Barking up the wrong tree

Can someone answer a question for me? I want to see if I've understood a
few things.

As I understand it, the higher the F number of a 'scope, the more
contrast it gives and it will be correspondingly better for planetary
viewing. Hence a 'scope of F11 will give better contrast than a same
sized 'scope that's F4. Since the F number is a ratio of the focal
length to the diameter of the primary, would using a barlow lens
effectively increase a 'scopes F number and hence contrast? Or in other
words, would a 20mm barlowed e/p be better for planetary work than a
10mm non-barlowed lens?

Sorry if this is me going waaaaaaay off kilter but it's something that's
puzzled me over the last few days.

Thanks,

Jim
--
AIM/iChat:JCAndrew2
"We deal in the moral equivalent of black holes, where the normal
laws of right and wrong break down; beyond those metaphysical
event horizons there exist ... special circumstances" - Use Of Weapons

"Jim" wrote in message
m...
Can someone answer a question for me? I want to see if I've understood a
few things.

As I understand it, the higher the F number of a 'scope, the more
contrast it gives and it will be correspondingly better for planetary
viewing. Hence a 'scope of F11 will give better contrast than a same
sized 'scope that's F4. Since the F number is a ratio of the focal
length to the diameter of the primary, would using a barlow lens
effectively increase a 'scopes F number and hence contrast? Or in other
words, would a 20mm barlowed e/p be better for planetary work than a
10mm non-barlowed lens?

Sorry if this is me going waaaaaaay off kilter but it's something that's
puzzled me over the last few days.


I would recommend using a scope designed for what you want to do, if only
because adding extra optics to the light path adds risk of optical
aberrations. Also, a long-focus telescope is much easier to design and make
to high image quality specs than a short-focus telescope (esp. off-axis).

In such situations as you describe, I would use the shorter FL eyepiece
rather than use a Barlow. My experience of most Barlows is that they
degrade the image slightly, add a bit of chromatic aberration, etc.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

"Jim" wrote in message
m...
Can someone answer a question for me? I want to see if I've understood a
few things.

As I understand it, the higher the F number of a 'scope, the more
contrast it gives and it will be correspondingly better for planetary
viewing. Hence a 'scope of F11 will give better contrast than a same
sized 'scope that's F4. Since the F number is a ratio of the focal
length to the diameter of the primary, would using a barlow lens
effectively increase a 'scopes F number and hence contrast? Or in other
words, would a 20mm barlowed e/p be better for planetary work than a
10mm non-barlowed lens?

Sorry if this is me going waaaaaaay off kilter but it's something that's
puzzled me over the last few days.


I would recommend using a scope designed for what you want to do, if only
because adding extra optics to the light path adds risk of optical
aberrations. Also, a long-focus telescope is much easier to design and make
to high image quality specs than a short-focus telescope (esp. off-axis).

In such situations as you describe, I would use the shorter FL eyepiece
rather than use a Barlow. My experience of most Barlows is that they
degrade the image slightly, add a bit of chromatic aberration, etc.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

Mike Dworetsky wrote:

I would recommend using a scope designed for what you want to do, if only
because adding extra optics to the light path adds risk of optical
aberrations. Also, a long-focus telescope is much easier to design and make
to high image quality specs than a short-focus telescope (esp. off-axis).

In such situations as you describe, I would use the shorter FL eyepiece
rather than use a Barlow. My experience of most Barlows is that they
degrade the image slightly, add a bit of chromatic aberration, etc.

Also, having given it a wee bit more thought, a Barlow doesn't double
the focal length of the 'scope - it doubles the focal length of the e/p,
yes? Hence it's not going to alter the F number at all. I'd need to whip
up an aperture mask to do that.

Thanks for the reply.

Jim
--
AIM/iChat:JCAndrew2
"We deal in the moral equivalent of black holes, where the normal
laws of right and wrong break down; beyond those metaphysical
event horizons there exist ... special circumstances" - Use Of Weapons

Mike Dworetsky wrote:

I would recommend using a scope designed for what you want to do, if only
because adding extra optics to the light path adds risk of optical
aberrations. Also, a long-focus telescope is much easier to design and make
to high image quality specs than a short-focus telescope (esp. off-axis).

In such situations as you describe, I would use the shorter FL eyepiece
rather than use a Barlow. My experience of most Barlows is that they
degrade the image slightly, add a bit of chromatic aberration, etc.

