Enviro-mentality disease hits Takahashi, the new scope

On Friday, 3 July 2015 14:27:04 UTC+2, Quadibloc wrote:
On Thursday, July 2, 2015 at 8:48:50 PM UTC-6, RichA wrote:

Unitron made 3 and 4 inch folded refractors, to save the length. They never
made apochromats. But, they did admit their f/15 refractors did not have the
resolution of the unfolded ones, which should be obvious since the folds are
not right angles.

It's unclear to me why a fold that is not a right angle should make any
difference to resolution. Perhaps some other attribute of the design would
explain that.

John Savard

The greatest likelihood is poor quality of the small [folding] flats. Perhaps they were slightly undersized and suffered from typically rolled off edges.

Unless a folded refractor has adequate internal shielding, stray light can easily bleed into unwanted areas of the light path. The Unitron folded refractors used quite a compact main tube which may have exacerbated the latter problem. Greater angles of reflection, with inevitably greater overall size, offer a better chance of shielding from stray light.

There may also have been thermal issues from having three optical components and their supports inside a compact, sealed tube. Having never had the chance to study one at first hand I am only guessing.

On Friday, 3 July 2015 11:53:11 UTC-4, Chris.B wrote:
On Friday, 3 July 2015 14:27:04 UTC+2, Quadibloc wrote:
On Thursday, July 2, 2015 at 8:48:50 PM UTC-6, RichA wrote:

Unitron made 3 and 4 inch folded refractors, to save the length. They never
made apochromats. But, they did admit their f/15 refractors did not have the
resolution of the unfolded ones, which should be obvious since the folds are
not right angles.

It's unclear to me why a fold that is not a right angle should make any
difference to resolution. Perhaps some other attribute of the design would
explain that.

John Savard

The greatest likelihood is poor quality of the small [folding] flats. Perhaps they were slightly undersized and suffered from typically rolled off edges.

Unless a folded refractor has adequate internal shielding, stray light can easily bleed into unwanted areas of the light path. The Unitron folded refractors used quite a compact main tube which may have exacerbated the latter problem. Greater angles of reflection, with inevitably greater overall size, offer a better chance of shielding from stray light.

There may also have been thermal issues from having three optical components and their supports inside a compact, sealed tube. Having never had the chance to study one at first hand I am only guessing.

It was a latch-ditch effort to stave-off the SCT juggernaut, make a portable refractor. It didn't work. I've seen a 3", never looked though one. However, in deference to Unitron, I had one of the nicest views of Jupiter though a normal length 4" of theirs.

On Saturday, 4 July 2015 08:06:12 UTC+2, RichA wrote:

It was a latch-ditch effort to stave-off the SCT juggernaut, make a portable refractor. It didn't work. I've seen a 3", never looked though one. However, in deference to Unitron, I had one of the nicest views of Jupiter though a normal length 4" of theirs.

There is nothing inherently wrong with a folded refractor provided the flats are of high enough quality and it is properly light baffled and collimated. Some incredibly expensive folded APO and achromatic refractors have been made and are still offered in Europe.

I was quoted £1k, $1.5kUS just for one suitable flat for folding an 8" F18 from D&G. At that price it would be very tempting to make one's own using three round Pyrex blanks in the traditional manner.

A less ambitious design would be to make an "elbow" refractor with a single flat placed somewhere beyond half the focal length. A large but decent elliptical Newtonian flat could then be utilised provided it was slightly oversized.

This arrangement could be mounted as an altazimuth Dob. With the focuser end of the "bent" OTA exiting the altitude bearing. The objective would need counterbalancing but the relatively short lever makes this quite easy.

A 2" star diagonal in the focuser would help to shorten the OTA just a little more and make for comfortable, seated viewing. Image orientation might be slightly confusing.

On Saturday, July 4, 2015 at 7:52:40 AM UTC-4, Chris.B wrote:
On Saturday, 4 July 2015 08:06:12 UTC+2, RichA wrote:

It was a latch-ditch effort to stave-off the SCT juggernaut, make a portable refractor. It didn't work. I've seen a 3", never looked though one. However, in deference to Unitron, I had one of the nicest views of Jupiter though a normal length 4" of theirs.

There is nothing inherently wrong with a folded refractor provided the flats are of high enough quality and it is properly light baffled and collimated. Some incredibly expensive folded APO and achromatic refractors have been made and are still offered in Europe.

