why fast newtonians?

[bambuko]

Why is it that (as it seems to me) majority of telescope manufacturers seem to prefer fast newtonians around f/5 ??

I can see the advantages to manufacturers/suppliers of smaller package (less material and cheaper freight - serious consideration when it comes to importing everything from China).

But as I see it (as a simple newb) fast ones have some disadvantages...

   tight collimation tolerances

   need for more frequent collimation

   intolerance of cheap wide angle eyepieces

   need to use short f/l eyepieces to yield large magnifications

   more difficult focusing

On the other hand slow reflectors seem

   very relaxed collimation tolerances

   and therefore once adjusted more likely to remain so for a longer time

   much easier on eyepieces

   only needs medium f/l eyepieces to achieve large magnification (easier eye relief)

   easier focusing

   crisper, higher contrast performance

OK, there are some disadvantages (including more cumbersome handling of longer tube).

I am sure there are other points to consider?

But why is it that so few people seem to be recommending slow reflectors? (or have I got it wrong?)

I now have one f/5 (6") and one f/7.3 (4,5") and I would love to try something like f/10 (6").

What do you reckon boys and girls?

[Alien 13]

I suppose its portability the higher f numbers are what MAKs are for.

[101nut]

What you have said fast newts are all user disadvantages, they make little or no difference to the manufacturer.

Big advantage to the manufacturer is the ability to produce large, accurate mirrors cheaply. In general, the longer the focal length the more difficult the mirror to figure and the more expensive the result.

From the user point of view a fast newt controls lots of light with a relatively lightweight package ... 

Arguably, a fast newt gives the biggest bang for the buck ...

AndyG

[jambouk]

Large field of view.

[michael.h.f.wilkinson]

For big Newtonian does not require a tall ladder to look through it.

[Optical Dob]

Under dark skies the large apertures enable us to see more faint objects. I think the manufacturers are just producing goods that astronomers want. Slow reflectors are often recommended on here for moon and planet observations. 

[Andy_E]

Why no one is producing a 10" f6 is truly beyond me. I seriously can't think of a better 'go anywhere do anything' telescope, plus it would be SO much easier on EPs! 

[John]

Big advantage to the manufacturer is the ability to produce large, accurate mirrors cheaply. In general, the longer the focal length the more difficult the mirror to figure and the more expensive the result.

Its the other way around actually. The faster the mirror, harder it is to get the correct figure on it. Making an accurate F/4.5 mirror is quite a lot harder than an F/6 one.

I was told a few years back by someone who used to be a dealer that shipping charges, being a fair chunk of the costs, drive the physical specs as manufacturers want to cram as many units into a shipping container as possible.

My F/5.3 12" newtonian is quite a bit more forgiving on collimation errors and eyepiece aberrations than my previous F/4.8 10" was. 

[rowan46]

its partly to do with trickle down from the larger scopes consider an f10 12" scope gives a tube length of about 9 feet. with a bigger mirror they need a shorter focal length to keep them a manageable size

[Naemeth]

Cost probably comes into it .

[bambuko]

Under dark skies the large apertures enable us to see more faint objects. I think the manufacturers are just producing goods that astronomers want. Slow reflectors are often recommended on here for moon and planet observations. 

why do you assume that slow=small aperture?

assume both are the same aperture and then compare advantages of fast ve slow, otherwise you are comparing apples with pears...

[bambuko]

...shipping charges, being a fair chunk of the costs, drive the physical specs as manufacturers want to cram as many units into a shipping container as possible....

I know this from my past working (with China) experience as well, hence my suspicion that we might be giving in to hype that is driven less by what we would benefit from and more by manufacturing/shipping considerations.

[Moonshane]

Big advantage to the manufacturer is the ability to produce large, accurate mirrors cheaply. In general, the longer the focal length the more difficult the mirror to figure and the more expensive the result.

surely assuming the same aperture it's more difficult/expensive to make a fast mirror than a slow mirror?

I like fast mirrors and two of my three scopes (all dobs) are f4 (12" and 16") as at this aperture I want max aperture, portability and available field. I also have a f11 scope (6") and in this scope I want fine detail and contrast on planets, lunar and tight double star images. this scope is more cumbersome and difficult to set up than my 12" scope.

collimation is a non consideration as it takes seconds and eyepieces even of mediocre quality still work in fast scopes but obviously top brands work that bit better.

I am considering a small (114-130mm) newt to make into a tiny truss dob for holidays when space is tight. This will be a replacement for binoculars and I want a ultra wide field; at darker sites even a small scope can create good viewing.

[101nut]

Its the other way around actually. The faster the mirror, harder it is to get the correct figure on it. Making an accurate F/4.5 mirror is quite a lot harder than an F/6 one.

