Opinions Wanted:SCTs -vs- Refractors

I'm finally ready to make the plunge for a decent $1000-$1500 range
scope. I have researched several makes of both SCTs and refractors,
and the choice is getting difficult. So, those of you having
experience with both types, I welcome your opinions on what you like,
what to look for, and what to buy.

thanks and happy holidays,

Caesar Garcia

SCTs provide lower (poorer) contrast than refractors of the same aperture.
The contrast loss associated with recreating the target object at the focal
plane is known as the CTF (Contrast Transfer Function). In a nutshell,
diffraction resulting from the aperture causes the light to "ripple", just
like water did in Young's experiment. In his case he used "slits", and in
our case we use a circle. In Young's experiment, waves (ripples) were
created on one side of the double slit aperture and then passed through the
slits. When they reached the other side, the waves interfered with one
another such that where they were in phase, they reinforced, and where they
were out of phase they cancelled. In our case, we get the same result, only
from a single circular aperture. The result is a circular diffraction
pattern. This pattern consists of a central disk and a set of concentric
rings (if the scope is properly collimated).

Given a target pattern of alternating dark and light lines, the extent and
characteristics of the diffraction pattern will have a direct affect on the
CTF. However, because of the conservation of energy, there is no actual loss
of light, so only contrast is affected, not overall brightness. Thus a 5"
SCT and a 5" refractor will have the same brightness, given the same
efficiency of light transfer. (The efficiency of light transfer, however,
will not be the same. A lens is more efficient than a mirror, and the SCT
has a three mirrors to deal with (if you count the diagonal), where a
refractor has only one, the diagonal).

The obstruction (secondary mirror) in the center of the aperture of a
typical reflecting telescope (Newt, SCT, MAK, MNT) adds a further result of
influencing the diffraction pattern such that more energy lands in the first
diffraction ring, than it does for an unobstructed scope. Hence, the
obstructed scope, using our light/dark line pattern, will spill even more of
the light from the light lines, over into the dark lines, having a further
deleterious effect on the CTF.

To compensate for this added loss of efficiency in the CTF brought on by the
obstruction, one needs the extra light and increased resolution of a larger
aperture. The latter primarily a result of the fact that a larger aperture
has a both smaller and brighter central disk in its diffraction pattern.

(Guessing here, but one might think that the larger aperture preserves more
energy in the central disk, thereby having a dimmer set of rings in its
diffraction pattern, with a resultant decrease in "spillage".)

In any event, the "tighter" central diffraction disk means less of the
target area is covered by a single diffraction pattern, and it follows that
the finer details one can see (improving resolution). So, while the central
obstruction is robbing light from the central disk and placing it into the
first diffraction ring (and thereby spilling light on to the adjacent point
in the image plane), the increase in resolution allows that adjacent point
to be seen better, regardless.

The rule of thumb is, that in order to equal the contrast of a refractor, at
a minimum an obstructed aperture will have to have as much more aperture as
they do central obstruction. With respect to contrast alone, obstructed
aperture - obstruction = unobstructed aperture, so an (typical) 8" SCT with
a 34% central obstruction will compare to a 5" refractor of similar optical
quality, and an 8" Newtonian with a 25% central obstruction (CO) will
compare to a 6" refractor.

Further, it has been stated that once you get the CO down to 20% and
smaller, the effect of the CO on the diffraction pattern is inconsequential,
and the obstruction no longer has a deleteriuos effect on contrast.

Having said all of that, refractors are more expensive per inch of aperture
than SCTs, and SCTs are more expensive per inch of aperture than Newtonian
reflectors.

Given $1000, one can either buy a larger aperture SCT, or a _much_ larger
aperture Newtonian, than a refractor. This gives you a scope that rivals the
refractor in contrast, but more importantly, makes deep sky objects more
accessible owing to the increase in light grasp. In the end, it is aperture
that brings deep sky objects into view.

Of course, if you're not interested in deep sky...., refractors can be a
coveted item.

