Top lenses versus top telescopes for telephoto work

IMO, a good apochromatic telescope while not possessing a fast f-ratio
will beat any camera lens when it comes to long tele work. For
instance, compare a Takahashi FSQ106 telescope against a 500mm Nikon
both operating around f5, the Takahashi scope shooting wide open, it
will easily beat the Nikon using a high resolution digital camera body.
The difference would be even more apparent using a Canon 1DsMkII with
its 16 megapixels. The Tak also supports 6x7 medium format. These
telescopes range in price from entry-level "semi-apos" costing $300 for
a 360mm f5.5 to over $4000 or more for a colour free 500mm-800mm f4-6
that can be "sped-up" via dedicated compressor lenses.

The reason these scopes have to be so good optically is that shooting
celestial objects with stars in the field is a severe test of optics.
Any residual off-axis aberrations show up in the star (point source)
images. Most of us have seen the flary, blurred out images at the edge
of a field of lights seen when night shots are taken with camera lenses
used wide open. These scopes can't allow that.

The reason these instruments haven't been made use of by more
photographers is that they don't operate (no IS, focus radically
different) like camera lenses. However, for wildlife photogs who use
blinds and wait for the opportunity to get good shots, they would be
ideal.

Interesting thing; In 1977, Modern Photography tested a 550mm f5.5
telescope put out by an American company called, "TeleVue." They put
it up against other long teles from Nikon, Canon, Leica, etc. It beat
them all. In fact, its MTF figures where so high they looked like those
from a macro lens. That telescope wouldn't compare to what is avalable
today from TeleVue or other makers of high end scopes.

The closest these instruments have come to integrating with the general
photographic community is though "Birders" who often use high-end
spotting scopes that they can attach their cameras. However,
"digiscoping" using P&S cameras and terrestial spotting scopes is not
the same as using a DSLR on a non-terrestrial telescope operating as a
"prime" lens.

The apochromatic telescope (otherwise known as an apochromatic
refractor) made its debut around the late 1970s with the introduction
of brands like the American Astro-Physics, Takahashi, Nikon and Pentax.
Nikon left the business some time ago and it's telescopes are highly
sought-after collector's items. Pentax is about to enter the American
market with its apos.
They make a 400mm f4 called the 100 SDUF II.

You will never see f2.8 or f2 apo telescopes, the need for diffraction
limited performance negates that possibility. But at f4-8, they
represent the pinnacle of imaging systems.

 

"Rich" <> writes:
>IMO, a good apochromatic telescope while not possessing a fast f-ratio
>will beat any camera lens when it comes to long tele work.

>The reason these scopes have to be so good optically is that shooting
>celestial objects with stars in the field is a severe test of optics.
>Any residual off-axis aberrations show up in the star (point source)
>images. Most of us have seen the flary, blurred out images at the edge
>of a field of lights seen when night shots are taken with camera lenses
>used wide open. These scopes can't allow that.

That's part of it. Another factor is that telescope objectives are
normally used as part of a system for visual observing, and the
magnification is changed by changing eyepieces. A typical good
refractor has a 2 inch focuser, and might be used with a very low-power
eyepiece (e.g. 55 mm FL) at about 10X magnification. At that power, it
is expected to produce an image nearly 2 inches in diameter with good
quality.

Or it may be used with a series of successively shorter focal length
eyepieces to give higher magnification. A 4 inch diameter objective is
capable of 200X useful magnification if the objective is diffraction
limited, and refractors are *expected* to be diffraction limited, at
least in the central portion of the image that would be visible at such
high power. This is somewhat comparable to finding a zoom lens that
performs well over a 20X zoom range.

>The reason these instruments haven't been made use of by more
>photographers is that they don't operate (no IS, focus radically
>different) like camera lenses. However, for wildlife photogs who use
>blinds and wait for the opportunity to get good shots, they would be
>ideal.

I've used a TeleVue Oracle on a film camera. This is a 3 inch diameter
"semi apo" - not as good as the 4" TeleVue apo refractor referred to,
but also a lot cheaper. It was sharp, but it's also quite a pain to use
compared to a camera lens: (These apply to most astro telescopes used
as camera lenses)

1. Focusing is by rack and pinion, which is really too quick to get
precise focus with a camera. (It's fine for telescope use, where
your eye itself provides some degree of final fine focus).

