Cassegrain Characteristics?

[jetstream]

In general what are the Pros & cons of these types of telescopes? I see quite a lot of people use Schmidt-Cassegrains & the like,appears they have a long focal length and was wondering what advantages these scopes offer.

[faulksy]

I think the cassagrains are great and do not regret buying mine at all, very portable amazing optics brilliant for most things. down sides need time to cool no big deal, narrow fov compared to a apo/refractor need a battery for optic heaters, that's it

[perfrej]

Usually long focal length and high F-numbers, mostly suitable for planetary stuff. Compact and usually quite good optics.

/p

[Naemeth]

Another downside to consider is that while the SCT OTAs may be light and portable, the mounts for them usually are not light (e.g. a C11 is about 20kg - you would need at least an NEQ-6 to mount it).

[YKSE]

More compact and lighter than reflectors of same aperture, especially for apertures up to 8", I think.

It's C14 weighs 20 kg according to celestron:

http://www.celestron.com/astronomy/celestron-c14-af-xlt-cge.html

While a light-weight 14" releflector weighs about 24kg.

http://www.teleskop-express.de/shop/product_info.php/info/p2565_Orion-UK-Carbon-Newtonian-CT14---350mm-F-4-5---Deluxe-version.html

[Naemeth]

More compact and lighter than reflectors of same aperture, especially for apertures up to 8", I think.

It's C14 weighs 20 kg according to celestron:

http://www.celestron...af-xlt-cge.html

While a light-weight 14" releflector weighs about 24kg.

http://www.teleskop-...xe-version.html

This is true, but for reflectors with apertures of 14"+ I'd far rather go for a dobsonian mount. You get the height you need and the mount doesn't take as much space up.

[ronin]

Another downside to consider is that while the SCT OTAs may be light and portable, the mounts for them usually are not light (e.g. a C11 is about 20kg - you would need at least an NEQ-6 to mount it).

If you had an 11 inch newtonian, or could get an 11 inch refractor, you would need an NEQ6 as well, so not any different.

They are basically a folded reflector, you get a long focal length in a short body and a diameter of whatever is stated.

[RT65CB-SWL]

When using a 90^o star diagonal east & west are reversed - north & south remain the same way up!

(the same rule applies with a Maksutov too!)

[Davey-T]

Everybody needs a wide field refractor a long fl refractor a big dob a fast imaging Newtonian a 10" SCT and probably a solar scope and probably something else but an SCT wouldn't be my first choice.

Dave

[damnut]

My favourite scope was a 10" lx 200. IMHO the best compromise for a large apperture scope. For those who think you cannot image through them, mine was originally owned by Pete Shah who produced some excellent images.

Rarely used the goto, a wide angle 2" 30mm and just move it on the forks. Yes a 70lb lump but a nice one, also like above have had 6" refractor, 5" refractor long focal length, 5" refractor short for grab & go, 102mm mak, 200 mm newt.... Favourite = the SCT!

[faulksy]

My favourite scope was a 10" lx 200. IMHO the best compromise for a large apperture scope. For those who think you cannot image through them, mine was originally owned by Pete Shah who produced some excellent images.

Rarely used the goto, a wide angle 2" 30mm and just move it on the forks. Yes a 70lb lump but a nice one, also like above have had 6" refractor, 5" refractor long focal length, 5" refractor short for grab & go, 102mm mak, 200 mm newt.... Favourite = the SCT!

+ 1 as well

[jetstream]

The info is great,a while ago I started looking at different scope types,but I sort of got "lost" after looking at all of them.So I picked a small refractor,with an eye toward imaging....I and love my scope,so easy to set up,easy on eyepieces,etc and for now have forgoten about imaging-total credit to those who have the knowledge,time and money to give us those excellent pictures.I almost bought the Meade Lx 200,but in the end I figured this scope was too advanced for me.Hopefully I can look thru one of these telescopes,lots of aperature in a short telescope....the design intrigues me!

[Dunkster]

This is true, but for reflectors with apertures of 14"+ I'd far rather go for a dobsonian mount. You get the height you need and the mount doesn't take as much space up.

