When is fast, fast?

[Ags]

I'm always thinking about what my next telescope will be. I'm leaning towards a plain dobsonian to get the cheapest big aperture that I'm still able to carry about easily. But I don't want a fast telescope that needs expensive eyepieces.

So is a 200mm telescope at F5.9 too fast for most affordable eyepieces? I can afford up to Baader Hyperions and the SkyWatcher 2" eyepieces in the same price range. I want my current investment in TS Planetaries to be usable in the new scope.

This is all just research - I'm not planning on actually buying another telescope for at least a year. I want to spend a full year exploring the sky with my Mak first.

[ollypenrice]

I think f5 would count as moderately fast and f6 very manageable in these terms. Below f5 is certainly fast. I daresay someone with the actual scopes/EPs in question will give a better answer. Our f4.1 20 inch Dob does vastly better with premium EPs. I have TeleVues. Even something like a Meade 4000 Plossl is pretty nasty at the edges.

Olly

[Ben Ritchie]

I'd agree with Olly, 'fast' to me implies f/4 or maybe f/4.5 - the f/5.9 of the 200P is nice as it's considerably less demanding on eyepieces, and I think most entry-level eyepieces still perform well.

[yeti monster]

My 10" SNT is f4 and I really can't say that it suffers from having the WA EPs used in it, other than they are 1.25" rather than 2". Perhaps my eye has yet to become spoilt by the high(er) end UWA EPs. Obviously the UWAs are of finer quality but in all reality, so long as you aren't putting the givaway sky watcher frosted glass EPs into your scope, you'll be rewarded with v. good to excellent views.

I'm of the opinion (although more than willing to be convinced to the contrary) that most of the bumf surrounding EPs is just that, and that there really is a case of diminishing returns on optical products.

However, those folk whom are ready to pay the extra for a particular brand to be embossed onto their toys (which appear to eminate from the same factory as "lesser" brands) are more than welcome to do so.

[ollypenrice]

It is certainly true that the law of diminishing returns applies to optical products to an extreme degree. I can't think of any area in which it applies more strongly, really. It is also true that recent relatively low cost wide angle EPs run the high end marques pretty close, whereas in the past this was not so. However, in our fast Dob, with a 'fair to middling' mirror, the difference between premium and budget EPs is not subtle, it is very marked and very obvious. I'd be the first to economize if this were not the case. A couple of visiting Ethos EPs (13mm) were downright sensational. Believe me.

Olly

[yeti monster]

This is why I am so willing to be convinced Olly. As I have only my limited experience to go off, I really need someone to force me to look through their scopes/EPs, and also to hold a gun to my head in order to have me try my EPs in their scopes (and vice-versa), thereby ensuring that I can form a really edjumicated opinion based on wide ranging experience.

[Ben Ritchie]

However, those folk whom are ready to pay the extra for a particular brand to be embossed onto their toys (which appear to eminate from the same factory as "lesser" brands) are more than welcome to do so.

The only direct example I can think of here is the William Optics UWANs and SkyWatcher Nirvana eyepieces, and I would agree that there seems little reason to buy the more expensive WO versions. As far as I am aware the likes of Pentax, TeleVue et al. aren't cloned, and do their own quality control.

[Mouse on the Moon]

I'm a newbie to this game, but I hope I can offer an insight from my own experience.

I bought a 6mm Celestron Omni Plossl new for £25 (I think). The experience was utterly miserable - like looking through a blurry dark pinhole. I'll be lucky to sell this eyepiece on fleabay for a tenner once I've paid fees and postage etc.

Bought a Televue 6mm second hand from SGL and the difference is night and day. Wonderful, bright sharp 60 degree views. The best bit is I'll be able to sell it back in a view years for £10-15 less than I paid. (might even break even)

An "investment" in a good second hand eyepiece makes financial as well as personal sense. You just have to be prepared to "tie up" your money in your eyepiece for the time you own it.

I've therefore taken the decision to spend a bit to assemble a set of second hand Televues that will give me the most viewing pleasure. Sure, I'lll spend a few hundred quid, but I will get almost all of that back! Like money in the bank

I'm not banging the drum for Televue particularly, as I'm sure the same applied to any "premium" second hand purchase of this type.

Hope this helps.

[ollypenrice]

Interesting thread, but to get back to the original question, at what sort of f ratio do you really start to 'need' (if that's the word!) premium or at least very good EPs. I only have one Newt so I can't be of any help.

