Paraboloid or Parabolic

[Ridgback (Dale)]

Hi All,

I have been reading and researching for some time, thank you for sharing a fantastic resource of knowledge on this website.

I am considering as my first scope - an 8" or 10" Dob, quoted as having a paraboloidal mirror. I understand from my school days what a parabolic shape is and what it does for optical reflections, but google only explains mathematically what a defines a paraboloid.

I understand the problem of a spherical mirror, can you explain why this shape is utilised, or is it just marketing ploy.
What is its useful application (max diameter)

All advice greatly appreciated


Dale

[mathewb]

Hi Dale

Paraboloid surfaces are used because they reflect light back to a point. When we put our secondary in the way of that reflected light we are efectively taking that large surface area of the primary mirror and then making it more condensed for our eyes to see. Its the reason why our Austar and Foxtel satellite dishes are shaped they way they are - they take the incoming signal then reflect it back onto the actual antenna which is really only about the size of a matchbox (well .. ok maybe a bit bigger but not much ! )

Hope this helps,
Mathew.

[Robh (Rob)]

Originally Posted by Ridgback

Hi All,

I have been reading and researching for some time, thank you for sharing a fantastic resource of knowledge on this website.

I am considering as my first scope - an 8" or 10" Dob, quoted as having a paraboloidal mirror. I understand from my school days what a parabolic shape is and what it does for optical reflections, but google only explains mathematically what a defines a paraboloid.

I understand the problem of a spherical mirror, can you explain why this shape is utilised, or is it just marketing ploy.
What is its useful application (max diameter)

All advice greatly appreciated


Dale

Hi Dale,

A Dob uses a Newtonian design, which has a curved parabolic primary mirror and a flat diagonal secondary mirror. The primary is more accurately described as a paraboloid surface, which is the three dimensional equivalent obtained by rotating a parabola about its axis. A parabola has the property that all incoming rays of light are focussed to the same point. Larger Newtonians need pretty solid mounts for tracking and photography. Even with small f numbers (focal ratios), Newtonians can become quite bulky and long over 12 inches of aperture.

A Schmidt-Newtonian or a Schmidt-Cassegrain uses a spherical mirror, which can be described as a spherical cap. A spherical mirror by itself will not focus incoming light rays. For these you need a front aspheric corrector plate to modify the path of incoming light rays. The advantage of Schmidt-Cassegrains are their compact design. Larger scopes, however, get quite heavy. Even a 10 inch OTA weighs more than 20 kg. They are more stable on computerised mounts and popular for astrophotography. Folded path, longer focal length scopes are more forgiving of less corrected eyepieces. However, true field of view will be smaller for a given eyepiece.

Regards, Rob

[Ridgback (Dale)]

Thanks for your help,

I am purely looking at Newts or Dob's

Since my first post I have located a single supplier, one particular manufacturer - advertising :

Dob with paraboloidal pyrex mirror
and
Dob (collapsible truss) with a parabolic mirror

Both have spec's 254mm, 1200 focal length (f/5 or f/4.7)

So with almost identical spec's from the same manufacturer I still do not understand the difference between "paraboloid" and "parabolic".

I would like to understand the affect it will have on my satisfaction with my investment. I appreciate that being able to visually identify the imperfections may be academic, but I would like to understand what the theoretical implications / limitations are in the differences.
I would like to be able to get into photography with just a mount upgrade.

If anyone can explain how they have different f/ratio with the same aperture and focal length I would appreciate it as the published concensus seems to be :focal ratio = focal length of telescope / aperture of telescope

Dale

[sopticals (Stephen)]

Originally Posted by Ridgback

Dob with paraboloidal pyrex mirror
and
Dob (collapsible truss) with a parabolic mirror

Both have spec's 254mm, 1200 focal length (f/5 or f/4.7)

So with almost identical spec's from the same manufacturer I still do not understand the difference between "paraboloid" and "parabolic".



If anyone can explain how they have different f/ratio with the same aperture and focal length I would appreciate it as the published concensus seems to be :focal ratio = focal length of telescope / aperture of telescope

Dale

Hi Dale,

same horse, just different ways of describing it.