2. There's no auto diaphragm. In fact, there's no diaphragm at all,
so you're always shooting wide open.

3. The optical tube is longer and more awkward than a camera lens
of comparable focal length.

>Interesting thing; In 1977, Modern Photography tested a 550mm f5.5
>telescope put out by an American company called, "TeleVue." They put
>it up against other long teles from Nikon, Canon, Leica, etc. It beat
>them all. In fact, its MTF figures where so high they looked like those
>from a macro lens. That telescope wouldn't compare to what is avalable
>today from TeleVue or other makers of high end scopes.

Dave

 

There was a craze about 25 years ago for using telescopes with camera
adapters in place of a long telephoto lens. Two problems limited the
popularity and the craze died out.

First, as you say, relative aperture was limited, so exposures were
slower in low light. This was not too big a deal, as these things HAD
to be used with tripods anyway. Secondly, they did not have adjustable
apertures- exposures HAD to be set by varying shutter speed only.

This later problem meant that the lenses could not be used with
automatic exposure, which was getting popular at the time. While auto
focus was not around yet, AE was, and many folks had already come to
depend on it by then.



Rich wrote:
> IMO, a good apochromatic telescope while not possessing a fast f-ratio
> will beat any camera lens when it comes to long tele work. For
> instance, compare a Takahashi FSQ106 telescope against a 500mm Nikon
> both operating around f5, the Takahashi scope shooting wide open, it
> will easily beat the Nikon using a high resolution digital camera body.
> The difference would be even more apparent using a Canon 1DsMkII with
> its 16 megapixels. The Tak also supports 6x7 medium format. These
> telescopes range in price from entry-level "semi-apos" costing $300 for
> a 360mm f5.5 to over $4000 or more for a colour free 500mm-800mm f4-6

 

In article <>,
"Don Stauffer in Minnesota" <> wrote:

> There was a craze about 25 years ago for using telescopes with camera
> adapters in place of a long telephoto lens. Two problems limited the
> popularity and the craze died out.
>
> First, as you say, relative aperture was limited, so exposures were
> slower in low light. This was not too big a deal, as these things HAD
> to be used with tripods anyway. Secondly, they did not have adjustable
> apertures- exposures HAD to be set by varying shutter speed only.
>
> This later problem meant that the lenses could not be used with
> automatic exposure, which was getting popular at the time. While auto
> focus was not around yet, AE was, and many folks had already come to
> depend on it by then.


You present a good summary. I have been using these lenses for a good 25
years now, first with film, now with digital.

With P&S digicams, auto focus and exposure works fine- just mount the
camera lens as close to the eyepiece as possible. The Coolpix series of
cameras with the 28mm threads on the lens (9xx, 4500) are the best way.

With SLR's, yes- you must manually adjust the shutter and focus and the
aperture is set by the exit pupil of the telescope. But cameras come
with light meters so I don't see the big deal. As you say, one must use
a tripod and set up the subject carefully, have a lot of light and
usually a remote shutter release. And often the photos will not be
tack-sharp.

But when you get one like this, it's all worth it:

http://www.netaxs.com/~mhmyers/cdjpgs/eagle1L.jpg

--
m-m

 

In article <>,
M-M <> wrote:

> when you get one like this, it's all worth it:
>
> http://www.netaxs.com/~mhmyers/cdjpgs/eagle1L.jpg


forgot to mention...

That photo was taken with a $200 Swift Telemaster spotting scope and a
Pentax K1000.
--
m-m

 

Rich wrote:
> IMO, a good apochromatic telescope while not possessing a fast f-ratio
> will beat any camera lens when it comes to long tele work. For
> instance, compare a Takahashi FSQ106 telescope against a 500mm Nikon
> both operating around f5, the Takahashi scope shooting wide open, it
> will easily beat the Nikon using a high resolution digital camera body.

