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Thread: Fast Scope Slow Scope?

  1. #1
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    Smile Fast Scope Slow Scope?



    I've read a lot about fast scopes and slow scopes and that some scopes are recommended for planetary viewing (slow)and others for deep sky viewing (fast). Why is this? I have been trying to figure it out for myself and this is where I am.

    Take as an example an aperture of 200mm

    An f/8 scope would require a tube length of 1,600mm (or 200mm x8)
    an EP of 4mm would give a magnification of 400x (or 1600/4)
    Is this good for planetary viewing because it will still give sharp views at high magnification? This is a slow scope?

    An f/4 scope would require a tube length of 800mm (or 200mm x4)
    an EP of 4mm would give magnification of 200x (or 800/4)
    with a 2x barlow it would bring the magnification to 400x also.

    So what is the difference between the f/8 and f/4? They have the same aperture and same light gathering ability so I think the view through both should be the same. Is the difference magnification? The f/8 can handle higher magnification better than the f/4 making it suited for planetary viewing? The f/4 with a shorter focal length and wide field of view is well suited to deep sky viewing as faint fuzzies are best viewed at low magnification? Is the f/4 scope good for photography because an f/4 needs less exposure time than an f/8? This is a fast scope?

    I know it's all questions but I have done some research and this is what I understand from what I've read so far but I could be well off the mark! At some point I would like to buy another scope and understanding what fast, slow, the f no and focal length mean will help me chose a scope which will suit my needs.

    Any help would be appreciated.

    Thank you

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    I'm going to approach this from the perspective of a visual observer - as you're aware, there are two contexts, including visual observation and imaging.

    For a visual observer, there are two considerations:

    - inherent image scale, which relates to the focal length, and indirectly to magnification

    - visual contrast

    For two instruments with identical aperture, but different focal lengths, the instrument with the longer focal length will have a larger focal ratio (f/#), while the instrument with less focal length will have a smaller focal ratio, as you've analytically determined with the math.

    The larger focal ratio brings gathered light into focus in a longer distance, which has some implications, like being more "forgiving" of optical aberration that may come about based on eyepiece quality and type. This is good for a visual observer, as the eye's integration capability, along with the brain's integration capability is all we have to "rid" the visual image of aberration.

    The longer focal length produces a larger image at the field stop (an internal point in space interior to the telescope) where the initial image is formed. The gathered light is spread over a larger area at the field stop, which improves image contrast. In turn, the larger image is less demanding for the eyepiece to form the image the eye sees - the eyepiece has two major components formed from individual lens elements - the first, the field lens forms the image from the telescope's field stop. The second, the "eye lens" forms the image seen at the eyepiece exit pupil by the observer's eye.

    If the initial image at the field stop is larger, with higher contrast (the gradient difference between light and dark parts of the image), the eyepiece has a less demanding job in terms of forming the final image the eye sees.

    The human eye detects edges and details better in the presence of higher contrast. The larger (slower) focal ratio improves contrast, which in turn improves the visual image of objects like planets, or objects like the moon which are not "point sources".

    The combination of larger initial image scale, contrast improvement, and magnification are favorable for planetary observation - which requires both lots of light gathering capability, as well as lots of magnification.

    "Fast" scopes, with low focal ratios are important for imagers, where mechanical considerations related to the telescope mount become limiting factors in imaging - the low, "fast" focal ratio allows shorter duration exposures to gather the same amount of light. Uncorrected, most fast systems have a number of optical aberrations, including coma, and other forms of spherical distortion. They also produce "bright" low contrast images, which our eye can't easily correct for, thus reducing the detail we see.
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    Thank you for your detailed reply AustinPSD. I've read it a couple of times and I think I understand it. Will mull it over and may come back to you with another question or two!

    Thank you

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    Carol,
    Look at it this way. For any given size eyepiece, the slow telescope with the longer focal length ... higher focal ratio .. will have a larger image and the fast telescope with the shorter focal length ... lower focal ratio .... will have a larger field of view. As you pointed out, by switching eyepieces you can get the same image size/field of view in either within some limits generally set by the human eye regarding field of view and atmospherics regarding magnification. This fast vs slow tag we give our scopes is from photography as the faster scope will require less exposure time.

