Enviro-mentality disease hits Takahashi, the new scope

On Sunday, 5 July 2015 01:22:31 UTC-4, Paul Schlyter wrote:
On Sat, 4 Jul 2015 17:42:43 -0700 (PDT), Quadibloc
wrote:
On Saturday, July 4, 2015 at 11:09:47 AM UTC-6, RichA wrote:


There is nothing stopping someone from making a "refractonian"
where the
refractor element replaces the top-end of the Newtonian and the
primary mirror
becomes a flat.

I absolutely agree that one should not tolerate central obstruction
unless one
absolutely has to - and in a refractor, one doesn't.
John Savard

In a reflector, a central obstruction isn't absolute necessary
either. William Herschel knew that.

But, a central obstruction of 15% or less apparently has no impact on the image that can be seen. Or so I've heard.

On Sunday, 5 July 2015 08:38:26 UTC+2, RichA wrote:
On Sunday, 5 July 2015 01:22:31 UTC-4, Paul Schlyter wrote:
On Sat, 4 Jul 2015 17:42:43 -0700 (PDT), Quadibloc
wrote:
On Saturday, July 4, 2015 at 11:09:47 AM UTC-6, RichA wrote:


There is nothing stopping someone from making a "refractonian"
where the
refractor element replaces the top-end of the Newtonian and the
primary mirror
becomes a flat.

I absolutely agree that one should not tolerate central obstruction
unless one
absolutely has to - and in a refractor, one doesn't.
John Savard

In a reflector, a central obstruction isn't absolute necessary
either. William Herschel knew that.

But, a central obstruction of 15% or less apparently has no impact on the image that can be seen. Or so I've heard.

A very small central obstruction can only be achieved by adopting a longer focus more typical of the modern, achromatic refractor.
Short, fast Newtonians need much larger flats than the more "ungainly" [out of fashion] longer ones or they no longer utilise their full aperture.

The demand is for compact instruments which are easy to store and quickly brought into use when the whim takes the owner.
Only the very keen and/or relatively affluent will afford and use a large reflector or refractor.
Protection from the weather becomes a serious headache.
Dismantling/re-assembly/re-collimation places serious hurdles between the observer and regular use.
Observatories are nice but bring their own thermal/humidity problems.

Most refractors suffer from lack of aperture. Only small ones are relatively cheap.
A small APO suffers from exactly the same lack of aperture regardless of cost or design.
Longer achromats [and APOs] help to reduce colour errors, work with cheap eyepieces with comfortably long eye relief to still provide high powers.
They also place the objective higher above ground's normal thermal effects in falling temperatures.

Offset paraboloids are not normal fare and are far more difficult to make well by mass production techniques.
They would need to be slow/long to reduce introduced errors by being offset.
Placing the observer under the objective, or at the tube opening, is a *very* bad idea.
The Springfield places the observer under the tube and forces a fixed eyepiece orientation.
Which might well cause problems with floaters in the [typically] older owner.

Fortunately, optical flats can now be made highly reflective and thermally stable to a [potentially] high optical standard.
So Herschel's struggles with low reflectivity speculum metals and constant re-polishing and re-figuring and re-collimating *in a very damp environment* are no longer necessary.

Almost every possible instrument design has been tried and all suffer from their own advantages and disadvantages in use.
An ultra-lightweight, all-plastic, unobstructed, flat field, zero colour refractor, of 10-12" aperture and only twice the length, would be quite a nice toy.
Except that it would need exotic eyepieces/Barlows to achieve higher powers.
Which are likely to need multiple elements with slight loss of ultimate resolution and light at each surface.

On Sat, 4 Jul 2015 23:38:25 -0700 (PDT), RichA
wrote:
I absolutely agree that one should not tolerate central
obstruction
unless one
absolutely has to - and in a refractor, one doesn't.
John Savard

In a reflector, a central obstruction isn't absolute necessary
either. William Herschel knew that.

But, a central obstruction of 15% or less apparently has no impact
on the image that can be seen. Or so I've heard.

That depends on how carefully you look, or on how sensitive your
camera is. Causal observers will gladly accept large central
obstructions.

On Sunday, July 5, 2015 at 2:37:43 AM UTC-4, RichA wrote:

Yes, I was around when the "boomers" got into astronomy. The ads of people sipping white wine, with blazing lights on, "observing," in a Celestron
catalog...But, achromats haven't changed, they still have the same colour
error and you are right, they are cheap, which allows a less discerning
clientele to buy them than those who bought Unitrons.

In the "old days" Jaegers sold refractor objectives in short focal lengths up to about six-inches aperture. Most "yuppies" (not all boomers are yuppies) weren't interested in building telescopes, but a few were interested in commercial telescopes for their "bling" factor and the "status" that might confer upon them in certain circles.

