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barbusg60

Aperture Or Focal Length

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Guys,

If I was interested in the moon and planets, which would be best?

A scope with 114mm aperture & a focal length of 910mm or one with an aperture of 130mm but a shorter focal length of 650mm?

Thanks in advance,

Rob.

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A larger aperture means the telescope has a higher light-gathering ability. A larger aperture is more important than a longer focal length, because magnification decreases the amount of light coming into your eye, and a smaller aperture will gather less light and so be more limited as to it's magnification.

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Bigger aperture will gather more light and also show more detail.

I suspect that the 114mm is also a spherical mirror at that focal length whereas the 130 will be paraboloid and more effective at focusing light to a point which is good for higher magnifications for planets

what telescopes are you considering?

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The above reply by Naemeth is quite correct but for planetary and Lunar work aparture is only really important for resolution (the ability to seperate fine detail). Far more important is focal length as this determines the size of the image formed at prime focus. It is that image that you look at with your eyepiece. So, basically, longer focal length means bigger image. As the Moon and planets are, generally speaking, quite bright the "better" scope would be the one of longer focal length.

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I personally would go for 130mm over a 114mm aperture, presumably you are looking at the small reflectors. I guess a lot depends on your budget and although I have a 130mm skywatcher branded reflector, you may be better off with a small mak but it would bring the cost up a bit, or maybe a 6" dob?

Also you say interested in the moon and planets, do you wish to view them or to take pictures of them? It makes a huge difference

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Thanks Guys,

I am looking for an affordable scope to view the moon and planets and would also like to try using a webcam in the future if possible.

I have been thinking of a Heritage 130p but spotted an Orion 4.5 dob or an Orion Starblast 6 although both of these are more expensive and I am not sure if they will offer anything the 130p doesnt.

I think you are correct about the 114mm Orion dob not having a parabolic mirror too.

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This is a tricky one as I am assuming that we are talking about a reflector rather than a refractor type scope. Aperture does mean greater light gathering capability and where there is light, there is information (resolution). Magnification decreases the amount of light because it stretches it but in your example, you are only wanting to look at brighter objects to start with than the more faint deep sky objects. Magnification (through focal length) is important and although the shorter scope will gather more light, using a 6mm eyepiece will yield a magnification of only x108, which of course is better that a pair of binos but when looking at planets, you really need to be using somewhere in the region x150 and upwards, otherwise these objects remain too small. There are smaller modestly priced eyepieces than 6mm but the choices start thinning out fairly quickly when you go beyond that, and so I would rather have 1" less aperture but have the ability to increase the magnification (longer tube) through a wider choice of eyepieces. Of course when we are talking about looking at planets with either of the above scopes, what we are realistically talking about is discerning some detail on Mars. Jupiter and Saturn. Mercury is to near the sun to view, Venus will be far to bright to view any detail with Pluto and Uranus only revealing themselves as very small faint discs of colour. The moon is so bright that any magnification that can help reduce the glare is actually a useful consequence and being so much closer, there is very little difference between what the apertures will show. What you really need is the 130mm with a longer focal length then you can have the best of both worlds.

James

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At around that focal length spherical is starting to not mater greatly but nevertheless the 130 heritage and the 6 inch orion will have better quality mirrors.

bigger focal length will give you higher magnification for planets than a shorter focal length scope if using the same eyepiece but if it was a choice between the 130 and the 114mm I would say 130 as better mirror and you can always use smaller focal length eyepiece to get high mags plus the bigger aperture toy have the higher your maximum usable magnification is.

But......

If you have space to store it and use it like a garden (or a car to transport it to a suitable site) have you you thought about a 150p skyliner dobsonian by sky watcher? About 200 pounds, good 6 inch mirror with 1200 mm focal length. It will be good on planets plus other targets if you fancy. Iwould have one myself if I had a garden. Depends if your budget can stretch to it....

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You can always get an eyepiece (perhaps barlowed) that will allow you to reach the maximum useful magnification, no matter what your focal length. So don't loose sleep over the focal length. The aperture and optical quality matter a lot more. The most you can squeeze out of the scope will be about 40x per inch (perhaps you'll get more, perhaps less, it depends on the optics and on how well your eyes tollerate small exit pupils. If floaters bother you then it may be unpleasant). So don't expect the 130 mm to give you more than about 200x. Probably you'll be operating closer to 150x. The 114mm will mean you have to settle for less power. Also, avoid the spherical mirrors. You want the parabolic.

