T-mount questions

In article ,
RichA wrote:

Just avoid the Opteka refractive lenses, unless you like working with
long, LONG focal ratios. Stick to the mirror lenses, then you have at
least a hope of getting something decent from it. Honestly, though,
go to Orion Telescopes and look at their Maksutov mirror-lens tube
assemblies and avoid the Opteka stuff.

Are you referring to Orion Telescopes, or do they also make a mirror lens
intended specifically for photography? I have seen the phrase "Orion
mirror lens" used elsewhere, but I can't find any such thing.

--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

On Jul 10, 7:46*am, William Hamblen
wrote:
On 2012-07-10, wrote:

On Jul 7, 11:43*am, (Paul Ciszek) wrote:

But the difference has
to be important, as the different adapters are going to be placing the
focal plane of the camera at different distances from the optics.

Making up for the "different distances" is done by focusing the camera
just as you would for any lens.

The photographic t-mount to camera adapter puts the front face of the
adapter 55 mm from the focal plane of the camera. *A simple 1-1/4"
to t-mount adapter for the telescope adds a few millimeters to that.
Most small refractors have enough back focus to work with that. *Most
newtonian reflectors do not. *Schmidt-cassegrainian telescopes have more
than enough back focus, and the manufacturers supply camera adapters that
are designed to work with their telescopes. *"Back focus" is the distance
from the end of the telescope to the focal plane of the telescope. *To get
your camera in focus the focal plane of the telescope has to coincide
with the focal plane of the camera. *If the camera focuses too far out
you can easily add an extension. *If the camera focuses too close in and
you run out of back focus you have to modify the telescope by shortening
the tube on a refractor or moving the primary mirror on a newtonian.

http://www.telescopeadapters.com/index.htm?TMINUS.htm

On Jul 10, 12:46*pm, (Paul Ciszek) wrote:
In article ,

RichA wrote:

Just avoid the Opteka refractive lenses, unless you like working with
long, LONG focal ratios. *Stick to the mirror lenses, then you have at
least a hope of getting something decent from it. *Honestly, though,
go to Orion Telescopes and look at their Maksutov mirror-lens tube
assemblies and avoid the Opteka stuff.

Are you referring to Orion Telescopes, or do they also make a mirror lens
intended specifically for photography? *I have seen the phrase "Orion
mirror lens" used elsewhere, but I can't find any such thing.

What kind of camera do you have? Specific model please. There are
short t-rings for DSLR and long t-rings for MIL cameras.

In article ,
wrote:
On Jul 10, 12:46*pm, (Paul Ciszek) wrote:
In article
,

RichA wrote:

Just avoid the Opteka refractive lenses, unless you like working with
long, LONG focal ratios. *Stick to the mirror lenses, then you have at
least a hope of getting something decent from it. *Honestly, though,
go to Orion Telescopes and look at their Maksutov mirror-lens tube
assemblies and avoid the Opteka stuff.

Are you referring to Orion Telescopes, or do they also make a mirror lens
intended specifically for photography? *I have seen the phrase "Orion
mirror lens" used elsewhere, but I can't find any such thing.

What kind of camera do you have? Specific model please. There are
short t-rings for DSLR and long t-rings for MIL cameras.

My Olympus OM-D E-M5 is a micro-four-thirds system camera. Allegedly,
the geometry of any micro-four-thirds camera should be the same.

--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

In article ,
RichA wrote:

Just avoid the Opteka refractive lenses, unless you like working with
long, LONG focal ratios. Stick to the mirror lenses, then you have at
least a hope of getting something decent from it. Honestly, though,
go to Orion Telescopes and look at their Maksutov mirror-lens tube
assemblies and avoid the Opteka stuff.

These telescopes:

http://www.telescope.com/Telescopes/...es/pc/1/14.uts

....are considerably longer focal length than what I had in mind
originally. I would like to be able to, for example, capture the
entire disk of the moon. Now, is there such a thing as a "collapser"
that can be used to decrease the magnification and reduce the f-number
of a telescope like this? I certainly wouldn't mind having a 1250mm
*and* a 625mm "lens". Orion sells something they call a "focal reducer",
but not for these scopes.

--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

On Wed, 11 Jul 2012 04:06:54 +0000 (UTC), (Paul
Ciszek) wrote:

These telescopes:

http://www.telescope.com/Telescopes/...es/pc/1/14.uts

...are considerably longer focal length than what I had in mind
originally. I would like to be able to, for example, capture the
entire disk of the moon. Now, is there such a thing as a "collapser"
that can be used to decrease the magnification and reduce the f-number
of a telescope like this? I certainly wouldn't mind having a 1250mm
*and* a 625mm "lens". Orion sells something they call a "focal reducer",
but not for these scopes.

Yes, you need a focal reducer. They are positive lenses that do just
the opposite of what barlows (negative lenses) do. A focal reducer
reduces the effective focal length of a telescope. There are generic
focal reducers, although many are made for specific scopes and correct
certain aberrations like field curvature or coma as well as reducing
the focal length.