While I agree that telescopes can beat out telephoto lenses,
they do so only at near infinity focus. Camera lenses are
a compromise in giving good image quality over a larger range
of subject distances. A apochromatic telescope will not do as well
on a closer frame filling subject, like birds. Then, in real
world situations, the manual focus, fixed aperture telescopes
would give poor overall performance "to get the image" than
modern autofocus lenses. And even so, the super telephoto
lenses are VERY close in quality to apochromatic telescopes, and
I bet it would be hard to tell the difference of say a FSQ106
versus a Canon 500 mm f/4 L at f/5.6. For example, look at the star
images on this photo of M31 taken with a Canon 500 mm f/4 at f/4:
http://www.clarkvision.com/gallerie...1/web/m31.c07.10.2005.Avg23-75s-v1.6-700.html
At f/5.6, star images of most of the field are barely over on pixel,
being limited by the blur filter of the camera.

M-M wrote:
> In article <>,
> "Don Stauffer in Minnesota" <> wrote:
>
>
>>There was a craze about 25 years ago for using telescopes with camera
>>adapters in place of a long telephoto lens. Two problems limited the
>>popularity and the craze died out.
>>
>>First, as you say, relative aperture was limited, so exposures were
>>slower in low light. This was not too big a deal, as these things HAD
>>to be used with tripods anyway. Secondly, they did not have adjustable
>>apertures- exposures HAD to be set by varying shutter speed only.
>>
>>This later problem meant that the lenses could not be used with
>>automatic exposure, which was getting popular at the time. While auto
>>focus was not around yet, AE was, and many folks had already come to
>>depend on it by then.
>
> You present a good summary. I have been using these lenses for a good 25
> years now, first with film, now with digital.
>
> With P&S digicams, auto focus and exposure works fine- just mount the
> camera lens as close to the eyepiece as possible. The Coolpix series of
> cameras with the 28mm threads on the lens (9xx, 4500) are the best way.
>
> With SLR's, yes- you must manually adjust the shutter and focus and the
> aperture is set by the exit pupil of the telescope. But cameras come
> with light meters so I don't see the big deal. As you say, one must use
> a tripod and set up the subject carefully, have a lot of light and
> usually a remote shutter release. And often the photos will not be
> tack-sharp.
>
> But when you get one like this, it's all worth it:
>
> http://www.netaxs.com/~mhmyers/cdjpgs/eagle1L.jpg

I agree that you can get nice images with a telescope, but the
piggyback through the lens adds a lot of flair and depends a lot
on the quality of the eyepiece and matching of field curvature
between camera lens and eyepiece. Compare this eagle image with
a super telephoto lens (costing several times more):
http://www.clarkvision.com/galleries/gallery.bird/web/eagle.c09.11.2004.JZ3F4717.b-700.html

The advantage of the super telephoto is the speed, autofocus, and
flexibility to get action images that would be difficult with
a telescope. (I own several telescopes, up to 12.5 inches aperture
Dobsonian and a 8-inch home-made Cassegrain). Before spending
$5,700 on a 500 mm f/4 I tried using a telescope, e.g. 900 mm f/6
Newtonian. I could get great images, but too bulky and close
focusing was a problem, and the closer the focus, the more
the image quality dropped.

Roger

 

Dave Martindale wrote:
> "Rich" <> writes:
> >IMO, a good apochromatic telescope while not possessing a fast f-ratio
> >will beat any camera lens when it comes to long tele work.
>
> >The reason these scopes have to be so good optically is that shooting
> >celestial objects with stars in the field is a severe test of optics.
> >Any residual off-axis aberrations show up in the star (point source)
> >images. Most of us have seen the flary, blurred out images at the edge
> >of a field of lights seen when night shots are taken with camera lenses
> >used wide open. These scopes can't allow that.
>
> That's part of it. Another factor is that telescope objectives are
> normally used as part of a system for visual observing, and the
> magnification is changed by changing eyepieces. A typical good
> refractor has a 2 inch focuser, and might be used with a very low-power
> eyepiece (e.g. 55 mm FL) at about 10X magnification. At that power, it
> is expected to produce an image nearly 2 inches in diameter with good
> quality.
>
> Or it may be used with a series of successively shorter focal length
> eyepieces to give higher magnification. A 4 inch diameter objective is
> capable of 200X useful magnification if the objective is diffraction
> limited, and refractors are *expected* to be diffraction limited, at
> least in the central portion of the image that would be visible at such
> high power. This is somewhat comparable to finding a zoom lens that
> performs well over a 20X zoom range.
>
> >The reason these instruments haven't been made use of by more
> >photographers is that they don't operate (no IS, focus radically
> >different) like camera lenses. However, for wildlife photogs who use
> >blinds and wait for the opportunity to get good shots, they would be
> >ideal.
>
> I've used a TeleVue Oracle on a film camera. This is a 3 inch diameter
> "semi apo" - not as good as the 4" TeleVue apo refractor referred to,
> but also a lot cheaper. It was sharp, but it's also quite a pain to use
> compared to a camera lens: (These apply to most astro telescopes used
> as camera lenses)
>
> 1. Focusing is by rack and pinion, which is really too quick to get
> precise focus with a camera. (It's fine for telescope use, where
> your eye itself provides some degree of final fine focus).