Also, having given it a wee bit more thought, a Barlow doesn't double
the focal length of the 'scope - it doubles the focal length of the e/p,
yes? Hence it's not going to alter the F number at all. I'd need to whip
up an aperture mask to do that.

Thanks for the reply.

Jim
--
AIM/iChat:JCAndrew2
"We deal in the moral equivalent of black holes, where the normal
laws of right and wrong break down; beyond those metaphysical
event horizons there exist ... special circumstances" - Use Of Weapons

On Sun, 10 Aug 2003 13:53:17 +0100, (Jim)
wrote:

Can someone answer a question for me? I want to see if I've understood a
few things.

As I understand it, the higher the F number of a 'scope, the more
contrast it gives and it will be correspondingly better for planetary
viewing. Hence a 'scope of F11 will give better contrast than a same
sized 'scope that's F4. Since the F number is a ratio of the focal
length to the diameter of the primary, would using a barlow lens
effectively increase a 'scopes F number and hence contrast? Or in other
words, would a 20mm barlowed e/p be better for planetary work than a
10mm non-barlowed lens?

Sorry if this is me going waaaaaaay off kilter but it's something that's
puzzled me over the last few days.

Thanks,

Jim

Jim, you are getting a bit confused :-)

Contrast is a function of the OTA design and also the quality of the
optics, not a function of the f-ratio (although increasing the
f-ratio may increase contrast through indirect means). Think of an f/8
APO refractor, an f/8 Newtonian, and an f/8 SCT. Contrast will be
highest for the refractor and lowest for the SCT - with the Newt
somewhere between. A lot of this has to do with the Newt and SCT
having large obstructions in the light path (secondary reflectors).
For reflectors, the smoothness of the reflecting surface is also very
important, a 'rough' surface (on the micro scale) causes light scatter
which decreases contrast. For lenses light scatter can occur at the
air/glass interface and anti-reflection coatings here can also improve
contrast. As a visual analogy you can use the Star Test where reduced
contrast can be seen as the shifting of light energy from the central
airey disk into the outer diffraction rings, APO refractors offer high
contrast and produce excellent star tests - pin-point stars and very
fine diffraction rings. A comparable view through an optically
excellent SCT will show brighter diffraction rings.

Using the Newt as an example, would increasing the f-ratio from from
f/5 to f/8 increase apparent contrast? Well yes it would - because of
two factors. Most importantly the size of the secondary required for
the f/8 scope could be significantly reduced, thus decreasing the
central obstruction. Secondly, in practical terms the f/8 optics are
easier to figure to extreme accuracy and so would tend to be of better
quality (you can, of course, pay enough money to overcome this
limitation and get any quality of optics you like!).

Finally, don't forget the role of the eyepiece. You want the least
number of air-glass surfaces possible, and the highest light
transmission. If you go to extremes the Tolles design has only two
air-glass surfaces, but most people tend to go for high-quality
Orthoscopics having just four. More complicated eyepieces (Radians for
example) have yet more glass elements but try to compensate (some
would say they fail) by using the best quality components and the best
anti-reflection coatings.

Oh, and a barlow really does increase the effective f/l of the primary
(objective or primary mirror). However, because it is yet another lens
to be inserted in the lightpath there is also an associated reduction
of contrast (and total light throughput) due to transmission and
reflective losses. Remember though, adding the barlow has *not*
reduced the size of the central obstruction (in a reflector) despite
increasing the apparent focal length. Again, quality of manufacture
can go some way to minimising transmission and reflective losses and a
TV Powermate will perform better than a cheaper model.

There are a whole bunch of caveats that apply here, you have hit on a
question that is argued over continuously.

ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

On Sun, 10 Aug 2003 13:53:17 +0100, (Jim)
wrote:

Can someone answer a question for me? I want to see if I've understood a
few things.

As I understand it, the higher the F number of a 'scope, the more
contrast it gives and it will be correspondingly better for planetary
viewing. Hence a 'scope of F11 will give better contrast than a same
sized 'scope that's F4. Since the F number is a ratio of the focal
length to the diameter of the primary, would using a barlow lens
effectively increase a 'scopes F number and hence contrast? Or in other
words, would a 20mm barlowed e/p be better for planetary work than a
10mm non-barlowed lens?

Sorry if this is me going waaaaaaay off kilter but it's something that's
puzzled me over the last few days.