HTH,
Stephen Paul

SCTs provide lower (poorer) contrast than refractors of the same aperture.
The contrast loss associated with recreating the target object at the focal
plane is known as the CTF (Contrast Transfer Function). In a nutshell,
diffraction resulting from the aperture causes the light to "ripple", just
like water did in Young's experiment. In his case he used "slits", and in
our case we use a circle. In Young's experiment, waves (ripples) were
created on one side of the double slit aperture and then passed through the
slits. When they reached the other side, the waves interfered with one
another such that where they were in phase, they reinforced, and where they
were out of phase they cancelled. In our case, we get the same result, only
from a single circular aperture. The result is a circular diffraction
pattern. This pattern consists of a central disk and a set of concentric
rings (if the scope is properly collimated).

Given a target pattern of alternating dark and light lines, the extent and
characteristics of the diffraction pattern will have a direct affect on the
CTF. However, because of the conservation of energy, there is no actual loss
of light, so only contrast is affected, not overall brightness. Thus a 5"
SCT and a 5" refractor will have the same brightness, given the same
efficiency of light transfer. (The efficiency of light transfer, however,
will not be the same. A lens is more efficient than a mirror, and the SCT
has a three mirrors to deal with (if you count the diagonal), where a
refractor has only one, the diagonal).

The obstruction (secondary mirror) in the center of the aperture of a
typical reflecting telescope (Newt, SCT, MAK, MNT) adds a further result of
influencing the diffraction pattern such that more energy lands in the first
diffraction ring, than it does for an unobstructed scope. Hence, the
obstructed scope, using our light/dark line pattern, will spill even more of
the light from the light lines, over into the dark lines, having a further
deleterious effect on the CTF.

To compensate for this added loss of efficiency in the CTF brought on by the
obstruction, one needs the extra light and increased resolution of a larger
aperture. The latter primarily a result of the fact that a larger aperture
has a both smaller and brighter central disk in its diffraction pattern.

(Guessing here, but one might think that the larger aperture preserves more
energy in the central disk, thereby having a dimmer set of rings in its
diffraction pattern, with a resultant decrease in "spillage".)

In any event, the "tighter" central diffraction disk means less of the
target area is covered by a single diffraction pattern, and it follows that
the finer details one can see (improving resolution). So, while the central
obstruction is robbing light from the central disk and placing it into the
first diffraction ring (and thereby spilling light on to the adjacent point
in the image plane), the increase in resolution allows that adjacent point
to be seen better, regardless.

The rule of thumb is, that in order to equal the contrast of a refractor, at
a minimum an obstructed aperture will have to have as much more aperture as
they do central obstruction. With respect to contrast alone, obstructed
aperture - obstruction = unobstructed aperture, so an (typical) 8" SCT with
a 34% central obstruction will compare to a 5" refractor of similar optical
quality, and an 8" Newtonian with a 25% central obstruction (CO) will
compare to a 6" refractor.

Further, it has been stated that once you get the CO down to 20% and
smaller, the effect of the CO on the diffraction pattern is inconsequential,
and the obstruction no longer has a deleteriuos effect on contrast.

Having said all of that, refractors are more expensive per inch of aperture
than SCTs, and SCTs are more expensive per inch of aperture than Newtonian
reflectors.

Given $1000, one can either buy a larger aperture SCT, or a _much_ larger
aperture Newtonian, than a refractor. This gives you a scope that rivals the
refractor in contrast, but more importantly, makes deep sky objects more
accessible owing to the increase in light grasp. In the end, it is aperture
that brings deep sky objects into view.

Of course, if you're not interested in deep sky...., refractors can be a
coveted item.

HTH,
Stephen Paul

SCTs provide lower (poorer) contrast than refractors of the same aperture.
The contrast loss associated with recreating the target object at the focal
plane is known as the CTF (Contrast Transfer Function). In a nutshell,
diffraction resulting from the aperture causes the light to "ripple", just
like water did in Young's experiment. In his case he used "slits", and in
our case we use a circle. In Young's experiment, waves (ripples) were
created on one side of the double slit aperture and then passed through the
slits. When they reached the other side, the waves interfered with one
another such that where they were in phase, they reinforced, and where they
were out of phase they cancelled. In our case, we get the same result, only
from a single circular aperture. The result is a circular diffraction
pattern. This pattern consists of a central disk and a set of concentric
rings (if the scope is properly collimated).