Olly

[Ben Ritchie]

Interesting thread, but to get back to the original question, at what sort of f ratio do you really start to 'need' (if that's the word!) premium or at least very good EPs.

Depends on your eyes and how much optical aberrations bug you, I think. There's a very heavy subjective element to eyepiece performance, and you'll never get agreement between people who don't mind the edge performance breaking down a bit and others who demand point stars to the edge of the FOV. Hence the endless flamewars on CloudyNights...

As with everything eyepiece related, the best solution is to try as much as possible and decide where your requirements and budget overlap

[johninderby]

From my own experience I've found that in an f/5.9 dob the TMB planetaries work almost as well as a premium eyepiece. In an f/5 dob they still work very well but distortions are becoming more evident, but not objectionable, and they're still nice to use. At f/4.6 the premium eyepieces are just simply in a different class.

It seems that anything faster than f/5 is the point where you really start to see the benefits of getting the premium eyepieces. You wouldn't think that there would be that much difference between f/5 and f/4.6 but the f/4.6 is far more demanding on eyepieces.

John

P.S.

Do you think you'd need premium eyepieces for this 32" f2.3 dob :D:D

[Mouse on the Moon]

Have to agree with Ben on that one.

I think the "eyepiece question" will run and run as visual observing is ultimately a purely subjective experience.

On the "fast" scope question I have a no-name 12.5mm plossl that gives a lovely view, so the only way is to try out a few combo's yourself!

[Moonshane]

I agree with Ben that this is a very subjective subject and 10 different people will give ten different answers.

I decided to go for Televue a while back as they are one of the better brands and will last a lifetime if looked after, also working well in an scope down to f4. Many suggest that eg Televue eyepieces are just not worth the money but in reality not all TV eyepieces are expensive. TV plossls are not much more than other plossls but for the area they show I understand are as good as any other TV eyepieces.

If you want a wider field of view eg 60 degrees plus then eyepieces start to get more expensive to make if you want good edge performance in a faster scope. Almost all eyepieces will work OK for most of the field in a decent f6 scope (eg the SW 200p dob) and it's only the outer edges where you will get a little distortion.

Even with the best eyepieces though you also get 'coma' in faster scopes (the faster the scope the more coma - it's a result of the curvature of the primary mirror) so I wouldn't worry if I were you. As an example, your 6mm TMB would provide a satisfying image in a 200mm f6. I also get a decent image in my f5.3 dob with my 8mm version of the same eyepiece.

my advice would be go for more aperture (as much as you can) and a dob is a good way to achieve this economically. then try the eyepieces you have and if you feel that there could be some improvement then buy (or borrow) a better quality version of what you have and see if it improves the view noticeably. if it does start saving and plan to sell what you have unless you want to build up a dual set for the two scopes you'll then have.

my own methodology is 1) decide what 'gap' I 'need' to fill. 2) buy a cheaper but good version (e.g. the TMB series) of what I 'need'. 3) Use this for a while 4) If I remain convinced I am going to use it frequently I start saving for the appropriate Televue version. 5) When I have the TV version I usually retain this and sell the other one thus reducing the overall cost of the 'permanent item'. Also if you buy and sell used items then you will lose less money if you resell the quality items.

Hope this helps and hasn't strayed too far off topic.

[LukeSkywatcher]

I dont mind if stars near the edge of my FOV are not sharp. I usually only look at the centre of my FOV.

[Ags]

Thanks everyone, it sounds like F5.9 would not be too fast. The quality of the view accross the whole field is important to me.

[jonstarrysky]

you should be fine at f5.9. This is not fast. My f4.7 exhibits marked coma without paracorr, dependent on the particular eyepiece to some degree. Eyepieces I view as a long term purchase anyway, they can travel as you upgrade scope. At around f5 and faster you'll be wanting paracorr unless if you dont mind stars looking like seagulls around the edge.

[yesyes]

I've always wondered why a lower focal ratio is referred to as "fast". What exactly makes it faster? I know the light cone has a different angle. But why "fast"?

[johninderby]

It comes fom camera lenses. Here's an explanation from Wikipedia.

Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is a fast lens because it delivers more light intensity (illuminance) to the focal plane, allowing a faster shutter speed. A smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity and requires a slower shutter speed.

A lens may be referred to as "fast" or "slow" depending on its maximum aperture compared to other lenses of similar focal length designed for a similar film format. Lens speed given by the minimum f-number, or alternatively maximum aperture diameter or maximum numerical aperture, is a useful quantitative way to compare similar lenses.

John

[yesyes]

Ah, I see. Thanks a lot for that. ;-)