telescope specs need explaining please

[black hole sun]

Hi all, please please could somebody in simple terms explain all the numbers attatched to telescopes and what is important and what is not. Ive read that basic consumer telescopes dupe you into thinking your buying a mini hubble....not the case obviously! My potential purchase is a Meade ETX90EC and the baffling info that needs interpreting is below. So i turn to the experts from a novice, thanks in advance for any help,Scott.

Optical design: Maksutov-Cassegrain Primary mirror diameter: 96mm (3.78") Clear aperture: 90mm (3.5") Resolving Power (arc secs.): 1.3 Focal length: 1250mm Focal ratio: f/13.8 Eyepiece: Super Plossl 26mm Maximum practical visual power: 325x Optical tube dimensions: 4.1" x 11"

[barkis]

The numbers on telescope labels vary according to the type of telescope. The numbers are very significant, as they represent the specification of the instrument.

Basically, one number indicates the lens or mirror diameter in millimetres

The letter f will be the telescopes focal length, usually in millimetres.

The focal ratio of the scope will be the larger number, divided by the smaller number. For example, if the f number, is 1000mm, and the mirror/lens diameter is 200mm, then simply by division, 1000/200= f5

The f5 can be reasonably assumed to be a fast system, with regard to photographic speed. It also represents a reasonably widefield scope, which will give a wide view of an object such as the Pleiades star cluster, depending on the power of the eyepiece used.

It's dfficult to incorporate all the relevant elements involved in one reply.

Ron.

[black hole sun]

Thanks Ron, appreciated. So f13.8 is good or not? am i right in thinking focal length 1250mm / mirror diameter 96mm? = f13.8? or have i got to sit at the back of the class?

[barkis]

The telescope you refer to in your post is a compound telescope, which is a reflecting telescope, it's design allows it's focal length to be encompassed in a short tube, because the light from the star or other object, passes through a front plate in the tube, which also holds a secondary mirror, which reflects the light bounced from the main mirror, back down the tube, through a hole in the main mirror, and on through the focuser and into the eyepiece.

So effectively, the beam of light is folded.

The particular specs. for that unit, would make it fairly limited as to what you could observe with it. Probably the best objects to use it on would be the moon, the brighter planets, and double stars. It would not perform well on Deep Sky objects.

[smoggy]

f13.8 will mean it is a slow telescope, though slow doesn't mean bad! There is quite a small field of view which makes it perfect for getting largeish images of the planets. A faster scope means you just see a larger area of space.

[barkis]

Thanks Ron, appreciated. So f13.8 is good or not? am i right in thinking focal length 1250mm / mirror diameter 96mm? = f13.8? or have i got to sit at the back of the class?

Yes, you got your sums right, take a bow .

[black hole sun]

Hha, thanks for the advice and happy i got my sums correct ;-)

If I want best for my money, around £300ish what you guys/gals recommend?

[sgazer]

If you want it computerised to find stuff quickly and the scope to track the object, the scope with max aperture in your price band is;

First Light Optics - Skywatcher Explorer 130P SynScan AZ GOTO

If you want to find stuff yourself and not have tracking, then you could move up in aperture to this (objects will be 2.3x brighter);

First Light Optics - Skywatcher Skyliner 200P Dobsonian

[barkis]

There is a phrase in astronomy that says "Aperture is king", meaning the bigger the objective, ie, the mirror or lens the better.

It doesn't always apply in this modern era, because technology has improved things a great deal. Having said that, I personally, (and others will differ on this,) would make sure that any sum I spent on a first telescope, would go towards buying the one with the largest objective, meaning the mirror or lens.

At least by doing that, and you will have to sacrifice other bells and whistles, but those fancy things won't let you improve on the object you are observing. A bigger lens will let you see more lunar craters. The more impressed you are with your first telescopes performance, the better the chance you will love astronomy, and stick with it. Many a newbie has been disillusioned because their first scope purchase was dissappointing and a big let down.

There are some great little scopes out there, don't get me wrong, but there are some duds too. Just be careful, money doesn't grow on trees as you well know.

Seek advice before you commit, there's plenty of help and advice available on SGL, just ask.

[mag10]

The three most important things in astronomy:

Aperture.

Aperture.

Aperture.

You can buy fancy doohickeys on your next telescope.

Your first instument should simply be the biggest you can afford.

And the simpler the better.

You learn more that way, and when comes the time to buy that 12" GOTO, you'll know how to align it properly.

It's like mathematics.

It's no use getting a calculator if you don't understand the basics of adding and substracting.

Go for the biggest scope your money will allow.

It's a no-brainer.

[Pete Presland]

The three most important things in astronomy:

Aperture.

Aperture.

Aperture.

You can buy fancy doohickeys on your next telescope.

"doohickeys" almost made me choke on my cornflakes

certainly aperture is very important,and bigger is normally better

[sgazer]

"doohickey - something unspecified whose name is either forgotten or not known". Never heard it until now!

