How not to design a small, 5" telescope

On Wednesday, February 17, 2016 at 10:04:25 AM UTC-7, wrote:
On Wednesday, February 17, 2016 at 10:50:52 AM UTC-5, Chris.B wrote:

c) I'll think of something...
Oh, yes! Obstruction.

TCTs "solve" that. Did you hear about the five-inch TCT that required TWO
eight-inch blanks?

But wouldn't it _still_ be cheaper than a 5-inch triplet apo?

John Savard

On Friday, February 19, 2016 at 1:13:17 PM UTC-5, Quadibloc wrote:
On Wednesday, February 17, 2016 at 10:04:25 AM UTC-7, wsne.. wrote:
On Wednesday, February 17, 2016 at 10:50:52 AM UTC-5, Chris.B wrote:

c) I'll think of something...
Oh, yes! Obstruction.

TCTs "solve" that. Did you hear about the five-inch TCT that required TWO
eight-inch blanks?

But wouldn't it _still_ be cheaper than a 5-inch triplet apo?


Irrelevant.

We are not comparing equal apertures. A good eight-inch Newt made from one the TCT's blanks would outperform the TCT in practice.

On Friday, February 19, 2016 at 3:06:18 PM UTC-7, wrote:
On Friday, February 19, 2016 at 1:13:17 PM UTC-5, Quadibloc wrote:
On Wednesday, February 17, 2016 at 10:04:25 AM UTC-7, wsne.. wrote:
On Wednesday, February 17, 2016 at 10:50:52 AM UTC-5, Chris.B wrote:

c) I'll think of something...
Oh, yes! Obstruction.

TCTs "solve" that. Did you hear about the five-inch TCT that required TWO
eight-inch blanks?

But wouldn't it _still_ be cheaper than a 5-inch triplet apo?

Irrelevant.

We are not comparing equal apertures. A good eight-inch Newt made from one
the TCT's blanks would outperform the TCT in practice.

Well, if one wishes to say that TCTs are absurd in some absolute sense, one
should compare them to every possible alternative.

They might be a better expenditure of money than a triplet apo of equal
aperture, even if they are a worse use of glass than a Newtonian of equal cost.

Which means that you could only laugh at TCTs if you laughed louder at triplet
apos. Well, maybe not. Because a 5" TCT is big and bulky, whereas apochromatic
refractors are popular not _just_ because they lack central obstruction - but
also because they're compact.

Some people have optics and telescope making as their hobby instead of
observing so much. Nothing wrong with that; they may discover things of use to
those whose interest is primarily observing.

John Savard

On Saturday, February 20, 2016 at 9:10:48 AM UTC-5, Quadibloc wrote:
On Friday, February 19, 2016 at 3:06:18 PM UTC-7, wsne... wrote:
On Friday, February 19, 2016 at 1:13:17 PM UTC-5, Quadibloc wrote:
On Wednesday, February 17, 2016 at 10:04:25 AM UTC-7, wsne.. wrote:
On Wednesday, February 17, 2016 at 10:50:52 AM UTC-5, Chris.B wrote:

c) I'll think of something...
Oh, yes! Obstruction.

TCTs "solve" that. Did you hear about the five-inch TCT that required TWO
eight-inch blanks?

But wouldn't it _still_ be cheaper than a 5-inch triplet apo?

Irrelevant.

We are not comparing equal apertures. A good eight-inch Newt made from one
the TCT's blanks would outperform the TCT in practice.

Well, if one wishes to say that TCTs are absurd in some absolute sense, one
should compare them to every possible alternative.

The potential existence of even more absurd designs doesn't make a particular TCT any less absurd.


They might be a better expenditure of money than a triplet apo of equal
aperture, even if they are a worse use of glass than a Newtonian of equal cost.

I think a well-executed 6-, 7- or 8-inch Newt would suit just about all purposes better than that 5-inch TCT.


Which means that you could only laugh at TCTs if you laughed louder at triplet
apos.

The apo has a niche. The typical TCT does not, apparently.


Well, maybe not. Because a 5" TCT is big and bulky, whereas apochromatic
refractors are popular not _just_ because they lack central obstruction - but
also because they're compact.

A Newtonian larger than the apo can also be compact. The apo will need a tall tripod.

On Saturday, February 20, 2016 at 12:28:34 PM UTC-7, wrote:
On Saturday, February 20, 2016 at 9:10:48 AM UTC-5, Quadibloc wrote:

They might be a better expenditure of money than a triplet apo of equal
aperture, even if they are a worse use of glass than a Newtonian of equal cost.

I think a well-executed 6-, 7- or 8-inch Newt would suit just about all
purposes better than that 5-inch TCT.

Then there's an 11-inch Newtonian, which, given a 1-inch diagonal (that's
probably a little too optimistic, so perhaps I should have said 12-inch) can be
_proven_ to equal a 5-inch triplet apochromat... just slap an apodizing mask on
the front.

John Savard

On Saturday, February 20, 2016 at 9:47:30 PM UTC-5, Quadibloc wrote:
On Saturday, February 20, 2016 at 12:28:34 PM UTC-7, wsne... wrote:
On Saturday, February 20, 2016 at 9:10:48 AM UTC-5, Quadibloc wrote:

They might be a better expenditure of money than a triplet apo of equal
aperture, even if they are a worse use of glass than a Newtonian of equal cost.

I think a well-executed 6-, 7- or 8-inch Newt would suit just about all
purposes better than that 5-inch TCT.

Then there's an 11-inch Newtonian, which, given a 1-inch diagonal (that's
probably a little too optimistic, so perhaps I should have said 12-inch) can be
_proven_ to equal a 5-inch triplet apochromat... just slap an apodizing mask on
the front.

The eight should be more than sufficient... No need to go to an eleven or apodizing mask.