That is your preference based on characteristics you value, which by definition are not objective statements!

The C14 is 20kg, within the 25kg load capacity for visual of the NEQ6 as quoted by FLO here http://www.firstligh...ro-synscan.html

The C11 is 12kg, as listed by Celestron here http://www.celestron...-a-xlt-cge.html

Height depends on the individual. With the tripod-based mounts used commonly for SCTs (Meade LX, Celestron CPC, any GEM...) the height can be adjusted to the preference of the observer. This is not a design feature of the Dobsonian mount, however the height of the eyepiece using a Dobsonian mount is dependent on the altitude of the object being viewed, whereas the height of an eyepiece using either an equatorially or alt-az mounted SCT is much less variable.

Likewise, the space taken up by the mount is fixed with a Dobsonian mount, but variable with any other type of mount. Typically the mount unscrews into multiple pieces, at least tripod and mount head be it equatorial or alt/az.

[Dunkster]

The info is great,a while ago I started looking at different scope types,but I sort of got "lost" after looking at all of them.So I picked a small refractor,with an eye toward imaging....I and love my scope,so easy to set up,easy on eyepieces,etc and for now have forgoten about imaging-total credit to those who have the knowledge,time and money to give us those excellent pictures.I almost bought the Meade Lx 200,but in the end I figured this scope was too advanced for me.Hopefully I can look thru one of these telescopes,lots of aperature in a short telescope....the design intrigues me!

The SVR-90T is supposed to be a fine scope from what I've heard

The most popular Cassegrain designs round here are the Schmidt (and modified Schmidt) Cassegrains and the Maksutov Cassegrains...

• Both use a primary mirror in much the same way as a newtonian reflector, so the weight issue is related to the size of the mirror (for any type of reflecting telescope)
  
• Both also use a corrector plate, relatively thin on the SCT but thicker and heavier on the Mak, correcting for spherical aberration, but can make them susceptible to dew if the thermals aren't controlled (or if the atmosphere is overly humid, like the UK )
  
• The down side with having both ends closed is that it traps air that for best results should be in equilibrium with the ambient temperature. That doesn't mean it can't be used while cooling! The internal thermal currents affect high magnification viewing, so common sense says use lower magnification in the meantime
  
• They tend to suffer with field curvature and a little coma too, the latter usually being fairly small because of the long focal ratios, however some recent modified designs have sought to reduce/eliminate one or both of those, but these are not cheap compared with bog-standard SCTs. Newtonians typically suffer badly with coma the faster they get (below f/5?), and people use coma correctors (at additional cost)
  
• The long focal length restricts the maximum field of view, as they are inversely related. If you want a wide field scope, use a small refractor In reality, most except the largest subjects will fit in the FOV as even the C11 can show almost 1 degree across, but focal reducers exist that shorten the effective focal length thus widening the field of view (depending on model, etc)
  
• The long focal length also means that high magnifications are achieved with longer focal length eyepieces, compared with faster focal ratio scopes. In these days of long eye relief Delos and friends available in short focal lengths, this is less significant
  

The biggest gripe folk seem to have about these things is price, which is dictated by the manufacturers/distributors, and likely related (at least in part) to their relative complexity to manufacture compared with some other designs.

[KaStern_Former_Member]

Hi Gerry,

do you wnat to know about Cassegrain telescopes or Schmidt-Cassegrain Telescopes?

They are very different allthout they may look relatively similar.

Cheers, Karsten

[jetstream]

Hi Gerry,

do you wnat to know about Cassegrain telescopes or Schmidt-Cassegrain Telescopes?

They are very different allthout they may look relatively similar.

Cheers, Karsten

Not sure I named post right,catadioptric may have been more appropriate,so I can't really answer your question because I don't know how Cass..& Scmidt Cass.. are made.If you are willing to describe the differences & characteristics of the types I would appreciate it.I briefly looked at schematics of them,however I chose a simple scope & kinda forgot about these designs.But now I'm interested again after seeing how many people use them.Muchas Gracias Karsten

[John]

Then there are maksutov-cassegrains, schmidt-newtonians and maksutov-newtonians. Confusing isn't it ?.

