser plus I was using  the new 14.5mm Orion
edge-on planetry eyepiece. I was like a kid in the candy store. This scope is so
much better than it was for such a small investment. Next moon I will try it
with a higher power eyepiece.
 Thanks
 gary saunders
 
 
 
 
 
 
i

--- On Tue, 11/10/09, William Hamblen <wrhamblen@...> wrote:


From: William Hamblen <wrhamblen@...>
Subject: Re: [Refractors] Meade LXD 75 AR6 Refractor
To: Refractors@yahoogroups.com
Date: Tuesday, November 10, 2009, 9:42 PM


 



bonview@verizon. net wrote:
> I did a collimation on this scope but when I look at the moon I have blue
light to the sun side and yellow to the dark side. Are the lens out with respect
to each other? Can I tweak this myself? Can this be tweak with this scope? I
take it that this scope has two air spaced lens.
Even with a reflector your eyepiece will give you blue on one side of
the Moon and yellow on the other when your eye is not in the center.
This is very noticeable with a low power eyepiece.

Bud







[Non-text portions of this message have been removed]

bonview@... wrote:
>  I did a collimation on this scope but when I look at the moon I have blue
light to the sun side and yellow to the dark side. Are the lens out with respect
to each other? Can I tweak this myself? Can this be tweak with this scope? I
take it that this scope has two air spaced lens.
Even with a reflector your eyepiece will give you blue on one side of
the Moon and yellow on the other when your eye is not in the center.
This is very noticeable with a low power eyepiece.

Bud

I did a collimation on this scope but when I look at the moon I have blue light
to the sun side and yellow to the dark side. Are the lens out with respect to
each other? Can I tweak this myself? Can this be tweak with this scope? I take
it that this scope has two air spaced lens.
    gary saunders

This is a rather loose "rule of thumb" and such a lens will clearly show
the out of focus blue and red on bright objects.  I personally like
Dimitroff and Baker's more strict "five" times rule.

"The focal-length of an achromatic objective with unobjectionable color
aberration is given with sufficient accuracy by the formula f = 5*D^2."

A 3" f/15 meets this criteria, although they note "Its color is evident
only when a large color range is visible, as in the case of a bright
object against a dark background."

The SPC102 (~f/9) I had here for a while showed an obvious halo around
Jupiter, as did my 5" f/15 Lohmann Brothers.

Tolerance for the out-of-focus color seems to vary with the individual.
Before the days of modern apochromats, the vanilla f/15 achromat had a
reputation for providing the best lunar and planetary views for the
aperture - in spite of the secondary color.

Clear skies, Alan

William Hamblen wrote:
> The effect of chromatic aberration on the image varies with the focal
> ratio of the telescope.  For an achromat using ordinary glass the rule
> of thumb is to have the focal ratio equal to roughly 3 times the
> aperture in inches.
> The Celestron Omni XLT 120mm is f/8.3 which is faster than the rule of
> thumb (f/14).
>
> If you are interested in deep sky aperture is very important.  You
> generally can get a newtonian that is twice as big as a refractor that
> cost the same.
>
> Bud
>
>

The effect of chromatic aberration on the image varies with the focal
ratio of the telescope.  For an achromat using ordinary glass the rule
of thumb is to have the focal ratio equal to roughly 3 times the
aperture in inches.
The Celestron Omni XLT 120mm is f/8.3 which is faster than the rule of
thumb (f/14).

If you are interested in deep sky aperture is very important.  You
generally can get a newtonian that is twice as big as a refractor that
cost the same.

