15 replies to this topic

[Enceladus96]

How do some cassegrains have focal lengths around 2500 when they seem smaller compared to a big dob? I have an 8 inch dob and that has a 1200 mm focal length but it’s giant compared to the size of a cassegrain. I know it’s a very beginner question, as I’m pretty new to the hobby but I just don’t understand how focal length is measured for cassegrains. It’s confusing to me! Thanks for the help.

[Jim Davis]

In an SCT, the focal length is from the back main mirror, to the front secondary mirror, to the focal point behind the scope.

 

 

In a Newtonian such as a Dob, the focal length is from the mirror, to the secondary, to the focal point on the side at the front.

 

 

The SCT path is longer. The point behind the SCT is to have a long focal length in a short tube.

 

 

[ButterFly]

An SCT has a curved secondary whereas a newt has a flat secondary.  A curved mirror acts like a lens with a bounce and magnification power.  A flat mirror is just a bounce with no magnification power.  The focal length of a newt thus does not depend on the position of the secondary - it's flat.  The SCT's focal length very much depends on the distance between the primary and the secondary - the secondary has power.

[TOMDEY]

The Cassegrain's convex Secondary Mirror (SM) is actually nothing more nor less than a reflective Barlow. It amplifies the focal length and magnification exactly the same way that a refractive Barlow Lens does. Cass Secondaries typically amplify/magnify 5x or more. This is a similarity that nearly all amateurs (and most professionals) are blissfully unaware of.

 

Here's a handy little formula that I'll bet few here have seen or used:

 

SM optical surface power expressed in diopters:

 

D = 2000/R

 

where R is the (negative) surface radius, expressed in millimeters

 

So its power is negative, and its amplification depends on its configuration/location relative to the telescope's Primary Mirror (PM).   Tom

[Garyth64]

Ok, one more explanation:

 

In a Cassegrain, or a Gregorian, or a SCT, the secondary amplifies the f-ratio of the primary.

 

The explanation in #2 is not correct, sorry.

 

The amplification is determined by two things, the distance of the secondary inside the focus of the primary, p, (or outside for a Gregorian), and the distance from the secondary to the final focus behind the primary, p'.

 

p'/p  = the amp

 

If the primary is an f/4, and the amp of the secondary is 5x, then the system becomes an f/20.

 

So you can get a lot of focal length in a short tube.

[Steve OK]

Here is the diagram I would draw when teaching about SCT's to high school students: 

 

click for larger view, if you want:

 

The narrow light cone at the final focus seems to be coming from a long way off, and the image formed there is the size it "should be" if the actual aperture (and objective, if this were a refractor) really was that far away.

 

Steve

[Susanna144]

How do some cassegrains have focal lengths around 2500 when they seem smaller compared to a big dob? I have an 8 inch dob and that has a 1200 mm focal length but it’s giant compared to the size of a cassegrain. I know it’s a very beginner question, as I’m pretty new to the hobby but I just don’t understand how focal length is measured for cassegrains. It’s confusing to me! Thanks for the help.

Thank you so much for asking this question!  It's something I thought I knew, but I knew so little!  The effective focal length is much greater than the light path, as Steve's diagram makes clear.

[rmollise]

Easy... While an f/10 (2000mm) SCT, for example, has a short little tube and an f/2 primary, it also has a 5x magnifying convex secondary.

[shark-bait]

The explanation in #2 is not correct, sorry.

It is correct but not as thorough. The light path and focal plane are in fact longer. I had no idea about the secondary differences between an SCT and Newt.

I love this forum!

Thank you for the thorough explanation.

[Garyth64]

It is correct but not as thorough. The light path and focal plane are in fact longer. I had no idea about the secondary differences between an SCT and Newt.

I love this forum!

Thank you for the thorough explanation.

 

No, it is not correct!  It is not even close.

 

Follow his explanation when looking at his sketch.  "In a SCT, the focal length is from the back of the main mirror to the front of the secondary, to the focal point behind the mirror."  That is wrong.

 

Steve showed a good representation and I will explain it once again.

 

In a SCT, the focal length is determined by the amplification of the secondary times the focal length, or f ratio of the primary.

 

 

The amp is a ratio of the distance from the secondary to the focal point behind the mirror, which I will call, p', divided by the distance of the secondary inside the focus of the primary, p.

