Why is it called "fast"?

[Stu]

5 minutes ago, JOC said:

So the larger the mirror for any given focal length (tube length) the 'faster' the telescope?

Yep, that's right.

[JOC]

11 hours ago, Alan64 said:

A 200mm f/6 Newtonian on a Dobson mount is an example of a "moderate" telescope; and barlows are oft employed with same, but still optional per the user's interests.

So the above is what I have - so this is sort of middle ground in telescopes? - a sort of 'jack of all' trades telescope which will probably be OK with any middle ground 'jack of all trades' Eye-pieces?  This does indeed sound the ideal sort of beginners instrument that SGL contributors pointed me towards, a fact which is confirmed by almost every thread I read including this one so I continue to be very pleased with my choice and thank you again SGL.

[ollypenrice]

43 minutes ago, JOC said:

So the larger the mirror for any given focal length (tube length) the 'faster' the telescope?

The tube length doesn't matter. A 'folded' light path as in an SCT makes for a short tube. But, yes, the larger the mirror or lens for a given focal length the faster the scope.

Olly

[JOC]

ollypenrice - so does that focal length include the distance from the main mirror back to the secondary mirror and up through the eyepiece optic as well as the distance from where the light enters the tube and strikes the primary mirror then?

[Stu]

22 minutes ago, JOC said:

ollypenrice - so does that focal length include the distance from the main mirror back to the secondary mirror and up through the eyepiece optic as well as the distance from where the light enters the tube and strikes the primary mirror then?

In a Newtonian, the light is not deviated from its effectively parallel rays until it hits the mirror. The figure of the mirror determines the focal length and that is basically the length from the mirror through to where the light is brought to a focal point within the focuser, so yes it includes the distances from secondary through to the focuser but no it doesn't include the distance from the top end down to the mirror. 

The situation is different in an SCT because the front corrector plate is where the scope starts to shape the light beam.

[ollypenrice]

Just now, JOC said:

ollypenrice - so does that focal length include the distance from the main mirror back to the secondary mirror and up through the eyepiece optic as well as the distance from where the light enters the tube and strikes the primary mirror then?

No, it does not include the eyepiece. The telescope's focal length is independent of that. The focal length is not concerned with the physical tube of the telescope, either.

The telescope's focal length measures the distance from the objective - which is either the primary mirror of a reflector or the front lens of a refractor - and the point at which that objective brings the light to focus. This distance does include any 'shunting' done by a secondary mirror and any changes to the focal length created by a secondary mirror which is not flat. (That is all there is to it in photography.)

In visual observing you add an eyepiece and, when you do this, you become interested in the effective focal length of the system - the system being the scope and eyepiece working together. The most useful thing to do, though, is divide the telescope's focal length by the eyepiece's focal length because this gives you the magnification. Magnification means 'something being made larger' but what are we making larger? In visual observing we can easily answer that question. We are magnifying the the size of the object as seen with the naked eye. Call that the 'natural size of the image,' if you like.  In photography, however, there is no 'natural size.' The size of the image projected onto film or chip depends on the focal length of the telescope. There is no natural or standard size so it is pointless to talk about magnification in imaging. We need to talk about plate scale or image scale.

Olly

 

[ollypenrice]

Just now, Stu said:

In a Newtonian, the light is not deviated from its effectively parallel rays until it hits the mirror. The figure of the mirror determines the focal length and that is basically the length from the mirror through to where the light is brought to a focal point within the focuser, so yes it includes the distances from secondary through to the focuser but no it doesn't include the distance from the top end down to the mirror. 

The situation is different in an SCT because the front corrector plate is where the scope starts to shape the light beam.

That's true, but the corrector plate is not really part of the objective. It is there to anticipate aberrations which will be introduced by the objective. I think that if you remove the corrector plate from an SCT it will still form an image at pretty much the same point as usual but not such a good one. (This reply does not come with a guarantee!! Where's Peter Drew???)

Olly

[Stu]

4 minutes ago, ollypenrice said:

That's true, but the corrector plate is not really part of the objective. It is there to anticipate aberrations which will be introduced by the objective. I think that if you remove the corrector plate from an SCT it will still form an image at pretty much the same point as usual but not such a good one. (This reply does not come with a guarantee!! Where's Peter Drew???)

Olly

Thanks Olly! I'm sure @Peter Drewwill be along soon to put me right

I guess I was considering my OMC200 which had a 4m focal length in a comparatively short tube so assumed that it was a three way folded path (i.e. down-up-down) which got to it that length?