Bud

Dan wrote:
> Rich,
> Thank you for your response. It really helps with making my decision on what
to buy. I do have a budget to consider so I may just go with an achromat for
now. Most of my observing is deep sky and I don't plan on doing any
astrophotography. I just didn't want to spend money and not know what I was
buying.
>
> Thanks again,
> Dan
>
> --- In Refractors@yahoogroups.com, "Rich" <astronut1001@...> wrote:
>
>> Dan;
>>
>> Normally speaking it is the level of color correction in a refractor.  An
achromat has the lowest level of color correction normally speaking and a well
designed Apo or Apochromat has the best.
>>
>> Technically speaking the achromat brings two colors to the same focus while
an apochromat brings three colors to the same focus.  The best apochromats also
try to minimize focal differences for all spectrum colors including the violet. 
A matter of careful design, glass materials choices and fabrication accuracy.
>>
>> ED in a scope name merely indicates that an ED glass was used in lens
construction.  This can allow much better than achromat performance but does not
guarantee such.  Usually you will see "ED Apo" in the description. I would be
nervous if I did not.  Also marketing types frequently label any non standard
dispersion glass as ED whether technically correct or not.
>>
>> Most achromats will show a purple fringe around bright objects such as the
moon or planets.  An Apo should not.  If the scope is to be used for
astrophotography get the Apo as it makes the job easier.
>>
>> BTW so far as I know most of the refractors sold by Celestron, Skywatcher and
Orion are the same optical tube assemblies except for the paint.  All are made
in China by Synta who now owns both Celestron and Skywatcher.  I am not sure but
some Orion ED or Apo scopes may not be Syntas but all of their achromats are so
far as I know.
>>
>> Rich Wood
>>
>>
>> --- In Refractors@yahoogroups.com, "Dan" <dan@> wrote:
>>
>>> I'm looking to buy a new refractor. I am considering the Celestron Omni XLT
120mm. Celestron also offers the XLT 102mm ED but for considerable more money
and a little less light gathering power. What I don't understand is the
difference between the lens. And then someone mentioned APO. So, what is the
difference between an achromat, APO and ED?
>>>
>>> Thanks in advance,
>>> -Dan

Doug,
I also wanted to thank you for your thoughts on the subject.

Dan

--- In Refractors@yahoogroups.com, Doug <truckstop.astronomer@...> wrote:
>
> Also, a true Apo is about more than absolute color correction. It's also
> designed for no spherical aberration, in fact, no aberrations to speak of.
> An ED can give you near absolute color correction, but not the other
> aberrations simultaneously. Apo's can be, and usually are, very expensive,
> ED's less so, and Achromats even less. It should be noted that medium
> f/ratio ED's and long f/ratio Achromats can give a near Apo color
> performance, at least for visual purposes.
> Doug
>
> On Sun, Nov 8, 2009 at 3:42 PM, Rich <astronut1001@...> wrote:
>
> >
> >
> > Dan;
> >
> > Normally speaking it is the level of color correction in a refractor. An
> > achromat has the lowest level of color correction normally speaking and a
> > well designed Apo or Apochromat has the best.
> >
> > Technically speaking the achromat brings two colors to the same focus while
> > an apochromat brings three colors to the same focus. The best apochromats
> > also try to minimize focal differences for all spectrum colors including the
> > violet. A matter of careful design, glass materials choices and fabrication
> > accuracy.
> >
> > ED in a scope name merely indicates that an ED glass was used in lens
> > construction. This can allow much better than achromat performance but does
> > not guarantee such. Usually you will see "ED Apo" in the description. I
> > would be nervous if I did not. Also marketing types frequently label any non
> > standard dispersion glass as ED whether technically correct or not.
> >
> > Most achromats will show a purple fringe around bright objects such as the
> > moon or planets. An Apo should not. If the scope is to be used for
> > astrophotography get the Apo as it makes the job easier.
> >
> > BTW so far as I know most of the refractors sold by Celestron, Skywatcher
> > and Orion are the same optical tube assemblies except for the paint. All are
> > made in China by Synta who now owns both Celestron and Skywatcher. I am not
> > sure but some Orion ED or Apo scopes may not be Syntas but all of their
> > achromats are so far as I know.
> >
> > Rich Wood
> >
> > --- In Refractors@yahoogroups.com <Refractors%40yahoogroups.com>, "Dan"
> > <dan@> wrote:
> > >
> > > I'm looking to buy a new refractor. I am considering the Celestron Omni
> > XLT 120mm. Celestron also offers the XLT 102mm ED but for considerable more
> > money and a little less light gathering power. What I don't understand is
> > the difference between the lens. And then someone mentioned APO. So, what is
> > the difference between an achromat, APO and ED?
> > >
> > > Thanks in advance,
> > > -Dan
> > >
> >
> >
> >
>
>
>
> --
> Doug
> Truckstop Astronomer
>
>
> [Non-text portions of this message have been removed]
>

Rich,
Thank you for your response. It really helps with making my decision on what to
buy. I do have a budget to consider so I may just go with an achromat for now.
Most of my observing is deep sky and I don't plan on doing any astrophotography.
I just didn't want to spend money and not know what I was buying.