BTW, with a 1250 mm lens, the Moon makes an image 11 mm across, which
will fit comfortably on the sensor of many cameras, including all
DSLRs.

On 11/07/2012 05:06, Paul Ciszek wrote:
In article ,
RichA wrote:

Just avoid the Opteka refractive lenses, unless you like working with
long, LONG focal ratios. Stick to the mirror lenses, then you have at
least a hope of getting something decent from it. Honestly, though,
go to Orion Telescopes and look at their Maksutov mirror-lens tube
assemblies and avoid the Opteka stuff.

These telescopes:

http://www.telescope.com/Telescopes/...es/pc/1/14.uts

...are considerably longer focal length than what I had in mind
originally. I would like to be able to, for example, capture the
entire disk of the moon. Now, is there such a thing as a "collapser"

A useful back of the envelope figure is that the moon subtends an angle
of just under 1/100 radian (1%) which means that a given focal length of
lens will form a prime focus image that is 1/100 of its focal length.

A 2m focal length is nice for 35mm slide and 1.6m is good for most DSLRs
(which have a nominal 1.4x ish scale factor) on the moon.

So you can do the sums to decide what will fit on your sensor.

that can be used to decrease the magnification and reduce the f-number
of a telescope like this? I certainly wouldn't mind having a 1250mm
*and* a 625mm "lens". Orion sells something they call a "focal reducer",
but not for these scopes.

To use a focal reducer the input light cone has to be larger than the
sensor area by the appropriate factor or you will have vignetting.

A 1.25" eyepiece tube will be too small to take one. A 2" might be OK.
My Meade one screws on the back plate instead of the eyepiece carrier.

You are asking very similar questions on rec.photo.digital - it would
help to give you better advice if you made it clear whether you are
looking for astronomical use, wildlife photography or other use.

--
Regards,
Martin Brown

On Wed, 11 Jul 2012 04:06:54 +0000 (UTC), (Paul
Ciszek) wrote:
These telescopes:


http://www.telescope.com/Telescopes/...es/pc/1/14.uts

...are considerably longer focal length than what I had in mind
originally. I would like to be able to, for example, capture the
entire disk of the moon. Now, is there such a thing as a
"collapser"
that can be used to decrease the magnification and reduce the
f-number
of a telescope like this? I certainly wouldn't mind having a 1250mm
*and* a 625mm "lens". Orion sells something they call a "focal
reducer",
but not for these scopes.

A Barlow lens could be used as such a "collapser", if you reduce the
distance between the eyepiece and the Barlow. Do some
experimentation! Try to decrease the distance to e.g. 1/3 of the
original distance.

On Tue, 10 Jul 2012 22:49:49 -0600, Chris L Peterson
wrote:
Yes, you need a focal reducer. They are positive lenses that do just
the opposite of what barlows (negative lenses) do. A focal reducer
reduces the effective focal length of a telescope.

Focal reducers can be negative as well as positive lenses. A Barlow
lens (should we call it a "focal enhancer"?) can be converted to a
focal reducer by simply reducing the distance between the eyepiece
(or the camera sensor) and the Barlow lens.

If you prefer to use a positive lens as focal reducer, or focal
enhancer, you can use one of your eypieces - this is called eyepiece
projection. Or you can use your camera's macro lens, if you have one.

The most notable difference between using a positive or negative lens
as a focal reducer or enhancer is that the positive lens rotates the
image 180 degrees while a negative lens doesn't do that. This is
really the same effect as in a Galilean telescope (which gives erect
images) versus a Keplerian telescope (which gives upside-down images,
this is the dominant telescope type today).

On Jul 10, 11:58*pm, (Paul Ciszek) wrote:
In article ,









wrote:
On Jul 10, 12:46*pm, (Paul Ciszek) wrote:
In article
,

RichA wrote:

Just avoid the Opteka refractive lenses, unless you like working with
long, LONG focal ratios. *Stick to the mirror lenses, then you have at
least a hope of getting something decent from it. *Honestly, though,
go to Orion Telescopes and look at their Maksutov mirror-lens tube
assemblies and avoid the Opteka stuff.

Are you referring to Orion Telescopes, or do they also make a mirror lens
intended specifically for photography? *I have seen the phrase "Orion
mirror lens" used elsewhere, but I can't find any such thing.

What kind of camera do you have? *Specific model please. *There are
short t-rings for DSLR and long t-rings for MIL cameras.

My Olympus OM-D E-M5 is a micro-four-thirds system camera.

Based on that info, I would conclude that you need the second of the
three links that you included in your original post since your camera
appears to be a DSLR and not an MIL.

Allegedly,
the geometry of any micro-four-thirds camera should be the same.

The MILs lack a mirror, prism and viewfinder, and are therefore
thinner and would need a different and much thicker t-ring to achieve
the 55mm distance from front of mount to sensor.