Thanks,

Jim

Jim, you are getting a bit confused :-)

Contrast is a function of the OTA design and also the quality of the
optics, not a function of the f-ratio (although increasing the
f-ratio may increase contrast through indirect means). Think of an f/8
APO refractor, an f/8 Newtonian, and an f/8 SCT. Contrast will be
highest for the refractor and lowest for the SCT - with the Newt
somewhere between. A lot of this has to do with the Newt and SCT
having large obstructions in the light path (secondary reflectors).
For reflectors, the smoothness of the reflecting surface is also very
important, a 'rough' surface (on the micro scale) causes light scatter
which decreases contrast. For lenses light scatter can occur at the
air/glass interface and anti-reflection coatings here can also improve
contrast. As a visual analogy you can use the Star Test where reduced
contrast can be seen as the shifting of light energy from the central
airey disk into the outer diffraction rings, APO refractors offer high
contrast and produce excellent star tests - pin-point stars and very
fine diffraction rings. A comparable view through an optically
excellent SCT will show brighter diffraction rings.

Using the Newt as an example, would increasing the f-ratio from from
f/5 to f/8 increase apparent contrast? Well yes it would - because of
two factors. Most importantly the size of the secondary required for
the f/8 scope could be significantly reduced, thus decreasing the
central obstruction. Secondly, in practical terms the f/8 optics are
easier to figure to extreme accuracy and so would tend to be of better
quality (you can, of course, pay enough money to overcome this
limitation and get any quality of optics you like!).

Finally, don't forget the role of the eyepiece. You want the least
number of air-glass surfaces possible, and the highest light
transmission. If you go to extremes the Tolles design has only two
air-glass surfaces, but most people tend to go for high-quality
Orthoscopics having just four. More complicated eyepieces (Radians for
example) have yet more glass elements but try to compensate (some
would say they fail) by using the best quality components and the best
anti-reflection coatings.

Oh, and a barlow really does increase the effective f/l of the primary
(objective or primary mirror). However, because it is yet another lens
to be inserted in the lightpath there is also an associated reduction
of contrast (and total light throughput) due to transmission and
reflective losses. Remember though, adding the barlow has *not*
reduced the size of the central obstruction (in a reflector) despite
increasing the apparent focal length. Again, quality of manufacture
can go some way to minimising transmission and reflective losses and a
TV Powermate will perform better than a cheaper model.

There are a whole bunch of caveats that apply here, you have hit on a
question that is argued over continuously.

ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

ChrisH wrote:

Jim, you are getting a bit confused :-)

I suspected as such, hence the subject title :-)

Contrast is a function of the OTA design and also the quality of the
optics, not a function of the f-ratio (although increasing the
f-ratio may increase contrast through indirect means).

I'm curious in that case - what difference does the f-ratio actually
make then? My understanding (ha!) is that it has something to do with
the angle of the light cone, ie the smaller the number the shorter the
cone. Is that right? In which case, what effect does that have on the
image?

Jim
--
AIM/iChat:JCAndrew2
"We deal in the moral equivalent of black holes, where the normal
laws of right and wrong break down; beyond those metaphysical
event horizons there exist ... special circumstances" - Use Of Weapons

ChrisH wrote:

Jim, you are getting a bit confused :-)

I suspected as such, hence the subject title :-)

Contrast is a function of the OTA design and also the quality of the
optics, not a function of the f-ratio (although increasing the
f-ratio may increase contrast through indirect means).

I'm curious in that case - what difference does the f-ratio actually
make then? My understanding (ha!) is that it has something to do with
the angle of the light cone, ie the smaller the number the shorter the
cone. Is that right? In which case, what effect does that have on the
image?

Jim
--
AIM/iChat:JCAndrew2
"We deal in the moral equivalent of black holes, where the normal
laws of right and wrong break down; beyond those metaphysical
event horizons there exist ... special circumstances" - Use Of Weapons

"Jim" wrote in message
...
Mike Dworetsky wrote:

I would recommend using a scope designed for what you want to do, if
only
because adding extra optics to the light path adds risk of optical
aberrations. Also, a long-focus telescope is much easier to design and
make
to high image quality specs than a short-focus telescope (esp.
off-axis).

In such situations as you describe, I would use the shorter FL eyepiece
rather than use a Barlow. My experience of most Barlows is that they
degrade the image slightly, add a bit of chromatic aberration, etc.

Also, having given it a wee bit more thought, a Barlow doesn't double
the focal length of the 'scope - it doubles the focal length of the e/p,
yes? Hence it's not going to alter the F number at all. I'd need to whip
up an aperture mask to do that.

Thanks for the reply.

Jim
--

Actually, it effectively halves the FL of the eyepiece. (Twice the
magnification.)

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)