Given a target pattern of alternating dark and light lines, the extent and
characteristics of the diffraction pattern will have a direct affect on the
CTF. However, because of the conservation of energy, there is no actual loss
of light, so only contrast is affected, not overall brightness. Thus a 5"
SCT and a 5" refractor will have the same brightness, given the same
efficiency of light transfer. (The efficiency of light transfer, however,
will not be the same. A lens is more efficient than a mirror, and the SCT
has a three mirrors to deal with (if you count the diagonal), where a
refractor has only one, the diagonal).

The obstruction (secondary mirror) in the center of the aperture of a
typical reflecting telescope (Newt, SCT, MAK, MNT) adds a further result of
influencing the diffraction pattern such that more energy lands in the first
diffraction ring, than it does for an unobstructed scope. Hence, the
obstructed scope, using our light/dark line pattern, will spill even more of
the light from the light lines, over into the dark lines, having a further
deleterious effect on the CTF.

To compensate for this added loss of efficiency in the CTF brought on by the
obstruction, one needs the extra light and increased resolution of a larger
aperture. The latter primarily a result of the fact that a larger aperture
has a both smaller and brighter central disk in its diffraction pattern.

(Guessing here, but one might think that the larger aperture preserves more
energy in the central disk, thereby having a dimmer set of rings in its
diffraction pattern, with a resultant decrease in "spillage".)

In any event, the "tighter" central diffraction disk means less of the
target area is covered by a single diffraction pattern, and it follows that
the finer details one can see (improving resolution). So, while the central
obstruction is robbing light from the central disk and placing it into the
first diffraction ring (and thereby spilling light on to the adjacent point
in the image plane), the increase in resolution allows that adjacent point
to be seen better, regardless.

The rule of thumb is, that in order to equal the contrast of a refractor, at
a minimum an obstructed aperture will have to have as much more aperture as
they do central obstruction. With respect to contrast alone, obstructed
aperture - obstruction = unobstructed aperture, so an (typical) 8" SCT with
a 34% central obstruction will compare to a 5" refractor of similar optical
quality, and an 8" Newtonian with a 25% central obstruction (CO) will
compare to a 6" refractor.

Further, it has been stated that once you get the CO down to 20% and
smaller, the effect of the CO on the diffraction pattern is inconsequential,
and the obstruction no longer has a deleteriuos effect on contrast.

Having said all of that, refractors are more expensive per inch of aperture
than SCTs, and SCTs are more expensive per inch of aperture than Newtonian
reflectors.

Given $1000, one can either buy a larger aperture SCT, or a _much_ larger
aperture Newtonian, than a refractor. This gives you a scope that rivals the
refractor in contrast, but more importantly, makes deep sky objects more
accessible owing to the increase in light grasp. In the end, it is aperture
that brings deep sky objects into view.

Of course, if you're not interested in deep sky...., refractors can be a
coveted item.

HTH,
Stephen Paul

"Stephen Paul" wrote in message
...
(Guessing here, but one might think that the larger aperture preserves
more
energy in the central disk, thereby having a dimmer set of rings in its
diffraction pattern, with a resultant decrease in "spillage".)

Good guess. The percentages are the same with aperture increase, but the
size of the airy disk decreases.

Clear Skies!

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
************************************************** **********

"Stephen Paul" wrote in message
...
(Guessing here, but one might think that the larger aperture preserves
more
energy in the central disk, thereby having a dimmer set of rings in its
diffraction pattern, with a resultant decrease in "spillage".)

Good guess. The percentages are the same with aperture increase, but the
size of the airy disk decreases.

Clear Skies!

