why fast newtonians?

[Moonshane]

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?

I don't really read reviews to be honest but can you point some out? a faster scope is more of an advantage for imaging than for visual as the image visually at e.g. 120x is approximately the same for a f5 scope as it is for a f8 scope.

assuming we are talking about two newtonians, and assuming we are talking about visual, I have no idea why reviewers would recommend one over the other unless they say why.

I would not think about focal ratio personally as it's just coincidental.

my choice of scope is goverened by:

budget

aperture - I want more where feasible

transport - I need to be able to get it in my car

eyepiece height - I want to observe on my feet not on a ladder

available field - faster is better assuming the scope matches the above

stability - longer tube for the same aperture = harder to maintain a stable image on the mount

to me other characteristics are largely irrelevant

[John]

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

I do read quite a lot of reviews and I've not noticed this either.

Above 8" and all the mass produced newtonians available, at least in the UK, seem to be F/5 or faster. Unless you are prepared for fork out the extra £'s to have a slower scope made then it's really a case of the only option rather than the preferred one.

[bambuko]

Thank you for all your contributions!

I am a lot clearer now on the options and choices.

It seems I am not the only newb with telescope focal length questions judging by similar threads on other boards:

Cloudy Nights

Astronomy forum

[Stargazer_00]

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! 

Totally agree with this.  I'd probably buy another 10" if someone made an F6 version.

[bambuko]

250mm (10 inch) f6.3 Optics set Professional Grade
1/6 pv wavefront. 1600mm focus
Secondary: minor Axis 50mm

Price:   £460.00

I am starting to save money now

[bambuko]

or alternatively:

VX10L Tube Assembly (£681)

VX10L Specification

Tube weight - 12kg

Tube length - 1550mm

Tube diameter - 280mm

Focal length - 1600mm

Focal ratio - f6.3

Primary mirror size - 250mm

Secondary mirror size - 50mm

[Peter Drew]

There are plenty of David Hinds 10" F6,3 optical sets out there, keep a look out. 

[John]

Keep a look out for used Orion Optics scopes as well - they cost a lot less used than they do new 

[RikM]

There is a clear difference in requirements for visual use and for imaging. I have two Newtonians and use them for both visual and astrophotography. For astrophotography, you want bright images in a short exposure time = fast optics please My preference for visual astronomy is fairly large faint things. So I want maximum field of view from as large an aperture as I can manage = fast optics please With these preferences in mind, the telescopes that suit me best are a 6" f/5 and a 10" f/4.7. I would in preference have a 12" f/4 but couldn't afford one and having compared side by side in my own back garden with a friends scope, can't justify the 2" upgrade.

If I was interested in high contrast, high magnification views of small objects, and was limited to short eye-relief eyepieces, then a longer focal ratio scope would be preferable. 8" f/8 or 10" f/6.3 but I'm not that interested.

[bambuko]

That's a very good point RikM, which I somehow missed when starting this thread.

The choice is indeed driven by personal preferences and what do you want to use the scope for.

[cotterless45]

Exactly, wish more folk would begin their quest by thinking of what they'd like to see.

Almost the same with buying a car, there's few that do everything and loads for special purposes.

Hopefully they all get from A to B,

Nick.

[jnb]

Another point in favour of fast scopes is that by reducing the focal length and thus tube length they are more tolerant of smaller mounts which, given that the mount can be the major part of the cost, gives them an advantage.

[LTB]

True newtonians are competing with the Dobs now.  As a result a 15" Newt would cost a fortune and require a humongous mount to run because of size and weight.   I think the industry has made a conscious decision to move to imaging with their NEWTS and keep them in the 6"-8" range which keeps them affordable and manageable.  If someone wants to do visual with a newtonian their is no better way to do it then with a dob that can get into the 15-20 inch range without breaking the bank or the back.  From what Im seeing its a conscious decision by nearly every manufacturer that these Newts just cant compete with Dobs to scale and so they have chosen to make good quality IMAGING Newts in the f4 range and let the dobs have the large aperature visual market.

[AlexB67]

or alternatively:

VX10L Tube Assembly (£681)

VX10L Specification

Tube weight - 12kg

Tube length - 1550mm

Tube diameter - 280mm

Focal length - 1600mm

Focal ratio - f6.3

Primary mirror size - 250mm

Secondary mirror size - 50mm

While that will be  great investment without a doubt, add to that the cost of a mount as well. A OOUK package will not be cheap new.  Keep an eye out second hand.  I have seen one or two go in no time whatsoever in the last few months, when they are reasonably priced. The chances of finding one near you having to collect etc. it will not that happen that often.  On top of that knowing what to look for in a second hand item to know whether it  is worth it or not makes that hard, but no doubt people here will offer help in that when you need it. 

I'd be inclined to perhaps have a look through one or two if you could, I think I would If I were to spend that much first time out if I'd never used a scope, so you know why you are buying what you are buying and what can be seen with it. 

Also, the cheaper mass produced Dob options are really not bad for a beginner by any means, but I don't deny that the OOUK will be better optically.   Whatever the case, that scope will be a great scope, no doubt about it.