[Adrian]

A second hand scope will make your money go further.

I would not recommend diving in on unknown scopes when you don't know what to look for, but http://telescopeoutlet.co.uk/ has good deals on used items and are the outlet for Astronomia (a fairly well regarded high street shop in Dorking). They also give a 12 month store warranty and a money back 2 week returns period.

They have the 5 inch goto version of the scope you are discussing for £319 or a 6 inch reflector for £249. Both would be excellent scopes to set out with.

Then, if you change your mind about astronomy, or want to upgrade, it has cost you less depreciation.

(I am not affiliated with Telescope Outlet, but several people here have bought from them and have had no problems)

[acey]

The most important specs:

1. Price

2. Weight

3. Aperture

4. Location

Many people think only about 1 and 3 end up with too much of 2, stuck in an inadequate 4 (e.g. a light-polluted back garden with an obstructed view).

The "best" scope is one that has an aperture of a few metres and is supported by a few hundred tons of steel and concrete on top of a mountain. For the rest of us, the "best" scope is one we'll find convenient enough to use on a regular basis. Getting decent views may involve transporting it. So think first about where you might be observing, then think about what scope you might use.

At a dark site a 90mm scope will show all the Messier objects. At a light-polluted one they can be a struggle with a 12". GOTO will aim the scope at the right spot more quickly but won't make invisible objects visible.

[studio1one]

I'm going to disagree with common consensus a little. The best first scope is the one you will use the most. Aperture is great but they can be big unwieldy beasts too and it can put some people off getting them out and setting up, although a Dob is nice and easy compared to an EQ mount. Unless you are reasonably big and strong or can leave it permanantly set up a 8" Newt can be quite tricky to mount and unmount onto an EQ mount. Not trying to put you off or anything but I think it is crucial to think about how dedicated to getting your scope set up you will be and if having a big heavy scope to lug aruond and set up will put you off observing. It's no good having masses of aperture if you can't be bothered to set it up and use it.

[obscura]

I concur with Studio1one. The bigger the aperture the more light and thus the more you may see. However, the downside can be the bulk and weight of newts and dobs particularly in the dead of a freezing night.

A question to ask is, what do you want and want to do with the scope?

Viewing, astrophotography? Each requires its own type of scope. Generally, viewing 6-8" dob/newt. astrophotography 80mm+ but both on a good equatorial mount. I think the ETX90 is an alt az which is not the best at sidereal tracking.

At the end of the day, its governed by budget, clear skies and horizon.

[black hole sun]

Wow! so much help and information, all appreciated. I think ill say what i would like and see if anyone can give me some links or info what to look for and approx price pay. The ETX90EC does not seem a popular choice from you experts out there so this is what i would like to do.......Im in love with the moon, been fascinated with its desolate landscape for years. Lost count how many times ive not watched the road and starred at a gorgeous moon in the sky! not clever i know! but would like to get to the surface of the moon as close as possible! I would also love to see the planets/rings and the other stuff out there im getting to grips with. Would rather find these myself rather than have a motor to point me, i think long term this will entrench my knowledge. Any ideas?....... and thank you all.

[Capricorn]

You don't say if you looked at a goto for any specific reason. Although these days people are getting to expect a computer system in most things. Scopes are no exception.

The ETX90 is the smallest Mak the Meade produce.

The field of view is narrow, but unless you go for an f/5 achro you are not going to get much choice. In the goto line.

The Meade is fairly straight forward to set up and get aligned. No, it doesn't do it for you and you do have to supply accurate and correct information.

As the field of view is narrow get a 32mm or 40mm eyepiece, don't worry about the image quality as you want it simply for the field of view when aligning the scope. Basically a cheap one will be fine.

Get the pre-alignment set up as good as possible, level the mount, level the scope, point the tube due North. These have to be as accurate as possible for the first alignment star to appear in the field of view.

Suggest you ignore the tales of woe, all scopes have tales of woe associated with them.

[AweSIM]

hi.. ive learnt so much from the pros here.. two months ago i was myself wondering which scope to buy.. i started out with the ETX too.. shifted to Celestron AstroMaster.. then from one scope to another.. finally ive purchased a skywatcher explorer 200pds.. with synscan mount coming up soon.. so i think i can better relate to your state of mind.. my advice..

- dont go for scopes like etx.. they're small (less aperture), slow and narrow field (high f-number).. heck it doesnt even have a proper tripod.. youd end up observing the moon only.. and maybe some planets which would appear as mere dots and youd lose interest quickly..

- dont go for very huge scopes.. portability is a BIG issue.. i thought 8" was nothing much.. but the scope ive ended up with is a monster.. its package is 18"x18"x48".. it barely fits in the back seat of my car.. (but i wouldnt part with it come what may =).. )..