You've probably run into a few such creatures, here and in real life.

On Sunday, July 5, 2015 at 3:25:02 AM UTC-4, Chris.B wrote:

Most refractors suffer from lack of aperture.

ALL telescopes suffer from a lack of aperture. It's just that the problem is MUCH easier to ameliorate by using Newtonian reflectors, especially Dobs, than with most other telescopes, with the possible exception of the 8-inch SCT.

On Saturday, July 4, 2015 at 1:09:47 PM UTC-4, RichA wrote:

Springfield design refractor?
There is nothing stopping someone from making a "refractonian" where the
refractor element replaces the top-end of the Newtonian and the primary mirror
becomes a flat. The secondary mirror could be mounted on a stalk glued on the
back surface of the objective or mounted on a traditional spider. Collimation
would either have to be permanent (glued stalk) with the large flat being the
tiltable component, or there would have to be a door in the scope tube to
permit collimation from the side. But then you'd have to wonder if building a
traditional Newtonian might just be a better idea, since the refractor would
be obstructed. Seems to me thought that the moment you introduce mirrors
(plural) any advantage to the refractor diminishes or disappears.

I wrote in another thread:

"A 12-inch f/6 Newt is probably worth the trouble, a 6-inch f/12 refractor is probably NOT worth the trouble."

On Saturday, July 4, 2015 at 1:02:40 PM UTC-4, RichA wrote:
On Saturday, 4 July 2015 09:03:55 UTC-4, wrote:

An 8-inch f/18 refractor is going to be an expensive, cumbersome monster no matter what you try. Fold it in half and one still has at least a six foot OTA with which to contend. There are potential collimation problems that might prove to be chronic. A flat large enough to fold such a scope is going to be expensive too. Mirrors tend to scatter more light....

Achromats make great small scopes... mid- to large-sized, not so much.

People are more tolerant of faster achros today than 30 years ago. No one
would have wanted to use a 120mm f/8.0 achromat in the time of Unitron owing
to colour error. Today, some are willing to tolerate 150mm f/5.0 achromats
which is beyond me.

Short focus achromats have become relatively cheap, so the "what the h***, I'll get one!" factor comes into play more often.

In the "old days" most people would have seen a comparably priced 8- to 10-inch Newtonian as a better use of scarce funds.

And of course, people have always tolerated such fast achromats in binocular form.

On Saturday, July 4, 2015 at 10:22:31 PM UTC-7, Paul Schlyter wrote:

In a reflector, a central obstruction isn't absolute necessary
either. William Herschel knew that.

That's certainly true, but off-axis designs have their own set of headaches,
which one might choose to avoid.

John Savard

On Sun, 5 Jul 2015 07:15:57 -0700 (PDT), Quadibloc
wrote:
On Saturday, July 4, 2015 at 10:22:31 PM UTC-7, Paul Schlyter wrote:

In a reflector, a central obstruction isn't absolute necessary
either. William Herschel knew that.

That's certainly true, but off-axis designs have their own set of
headaches,
which one might choose to avoid.
John Savard

There are reflectors without central obstructions which aren't
off-axis designs. Decades ago, the ATM column in S&T described a
Gregorian reflector where both the primary and the secondary mirrors
were cut in half and then moved away sideways from one another by an
amount just enough to make that telescope a big binocular Gregorian
reflector with no obstruction in the optical path. The entrance
apertures of each half of that binocular scope will then of course be
a semi-circle rather than a full circle, but that scope was
nevertheless an interesting example of a reflector without any
central obstruction.

On Sunday, 5 July 2015 11:25:32 UTC-4, Paul Schlyter wrote:
On Sun, 5 Jul 2015 07:15:57 -0700 (PDT), Quadibloc
wrote:
On Saturday, July 4, 2015 at 10:22:31 PM UTC-7, Paul Schlyter wrote:

In a reflector, a central obstruction isn't absolute necessary
either. William Herschel knew that.

That's certainly true, but off-axis designs have their own set of
headaches,
which one might choose to avoid.
John Savard

There are reflectors without central obstructions which aren't
off-axis designs. Decades ago, the ATM column in S&T described a
Gregorian reflector where both the primary and the secondary mirrors
were cut in half and then moved away sideways from one another by an
amount just enough to make that telescope a big binocular Gregorian
reflector with no obstruction in the optical path. The entrance
apertures of each half of that binocular scope will then of course be
a semi-circle rather than a full circle, but that scope was
nevertheless an interesting example of a reflector without any
central obstruction.

I can't believe any non-obstructed reflector system allows you to get off "scott-free" from problems. I remember Orion's oblique unobstructed 6" thinking, "the light is hitting the mirror surface at an angle other than perpendicular or 45 degrees, so what is the penalty?" Of course, they never said.