Good luck with whatever you buy   

[jnb]

True newtonians are competing with the Dobs now.  As a result a 15" Newt would cost a fortune and require a humongous mount to run because of size and weight.   I think the industry has made a conscious decision to move to imaging with their NEWTS and keep them in the 6"-8" range which keeps them affordable and manageable.  If someone wants to do visual with a newtonian their is no better way to do it then with a dob that can get into the 15-20 inch range without breaking the bank or the back.  From what Im seeing its a conscious decision by nearly every manufacturer that these Newts just cant compete with Dobs to scale and so they have chosen to make good quality IMAGING Newts in the f4 range and let the dobs have the large aperature visual market.

Is the cost argument quite so clear cut? For example a skywatcher 12" f5 dob with goto costs about £1250 while a skywatcher 12" f5 on NEQ6 pro costs £1550. Not such a huge difference really.

[Laurie61]

I must admit to have been slightly disappointed, when searching for a 12" newt, that there were no longer f scopes available 'off the shelf'. I decided that It was most likely down to cost, portability, usability ect. There is also a liking of ever faster DOB's in the States which might have something to do with it  . You will quite often read the comment's, regarding a nice eyepiece height, as being, seemingly, the single most important design element for a DOB?.  When I would have thought it was the view, when you got there, that matters 

Based on eyepiece height criteria what focal length can be gotten away with? Well a popular 16" scope is manufactured with an f4.4, 1800 mm focal length, using this as a workable benchmark what would happen if you maintained 1800 mm focal length across the range.

A 14" scope would be f 5.1 ish, a 12" would be f 6, a 10" f 7.2 and an 8" f 9.

I think some, far sited, manufacturer needs to introduce the '1800' range of visual Newtonian's          

[bambuko]

Helps if you 6' 4" like myself

With a little step I could easily get away with 100" long tube.

[AlexB67]

Laurie, I recall when I was dreaming about ideal Dobs  what you mention is what I was asking for also in another thread not  long ago. Why SW for example never make such a thing as a mass produced scope in the slower ratios.  I am sure there wold be a market for it, but I think the suspicion by a few was that perhaps overall costs including shipping may a limiting factor, considering companies like SW export to many places, and when profit margins are that marginal they need to optimise for that. 

Suffice it to say if I'd had the option to by my flex dob in f/6 or so I would have gone for it in a heartbeat   Tube is still short enough for my height, and pressed down it would still fit in the same car too and have that transport benefit .  Coming to think of it, if flex dobs are that much more compact in a container when shipped and if that is a substantial factor in overall cost and profits, perhaps it is one of the reasons bigger Dobs like the 12 inch solid tubes have been discontinued ??  well, I can only dream to think that it would be a good reason to make a longer focal length one, seeing they can fold down, as long as they can keep it rigid enough it would not add that much.

It will be  interesting to see what the next gen Dobs from companies like SW bring to the table in future, and what designs they come up with to keep the costs down.  Perhaps even more foldable designs that are quick to set up.

[laowho]

Yes Laurie,

It does seem that way, doesn't it, almost a knee-jerk reaction out of fear that if someone gets a scope requiring a step stool they'll magically stop using it, thus confirming the inviolable principle "the best scope is the one you'll use." It's twisted, tortuous logic sometimes, or even a "dumbing down." If there are specific reasons to prefer fast, for AP or richest field use, that's one thing, but I don't see an absolutely mandatory convenience as rising to the level of real criteria upon which to make the whole determination. Maybe if you factor in mobility...but still...as you say, it's the lack of options and discussion of the relative merits that for myself is the most off-putting.

[John]

I was very lucky to find my Orion Optics 12" F/5.3 newtonian at a very reasonable price and not too far away from me. Shane (Moonshane) designed and built a mount for me modeled on the Orion Optics dob mounts and which put the eyepiece at the height I wanted it at as I like to stand when observing. So it's a "bespoke" scope in that respect although 5.3 is a standard focal ratio for Orion Optics 12" newtonians. 

When the scope is on the equatorial platform and I'm viewing something in the zenith region I do need a low step to reach the eyepiece - literally just 4 inches so I'm unlikely to get vertigo  

[AlexB67]

Perhaps your wife wears stiletto heals, you may just avoid those steps,  though you may get some complaints when they are missing :0)

[John]

Perhaps your wife wears stiletto heals, you may just avoid those steps,  though you may get some complaints when they are missing :0)

Perhaps I ought to get some 1970's style boots to wear while observing 

Here you go - "zenith boots". I'll have a word with Steve at FLO and get him to stock them 

[Laurie61]

Those boots are fine, up to a point but for the longer f scope in your life you need these. 

[John]

Those boots are fine, up to a point but for the longer f scope in your life you need these. 

tffspockjetboots.jpg

[bambuko]

You kinky lot belong in the lounge not here, corrupting the newbs with all this talk of high heels and leather - and I thought that astronomy was wholesome hobby...