Focal lengths

[piff]

Given a choice between two telescopes of equal aperture - and therefore presumably capable of supporting equal maximum useful magnification - what are the advantages/disadvantages of choosing one with  a longer or shorter focal length?

[johninderby]

Essentially the longer the focal lentgh the smaller the field of view, The longer focal lengths are particularily suited to lunar and planetary observing and the shorter focal lengths with their wider field of view for DSOs.

No reason you can’t use any scope for any purpose but it’s just that some are more suited to certain types of observing.

Also With the longer focal lengths you can get away with using cheaper eyepieces. The shorter the focal length the more demanding they are of optical quality.

Edited October 28, 2019 by johninderby

[Carbon Brush]

A short focal length also makes more demands on telescope glass quality and figuring, whether reflector or refractor.

If you are thinking of a particular scope type, or family of scopes, then members might add comments on their experiences.

[joe aguiar]

 john explained good I can agree with his post so I guess ill add then it becomes portabililty then.

here are example:

you can get a 4" f/5 refractor will have lots colour but its small light and portable or u can get a 4"f/10 which is a lot longer bit heavy BUt will have much better colour. so whats important to you better quality image and colour correction or being portable.

Now since Ed/apo are  much cheaper compared to 15 to 25 years ago u have other options now.

you can also get a 4"f/8 ed with 51 glass and its shorter over the f/10 but gives same colour correction maybe even a tad better but th ed is bit shorter.

if you get a 4" with 53 glass you can probably get better colour correction of it being f/7, even smaller package.

then you can also get a triplet apo and make it f6 and it will have better colour then the f/10 version  and be 40% smaller

so you can have small package with good colour and sharp images like a ST tube BUt it will cost more something a lot more depending which one . if you cant afford the new ed/apo cause of the price then the old way was to get a acro but longer like f/10 to f/12 sometimes even more but its becomes too long which is why we are going other way smaller packages better glass BUT cost more.

joejaguar

Edited October 28, 2019 by joe aguiar

[Stu]

It's a balance of different factors depending on what your preferences are. A shorter focal length scope will likely suffer from field curvature, and if it is fast it will be more demanding on eyepieces ie will need more expensive eyepieces in order to maintain good edge correction particularly for 82 or 100 degree afov.

You can generally reach the higher powers in a short focal length scope by either short focal length eyepiece or using a Barlow, but a long focal length scope will struggle to reach wide fields, being limited in just how wide they will go even with say a 55mm Plossl. They do tend to excel on lunar, planetary and smaller DSOs though.

Horses for courses really, but I guess that's why scopes around f7 are popular, particularly refractors because they are good at most things.

[piff]

Thanks guys. I am, actually tossing up between two Skywatchers, both 102 mm aperture. One is a refractor with a focal length of 500 mm and the other is a Mak with a focal length of 1300 mm - the latter being only a fraction of the size and looks very convenient as a "gab and go" scope but I am concerned whether this would have equal viewing quality

[johninderby]

The mak will be far superior to to the refractor in optical quality and will take higher mag. However it has a narrow field of view. Ideal for Lunar / planetary but the short focal length refractor will be better for low power widefield views. 
 

It all,comes down to what you want to look at.

[John]

37 minutes ago, piff said:

Thanks guys. I am, actually tossing up between two Skywatchers, both 102 mm aperture. One is a refractor with a focal length of 500 mm and the other is a Mak with a focal length of 1300 mm - the latter being only a fraction of the size and looks very convenient as a "gab and go" scope but I am concerned whether this would have equal viewing quality

Those scopes compliment each other rather than compete. Ideally you would own both and then have a nice wide field scope for the deep sky and a nice scope capable of sharp planetary views at high magnifications. 

I realise though that going for both is not likely to be feasible

[Stu]

Yes, the frac is, I assume an achromatic doublet (Skywatcher 102mm f5?) Which would show plenty of false colour at mid to high powers but would be very nice for widefield viewing. The Mak would be free of any false colour and great for lunar and planetary, so it really does depend what floats your boat

 

[Louis D]

47 minutes ago, piff said:

Thanks guys. I am, actually tossing up between two Skywatchers, both 102 mm aperture. One is a refractor with a focal length of 500 mm and the other is a Mak with a focal length of 1300 mm - the latter being only a fraction of the size and looks very convenient as a "gab and go" scope but I am concerned whether this would have equal viewing quality

If you can stretch to a 102mm ED refractor, it will easily outperform the 102mm Mak and still have a much wider field of view.

[joe aguiar]

I guess the mid point best of both worlds is a 102 refractor at f8 to f10

Even at f10 the fov is medium better than f13 compared to the mak.

The f8 version is very common to find less colour than f5 but still semi wide fov.

Joejaguar 

[Rob Sellent]

Regarding focal length.

Ignoring aperture, light gathering capacities, aberrations and just focusing solely on the eyepiece's field of view the f12.7 Mak will give a slightly tighter field of view than your f10 Nexstar and the f5 frac will give a similar field of view to your Celestron f5.7 Travel Scope.

12 hours ago, piff said:

what are the advantages/disadvantages of choosing one with  a longer or shorter focal length?

If you still have the aforementioned scopes, maybe it'll be worth your time comparing their focal lengths' strengths and weaknesses. If possible over the coming weeks point the two scopes at, say, the Moon, Orion Nebula and M36 in Auriga. You'll probably notice the 8" f10 has a narrower field of view but can take magnification very well, whilst the 2.8" f5.7 opens up the sky view but doesn't suit high magnifications.

Edited October 28, 2019 by Rob Sellent

[Stu]

10 hours ago, joe aguiar said:

I guess the mid point best of both worlds is a 102 refractor at f8 to f10

If it's an achromatic scope then yes, an f10 is a good bet. The Tal 100R is a prime example of this. Not that expensive (only available used now) but with an excellent reputation.