    One thing your calculations of tube length omitted was catadioptric telescopes. The tube length of a catadioptric telescope is much shorter than its focal length which is only one of the many reasons Schmidt Cassegrain and Maksutov Cassegrain telescopes are so popular in amateur astronomy.

    You will notice a lot of fast f/5 telescopes on the market today that are sold as visual observation telescopes only. One very noticeable group are Dobsonian mounted telescopes. As you pointed out the tube length of a f/10 telescope is much longer than that of a f/5 scope. Thus, practical design considerations regarding physical use and size of the telescope, not its performance for visual observations, tend toward shorter focal lengths in DOB telescopes.

    The almost "standardization of f/5" for dobs comes about because that particular focal ratio provides the shortest tube length practical while readily allowing a diagonal to divert the light beam's focus outside the tube where it can be viewed. This characteristic is also why these Newtonians don't have the back focus needed for many aspects of work amateur astronomers do and is another reason for the popularity of catadioptric telescopes as well as growing popularity of refractors.

    The economic and physical attributes of this "short tube" characteristic also drives the design and marketing for visual observation of short tube refractors and Newtonian scopes. As Austin said, these characteristics are not based upon design for visual observation with the human eye but for other reasons; in my opinion, mostly economic but, in the case of refractors, also portability aspects.

    As Austin pointed out, fast scopes are better suited for photography and slow scopes for visual observation with the human eye. Medium speed scopes, around f/8 to f/10, tend to be great all around scopes suitable for both visual observation and photography which is yet another reason for the popularity of Schmidt Cassegrain telescopes.

    None of this is all black or white as many shades of gray are inbetween. In the end, it generally boils down to personal preferences.
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    Hi Sxinias,

    I have you to thank for making me think about this. In an earlier post you made this comment:-

    "The Orion Optics OD 200S f/6 telescope brings out the engineer in me. Me, I'd perfer the f/8 OD 200L but at 1.5 meters the tube is getting a little on the long side."

    This made me stop and think, why would you prefer the f/8 to the f/6? That go me doing a bit of research. Having had time to think about it, maybe a Dob is not the type of scope I want after all.... still thinking about that one! I will take your comments on board and check out the Catadioptric Scopes you mentioned. But I'm now in a much better position to make a choice. One more quick question how does a slower scope affect visual observation of DSO?

    Thank you


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    Quote Originally Posted by Carolgentle View Post
    Hi Sxinias,

    I have you to thank for making me think about this. In an earlier post you made this comment:-

    "The Orion Optics OD 200S f/6 telescope brings out the engineer in me. Me, I'd perfer the f/8 OD 200L but at 1.5 meters the tube is getting a little on the long side."

    This made me stop and think, why would you prefer the f/8 to the f/6? That go me doing a bit of research. Having had time to think about it, maybe a Dob is not the type of scope I want after all.... still thinking about that one! I will take your comments on board and check out the Catadioptric Scopes you mentioned. But I'm now in a much better position to make a choice. One more quick question how does a slower scope affect visual observation of DSO?

    Thank you


    Carol
    There'll be a lot of opinion on this - it is mostly subjective because of differences in our individual visual system based on age, eye health, visual acuity and "seeing training"...

    If we break DSO's into nebulae, globular clusters, open clusters, and galaxies, some things become apparent

    - many of these objects have low surface brightness - while their absolute magnitude, or the magnitude of any single object like a star within a nebula might be bright, because the object in general is large and diffuse, its apparent brightness is visually fairly low

    - some DSO's are relatively large in terms of their angular size - to observe the entirety of some of them, we need a large field of view

    - for DSO's that are mainly composed of point sources (clusters), we need resolving power to visually "separate" the individual elements within the object, and in some cases large FOV, and lots of aperture - these in some sense work against each other

    - for most DSO's, the human eye can't resolve color or structural detail for a variety of reasons, no matter how much contrast the observing instrument provides

    - some DSO's are very far away, and even though they are vast in actual scale, have very small angular size, and in some cases behave more like a point source than a diffuse object (galaxies)

    What all this suggests is back to the notion that it is hard to find a single instrument that can best support viewing all types of DSO's (visual or otherwise).

    A big Dob is a good example of a instrument designed for DSO's - the large aperture gives it lots of light gathering ability, needed for all DSO's, and the long focal length provides good initial image scale and supports high magnification. The "light cannons" in the f/4 range give the most spectacular views.