I was quoted £1k, $1.5kUS just for one suitable flat for folding an 8" F18 from D&G. At that price it would be very tempting to make one's own using three round Pyrex blanks in the traditional manner.

A less ambitious design would be to make an "elbow" refractor with a single flat placed somewhere beyond half the focal length. A large but decent elliptical Newtonian flat could then be utilised provided it was slightly oversized.

This arrangement could be mounted as an altazimuth Dob. With the focuser end of the "bent" OTA exiting the altitude bearing. The objective would need counterbalancing but the relatively short lever makes this quite easy.

A 2" star diagonal in the focuser would help to shorten the OTA just a little
more and make for comfortable, seated viewing. Image orientation might be
slightly confusing.

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.

On Saturday, 4 July 2015 09:03:55 UTC-4, wrote:
On Saturday, July 4, 2015 at 7:52:40 AM UTC-4, Chris.B wrote:
On Saturday, 4 July 2015 08:06:12 UTC+2, RichA wrote:

It was a latch-ditch effort to stave-off the SCT juggernaut, make a portable refractor. It didn't work. I've seen a 3", never looked though one. However, in deference to Unitron, I had one of the nicest views of Jupiter though a normal length 4" of theirs.

There is nothing inherently wrong with a folded refractor provided the flats are of high enough quality and it is properly light baffled and collimated. Some incredibly expensive folded APO and achromatic refractors have been made and are still offered in Europe.

I was quoted £1k, $1.5kUS just for one suitable flat for folding an 8" F18 from D&G. At that price it would be very tempting to make one's own using three round Pyrex blanks in the traditional manner.

A less ambitious design would be to make an "elbow" refractor with a single flat placed somewhere beyond half the focal length. A large but decent elliptical Newtonian flat could then be utilised provided it was slightly oversized.

This arrangement could be mounted as an altazimuth Dob. With the focuser end of the "bent" OTA exiting the altitude bearing. The objective would need counterbalancing but the relatively short lever makes this quite easy.

A 2" star diagonal in the focuser would help to shorten the OTA just a little
more and make for comfortable, seated viewing. Image orientation might be
slightly confusing.

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.

On Saturday, 4 July 2015 07:52:40 UTC-4, Chris.B wrote:
On Saturday, 4 July 2015 08:06:12 UTC+2, RichA wrote:

It was a latch-ditch effort to stave-off the SCT juggernaut, make a portable refractor. It didn't work. I've seen a 3", never looked though one. However, in deference to Unitron, I had one of the nicest views of Jupiter though a normal length 4" of theirs.

There is nothing inherently wrong with a folded refractor provided the flats are of high enough quality and it is properly light baffled and collimated. Some incredibly expensive folded APO and achromatic refractors have been made and are still offered in Europe.

I was quoted £1k, $1.5kUS just for one suitable flat for folding an 8" F18 from D&G. At that price it would be very tempting to make one's own using three round Pyrex blanks in the traditional manner.

A less ambitious design would be to make an "elbow" refractor with a single flat placed somewhere beyond half the focal length. A large but decent elliptical Newtonian flat could then be utilised provided it was slightly oversized.

This arrangement could be mounted as an altazimuth Dob. With the focuser end of the "bent" OTA exiting the altitude bearing. The objective would need counterbalancing but the relatively short lever makes this quite easy.

A 2" star diagonal in the focuser would help to shorten the OTA just a little more and make for comfortable, seated viewing. Image orientation might be slightly confusing.

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.

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

On Saturday, July 4, 2015 at 11:02:40 AM UTC-6, RichA wrote:
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.

Well, the telescope market has changed considerably since the time of Unitron.

a) Telescopes can be made in China, more cheaply.

b) Improved glasses have made apochromats available, and this, plus some
wealthier amateur astronomers, has meant more people out there are willing to
pay for the benefits of a refractor over a reflector - in a number of different
combinations of features.

John Savard

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.

On Saturday, 4 July 2015 20:45:07 UTC-4, Quadibloc wrote:
On Saturday, July 4, 2015 at 11:02:40 AM UTC-6, RichA wrote:
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.

Well, the telescope market has changed considerably since the time of Unitron.

a) Telescopes can be made in China, more cheaply.

b) Improved glasses have made apochromats available, and this, plus some
wealthier amateur astronomers, has meant more people out there are willing to
pay for the benefits of a refractor over a reflector - in a number of different
combinations of features.

John Savard

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.