The Oracle is a 16 year old telescope. Most apos now use two-speed
focusers for course and fine focus so that issue is resolved. But they
can't focus fast like a collar focusing lens or an AF lens.

>
> 2. There's no auto diaphragm. In fact, there's no diaphragm at all,
> so you're always shooting wide open.

That is something they could implement, but most observers want full
aperture.
>
> 3. The optical tube is longer and more awkward than a camera lens
> of comparable focal length.

Yes, these kinds of scopes don't incorporate (generally) elements
further down the tube to increase focal lengths, but generally rely on
the main lens (objective) to provide the focal length. Additionally,
few are below f5 (Pentax makes a 400mm f4 photographic model
and Takahashi makes a Sky 90 which is a 450mm f5). Both are fairly
compact.

 

Roger N. Clark (change username to rnclark) wrote:
> Rich wrote:
> > IMO, a good apochromatic telescope while not possessing a fast f-ratio
> > will beat any camera lens when it comes to long tele work. For
> > instance, compare a Takahashi FSQ106 telescope against a 500mm Nikon
> > both operating around f5, the Takahashi scope shooting wide open, it
> > will easily beat the Nikon using a high resolution digital camera body.
>
> While I agree that telescopes can beat out telephoto lenses,
> they do so only at near infinity focus. Camera lenses are
> a compromise in giving good image quality over a larger range
> of subject distances. A apochromatic telescope will not do as well
> on a closer frame filling subject, like birds.

In a situation like that, what you can do is stop down the scope
slightly to eliminate an
additional spherical aberration that might be encountered due to the
less than infinity focus.
But also, this issue might not be a major one for a small (80-100mm
aperture scope) and then only at very short distances. Questar
Corporation used to supply a 1/2" wide diaphram to stop down their 90mm
wide mirror-lens scopes when using them under object distances of 25
feet or less.

Then, in real
> world situations, the manual focus, fixed aperture telescopes
> would give poor overall performance "to get the image" than
> modern autofocus lenses. And even so, the super telephoto
> lenses are VERY close in quality to apochromatic telescopes, and
> I bet it would be hard to tell the difference of say a FSQ106
> versus a Canon 500 mm f/4 L at f/5.6. For example, look at the star
> images on this photo of M31 taken with a Canon 500 mm f/4 at f/4:
> http://www.clarkvision.com/gallerie...1/web/m31.c07.10.2005.Avg23-75s-v1.6-700.html
> At f/5.6, star images of most of the field are barely over on pixel,
> being limited by the blur filter of the camera.