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
************************************************** **********

"Stephen Paul" wrote in message
...
(Guessing here, but one might think that the larger aperture preserves
more
energy in the central disk, thereby having a dimmer set of rings in its
diffraction pattern, with a resultant decrease in "spillage".)

Good guess. The percentages are the same with aperture increase, but the
size of the airy disk decreases.

Clear Skies!

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
************************************************** **********

You have to ask yourself which of the following 2 attributes are more
important to you, pure optics or portability and ease of use. The reflector
has the slight advantage in optical performance in that the clear aperture
is a bit greater because of the larger central obstruction of the SCT.
Images in a Newt will appear slightly brighter than in an equal size SCT.
This is not to say, however, that the SCT performance is poor by comparison.
Now for the portability and ease of use...

The SCT has got the advantage hands down here, and it's a big advantage.
Since the focal length of the SCT is folded, the typical OTA is usually a
fraction the size of a Newt of the same aperture. This results in less
weight and less bulk, which contribute to ease of set up, ease of use and
ease of storage. Finally, the SCT is usually outfitted with full
electronics (tracking, GOTO, etc). Since it is not feasible to put large
amateur Newts on anything other than a Dobson mount, the reflector (10" and
larger) is usually devoid of electronics, including tracking.

Al
"Caesar Garcia" wrote in message
om...
I'm finally ready to make the plunge for a decent $1000-$1500 range
scope. I have researched several makes of both SCTs and refractors,
and the choice is getting difficult. So, those of you having
experience with both types, I welcome your opinions on what you like,
what to look for, and what to buy.

thanks and happy holidays,

Caesar Garcia

You have to ask yourself which of the following 2 attributes are more
important to you, pure optics or portability and ease of use. The reflector
has the slight advantage in optical performance in that the clear aperture
is a bit greater because of the larger central obstruction of the SCT.
Images in a Newt will appear slightly brighter than in an equal size SCT.
This is not to say, however, that the SCT performance is poor by comparison.
Now for the portability and ease of use...

The SCT has got the advantage hands down here, and it's a big advantage.
Since the focal length of the SCT is folded, the typical OTA is usually a
fraction the size of a Newt of the same aperture. This results in less
weight and less bulk, which contribute to ease of set up, ease of use and
ease of storage. Finally, the SCT is usually outfitted with full
electronics (tracking, GOTO, etc). Since it is not feasible to put large
amateur Newts on anything other than a Dobson mount, the reflector (10" and
larger) is usually devoid of electronics, including tracking.

Al
"Caesar Garcia" wrote in message
om...
I'm finally ready to make the plunge for a decent $1000-$1500 range
scope. I have researched several makes of both SCTs and refractors,
and the choice is getting difficult. So, those of you having
experience with both types, I welcome your opinions on what you like,
what to look for, and what to buy.

thanks and happy holidays,

Caesar Garcia

You have to ask yourself which of the following 2 attributes are more
important to you, pure optics or portability and ease of use. The reflector
has the slight advantage in optical performance in that the clear aperture
is a bit greater because of the larger central obstruction of the SCT.
Images in a Newt will appear slightly brighter than in an equal size SCT.
This is not to say, however, that the SCT performance is poor by comparison.
Now for the portability and ease of use...

The SCT has got the advantage hands down here, and it's a big advantage.
Since the focal length of the SCT is folded, the typical OTA is usually a
fraction the size of a Newt of the same aperture. This results in less
weight and less bulk, which contribute to ease of set up, ease of use and
ease of storage. Finally, the SCT is usually outfitted with full
electronics (tracking, GOTO, etc). Since it is not feasible to put large
amateur Newts on anything other than a Dobson mount, the reflector (10" and
larger) is usually devoid of electronics, including tracking.

Al
"Caesar Garcia" wrote in message
om...
I'm finally ready to make the plunge for a decent $1000-$1500 range
scope. I have researched several makes of both SCTs and refractors,
and the choice is getting difficult. So, those of you having
experience with both types, I welcome your opinions on what you like,
what to look for, and what to buy.

thanks and happy holidays,

Caesar Garcia