Thanks again,
Dan

--- In Refractors@yahoogroups.com, "Rich" <astronut1001@...> wrote:
>
> Dan;
>
> Normally speaking it is the level of color correction in a refractor.  An
achromat has the lowest level of color correction normally speaking and a well
designed Apo or Apochromat has the best.
>
> Technically speaking the achromat brings two colors to the same focus while an
apochromat brings three colors to the same focus.  The best apochromats also try
to minimize focal differences for all spectrum colors including the violet.  A
matter of careful design, glass materials choices and fabrication accuracy.
>
> ED in a scope name merely indicates that an ED glass was used in lens
construction.  This can allow much better than achromat performance but does not
guarantee such.  Usually you will see "ED Apo" in the description. I would be
nervous if I did not.  Also marketing types frequently label any non standard
dispersion glass as ED whether technically correct or not.
>
> Most achromats will show a purple fringe around bright objects such as the
moon or planets.  An Apo should not.  If the scope is to be used for
astrophotography get the Apo as it makes the job easier.
>
> BTW so far as I know most of the refractors sold by Celestron, Skywatcher and
Orion are the same optical tube assemblies except for the paint.  All are made
in China by Synta who now owns both Celestron and Skywatcher.  I am not sure but
some Orion ED or Apo scopes may not be Syntas but all of their achromats are so
far as I know.
>
> Rich Wood
>
>
> --- In Refractors@yahoogroups.com, "Dan" <dan@> wrote:
> >
> > I'm looking to buy a new refractor. I am considering the Celestron Omni XLT
120mm. Celestron also offers the XLT 102mm ED but for considerable more money
and a little less light gathering power. What I don't understand is the
difference between the lens. And then someone mentioned APO. So, what is the
difference between an achromat, APO and ED?
> >
> > Thanks in advance,
> > -Dan
> >
>

Also, a true Apo is about more than absolute color correction. It's also
designed for no spherical aberration, in fact, no aberrations to speak of.
An ED can give you near absolute color correction, but not the other
aberrations simultaneously. Apo's can be, and usually are, very expensive,
ED's less so, and Achromats even less. It should be noted that medium
f/ratio ED's and long f/ratio Achromats can give a near Apo color
performance, at least for visual purposes.
Doug

On Sun, Nov 8, 2009 at 3:42 PM, Rich <astronut1001@...> wrote:

>
>
> Dan;
>
> Normally speaking it is the level of color correction in a refractor. An
> achromat has the lowest level of color correction normally speaking and a
> well designed Apo or Apochromat has the best.
>
> Technically speaking the achromat brings two colors to the same focus while
> an apochromat brings three colors to the same focus. The best apochromats
> also try to minimize focal differences for all spectrum colors including the
> violet. A matter of careful design, glass materials choices and fabrication
> accuracy.
>
> ED in a scope name merely indicates that an ED glass was used in lens
> construction. This can allow much better than achromat performance but does
> not guarantee such. Usually you will see "ED Apo" in the description. I
> would be nervous if I did not. Also marketing types frequently label any non
> standard dispersion glass as ED whether technically correct or not.
>
> Most achromats will show a purple fringe around bright objects such as the
> moon or planets. An Apo should not. If the scope is to be used for
> astrophotography get the Apo as it makes the job easier.
>
> BTW so far as I know most of the refractors sold by Celestron, Skywatcher
> and Orion are the same optical tube assemblies except for the paint. All are
> made in China by Synta who now owns both Celestron and Skywatcher. I am not
> sure but some Orion ED or Apo scopes may not be Syntas but all of their
> achromats are so far as I know.
>
> Rich Wood
>
> --- In Refractors@yahoogroups.com <Refractors%40yahoogroups.com>, "Dan"
> <dan@...> wrote:
> >
> > I'm looking to buy a new refractor. I am considering the Celestron Omni
> XLT 120mm. Celestron also offers the XLT 102mm ED but for considerable more
> money and a little less light gathering power. What I don't understand is
> the difference between the lens. And then someone mentioned APO. So, what is
> the difference between an achromat, APO and ED?
> >
> > Thanks in advance,
> > -Dan
> >
>
>
>



--
Doug
Truckstop Astronomer


[Non-text portions of this message have been removed]