Focal Ratio

[Naemeth]

We all know that a fast scope is one with a focal ratio of F/5 or lower (F/4, F/3.5 etc...), and a slow scope is say F/8 or higher (F/9, F/10 etc.), but how low can you go without being ridiculous?

For instance, could one make a "mini" 5" F/1 Dobsonian with coma sufficiently corrected?

Could one make a 5" F/40 Dobsonian with a sufficiently collapsible tube?

If you were to go to extremes, what would each suffer from?

I think it would be brilliant to have a 6" F/1 Dobsonian that you could easily fit in a backpack, or to have a F/20 or so Dobsonian that makes an excellent "Planet Killer" scope and with collapsible truss tubes, it could actually be transportable compared to a similar refractor.

[FarSide]

It's a good idea in theory, (an F1) but for practicality you'd spend every waking moment collimating the thing; even in your sleep, especially if it was used for a grab and go. F2 Taks have to recollimated constantly, can't imagine and F1's requirements.

[JamesF]

The "hyperstar" imaging system for the Celestron SCTs replaces the secondary mirror, using only the corrector, primary and a set of correcting lenses to generate an image. The effective focal length of the SCT depends on the model, but I think it reaches f/1.9 for some of them.

James

[moriniboy]

At Salford Astro they have a refractor based on an old submarine periscope, I would estimate it to be nearly F20, will confirm next time I visit.

Never looked through it so I can't comment on how good it is.

[astrofox]

f4 is fast, f3 is just about do able in Newtons. Like camera lens lowering the f number is a trade off against optical performance ..... and money.

[Mike73]

I think it would be brilliant to have a 6" F/1 Dobsonian that you could easily fit in a backpack,

I think Umadog has a 10" Sumerian that is small enough to take as carry on when you fly, pretty impressive when you think how much light a 10" primary can catch!

[JamesF]

Imagine a 6" f/20 though. It would be ten feet long. Granted it would be possible to make a static scope, but to make one you could travel around with I think you'd have a pig of a time keeping collimation.

James

[FarSide]

Imagine a 6" f/20 though. It would be ten feet long. Granted it would be possible to make a static scope, but to make one you could travel around with I think you'd have a pig of a time keeping collimation.

James

Hershel did it. Granted it was 40 feet long, but he did it.

[JamesF]

Hershel did it. Granted it was 40 feet long, but he did it.

I bet he never took it on an aeroplane though

James

[GlassWalker]

I think fast refractors are an area that isn't well served. There's a few around f/2.8 for loadsamoney. I wonder if there's something possible between the photographic world and astrographs, based on a photo lens but without having to correct for close focus should reduce complexity and cost significantly, and make it easier to maintain high performance.

The hyperstars are interesting, using corrective optics to give systems around f/2. If only they didn't come with the disadvantages of a cat system. Having the focal point in front of the device is also inconvenient.

In an optical theory book, there was a f/0.5 system shown, but unfortunately you have a spherical focal plane that's slap bang in front of the mirror, so practicality isn't a strong point of that.

[Stu]

My mak is f20, with a 4m focal length . Nice compact tube, it eats globs for breakfast

Stu

[FarSide]

planet killer right there.

[Stu]

Yep

Stu

[Capricorn]

For some reason I think that the IoA at Cambridge University have an f/1 reflector.

Just something I heard in a conversation, nicknamed the punch bowel ??

Not 100% sure and if they have then no idea what it is used for.

Any other Cambridge members know about this ?

[ollypenrice]

For me the ultra fast Newtonian is a great idea since going ever bigger (20 inch, 25 inch, 35 inch...) doesn't get me where I want to be because the focal length keeps going up with the aperture. You just end up looking at smaller and smaller objects which remain faint fuzzies.

What I want to do is see larger objects with an enormous light grasp. M33, M51...

The biggest problem, I suppose, is not collimation but the size of the secondary which will reach such proportions as to cripple the contrast.

The serious quality fast astrograph makers seem to stick at F2.8 - like the Tak Epsilon. I do wish someone would make a better engineered Hyperstar from the ground up, though, not relying on the moving mirror for focus.

Olly

[Naemeth]

For me the ultra fast Newtonian is a great idea since going ever bigger (20 inch, 25 inch, 35 inch...) doesn't get me where I want to be because the focal length keeps going up with the aperture. You just end up looking at smaller and smaller objects which remain faint fuzzies.

What I want to do is see larger objects with an enormous light grasp. M33, M51...

The biggest problem, I suppose, is not collimation but the size of the secondary which will reach such proportions as to cripple the contrast.

The serious quality fast astrograph makers seem to stick at F2.8 - like the Tak Epsilon. I do wish someone would make a better engineered Hyperstar from the ground up, though, not relying on the moving mirror for focus.