Good image, because these lenses are used only at prime focus, and are
close enough to diffraction limited to avoid any visible issues at that
kind of magnification. But, unlike the camera lens, the telescope's
focal length can be increased significantly without any noticeable loss
of correction, contrast, etc. In-case the target needed this.
Downside being the increase in focal ratio (speed) of course.
>
> M-M wrote:
> > In article <>,
> > "Don Stauffer in Minnesota" <> wrote:
> >
> >
> >>There was a craze about 25 years ago for using telescopes with camera
> >>adapters in place of a long telephoto lens. Two problems limited the
> >>popularity and the craze died out.
> >>
> >>First, as you say, relative aperture was limited, so exposures were
> >>slower in low light. This was not too big a deal, as these things HAD
> >>to be used with tripods anyway. Secondly, they did not have adjustable
> >>apertures- exposures HAD to be set by varying shutter speed only.
> >>
> >>This later problem meant that the lenses could not be used with
> >>automatic exposure, which was getting popular at the time. While auto
> >>focus was not around yet, AE was, and many folks had already come to
> >>depend on it by then.
> >
> > You present a good summary. I have been using these lenses for a good 25
> > years now, first with film, now with digital.
> >
> > With P&S digicams, auto focus and exposure works fine- just mount the
> > camera lens as close to the eyepiece as possible. The Coolpix series of
> > cameras with the 28mm threads on the lens (9xx, 4500) are the best way.
> >
> > With SLR's, yes- you must manually adjust the shutter and focus and the
> > aperture is set by the exit pupil of the telescope. But cameras come
> > with light meters so I don't see the big deal. As you say, one must use
> > a tripod and set up the subject carefully, have a lot of light and
> > usually a remote shutter release. And often the photos will not be
> > tack-sharp.
> >
> > But when you get one like this, it's all worth it:
> >
> > http://www.netaxs.com/~mhmyers/cdjpgs/eagle1L.jpg
>
> I agree that you can get nice images with a telescope, but the
> piggyback through the lens adds a lot of flair and depends a lot
> on the quality of the eyepiece and matching of field curvature
> between camera lens and eyepiece. Compare this eagle image with
> a super telephoto lens (costing several times more):
> http://www.clarkvision.com/galleries/gallery.bird/web/eagle.c09.11.2004.JZ3F4717.b-700.html
>
> The advantage of the super telephoto is the speed, autofocus, and
> flexibility to get action images that would be difficult with
> a telescope. (I own several telescopes, up to 12.5 inches aperture
> Dobsonian and a 8-inch home-made Cassegrain). Before spending
> $5,700 on a 500 mm f/4 I tried using a telescope, e.g. 900 mm f/6
> Newtonian. I could get great images, but too bulky and close
> focusing was a problem, and the closer the focus, the more
> the image quality dropped.
>
> Roger

Yes, I can't see someone hauling a Newtonian scope into the field to
shoot birds, but it would make for an interesting conversation amongst
the others there!
But you can buy a refractor from TeleVue or TMB with an f5 focal ratio
and 500mm focal length that would make an interesing comparison.
http://www.televue.com/engine/page.asp?ID=297

 

"Don Stauffer in Minnesota" <> writes:

>Secondly, they did not have adjustable
>apertures- exposures HAD to be set by varying shutter speed only.

>This later problem meant that the lenses could not be used with
>automatic exposure, which was getting popular at the time. While auto
>focus was not around yet, AE was, and many folks had already come to
>depend on it by then.

That depends on the camera. In aperture priority mode, you set the
aperture and the camera determines shutter speed from the light meter
reading and how much it thinks the lens will stop down (if the meter
reading is taken before exposure). To work with a lens that has no
diaphragm, you just have to convince the metering system that the
shooting aperture is the same as the viewing aperture.

I know my old Minolta X-570 could do it. I'm sure the X-700 would too,
since it was a superset of the X-570. These cameras actually metered
off the film after the lens had stopped down to taking aperture.

Dave

PS: the "meter at shooting aperture" behaviour once caused me some
puzzlement for a while. I once shot a bunch of photos at a world's
fair using a lens whose diaphragm was stuck open. I didn't figure this
out until well after the fair. The camera noticed the bright "stopped
down" image and used an actual shutter speed that was much higher than
what it indicated it was going to use, based on the aperture ring
position. In the end, most of the exposures were pretty good, not
grossly overexposed like you'd expect from a stuck diaphragm. But I was
shooting with far less DOF than I expected to have, and some subjects
ended up out of focus because of that. I didn't understand why until I
noticed the stuck aperture; the camera doesn't record the actual shutter
speed it used anywhere.

