Trying to get the terminology straight

[smolloy]

Hi all,

I'm very new -- just starting really -- and am spending some time trying to figure out what sort of scope I should get. I'm starting to get a little lost in the terminology, and would really appreciate a few corrections from you.

As far as I can tell telescopes are split into two groups -- refractors and reflectors. The reflectors are then split into two groups:

1/ Cassegrains: The light travels through the tube three times. Once to get to the primary mirror. A second time to get to the secondary, and a third time to exit through a hole in the primary.

2/ Newtonians: The light only travels through the tube twice. Once to the primary, and then a second time to the secondary to be reflected out at 90 degrees.

(Is it true that the folded light path will make a Cassegrain smaller, but will cause it to lose visual contrast for planetary viewing?)

All of the other terminology indicates a modification on these basic types.

1/ Schmidt: A small corrective refracting lens is added to the light path of a Cassegrain or Newtonian to cancel the aberrations that are created by the primary mirror.

2/ Maksutov (sp?): A modification similar to the Schmidt type for the same reason.

3/ Dobsonian: A Newtonian on a mount that is designed to be cheaply constructed (where "cheaply" is not the same as "badly"!).

"Light bucket" refers to a reflector with a big aperture (> 8").

Also, there are two types of mounts for a scope:

1/ Alt-azi: Naively the most obvious type of mount. Easy and cheap to construct, but not as easy for tracking objects as they move across the sky.

2/ Equatorial: Properly aligned with the motion of the "celestial sphere". Tricky to understand, and expensive to construct, but simpler to track with.

I have lots of other questions (eye-pieces, binoculars, collimation, ...), but I'll stop here. Please let me know what I have got wrong, and what I have missed. Thanks!

[glowjet]

You are basically correct, telescopes with a folded light path with a combination of mirrors and lenses are generally termed Catadioptric of which the two popular are the Schmidt Cassegrain and the Maksutov Cassegrain, the difference with the Mak is the large meniscus correcting lens at the scopes aperture which allows for a smaller secondary than the schmidt`s this gives slightly better resolution for such as Planetary observing. You could do no better than ask Father Christmas for a copy of the Back Yard Astronomers Guide an excellent book full of such information

John.

[John]

Your research is delivering good results !

As well as the main scope types there are numerous sub-types and hybrid designs to add to the confusion.

One which you will see mentioned is the Maksutov-Newtonian (Maksutov was a Russian optician) which combines a front meniscus lens with a newtonian style primary and secondary mirror. The light is brought to focus at the side of the tube, also like a newtonian. Excellent and versatile scopes !. To avoid confusion I tend to refer to Mak-Cass's or Mak-Newts rather than just Maks.

Eyepiece designs will keep you busy for days !.

[themos]

Hi all,

(Is it true that the folded light path will make a Cassegrain smaller, but will cause it to lose visual contrast for planetary viewing?)

No, I don't think so. It's not the folded path that makes a difference to contrast but the central obstruction (which almost all mirror telescopes have). Note that it's not just that you're losing some light from the obstruction: the obstruction also scatters light and that's what messes up your contrast.

All of the other terminology indicates a modification on these basic types.

1/ Schmidt: A small corrective refracting lens is added to the light path of a Cassegrain or Newtonian to cancel the aberrations that are created by the primary mirror.

2/ Maksutov (sp?): A modification similar to the Schmidt type for the same reason.

3/ Dobsonian: A Newtonian on a mount that is designed to be cheaply constructed (where "cheaply" is not the same as "badly"!).

"Dobsonian" is really a mount design and has nothing to do with optics so it doesn't fit in this list.

It might help if you first learned about all the possible aberrations you can get with an optical system:

Starizona's Telescope Basics

Once you get your head round those, you can then start classifying catadioptrics according to

1. front correcting element
   
2. shape of primary
   
3. shape of secondary
   
4. additional correctors
   

So,

1. None
   
2. Spherical
   
3. Flat
   
4. None
   

is your simplest Newtonian design which has spherical aberration: basically, it can't properly focus anything!

1. None
   
2. Parabolic
   
3. Flat
   
4. None
   

is a parabolic Newtonian design that only has coma: it can focus a star right in the middle but not near the edge.

1. Schmidt corrector plate
   
2. Spherical
   
3. Flat
   
4. None
   

is your Schmidt Newtonian which doesn't have spherical aberration but has some coma, less than the parabolic Newt.

And so on.

[cantab]

Usually three groups are used. Refractors that use only lenses (excluding star diagonals, which are optional), reflectors that use only mirrors (excluding the eyepiece), and catadioptric telescopes that use both.

The "classical Cassegrain" is rarely seen nowadays; the configuration is invariably used with a Schmidt or Maksutov corrector in a Schmidt-Cass or Mak-Cass telescope (both catadioptric telescopes). The correcting lenses in these aren't "small", they're the size of the telescope aperture. (There are a few other designs that use a small corrector near the eyepiece; telescopes using such designs tend to be rubbish!)

On the refractor side of things, we distinguish achromatic refractors, that have some chromatic aberration (halos round bright objects, and less crisp detail) but much less than primitive telescopes, and apochromatic refractors that have almost no chromatic aberration.

[smolloy]

Thanks guys. Looks like I'm getting close, but still have a lot to learn.

Themos, your way of splitting every catadioptric into four questions is quite nice, and really helps me think about it. The starizona link was really useful as well.

Thanks cantab for pointing out that the additional lenses in Schmidts and Maks aren't all that small. I thought I remembered seeing a diagram that showed them as very small additions to the secondary, so it's good that you pointed out my mistake.

I remember reading somewhere that Cassegrains aren't recommended for planetary viewing since the secondary mirror can reduce the contrast, and that Newtonians are better for that (because of a smaller secondary?), but maybe I'm mis-remembering that as well....

Thanks all. I really appreciate your time. More advice & info is always welcome

[themos]

Cassegrains aren't recommended for planetary viewing since the secondary mirror can reduce the contrast, and that Newtonians are better for that

For contrast, you can't beat a refractoror an off-axis reflector (no obstruction at all) . The latter design is the one that can scale up to large apertures but they usually require a very "slow" design to control aberrations. Luckily, that's exactly what you want with the planets, so an off-axis folded path reflector design should be a great planetary telescope.