Mak-Newts

On Tuesday, July 19, 2016 at 12:13:53 PM UTC-7, Chris L Peterson wrote:
On Tue, 19 Jul 2016 11:33:19 -0700 (PDT),
wrote:

Well, my C 11 SCT, does have a large airy disk around stars for sure!
More magnified the more it shows.
So, you don't think a camera would record that?

The physical size of the Airy disk is determined by the focal ratio of
the telescope, and the resulting apparent size by the magnification
(or the pixel scale in the case of imaging). The central obstruction
doesn't change that. A large central obstruction does place a little
more energy into the outer rings, but it's unlikely that's really
going to make the stars look bigger (and it would certainly be very
subtle). When imaging, we almost never see diffraction rings around
stars at all, because seeing is the dominant factor in reducing
resolution.

Nearly all professional telescopes use RC optics these days, which
have very large obstructions. Resolution is not impacted, and for
imaging the effect on the MTF is generally very minor compared with
the other advantages of the design.

Well, in my 4" apo f/8.9 I have to look for any the airy disk, in my C-11 f/10 33% OC, pokes my eye out.
Maybe you should read Suiter's book, Star Testing Astronomical Telescopes !

A useful book for telescope testing, but not the best for
understanding optics. I'd suggest something like _Telescope Optics_ by
Rutten and van Venrooij. This will provide a nice discussion about
Airy disks. While both the scopes you mention have similar sized Airy
disks (12 um for the apo, 14 um for the SCT), how they will appear in
the EP depends on the magnification. What eyepieces are you using with
the two scopes?

If you're viewing or imaging at a high enough magnification to see the
Airy disk, you're generally exceeding the resolution of the system
anyway. That is, you have empty magnification. The optical resolution
is pretty much defined by the size of the Airy disk, not the amount of
energy that is shifted into outer rings.

The CO does not impact basic resolution, it impacts the MTF, which
means that image contrast is impacted- usually negatively, but not
always. However, contrast isn't important when imaging, which is why
CO is not considered very important with instruments intended
primarily for imaging.

Well, don't want to become a optical scientist at my age, plus I have lot of other things to do.
But, has to be a reason why the preference for refractors over SCT's or Newts?

On Tue, 19 Jul 2016 19:41:30 -0700 (PDT), StarDust
wrote:

Well, don't want to become a optical scientist at my age, plus I have lot of other things to do.
But, has to be a reason why the preference for refractors over SCT's or Newts?

I've observed no such general preference. Each has its place, and
people choose the type of scope that works best for them. I do think
that the majority of images are made using some form of reflecting
optics, however.

On Tue, 19 Jul 2016 19:31:45 -0700 (PDT), StarDust
wrote:

You are MISTAKEN! The Airy Disk in your refractor is almost THREE TIMES the diameter it is in your 4."!! Please, make it stop!!

Than, I'm must be blind?

No, just confused. Or incorrectly assuming that some optical defect is
related to the CO.

On Tuesday, July 19, 2016 at 8:53:42 PM UTC-7, Chris L Peterson wrote:
On Tue, 19 Jul 2016 19:41:30 -0700 (PDT),
wrote:

Well, don't want to become a optical scientist at my age, plus I have lot of other things to do.
But, has to be a reason why the preference for refractors over SCT's or Newts?

I've observed no such general preference. Each has its place, and
people choose the type of scope that works best for them. I do think
that the majority of images are made using some form of reflecting
optics, however.

I think, because many amateur astronomers have their own back yard observatory where he can have a larger instrument set-up permanent.
But others, looking for portability and high quality instrument, refractor is the way to go! Refractor gives very high magnification, also low power wide field views.

On Tuesday, July 19, 2016 at 8:54:45 PM UTC-7, Chris L Peterson wrote:
On Tue, 19 Jul 2016 19:31:45 -0700 (PDT),
wrote:

You are MISTAKEN! The Airy Disk in your refractor is almost THREE TIMES the diameter it is in your 4."!! Please, make it stop!!

Than, I'm must be blind?

No, just confused. Or incorrectly assuming that some optical defect is
related to the CO.

Don't think so, you don't fool me, I a have and use both type instruments, refractor and SCT. Actually, owned a few SCT's, sold them all, except my C-11.