The mak-newts are my favourite - capable to close to apochromatic refractor performance but for a lot less £'s

[Cjg]

Everybody needs a wide field refractor a long fl refractor a big dob a fast imaging Newtonian a 10" SCT and probably a solar scope and probably something else but an SCT wouldn't be my first choice.

Dave

And a partner that understands the "need" for 5/6 scopes too

Chris

[Cjg]

Gerry, to add to your original question, the Schmidt-Cassegrains tend to be "dew magnets" if you get one, chances are that you'll need at the very least a decent dew shield and or heaters. I have the 5 on the Celestron SE mount and love it. Long focal length in a short and stubby tube that fits in a backpack. Great views and can usually manage the whole lot from car boot to observing pad in one trip. I leave mine pointing downwards on the mount with nothing on the back for a few minutes to cool down before use.

The only "gripe" I have with mine is that the lens cap is useless and falls off whenever the scope is pointed downwards..Bigger OTA's in the same line have a better lens cap.

At astronomics.com, they are almost the price in dollars that we pay in ££, so very good value.

Good luck,

Chris

[F15Rules]

Hi Gerry,

Just to introduce another option on the compound design, how about a Maksutov?

I have an Intes Alter M603..6" aperture, F10. Very portable, very high quality optics (rivalling some very good refractors when properly set up and cooled down), apochromatic images, and very versatile. Hold collimation well.

Great Russian build quality and can be upgraded with a retro fit crayford for high power sensitive focusing (I've added one to mine).

cheers

Dave

[KaStern_Former_Member]

Hello Gerry,

here are two links with some information on Schmidt-Cassegrain-Telescopes (SCT) and Cassegrain-Telescopes:

http://en.wikipedia.org/wiki/Schmidt%E2%80%93Cassegrain_telescope

http://en.wikipedia.org/wiki/Cassegrain_reflector

Both share some properties, but differ inothers.

The SCT does have a spherical primary mirror and a spherical secondary mirror,

plus a Schmidt plate with one flat surface and one aspherical surface

The Cassegrain Telescope does have a paraboloidal primary mirror and a hyperboloidal secondary mirror.

Bot aspheres are more dificult to make than a spherical surface, especially the strongly aspheric convex secondary.

The SCT has strong off axis coma, the classical Cassegrain has coma

as strong as a parabolidal Newtonian with the same f/number.

Nowadays you will not find a classical Cassegrain,

allthough some mirror makers will make the mirrors on request.

Takahashi did make the CN 212 reflector,

but it has been dicontinued.

Cheers, Karsten

[dweller25]

Karsten, why does you Avatar show you as banned ??

[dweller25]

Hello Gerry,

Catadioptrics are great and my favourite type of scope as they are short so I can sit down to observe.

It's very important to have some kind of active cooling to get then to work at their best.

As John mentioned earlier the Intes Micro Maksutovs are excellent.

Good luck in your search.

[ollypenrice]

In my opinion the Schmidt Cassegrain is the classic 'beginner's mistake.' I'm allowed to say that because I made it twice!

The SCT has certain specific advantages and if you need those advantages then it will delight you. If you don't need them, don't buy an SCT however sexy they look.

In favour;

Physically compact but heavy.

Good (but beatable) planetary views.

Stunning planetary imaging. See Damian Peach, the best in the world.

Good deep sky views at high powers.

Easy collimation.

Sealed tube.

Against,

Dew.

Limited field of view. The FL is too long to give you a wide choice of targets.

Very tricky for deep sky imaging. Too many reasons to list, but don't start DS imaging with one of these.

Moving mirror focus. (Infuriating for any kind of imaging.)

Very low residual value. (Or buy second hand and count this as a 'For!')

Gross overselling on the part of their advertizers. There is more nonsense per square inch in SCT adverts than for any other astro product.

Olly

(I do have a 10 inch SCT, by the way.)

[perfrej]

Very good summary, Mr Penrice! I second that. My "SCT mistake" was an ETX-125. Totally useless in my view (no offense meant). Planetary and Hyperstar is OK.

/p