Olly

I see what you are saying the size of the secondary needs to be bigger with a faster focal ratio, but, a 20" Newtonian at F/1 (presumably with a 35% obstruction), is still around 13" effectively, with a focal length of 508mm, still gives a big field, of say 5 Degrees with a 31mm eyepiece...but... that would give an exit pupil of 31mm, which is clearly crazy.

I don't think a Mak design is ideal for Planetary, slow focal ratio of Dobsonians allow a very small obstruction (less than 15%), giving more contrast for Planets, at the price of course of portability.

[Moonshane]

it's all about compromise of cost and design I suppose as well as usability. I have a 6" f11 newt and this is about at the limit of stability but gives superb results. I think any longer focal length would produce a tube that flexes too much unless you used very expensive (or very heavy) materials. the scope on its own stands at about 5 feet tall so it would be possible maybe to get to f12 or f13 without too much of a problem though.

unless talking about larger apertures (as Olly points out) I don't think there's any big issues (or gaps in products) for visual astronomy but imaging may be very different ("I know nothing....").

[Naemeth]

I suppose it's do-able with enough money, isn't the new E-ELT an F/1 Newtonian?

[Stu]

As you say, everything is a compromise. The mak has good aperture (8"), 21% central obstruction and well designed baffling, the contrast is very good. It is in a useable tube size which makes it relatively portable. It gives very good performance on planets, globulars and other smaller dso's.

It needs plenty of cooling, it does require collimating every now and then, and is quite sensitive to seeing.

They are not made any more so I doubt I will ever sell mine, it's very good at what it does well but I wouldn't even attempt to claim it is the perfect scope.

Stu

[FarSide]

I see what you are saying the size of the secondary needs to be bigger with a faster focal ratio, but, a 20" Newtonian at F/1 (presumably with a 35% obstruction), is still around 13" effectively, with a focal length of 508mm, still gives a big field, of say 5 Degrees with a 31mm eyepiece...but... that would give an exit pupil of 31mm, which is clearly crazy.

I don't think a Mak design is ideal for Planetary, slow focal ratio of Dobsonians allow a very small obstruction (less than 15%), giving more contrast for Planets, at the price of course of portability.

I think the biggest weakness of a Dob is portability, I have a 10" solid tube, i'm 27 and i can barely carry the thing to my NEQ6. If there was anyway of building a Dob from lighter materials that would be fantastic, but a 20" FG Dob would cost a blooming fortune, and the truss tube poses more problems than advantages. Personally, i try to balance portability with optical performance. At the end of the day i think very fast reflectors like the Epsilon is at the moment at the best ends of out amateur capabilities at the moment, and though not portable, really, we can put up with fracts for that.

[FarSide]

And when i mean the Tak isn't portable i mean because of colimation.

[Moonshane]

I consider my 16" f4 dob portable. I can get it into the back of my small hatchback and it's a solid tube too. that said, I am considering making a truss version for the optics which will make it even more portable for star parties and a truss unit for a 12" f4 mirror set I am buying to act as a travel scope when camping with the family etc. for me an f4 dob is the height of portability and compromise if built correctly.

[laser_jock99]

The biggest problem, I suppose, is not collimation but the size of the secondary which will reach such proportions as to cripple the contrast.

The serious quality fast astrograph makers seem to stick at F2.8 - like the Tak Epsilon. I do wish someone would make a better engineered Hyperstar from the ground up, though, not relying on the moving mirror for focus.

Olly

Having the imaging device in front of a single hyperbolic mirror (with small correcting lenses) is the only way very fast imaging systems can be achieved. Curvature of the focal plane is the hardest problem to correct as this involves the use of complex, aspheric optics (although these are nowadays much easier to produce on CNC 3 axis lens generating machines in low volumes). The other way is to bend the imaging plane but I'm not aware of any spherical CCD's yet! In the old days of film photography the prefered method with super fast astrographs was bend the film in a curved film holder.

[cs1cjc]

For visual, the only reason for a fast system is to make it compact, for example a big dobsonian which can be used without a ladder. Such systems are demanding for eyepieces and collimation, and a coma corrector is needed.

For imaging you can gather a lot of light and cover a wide area. The new TEC 300VT, 300mm f/1.44 astrograph giving a 7-degree field is astonishing! See:

http://www.baader-planetarium.de/tec/download/tec-vt_300mm_7deg.pdf

[laser_jock99]

For visual, the only reason for a fast system is to make it compact, for example a big dobsonian which can be used without a ladder. Such systems are demanding for eyepieces and collimation, and a coma corrector is needed.

For imaging you can gather a lot of light and cover a wide area. The new TEC 300VT, 300mm f/1.44 astrograph giving a 7-degree field is astonishing! See:

http://www.baader-planetarium.de/tec/download/tec-vt_300mm_7deg.pdf

I'll bet that's not cheap! .......back in the real world, the way forward for ultrafast, affordable imaging systems has to be the Boren-Simon type 'super Newt' I would have thought?