Apologies for that, you're correct of course. I blame the fumes from today's dodgy fungus foray collection!

AndyG

[laowho]

Faster reflectors cater to portability and ease/comfort of use (no ladder) and(?) cheaper shipping (had never considered that--nice catch). They also give the widest possible FOV (richest field telescopes). We went f/6 in spite of the ladder requirement to give our optician a break for the deal he was giving us, knowing it'd be easier to figure. Then we learned of all the other advantages noted by the OP and others, and supposedly Carl Zambuto has said that "the magic begins at f/8." We gave up some FOV for easier/better collimation, better figure and match, better focus range, smaller secondary obstruction, better contrast detection for lunar/planetary viewing, lower susceptibility to defocusing, and better EP performance and without the need for added glass (whether in the EP or in a Paracorr). The choice was a no-brainer for us. Interestingly, the GSO/Synta mirrors advertise "diffraction limited" primaries at .84 Strehl but aren't accounting for the central obstruction which brings the system down below diffraction limited. But at f/6 they might easily produce primaries at .92 Strehl which, when calculated with the CO, would produce a truly diffraction limited scope, optimized for most places on most nights. But yes, there's yet another reason to recommend f/6 or even slower. Nice post.

[bambuko]

and... just to add to the discussion, an interesting article: Telescope Focal Length: Pros and Cons by Chuck Hawks and Gordon Landers

 

[swamp thing]

Collimation is more to do with mirror weight than speed of optics.

The bigger and heavier the mirror the more often it requires adjusting regardless of focal ratio.

Small lightweight mirrors seldom require adjusting regardless of speed. Big heavy mirrors need checking every observing session.

[bambuko]

Again... you are comparing big heavy mirrors with small lightweight mirrors

For this discussion we should assume identical mirrors and compare how will they behave in slow and fast reflector.

Slow will be quite tolerant, fast will be far less tolerant and that has nothing to do with either size or weight of the mirror?

[Moonshane]

collimation is a non-issue. you can have it done well before the mirror is cooled even if you take out all your optics first and lay them on the ground next to the scope.

[laowho]

Collimation is more to do with mirror weight than speed of optics.

The bigger and heavier the mirror the more often it requires adjusting regardless of focal ratio.

Small lightweight mirrors seldom require adjusting regardless of speed. Big heavy mirrors need checking every observing session.

Interesting. Hadn't ever thought of this aspect of "regular collimation needs," and the cell would factor in for this as well. However, the primary axial error tolerance is TWICE as great for f/6 than it is for f/5, and so when collimation IS desired/warranted, it really is that much easier, and as a correlative, the need for collimation should therefore also arise less frequently.

[bambuko]

I know it is the least of issues (you are right)

But it is one of the factors, especially when it comes to beginners - slow scope, very tolerant of collimation errors will be more stable and better suited for a first scope (as far as I am concerned).

Anyway... let me restate parameters for this discussion:

two reflector telescopes

identical aperture

different focal length (slow ve fast)

compare and discuss reasons why is it that all the reviews and recommendations seem to point to fast ones as preferred option?

[bambuko]

Large field of view.

yes... as far as I understand it you are indeed correct when you talking about true field of view, but consider this:

to achieve the same magnification in the slow telescope you will need an eyepiece with longer focal length than what you would need in the fast telescope.

So, you also need to take into consideration an apparent field of view of eyepieces involved?

Of course I am only a newb and might be getting confused

[jambouk]

What question(s) are we trying to answer, i'm confused; it really doesn't rake much

James

[John]

My understanding is that the "sweet spot" around the optical axis where the scope is in good enough collimation to achieve diffraction limited performance gets smaller as focal ratio gets faster. At F/8 it's a relatively tolerant 11mm but at F/5 it's down to 2.8mm. I think this is the primary axial error tolerance that laowho refers to.

I agree that thicker, heavier optical components will pose more mechanical challenges hold in alignment so this would be a factor too. They will take longer to cool as well.

It's the mass produced and imported newtonians where we have to accept what the manufacturers are prepared to serve us with though and in return for that, the price is kept low. "Any colour you like as long as it's black" as Henry Ford is reputed to have said (though might not have !).

 Other focal ratios are available from manufacturers such as the UK's Orion Optics but, as is often the case, there is additional cost associated with something a little more "bespoke". Orion Optics mirrors are usually thinner than other brands too.

[ronin]

Try looking through a 10" f/8 newtonian at anything near the zenith, the eyepiece will be too high, f/10 even worse.

In effect any eyepiece higher then around the 60" mark will mean it is unusable to some people, so the maker loses sales and another making 10" f/6 ones gets the missed sales and whatever they may have picked up.

Suspect that manufacturers might well prefer to make slightly slower mirrors and newtonians (cost less and better accuracy), but no good if people cannot use what is made.