Telescope focal lengths

[milkyjoe]

I'm trying to decide whether to purchase either a celestron nextar 4se or a celestron 102 slt. I have approx £350 to spend and can't really afford anymore. I have been review after review after review on these two scopes, but I am still no closer to deciding; because each scope seems to have it's own advantages and disadvantages, and I cannot find an equilibrium!

I suppose the first question I should be asking myself is 'what do I want to use it for?', looking at the moon and the planets or dso's... or both. I would like the best of both worlds, but it is evident that this is going to be impossible.

Rather than asking (like every newbie does) 'what can i expect to see?', I would like to ask, firstly, what is the difference between a fast focal length scope and a long focal length scope..., and second, how good the image quality would be for each type of scope? (I know I'm going to open up a can of worms with that question - lol).

I think if I can get a better idea of what each type of scope is capable of I can finally make up my mind and buy one, otherwise my brain will explode with information overload!

Both scopes are quite evenly matched, both have 102mm of aperture, but one is a mak and the other a refractor. Both are computerised.

Any advice would be appreciated.

Thank you.

[NGC 1502]

Hi, the Celestron 102, with its short focal length (fast) would be better for low to

medium power, more suitable for deep sky.

The 4se, with its longer focal length, would be better for moon, planets, double

stars etc as you don't need a wide view for these.

I'm afraid I can't comment on the "go to" capability of these scopes.

Hope this helps, Ed.

Edit : Personaly, I think that if you are new to astronomy, then the 4se would be most suitable, as you

probably want to see Saturn's rings, Jupiter & its moons etc, & the 4se would be better for this.

You can point any type of telescope at any type of object, but they vary as to what each type is

most suited. Cheers, Ed.

[fatwoul]

I may be speaking above my post count, but I'm not sure I understand what you mean by a fast focal length.

Focal lengths can be long or short, making the field of view narrow or wider respectively, but the "speed" of the scope is dictated, surely, by the aperture; the size of the objective relative to that focal length.

My experience is photographic, not optical, so I end up thinking of telescopes in those terms.

The 102SLT is f/6.5, making it 2 stops brighter (faster) than the f/13 4SE. The 102's focal length is half that of the 4SE, so can cover a larger portion of the sky.

So it seems to me that the 102 would be good for dimmer objects that cover more sky, such as nebulae, and the 4SE would be good for planets.

If it was me, I'd choose the 102. I'd shove a x2 barlow on it for observing the moon or planets at a similar focal length and f-number to the 4SE, but use it without for viewing faint fuzzies.

The 1300mm focal length of the 4SE would put me off, because it is longer than the suggested 1000mm max focal length that most mounts I've used are reliably capable of holding steady.

Sorry for the vague answer from this newbie.

EDIT: You've had two answers and they say the opposite thing! Maybe you should find a coin...

[steppenwolf]

I agree with Ed's assessment but would add that the 4SE will produce views without colour fringing whereas the 102, being an achromat, will deliver views with chromatic aberrations which might spoil your appreciation of what you see.

Sadly there is no 'one size fits all' with telescopes but I guess that the 102 would be the more flexible as you can relatively easily ramp up the magnification for Lunar and planetary work as well as using it as a widefield instrument for DSOs.

[barkis]

I can appreciate your dilemma MJ.

The thing that I always think about when someone is contemplating their first telescope, is why, when on a limited budget, (and please don't take offence at that, I don't mean to be), they insist on buying new. If you look around the for sale boards of the populat Astro Forums, you will find some very decent instruments on offer, which will suit you perfectly.

What is wrong with a 200mm f5 reflector in a Dob. mount?

You could buy one easy with the money you propose to spend, and still have some left for a couple of decent eyepieces.

To me, an 8" f5 is a superb scope, and will deliver splendid views of many DSO's, and the moon and major planets under the right seeing conditions.

Just my two pennorth, but of course it's your money, and you must do what you like, having weighed up all the input you receive.

Good Luck

Ron.

[John]

I'd echo what Ron says - do have a good look around at the other options that are available within your budget.

You are paying a good proportion of your budget on the computerisation aspect when it's the optics that you actually look though. £350 will buy you a lot of scope with a lot more observing potential if you can look beyond those with computerised mounts.

[LukeSkywatcher]

All i know (or think i know) about focal lengths is that the shorter the f/l means that the light from celestial objects arrives at the EP quicker. So scopes of say f5 are FAST and scopes of say f11 are SLOW.

What i cant understand is why scopes of f5 are GREAT for observing certain celestial objects while scopes of say f11 are better for observing others.

Is it a question of DISTANCE?

