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Why have fast scopes ?

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Hi All

I was just wondering how come we have fast scopes? Fast scopes are often so demanding on EP's that the outer 15% in most EP's are less than desirable so the larger FOV is some what questionable to it's effectiveness. More often than not a fast newt is huge in comparison to a similar aperture SCT or MAK and the longer length often means they can be more demanding on mounts. I understand a fast scope is advantageous to imagers but most scopes are sold with the visual astronomer in mind. Would it not be ideal to keep scopes that suffer aberrations to a set focal length that offers wide FOV but also are less demanding on the average persons selection of eyepieces ? Even with what most would consider a crown jewel in eyepieces there is no escaping the demands such a scope can place. I just often find it odd that a cheaper newt holds it's resale value reasonably well in comparison to a slower more expensive scope despite you having to spend a small fortune on EP's trying to combat aberrations. Another question is how come SCT's & MAK's always seem to have such long focal lengths? Is there no way they could be shortened by the manufacturer to offer wider FOV ?

Reading many reviews that suggest the vast majority of EP's work well in scopes f/7 and slower why not have this as a bench mark and offer faster scope to those who want to image or find aberrations less of an issue. Or am I missing some thing ?

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The reasons, IMO that manufacturers produce fast newts is.

#1. Packaging and shipping.

Have you noticed how most commercial reflectors are around 4' in length. ie 6" f/8, 8" f/6, 10" f/4.5 all around a 4' tube length. This isn't done for the laugh of it it's to do with packaging and shipping costs.

It's cheaper for big manufacturers to make faster mirrors than to start shipping different size containers.

#2 is mounting them.

Long focal length newts need considerably stronger mounts than there short tube equivalents because you're placing the mirror further from the fulcrum it puts considerably more strain on the mount. Can you imagine how large an EQ would need to be to hold a 70" long 10" scope.:)

#3 eyepiece height

On large Dobs who wants to be standing on a step ladder all the time? Because you will be if your mirror has a long focal length.

A 12" f/7 is huge 7' long a 16":icon_eek: well you get the idea.

IMO the demand for bigger and faster newts is only going to increase as new lightweight ways of mounting giant optics is realised. And let's be honest the views are great through wide field scopes.;)

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For newts, i imagine it's to keep the size down. It makes 10-12" newts just about manageable for everyday cars, and means you dont need a stepladder to use them.

So yeah, probably a physical requirement rather than a choice, i'm sure most people would love to sit around f7, but it would limit the amount of aperture the average joe could fit into his life.

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There's so many points to consider here, so let's take it one type at a time.


f/4 newts are obviously primarily for imaging, but most dobsonians over 16" are mostly about f/4 otherwise the eyepiece would be inconveniently high. The obvious drawback is you need the very best eyepieces.

f/5 newts are popular in smaller sizes as they are versatile and reasonably sized. Many use f/5s as all-rounders as they are still well suited to imaging. They are FAR less demanding on eyepieces than f/4.

f/6 and up newts are less common because in sizes over 6" they get very long and bulky. I have a 12" f/6 scope which is fine with most eyepieces, but is enormous. It's as tall as my 16" f/4.5 was, and the only reason I can put up with it is because I'm 6'5". 8" dobs are often f/6 to bring the eyepiece up to a more reasonable height. Of course the other reason is maintaining the widest possible field of view for that aperture - beyond f/7 you can no longer achieve the maximum exit pupil.


Often in the f/7 range as that is a good balance between reasonable speed for imaging and tube length on the one side, and ease of manufacture and optical quality on the other. Faster refractors will either cost more or be optically inferior to slower. Slower refractors are respected by observers, but their downside is bulk and unsuitabilty for imaging.


SCTs and MAKs bounce the light around inside to make a long focal length in a short package. They can't really be made faster because their primary mirrors are usually about f/2 for an f/10 final system.

So in the ideal f/6-8 range, which gives a good balance between undemanding and size, you're stuck with refractors or newts under 12", or a handful of exotic mirror scopes...


Edited by Andrew*
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With the exception of dobsonians, I would imagine that now most scopes are sold for the imager in mind ; and portability too.

I thought this was the reason behind the PDS range ?

I understand the points being made but I would have just hoped for a better reason why my bank account is being emptied by the need of better eyepieces.

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I thought this was the reason behind the PDS range ?

I understand the points being made but I would have just hoped for a better reason why my bank account is being emptied by the need of better eyepieces.

Need ... or want? :)

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