Dave

 

Rich wrote:

> Roger N. Clark (change username to rnclark) wrote:
>
>>Rich wrote:
>> > IMO, a good apochromatic telescope while not possessing a fast f-ratio
>> > will beat any camera lens when it comes to long tele work. For
>> > instance, compare a Takahashi FSQ106 telescope against a 500mm Nikon
>> > both operating around f5, the Takahashi scope shooting wide open, it
>> > will easily beat the Nikon using a high resolution digital camera body.
>>
>>While I agree that telescopes can beat out telephoto lenses,
>>they do so only at near infinity focus. Camera lenses are
>>a compromise in giving good image quality over a larger range
>>of subject distances. A apochromatic telescope will not do as well
>>on a closer frame filling subject, like birds.
>
> In a situation like that, what you can do is stop down the scope
> slightly to eliminate an
> additional spherical aberration that might be encountered due to the
> less than infinity focus.

But stopping down is a problem because you lose light and then
can not capture action. The point of the super telephotos
is to provide great image quality wide open over a large
range of distances.

> But also, this issue might not be a major one for a small (80-100mm
> aperture scope) and then only at very short distances. Questar
> Corporation used to supply a 1/2" wide diaphram to stop down their 90mm
> wide mirror-lens scopes when using them under object distances of 25
> feet or less.

But Questars were optically too slow for wildlife action photography.
>
> Then, in real
>
>>world situations, the manual focus, fixed aperture telescopes
>>would give poor overall performance "to get the image" than
>>modern autofocus lenses. And even so, the super telephoto
>>lenses are VERY close in quality to apochromatic telescopes, and
>>I bet it would be hard to tell the difference of say a FSQ106
>>versus a Canon 500 mm f/4 L at f/5.6. For example, look at the star
>>images on this photo of M31 taken with a Canon 500 mm f/4 at f/4:
>>http://www.clarkvision.com/gallerie...1/web/m31.c07.10.2005.Avg23-75s-v1.6-700.html
>>At f/5.6, star images of most of the field are barely over on pixel,
>>being limited by the blur filter of the camera.
>
> Good image, because these lenses are used only at prime focus, and are
> close enough to diffraction limited to avoid any visible issues at that
> kind of magnification. But, unlike the camera lens, the telescope's
> focal length can be increased significantly without any noticeable loss
> of correction, contrast, etc. In-case the target needed this.
> Downside being the increase in focal ratio (speed) of course.

Super telephotos are designed to still give good image quality
with teleconverters. Here is a star field with a 500 f/4
and 1.4x TC:
http://www.clarkvision.com/gallerie...eb/m45-700MM-8534-8561_C16B-add27-v3-800.html
The stars a still very good.

> Yes, I can't see someone hauling a Newtonian scope into the field to
> shoot birds, but it would make for an interesting conversation amongst
> the others there!
> But you can buy a refractor from TeleVue or TMB with an f5 focal ratio
> and 500mm focal length that would make an interesing comparison.
> http://www.televue.com/engine/page.asp?ID=297

I have compared image quality of some of the apochromatic telescopes
with my 500 f/4. The ones that have great performance, comparable
to the 500 f/4 L telephoto, are, guess what, as expensive as the
500 mm f/4 lens. You get what you pay for. The reason I looked
into this is that I have one undesirable problem with the
500 f/4: the aluminum tube changes length during long night
exposures (e.g. over 20 minutes). A PLASTIC TUBE WOULD BE BETTER!
;-) But for wildlife photography in rough environments, the
metal tube is great.

Roger

 

Roger N. Clark (change username to rnclark) wrote:
> Rich wrote:
>
> > Roger N. Clark (change username to rnclark) wrote:
> >
> >>Rich wrote:
> >> > IMO, a good apochromatic telescope while not possessing a fast f-ratio
> >> > will beat any camera lens when it comes to long tele work. For
> >> > instance, compare a Takahashi FSQ106 telescope against a 500mm Nikon
> >> > both operating around f5, the Takahashi scope shooting wide open, it
> >> > will easily beat the Nikon using a high resolution digital camera body.
> >>
> >>While I agree that telescopes can beat out telephoto lenses,
> >>they do so only at near infinity focus. Camera lenses are
> >>a compromise in giving good image quality over a larger range
> >>of subject distances. A apochromatic telescope will not do as well
> >>on a closer frame filling subject, like birds.
> >
> > In a situation like that, what you can do is stop down the scope
> > slightly to eliminate an
> > additional spherical aberration that might be encountered due to the
> > less than infinity focus.
>
> But stopping down is a problem because you lose light and then
> can not capture action. The point of the super telephotos
> is to provide great image quality wide open over a large
> range of distances.