I have 3 scopes. Two of which are f5.? (70mm,130mm)and the other is f11(90mm). I seriously can not detect any difference between them all apart from the brightness of images. The 130mm gives me the brightest images because it grabs more light. The 90mm gives me less bright views then the 130 and the 70mm gives me the least brightest views..........................but regarding size of object they are all about the same.

Just thought of something: my 70mm scope has a f/l of 400mm, my 90mm scope has a f/l of 1000mm and my 130mm scope has a f/l of 650mm.

Honestly i care not about the numbers,figures or maths or scopes.

I am happy once i get a crisp clear view of whatever. If i dont.................i put it down to "seeing" and move on.

Screw the science...............enjoy the hobby.

[milkyjoe]

Thanks for all your comments guys.

I hear what you're saying Ron, but to be honest, the reason I want to start small and with a computerised mount is so that it can introduce me relatively quickly to astronomy and if I get bored, I know I haven't blown a small fortune on a scope! I have read reviews about users who have 8", 10" dobs, but get frustrated because they can't find anything to look at.

'fatwoul' - that was my thinking about the 102, I can use it for dso's and planets using a 2x barlow. The only thing that put me off was the fact that the scope suffers from purple fringing. Can you buy filters to combat the fringing?

What make of eye pieces would you recommend?

Almost forgot, how easy is it to setup a digital camera to this scope, I have a Panasonic TZ7?

Thanks.

[fatwoul]

All i know (or think i know) about focal lengths is that the shorter the f/l means that the light from celestial objects arrives at the EP quicker. So scopes of say f5 are FAST and scopes of say f11 are SLOW...

The light doesn't arrive at the eyepiece quicker in a fast scope. Isn't "fast and slow" a reference to length of exposure required to produce an image on a piece of film? A fast scope (low f/number, large "aperture"), exposes film (or a CCD) more quickly than a slow scope with a higher f/number / smaller "aperture".

For example, an f/5 scope would require a quarter the time to expose film to the same amount of light as an f/10 scope.

[fatwoul]

...'fatwoul' - that was my thinking about the 102, I can use it for dso's and planets using a 2x barlow. The only thing that put me off was the fact that the scope suffers from purple fringing. Can you buy filters to combat the fringing?

Can you get to any clubs and have a look at some scopes? Maybe the fringing isn't as bad as reviews might have suggested, or perhaps it won't bother you as much as you might expect.

[Ags]

I have a 4se. The F ratio of the objective is only relevant for astrophotography and for purchasing eyepieces (small F ratio means more expensive eyepieces). Higher F ratios such as the 4se also have less optical distortions like coma, as the optics are bending the incoming light less (which is also why higher F ratios tolerate more eyepieces).

For visual observing the brighness of an image is determined by the aperture and the magnification used - with lower mag giving brighter images. F ratio is irrelevant.

So the SLT is not better at DSOs - it will be equal to the similar-sized aperture mak 4se. On the other hand the Mak will have less color fringing and a better focal length for viewing planets and the moon. The only thing the SLT will do better at is wide-field views as my Mak cannot give a lower magnification than approx 40x giving a 1.3 degree field of view. The SLT will give more than double that which will be great for viewing some clusters and star fields.

Another thing to consider - the 4se mount is more stable (important for higher power viewing of planets) and it has a built-in wedge so you can polar-align and try out some very basic astrophotography. You can try afocal photography or you could piggy-back a DSLR.

[milkyjoe]

I have a 4se. The F ratio of the objective is only relevant for astrophotography and for purchasing eyepieces (small F ratio means more expensive eyepieces). Higher F ratios such as the 4se also have less optical distortions like coma, as the optics are bending the incoming light less (which is also why higher F ratios tolerate more eyepieces).

For visual observing the brighness of an image is determined by the aperture and the magnification used - with lower mag giving brighter images. F ratio is irrelevant.

So the SLT is not better at DSOs - it will be equal to the similar-sized aperture mak 4se. On the other hand the Mak will have less color fringing and a better focal length for viewing planets and the moon. The only thing the SLT will do better at is wide-field views as my Mak cannot give a lower magnification than approx 40x giving a 1.3 degree field of view. The SLT will give more than double that which will be great for viewing some clusters and star fields.

Another thing to consider - the 4se mount is more stable (important for higher power viewing of planets) and it has a built-in wedge so you can polar-align and try out some very basic astrophotography. You can try afocal photography or you could piggy-back a DSLR.