    The human eye can adapt over focal ratios from about f/3.3 to about f/18, giving reasonable image contrast in most cases for a DSO, because they are not as bright in terms of apparent magnitude like the Moon or the planets.

    Another contender, and "slower" in general than a Dob is the SCT.

    The SCT (catadioptric) instrument comes closest to being the jack of all trades, because its optical system is flexible.

    Most SCT's have a base focal ratio of f/10. By using focal reducers, we can "adjust" the optics to f/6.3, f/3.3, and f/2 (Celestron/Starizona Hyperstar), or with custom reducers to points in between. We can also increase the effective focal ratio with a tele-extender, or Barlow, or a combination of these devices.

    This flexibility lets one "tune" or configure the telescope for a particular type of object. The focal reducers increase the "speed" of the scope and increase its field of view - the f/6.3, f/3.3 and f/2 reducers are primarily intended for use with imaging gear, but in some cases can be used visually. There are "nose-piece" reducers that fit into the optical train differently that are more common for visual use.

    The base focal ratio of f/10 provides effective contrast for visual use, and the relatively long focal length combined with a Barlow and short focal length eyepiece can produce lots of magnification. This is good for planetary observing.

    The flexibility is one of the reasons an SCT is in widespread use - it also shares of of the economic benefit of the reflector design (aperture per unit of cost), it is physically compact, and optically easy to "correct" for most applications.
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    How does a slower scope impact the view of a DSO? This just depends upon the DSO. Some open clusters like M45 cover a large area and do not fit entirely within the field of view of a f/8 or slower scope. This does not mean that you can not view them, but just don't get to see it all at once. Fortunately, there are not too many such objects so for nearly all DSOs using a slower scope has little impact at all. On the other hand, there are just as many or more DSOs that benefit from the larger imaging capability of a slow scope such globular clusters, M42, etc. where magnification improves the view.

    My personal opinion …. others will disagree ….. is that the view of very few objects are enhanced by the large field of view offered by fast (f/5) scopes in comparison to slower (f/8 – f/10) scopes having the same aperture.

    As you investigate scopes, don't forget other attributes such as aperture are important. While it is true I would perfer a f/8 scope over a f/6 one, if the f/6 scope had a larger aperture then the story may very well be different. I certainly would not select a 90mm f/10 scope over a 130mm F/5 scope.
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    Joe and Austin have covered a lot of bases here Carol , so I'll not cover any more !

    What I will say is that folk who really get the bug , end up getting more than one scope . Methinks you have gotten the bug !

    If space is at a premium and lets face it , in Scotland it is at a premium , where all squashed up in the central belt . This affects our properties greatly . Would be great to have a 30 odd inch lightbridge but where do we put it ?

    This is the reason I ended up getting a SCT . Size and space. I did have a 8" Newtonian, still miss it at times , ended up selling it . Why ? Its size . Cumbersome to set up every time, esp with our finicky skies. If I had an Obsy ( in planning , hehe ) then I could have left it there .

    As has been stated already , the Jack of all trades is the SCT. Can be used visually and for astrophotography. Mine is a f10 ( 9.25" mirror ) the contrast is definitely not as bright as the the f5 Newt I sold but thats maybe not a bad thing , especially when looking at a big ol' bright Moon !

    SCT's tend to be more manageable than long tubed Newt's, offer longer focal lenghts and can be tailored to suit the needs of there usage ( reducers etc ) . Easier to store away , simple enough to take in a car to a dark sky site, dont catch the wind as much.

    One thing that I would also consider , is a GEM mount , especially if you want to venture in the realms of Astrophotography.

    Refractor scopes ? Great for contrast at lower f# as said , smaller in size ( usually , unless you are a Lotto winner ! ), and great glass ( ED ones, doublets and triplets ) Great for wide field imaging and viewing. Not so good on Planetary though due to shorter focal lenght.

    So , as you will see , there is a lot of factors in this hobby of ours . Horses for course as it were.

    My advice would always be to decide what you want to ''look/do'' , then set a budget .
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    Besides the medium quality of my optics (barlow ad EP) this is why I can't see jupiter and saturn sharper in my 130 f/5?

    Saludos and clear skies!

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    Thank you all for your excellent advice. I'm just on my way out (local astronomy club meeting) so will have get back to you with some questions later. You're right Pete I do have the bug & know that AP will be the next natural step for me. Got lots to think about!

    Cheers

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