True, in terms of speed, there are no telescopes to match a fast
telephoto (f2.8, etc).
>
> > But also, this issue might not be a major one for a small (80-100mm
> > aperture scope) and then only at very short distances. Questar
> > Corporation used to supply a 1/2" wide diaphram to stop down their 90mm
> > wide mirror-lens scopes when using them under object distances of 25
> > feet or less.
>
> But Questars were optically too slow for wildlife action photography.

And yet people did use them for that. Questar's Field Model, they even
had one with
electric high speed focusing. But, were talking 1350mm at a minimum
for the 90mm model
so that is a long tele. However, Questar's 700mm mirror camera lens
was the best mirror lens of all time, in terms of definitiion and its
speed was f8 and it wasn't a telescope.

> >
> > Then, in real
> >
> >>world situations, the manual focus, fixed aperture telescopes
> >>would give poor overall performance "to get the image" than
> >>modern autofocus lenses. And even so, the super telephoto
> >>lenses are VERY close in quality to apochromatic telescopes, and
> >>I bet it would be hard to tell the difference of say a FSQ106
> >>versus a Canon 500 mm f/4 L at f/5.6. For example, look at the star
> >>images on this photo of M31 taken with a Canon 500 mm f/4 at f/4:
> >>http://www.clarkvision.com/gallerie...1/web/m31.c07.10.2005.Avg23-75s-v1.6-700.html
> >>At f/5.6, star images of most of the field are barely over on pixel,
> >>being limited by the blur filter of the camera.
> >
> > Good image, because these lenses are used only at prime focus, and are
> > close enough to diffraction limited to avoid any visible issues at that
> > kind of magnification. But, unlike the camera lens, the telescope's
> > focal length can be increased significantly without any noticeable loss
> > of correction, contrast, etc. In-case the target needed this.
> > Downside being the increase in focal ratio (speed) of course.
>
> Super telephotos are designed to still give good image quality
> with teleconverters. Here is a star field with a 500 f/4
> and 1.4x TC:
> http://www.clarkvision.com/gallerie...eb/m45-700MM-8534-8561_C16B-add27-v3-800.html
> The stars a still very good.

At 700mm that is pretty decent.

>
> > Yes, I can't see someone hauling a Newtonian scope into the field to
> > shoot birds, but it would make for an interesting conversation amongst
> > the others there!
> > But you can buy a refractor from TeleVue or TMB with an f5 focal ratio
> > and 500mm focal length that would make an interesing comparison.
> > http://www.televue.com/engine/page.asp?ID=297
>
> I have compared image quality of some of the apochromatic telescopes
> with my 500 f/4. The ones that have great performance, comparable
> to the 500 f/4 L telephoto, are, guess what, as expensive as the
> 500 mm f/4 lens.

Apochromatic telescopes are generally considered on a "price per inch
of aperture" basis.
So, a typical high-end one costs over $1000 per inch. A TeleVue 500mm
f5 is about $3800.00. Some cost more, some less. TMB now has a 130mm
aperture "signature" series that costs about $3800 for a 780mm f6 scope
with superb optics, around $750/inch.
A bargain today. Pentax's SDUF costs $3000 for a 400mm f4 lens and it
will support a medium format film size. Inexpensive apos (from China)
run about $100-$400 inch depending on the scope size.

You get what you pay for. The reason I looked
> into this is that I have one undesirable problem with the
> 500 f/4: the aluminum tube changes length during long night
> exposures (e.g. over 20 minutes). A PLASTIC TUBE WOULD BE BETTER!

Not really, the plastic would actually contract more as it got cold, it
would just take alot longer to do it. In other words, it would
probably not even stabilize during the night.

> ;-) But for wildlife photography in rough environments, the
> metal tube is great.

Alot of photogs seem to tape or wrap their lenses. PBS had a guy on
shooting Yellowstone in winter. He took his Nikon 500mm and stuck it
face down in the snow(!) while he used another lens on his camera!
But, the lens cap was on it.