The 4SE was the first scope I was interested in to begin with, but then I heard about the 102 SLT and the Sky Watcher Startravel 102 SynScan AZ Goto. I liked the idea of an F/5 scope so that not only could I look at the planets (with a 2x barlow) but also look at a few DSO's as well due to the 102's wider field of view, as you stated, but now with each reply I get to this thread, I'm finding myself going back and forth between each scope!

I realise that the 102's (SLT & AZ Goto) are refractors and will suffer from some chromatic abberation, but it can't be that bad or no-one would buy one. The thing that bothers me is that I simply don't know what to expect with each type of scope, which is hindering my thought process in choosing a scope to purchase! :-\

So, to recap then...

I would like to look at the planets and try some astro photography as well at some point, but I would also like to look at nebulae too. Would the 4SE bet better to mount a compact camera to than the SLT/SkyWatcher scopes?

One question I need to ask is what options are there for people like me who wear eye glasses when using a scope?

Decisions, decisions...

Regards.

[Capricorn]

I think we should drop the terms fast and slow when using a scope for visual use. The term is relevant to photography.

MJ you are spending way too much time thinking about a scope. I suspect you have a good idea which one you would like to have, so buy it.

Talking about it here or elsewhere doesn't get a single second of observing done.

If the 4SE, is it then if close enough ask A-B about hers. She knows what she is talking about.

For astrophotography you really need an equitorial mount, not an Alt/Az, and as none mention are on an EQ but are all Alt/Az they are not suited to AP.

[milkyjoe]

I think we should drop the terms fast and slow when using a scope for visual use. The term is relevant to photography.

MJ you are spending way too much time thinking about a scope. I suspect you have a good idea which one you would like to have, so buy it.

Talking about it here or elsewhere doesn't get a single second of observing done.

If the 4SE, is it then if close enough ask A-B about hers. She knows what she is talking about.

For astrophotography you really need an equitorial mount, not an Alt/Az, and as none mention are on an EQ but are all Alt/Az they are not suited to AP.

You are right Capricorn, I am spending way too much time reading, it's driving me bonkers!

I would like to buy the 102 SLT or the SkyWatcher 102 AZ GOTO, but I am concerned about the purple fringing, and whether I will be disappointed if I buy the scope and I don't like it: I'm losing my marbles!! ha-ha

I've read that review of A-B's 4SE, it was very interesting.

[Ags]

The 4se is better for astrophotography because it can be polar-aligned using a built in wedge (converting the alt-az to kind-of equatorial mount). This means it can track the stars' motion without the stars drifting about. The SLT mount will correctly track a point in the sky - but the view will slowly rotate around the point being tracked - bad news for longer exposures. I think anyone really into astrophotography would say that neither of these two telescopes is much of a basis for astrophotography though.

Most DSOs are not that big. The 4se is fine for viewing globular clusters, double stars, planetary nebulae and so on. I've even seen a few galaxies in the virgo cluster a couple of months back - but only as faint smudges (you would need a huge telescope to see more). Only a few of the brighter DSOs are too big. The only time I have wanted a wider field of view has been when trying to look at open clusters like Praesepe - I don't enjoy looking at these bigger objects in the 4se as they are much bigger than my field of view.

If DSOs are your main interest then you should look to getting a larger aperture as the 4se only has enough light grasp for the very brightest DSOs.

[Ags]

Regarding eye glasses - you need to look for eyepieces with 'long eye relief'. My experience as an glass-wearer is that the eye relief is never long enough, on my budget anyway!

[angusb1]

If you're on a tight budget (like me ;-)) you could keep an eye out on eBay, astrobuysell and the for sale section of this forum for some secondhand gear. That way if you decide you want something different you can sell it for roughly what you paid for it and then all its cost you is the postage. Like Capricorn said if you want to do photography you need an equatorial not an alt-az mount. Your budget will get you this if you buy secondhand. Good luck!

[fatwoul]

I think we should drop the terms fast and slow when using a scope for visual use. The term is relevant to photography...

Exactly. It just seems to confuse people unnecessarily, too.

[gareththegeek]

Can you buy filters to combat the fringing?

You can buy filters such as Achromat & Semi-Apo filters - Baader Fringe Killer Filter

Can't comment on how effective they are though, I have never used one.

[ollypenrice]

Lets sort out fast and slow first. These terms refer neither to focal length nor to aperture, they refer exclusively to the reltionship between the two - the focal ratio.

To find this you just divide the focal length by the aperture. A 100mm aperture with a focal length of a 1000mm is f10. (Slow) The same aperture with a focal length of 500mm is f5. (Fast.)

As has been said, the term is mainly relevant to photography. In visual use you don't use the native focal length because you put in an eyepiece to extend it. Within reason this means you can get most magnifications with a fast or a slow sope. You just change the EP.