- dont go for a fancy scope with goto mount etc.. go for a DOB.. simple.. convenient.. DYI.. perfect for observation.. once you get to astrophotography stage then the games different..

- figure out what your love is for? DSOs or planets.. if its DSOs.. slow scopes like etx and i think most SCTs wouldnt cope well with that.. their field of view is too narrow and most DSOs wont fit in that FOV at all.. if its planets then slow scopes are your best bet.. but bigger aperture is a must for both.. but not too big.. =)..

- listen to the people here.. they have nothing to gain from telling you if your choice is a bad one.. they have more experience.. they have multiple scopes that i dream of every night.. they know their scopes in and out while ive only seen pictures of it.. so if someone who owns a 200p tells me its trash.. i should listen.. no matter how much im dying to lay my hands on one.. =).. (its not trash btw =))..

- lastly.. and MOST IMPORTANTLY.. wait.. i can not stress the importance of this word.. do NOT jump into buying if youre getting really impatient.. youd regret it.. wait.. hang out around the forum.. read other peoples posts.. read the stickys.. post questions.. gain knowledge..

the kit (my first one) ive opted for isnt what id call a beginner scope.. but i had a problem that most people on this forum dont.. i live in pakistan and theres no second hand market here for scopes.. once you buy its yours to keep.. and importing scopes here is a big issue too.. i cant go upgrading.. so i had to build a kit thatll stick with me for a long time to come.. that doesnt apply to everyone..

awesim..

pakistan..

[EDIT]..

dont stick to one scope that youve seen online.. ask people.. explore.. look at more scopes.. ask people which is better.. then have the winner compared with another one.. keep going till youre DEAD SURE that the kit youre aiming for is going to:

- fulfill your expectation..

- not cost more than you have budget for..

- is going to fit in your cars back seat.. =)..

about the budget.. i started out with an expectation of 400 EUR or something.. as i started exploring this forum.. i found it was inadequate.. right now.. just the scope and a few accessories have cost this much.. the mount is gonna cost even more.. if i had spent 400 only for the whole kit.. i surely wouldve been disappointed.. ps.. thanks to all the forum members whove helped me in my quest.. i owe it all to you guys.. =)

[barkis]

Scott,

if you live in a large town, the liklihood is it will have an Astronomical Society, If so it will also likely have a website, and it's Secretary will have an Email address. Drop him an email, and ask if you could pop into their next meeting. Chances are, the members will own many different scope types, and in time, you may get some hands on experience, which will give you something to conjure with.

You can then make a more informed choice. If it is a little more expensive than your budget, save a bit more, it will be worth it.

Ron.

[AweSIM]

@barkis.. i strongly second that.. =).. very well said.. both about societies.. and saving more..

[Vlad]

Quality is king. Whatever you buy, make sure it's tested before you part with any money.Top quality optics on a sensible mount, something you can EASILY lift without a bag of spanners. Quality optics, quality optics. Something around 127mm up to 150mm is more than enough. Focal length around a metre. Equatorial mount, not a dobsonian. 57 years at it, that's what I have and it is superb. Fancy bits may come later, but this telescope will last a lifetime. 150mm Newtonian or 127 refractor is all the scope you will need for many years.

[black hole sun]

Great advice, thank you. Think i will shop further, i have posted on the other section regards societies/ meetings etc so hopefully somebody will come back to me. Hoping to meet up or chat with like minded people to gain further knowledge and pheraps make some friends along the way.

[DoctorD]

Hi

Numbers are important:

Aperture (diameter of lens or mirror) - the bigger this is the dimmer the objects you will be able to see given suitable sky conditions. At some point light pollution wins and you cannot see any dimmer because the sky is brighter than the object.

Focal length affects image size for a given eye piece - longer focal length the bigger the image (magnification).

However the field of view (I.e. How much of the sky you see depends upon the field of view of the eye piece and the magnification. Greater magnification the smaller the view.

A 200mm f1200mm dob and 102mm f1200 Mak (like mine) will give the same field of view through the same eye piece - however the 200mm aperture of the dob will reveal more detail and make dimmer stars visible.

Aperture also affect the resolution of the scope - the ability to see detail

in the view, however this is also affected by the seeing that us the quality if the air and how much turbulence is present.

As has been already said, the size and weight if the scope will affect your willingness to set up for a viewing session (especially once the initial novelty has worn off).

I believe there is too much of a good thing (aperture) for a beginner - what too much is really depends on you.

I'm happy with my little Mak on an AZ GOTO mount - I can always get that 200 or 250mm dob later.

Hope this helps

Paul

[black hole sun]

Thank you Paul, all advice welcome. Still reading the glossary of terms as i will read feedback and be bobbing along fine, nodding my head, then its 'brick wall' time when abbreviations are thrown in with terms i do not understand yet being a novice. The more the challenge the more determined. Cheers.