 

p'/p = amp

 

Let's make an example:

 

The primary is an 8" f/3, so it has a f.l. of 24".  The secondary in this example is placed 8" inside the focus of the primary.  This distance is, p.

The light is reflected back down from the secondary and thru the primary to a new focal point.  This distance is p', and is 32".

 

So, 32/8 = 4.  The amp of the secondary is 4 times.  Since the primary is an 8" f/3, the system becomes a f/12.  (f/3 x 4 = f/12)  The system would have a focal length of 96".

 

If I use my numbers as an example as what is explained in #2, then the distance from the primary to the secondary would be 16".  The distance from the secondary to the focal point behind the primary would be 32".  So what he is saying is that 16" + 32"  = 48" and that is the focal length of the system.(?)  No, that is all wrong.

 

It is as I explained, twice.

Edited by Garyth64, 01 July 2021 - 07:51 PM.

[shark-bait]

No, it is not correct! It is not even close.

Follow his explanation when looking at his sketch. "In a SCT, the focal length is from the back of the main mirror to the front of the secondary, to the focal point behind the mirror." That is wrong.

Steve showed a good representation and I will explain it once again.

In a SCT, the focal length is determined by the amplification of the secondary times the focal length, or f ratio of the primary.

example 2a.jpg

The amp is a ratio of the distance from the secondary to the focal point behind the mirror, which I will call, p', divided by the distance of the secondary inside the focus of the primary, p.

p'/p = amp

Let's make an example:

The primary is an 8" f/3, so it has a f.l. of 24". The secondary in this example is placed 8" inside the focus of the primary. This distance is, p.
The light is reflected back down from the secondary and thru the primary to a new focal point. This distance is p', and is 32".

So, 32/8 = 4. The amp of the secondary is 4 times. Since the primary is an 8" f/3, the system becomes a f/12. (f/3 x 4 = f/12) The system would have a focal length of 96".

If I use my numbers as an example as what is explained in #2, then the distance from the primary to the secondary would be 16". The distance from the secondary to the focal point behind the primary would be 32". So what he is saying is that 16" + 32" = 48" and that is the focal length of the system.(?) No, that is all wrong.

It is as I explained, twice.

Not questioning your explanation as I am confident that it could not be explained better a third time. Steve’s diagram is a great illustration for sure.

I see what you’re saying about the diagram and his statement.

[carolinaskies]

Here is a better pictorial for comparing a Newtonian simple focal length to an SCT's compound variable focal length. 

Most people do not understand how an SCT's spherical mirror system works.  The secondary mirror is a variable multiplier where at a specific distance the manufacturer dictates the focal ratio of the system. 

For the classic SCT this stated focal ratio results in a distance where the optics are the most well corrected for coma and other aberrations.  

[shark-bait]

Here is a better pictorial for comparing a Newtonian simple focal length to an SCT's compound variable focal length. 

Most people do not understand how an SCT's spherical mirror system works.  The secondary mirror is a variable multiplier where at a specific distance the manufacturer dictates the focal ratio of the system. 

For the classic SCT this stated focal ratio results in a distance where the optics are the most well corrected for coma and other aberrations.  

focal length Newt vs SCT.png

So in a RASA, the secondary is replaced with a lens element.  This element obviously is not a multiplier but perhaps a reducer?

[carolinaskies]

The RASA lens is a flatner, remember it's dealing with an F/2 curve.

[Garyth64]

Here is a better pictorial for comparing a Newtonian simple focal length to an SCT's compound variable focal length. 

Most people do not understand how an SCT's spherical mirror system works.  The secondary mirror is a variable multiplier where at a specific distance the manufacturer dictates the focal ratio of the system. 

For the classic SCT this stated focal ratio results in a distance where the optics are the most well corrected for coma and other aberrations.  

focal length Newt vs SCT.png

I've seen a lot of drawings, photos, and explanations of how light travels in a SCT or a Cassegrain, but never one quite like that.

Edited by Garyth64, 02 July 2021 - 12:55 PM.

[smasraum]

Lots of great, interesting info in this thread.

 

It's great to know that it's more than just the third leg of the folded path, but also the magnification of the secondary.  Thanks everyone!