I love refractors but fast refractors are very hard to make and expensive as in 'you might not believe it.' (An 85mm Takahashi FSQ is about three grand. Barking. (I still bought one though!)

A low cost fast refractor will have a lot of false colour and I would go for systems cheaper to make, like Newtonians or SCTs/Maks.

I would spend as much as possible on the optics and forget the fancy electronics on the mount. A telescope is for looking through. Optics are everything.

I agree that second hand is good and if you change your mind there is little or no depreciation.

Olly

[milkyjoe]

Lets sort out fast and slow first. These terms refer neither to focal length nor to aperture, they refer exclusively to the reltionship between the two - the focal ratio.

To find this you just divide the focal length by the aperture. A 100mm aperture with a focal length of a 1000mm is f10. (Slow) The same aperture with a focal length of 500mm is f5. (Fast.)

As has been said, the term is mainly relevant to photography. In visual use you don't use the native focal length because you put in an eyepiece to extend it. Within reason this means you can get most magnifications with a fast or a slow sope. You just change the EP.

I love refractors but fast refractors are very hard to make and expensive as in 'you might not believe it.' (An 85mm Takahashi FSQ is about three grand. Barking. (I still bought one though!)

A low cost fast refractor will have a lot of false colour and I would go for systems cheaper to make, like Newtonians or SCTs/Maks.

I would spend as much as possible on the optics and forget the fancy electronics on the mount. A telescope is for looking through. Optics are everything.

I agree that second hand is good and if you change your mind there is little or no depreciation.

Olly

That's very good advice olly. I've been going round in circles for the past week trying to decide which type of scope to buy, but in the end, I just couldn't make up my mind! :-\

I will stay away from refractors based on your sound comments about purple fringing - don't want any of that do we! lol

I (think) I've come to the conclusion that I am either going to buy the Celestron Nexstar 4SE, the 127 SLT or the Sky Watcher 127 Synscan AZ GOTO..., but which one is best; are they all made by Synta?

OR maybe a scope without the GOTO system, but me being a complete novice and all, it might come in handy. What do you think?

Regards

Richard.

[brantuk]

You can allways get a slow scope and use a focal reducer for those times you want a fast scope. E.g. A slow f10 scope can be turned into a faster f6.3 scope with a reducer. Quality optics will be important though if you do that.

Hope that helps

[ollypenrice]

Brantuk makes a good point about reducers though it applies mainly to SCTs since reducers for refractors don't reduce by much. They are intended for imaging.

GoTo? Well, here are some points that I have seen on the forum and some ideas of my own.

- Some people do not get GoTo to do what it says on the tin and ask a lot of questions about how to get them to work. That is a straightforward fact, though what is not clear is how many people have this trouble. It may only be a few. How would we know?

- The vast databases of small GoTo scopes greatly exceed the 'eyesight' of the telescope. You GoTo but don't see!

- Electronics are not all that reliable in the long term. Optics last for years and years.

- Part of the fun (and frustration) of astronomuy is learning your way around.

- But if you are light polluted then it is murder!

I would like a GoTo 20 inch scope becasue when it goes to some feeble little unfindable thing I will be able to see it. I would not buy a small aperture GoTo personally because the things within its grasp are fairly easy to find.

As an imager I am often, even usually, imaging things I have no hope of seeing in any telescope that has ever been made, so GoTo is a huge boon. (Getting a bigger scope doesn't help because as the focal length increases the field of view diminishes and the large faint object will no longer fit.)

There is no answer to this. You have to decide what is right for you. On a budget I would get the biggest and best optics I could on the simplest mount. Enter the Dobsonian.

The main thing is the beauty of the night sky. Don't get sidetracked from that!

Olly

[Space Cowboy]

For £350 with GOTO and imaging capabilities (planets) Skymax 127 is the one you want! If you want to view DSOs in any detail then its got to be at least an 8" Dobsonian....they are about £270 new but you could easily get a used 10" for less than £350......BUT you wont have tracking for imaging or GOTO.

Its all very well experienced members saying don't get a GOTO because its wasting money but for a beginner it makes a huge difference. What good is a wacking big scope if you can't find anything??????

[Ags]

I agonized over my first scope for a long time, and worried for months afterwards that I may have made a mistake. Now I am happy with my 4se. I think you would be too. You would also be happy with any of the other telescopes you are looking at! They are all good - at least, members of this forum own them and are happy with their purchase...

Just go to the FLO website blindfolded and keep clicking until your credit card starts smoking! ;-)