[ollypenrice]

13 minutes ago, Stu said:

Thanks Olly! I'm sure @Peter Drewwill be along soon to put me right

I guess I was considering my OMC200 which had a 4m focal length in a comparatively short tube so assumed that it was a three way folded path (i.e. down-up-down) which got to it that length?

Interesting. The ray diagrams for catadioptrics http://www.astronomyasylum.com/telescopeopticstutorial.html consistently show the incident rays as still parallel after passing through the corrector plate so I conclude (rightly? wrongly?) that the focal length still 'starts' at the primary.

Edit: I think your light path is not down-up-down but only up-down. However, your secondary is not flat so it radically changes the focal length.

Olly

[Stu]

Looks like you are right Olly. SCT focal length calculations are quite complex though aren't they?

https://starizona.com/acb/basics/equip_telescopes_scts.aspx

[JOC]

Wow, there is a lot to all this.  I didn't realise that the focal length didn't include the distance from where the light entered the tube, but I guess when I think about it as light essentially is given to go in straight lines you could effectively measure to the star itself with a long enough ruler.  It def. makes more sense that the measuring starts at the mirror that first collects the light.  So where are the measurements taken from given that the large 8" mirror in my telescope is curved - are the measurements taken from the middle or the edges of the curve?

[laser_jock99]

Just to complicate things further 'fast' scopes can be made 'ultra fast' by adding a telecompressor:

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4685_ASA-2-inch-Newton-Coma-Corrector-and-0-73x-Reducer-for-Astrophotography.html

Add one to an F4 Newtonian scope and you get an F2.9 instrument - i.e. the same apperture but shorter focal length.

[Peter Drew]

I'm still just about awake so I hope I don't add to the confusion. Stu and Olly are both correct in the main with what they've said. It's the compound telescopes that cause problems. The majority of the SCT's have F2 primaries which controls the tube length, the usual effective focal length of F10 results from the 5X amplification of the convex secondary mirror, think 5X Barlow for a similar effect. The system works at an EFFECTIVE F10 because the modification to the convergence of the light path "tricks" the eyepiece to think it is from a F10 objective. The correctors of SCT's and Maks don't have a meaningfull focal length and as such don't introduce CA if properly fashioned, they just introduce spherical aberration equal and opposite to that of the primary.  Bit late for this sort of thing.  

[ollypenrice]

9 hours ago, Peter Drew said:

I'm still just about awake so I hope I don't add to the confusion. Stu and Olly are both correct in the main with what they've said. It's the compound telescopes that cause problems. The majority of the SCT's have F2 primaries which controls the tube length, the usual effective focal length of F10 results from the 5X amplification of the convex secondary mirror, think 5X Barlow for a similar effect. The system works at an EFFECTIVE F10 because the modification to the convergence of the light path "tricks" the eyepiece to think it is from a F10 objective. The correctors of SCT's and Maks don't have a meaningfull focal length and as such don't introduce CA if properly fashioned, they just introduce spherical aberration equal and opposite to that of the primary.  Bit late for this sort of thing.  

A fine performance from a tired man! Thanks Peter.

10 hours ago, JOC said:

Wow, there is a lot to all this.  I didn't realise that the focal length didn't include the distance from where the light entered the tube, but I guess when I think about it as light essentially is given to go in straight lines you could effectively measure to the star itself with a long enough ruler.  It def. makes more sense that the measuring starts at the mirror that first collects the light.  So where are the measurements taken from given that the large 8" mirror in my telescope is curved - are the measurements taken from the middle or the edges of the curve?

Excactly. We don't extend the focal length when we pull out the dewsheild!

The whole point of a good mirror is that, from every point on its surface, the light will be brought to the same focal plane. So the distance from the edge of the mirror or from its centre to the focal plane is the same. Here's a rather remarkable fact about radio telescopes: you can make a dish to 'reflect' radio waves to a sensor at the point of focus, just as is the case with a reflecting telescope. But  if you put an array of antennae on a flat surface and connect them by wires of the same length to a sensor then it will function like a dish. The purpose of the dish or the mirror is to maintain the same distance between the collecting surface and the sensor for all incident rays. (I'm remembering this from some years ago when I was fighting a losing battle with a radio astronomy course so I'm open to correction if necessary!)

Olly

 

[Astro Imp]

What an interesting thread.

Just shows how by asking what appears at face value to be a fairly simple question the wealth of knowledge of members brings to light all sorts of unexpected (to me) answers.