On Tuesday, July 19, 2016 at 9:53:42 PM UTC-6, Chris L Peterson wrote:
On Tue, 19 Jul 2016 19:41:30 -0700 (PDT), StarDust
wrote:

Well, don't want to become a optical scientist at my age, plus I have lot of other things to do.
But, has to be a reason why the preference for refractors over SCT's or Newts?

I've observed no such general preference. Each has its place, and
people choose the type of scope that works best for them. I do think
that the majority of images are made using some form of reflecting
optics, however.

Since Schmidt-Cassegrains are more compact, and Newtonians are less expensive,
for refractors to exist at all, there needs to be a reason to prefer them - and
the lack of central obstruction, while it improves image quality only very
slightly, can be taken as at least part of the reason.

After all, it's only polite to avoid gratuitously dragging in "Rolex watches"
and so on.

John Savard

On Wednesday, July 20, 2016 at 4:43:43 AM UTC-7, Quadibloc wrote:
On Tuesday, July 19, 2016 at 9:53:42 PM UTC-6, Chris L Peterson wrote:
On Tue, 19 Jul 2016 19:41:30 -0700 (PDT), wrote:

Well, don't want to become a optical scientist at my age, plus I have lot of other things to do.
But, has to be a reason why the preference for refractors over SCT's or Newts?

I've observed no such general preference. Each has its place, and
people choose the type of scope that works best for them. I do think
that the majority of images are made using some form of reflecting
optics, however.

Since Schmidt-Cassegrains are more compact, and Newtonians are less expensive,
for refractors to exist at all, there needs to be a reason to prefer them - and
the lack of central obstruction, while it improves image quality only very
slightly, can be taken as at least part of the reason.

After all, it's only polite to avoid gratuitously dragging in "Rolex watches"
and so on.

John Savard

Wonder why Co.'s like Astrophysics are still in business still and people paying premium price for those refractors and value goes up?

On Wed, 20 Jul 2016 04:43:41 -0700 (PDT), Quadibloc
wrote:

I've observed no such general preference. Each has its place, and
people choose the type of scope that works best for them. I do think
that the majority of images are made using some form of reflecting
optics, however.

Since Schmidt-Cassegrains are more compact, and Newtonians are less expensive,
for refractors to exist at all, there needs to be a reason to prefer them - and
the lack of central obstruction, while it improves image quality only very
slightly, can be taken as at least part of the reason.

Of course... although not really relevant in the case of imagers.

On Wed, 20 Jul 2016 05:09:41 -0700 (PDT), StarDust
wrote:

On Wednesday, July 20, 2016 at 4:43:43 AM UTC-7, Quadibloc wrote:
On Tuesday, July 19, 2016 at 9:53:42 PM UTC-6, Chris L Peterson wrote:
On Tue, 19 Jul 2016 19:41:30 -0700 (PDT), wrote:

Well, don't want to become a optical scientist at my age, plus I have lot of other things to do.
But, has to be a reason why the preference for refractors over SCT's or Newts?

I've observed no such general preference. Each has its place, and
people choose the type of scope that works best for them. I do think
that the majority of images are made using some form of reflecting
optics, however.

Since Schmidt-Cassegrains are more compact, and Newtonians are less expensive,
for refractors to exist at all, there needs to be a reason to prefer them - and
the lack of central obstruction, while it improves image quality only very
slightly, can be taken as at least part of the reason.

After all, it's only polite to avoid gratuitously dragging in "Rolex watches"
and so on.

John Savard

Wonder why Co.'s like Astrophysics are still in business still and people paying premium price for those refractors and value goes up?

Because they make fine instruments with very high optical and
mechanical quality?

On Wed, 20 Jul 2016 03:35:26 -0700 (PDT), StarDust
wrote:

On Tuesday, July 19, 2016 at 8:53:42 PM UTC-7, Chris L Peterson wrote:
On Tue, 19 Jul 2016 19:41:30 -0700 (PDT),
wrote:

Well, don't want to become a optical scientist at my age, plus I have lot of other things to do.
But, has to be a reason why the preference for refractors over SCT's or Newts?

I've observed no such general preference. Each has its place, and
people choose the type of scope that works best for them. I do think
that the majority of images are made using some form of reflecting
optics, however.

I think, because many amateur astronomers have their own back yard observatory where he can have a larger instrument set-up permanent.
But others, looking for portability and high quality instrument, refractor is the way to go! Refractor gives very high magnification, also low power wide field views.

Portability is a factor, yes. Refractors tend to be shorter focal
length instruments, which with their range of optical corrections
makes them popular for wide field imaging.