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If you have space to store it and use it like a garden (or a car to transport it to a suitable site) have you you thought about a 150p skyliner dobsonian by sky watcher? About 200 pounds, good 6 inch mirror with 1200 mm focal length. It will be good on planets plus other targets if you fancy. Depends if your budget can stretch to it....

I agree one of these is the scope I would pick if I were new to astronomy today.

http://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-150p-dobsonian.html

Great all round performer, cheap, easy on eyepieces, extremely portable, set up time is in seconds, it'll hold collimation unless you throw it down the stairs, and tough as old boots.:)

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The focal length that determines magnification is the 'effective focal length' of eyepiece and scope. In visual use it really makes little difference how you arrive at this effective focal length. It can be with a long FL scope and short FL eyepiece or it can be the reverse. While theory might suggest that it is best to go for a long FL scope, the reality is that a number of other things matter more. I would go for aperture and a parabolic mirror myself. On a tight budget I'd always go for a Dobsonian mount as well because at planetary magnifications you want stability first.

Olly

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Might I suggest that if your main interest is solar system objects that a fl10 refractor would be the best tool for the job. If this is the avenue that you do decide to go down then I again would suggest a TAL 100RS at the cheaper end of the market. (the TAL 100RS is not a budget scope BTW) You can pick these wonderful scopes up for as little as £150 second hand.

Edited by Caldwell 14

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Resolving power is important. You can't magnify detail that isn't there. So size matters, but so does quality of the optics. What you would want for planetary work is plenty of resolution with decent contrast.

Focal length isn't so important as any scope is capable of offering the required magnification.

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Assuming similar quality & that we're talking reflectors i'd be suprised if there was much to choose in the views between the 114mm & 130mm.

The 130mm f5 will have a larger %~wise secondary lowering contrast somewhat c/w the 114mm offsetting it's marginally greater resolving power.

One thing to consider is the ease of focus, i've found acheiving crtical focus particularly important on the planets.

Ease of focusing varies inversely with the square of the focal ratio so the 114 f8 will be over twice as "easy" to focus as the 130 f5 without resorting to fancy focusers.

FWIW. i'd be tempted to save up a bit longer & go for a 4" f10 refractor or a 6" f8 reflector if it's the moon & planets you're chasing.

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+1 on what Olly is saying. Unless we're talking cheap refractors, the focal length or focal ratio aren't what will be limiting performancing on planets. The other stuff will matter more. Modern eyepiece designs offer good eye relief at short focal lenghths and a good barlow will not degrade the view in any way.

Edit: if you can swing a good 8" Dob (perhaps second hand) then the planetary detail will thank your for it.

Edited by umadog

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Personally I feel that for high brightness/high contrast visual observing, i.e. of planets and moon, the key factors in approximate order of importance (to me anyway!) are collimation, optical quality (of eyepieces and telescope - more the former though), focal ratio and aperture.

For me though, I suspect many people will get 'bored' (not really the right word) quickly with planetary and lunar observing and want to see other objects like galaxies, clusters and nebulae. For such objects the requirements are somewhat different (again for visual) with aperture, collimation, optical quality and focal ratio in order of preference (again for me).

I have three scopes. My best ever planetary views (far better than a 100mm f9 ED scope I compared with) were with my 16" f4 scope masked to 170mm f11. Next best were with my 6" f11 scope and third place my 6" f5 scope (all dobsonians). In my experience, the views through all three scopes were superior to a 100mm f9 ED scope I observed through and I could easily see Cassini division, planetary banding and several moons with all three scopes when observing Saturn recently at mags of 125x approximately.

If you are interested in visual observing (or occasional webcam imaging) then I'd suggest the largest aperture you can justify/carry/afford/store as this will provide the ability to see many more observing targets even if you suffer light pollution at home.

Everyone with have a slightly different opinion on this sort of thing but for me, nothing compares to the view through a big dob, no matter what you are looking at.

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  • There is on ebay a Saxon 8" Dob which if you look at is a skywatcher flextube in a white dress. Might be worth a watch as it's start price is £150. Look for Saxon Dobsonian on ebay. I'm tempted myself with it being a flex for easy transport.

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Like Moonshane, my best views have been through my 18" f/4. I haven't tried stopping it down, though. I keep meaning to give it a shot but I'm happy with full aperture views so I've not yet done so.

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