Beginner Question - Clarity?

[AlmostOhm]

Hi all,

I'll apologise in advance for what could be a really silly question...

What is the clarity/detail like on hobbyist type telescopes and is there a specific type which is better at this?

I might be using incorrect terminology, but what I mean is the ability to read a book from a few hundred of meters away (this is an example)- is this possible?

I've tried to use the internet to search for this, but I'm probably not searching the correct terms or misunderstanding the answers if they are there.

Thank you - AO

[vlaiv]

Hi and welcome to SGL.

Yes, that is somewhat wrong terminology, but I sort of understand what you are asking, so I'll try to answer the best I can.

There are two major things that telescope does for us - one is to gather light and other is to provide magnification.

If you want to know if a book can be read from certain distance - then you are interested in magnification part of telescope operation.

Simplest way to explain this would be - telescope can magnify up to certain magnification that depends on telescope size. Actual useful magnification will depend on you as observer and how sharp is your vision, but general rule is to say multiply aperture size in millimeters with x1-x2.

For example - 4" or 100mm telescope can magnify up to say x200 usefully. This does not mean that you can't magnify up to say x400 with such telescope - it just means that image will start to be larger but blurry without additional detail.

Back on the book example - with simple trigonometry, we can see that book at 200 meters when magnified x200 will look like book at 1m away. If you can read it at 1 meter away - then yes, you'll be able to read it at 200 meters with telescope that provides x200 magnification.

Things are a little bit different when doing astronomy - because there is atmosphere in the way (its a bit like looking over fire or over hot asphalt road in summer - everything shimmers and blurs), so maximum useful power will depend on how stable atmosphere is - but also what type of object you are looking at.

This brings us to last part of answer - are all telescopes equal. To answer that - you must understand too much magnification part and how it affects things. That blur that I mentioned at high magnification is actually loss of contrast (at certain spatial frequencies - a bit technical stuff here really). It will affect different things differently.

Text in a book is high contrast detail - it is black on white - it does not get more contrasty than that. Planetary detail is not like that. Lunar detail is not like that. It is often subtle difference in contrast and color and any blurring will hide such detail.

In principle - all telescopes that are diffraction limited - will go like 95% in sharpness / power, but last few percent will be determined by quality of optics. Better optics will simply give slightly sharper views at those high powers - and can reveal those subtle features.

It is really hard to explain the difference until you start looking thru a telescope, or even better - compare two telescopes side by side. This difference is either very big or hardly noticeable at all - depends on how you look at it. If someone without much experience takes a look - image will be pretty much the same in two scopes, but experienced observer will see small crater on the moon in one scope and fail with the other (inexperienced observer simply won't see it in either scope) - and that will present major difference - crater is either seen or not.

It is the same difference in scopes - just described differently by different people, so personal experience is the key there.

[AstroMuni]

44 minutes ago, AlmostOhm said:

What is the clarity/detail like on hobbyist type telescopes and is there a specific type which is better at this?

Welcome to SGL.
Vlaiv has given you a comprehensive explanation above. I am guessing you are thinking of using a telescope for observing the skies and not for birdwatching etc. If the latter then I would advise looking at binoculars first. A simple 10x50 will get you great views of terrestrial as well as stars.

Assuming its the former, it would be good to know answers to a few questions before we can assist you with your query.

- what is your budget

- are you interested in viewing or taking pictures

- are you interested more in planets/moon or the deep sky objects (DSO) like galaxy, nebula etc.

- are you able to readily setup a scope in your garden or do you have to carry it a distance to get to a clear and less light polluted site.

EDIT: And to add to vlaiv's explanation. The biggest constraint we have is light pollution. Most of us live in urban or semi urban areas and are surrounded by lights. So even assuming that you and I have the same scope, the viewing experience would differ depending on where we both live.

Edited December 19, 2022 by AstroMuni

[NGC 1502]

10 hours ago, AlmostOhm said:what I mean is the ability to read a book from a few hundred of meters away (this is an example)- is this possible?

Your best bet is to move much closer to the book😁

Perhaps a more useful answer- The best definition at the lowest price will be given by a Newtonian reflector. A 6” or 8” Dobsonian reflector is stunningly good value.  If it’s to be your only scope then it’s worth paying the extra for the 8”.

[allworlds]

Resolution is the term you are looking for.

Because light is a wave, a telescope (or even our unaided eye) will focus a perfect point source to a small disc of light called the Airy disc. Therefore two point sources that are too close together can't be distinguished from a single source.

The angular resolution of a telescope with perfect optics, in radians, is approximately equal to the wavelength of light divided by the diameter of the telescope's main mirror or lens.

As a practical formula, Dawes' limit is:

R = 116/D

Where R is the resolution in arc seconds and D is the aperture in mm.

(Sidenote. Put in 3 mm, for the typical size of our eyes' pupils in daylight, and you get R = 39 arc seconds. The actual resolution of healthy human vision is about 60 arc seconds. So our eyes are not far off the limits imposed by the laws of physics and their own size.)

You then need sufficient magnification for your own eye to see the detail the telescope resolves. and furthermore you'd like the detail to be clear, not just barely visible. So the rule of thumb is that maximum useful magnification is 2x per mm of aperture. Too much magnification and the image becomes dim and obviously blurry.

And you also need sufficient optical quality and precision. In reality no telescope has perfect optics. But a decent parabolic Newtonian reflector, Schmidt-Cassegrain, or apochromatic refractor will all come close in the centre of the view. The Newtonian reflector is the cheapest for a given aperture - remember, more aperture means better resolution.

Here are simulated eyepiece views of Saturn at 200x in a 60 mm scope ("too much magnification") and at 200x in a 200 mm scope. You can see how the larger aperture scope gives a brighter and crisper image. (The simulation isn't perfect but it gets the idea right).

https://imgur.com/a/uUGmbZA

[AlmostOhm]

Hi all - thank you for all your replies - very informative!

Myself and my partner would just like to explore the stars, planets, ISS.. etc.. just a general nosey at what's out there!

I'm quite flexible as far as budget is concerned but also mindful it might end up in the corner of a room after a few months, so I'd like to find something that is a decent introduction. Maybe £200-£400?

Ability to take pictures would be awesome, but not essential. Also, portability isn't essential.

Thanks again all!

[vlaiv]

34 minutes ago, AlmostOhm said:

Myself and my partner would just like to explore the stars, planets, ISS.. etc.. just a general nosey at what's out there!

Stars - check

Planets - check

ISS - not so much. The thing zooms around very fast and best you can hope is to have it fly thru your field of view in less than a second on medium power. Much faster on high power (fraction of a second).

Other interesting things - check (galaxies, nebulae, clusters ...) - check, although those require darker skies.

39 minutes ago, AlmostOhm said:

Maybe £200-£400?

Something like this is very serious starter scope:

https://www.firstlightoptics.com/ursa-major-telescopes/ursa-major-8-f6-dobsonian.html

(in fact - it is scope for life, most people that get one of these - keep it around)

It has a lot of aperture so it will give you plenty of light and ability to resolve things (x200-x300 is piece of cake for this kind of scope).

There are several drawbacks:

- it is heavy and bulky (do look up 8" newtonian on youtube to get the idea how large thing is compared to average person)

- it requires some adjustment from time to time (it is called collimation - or aligning mirrors for best image - there are tutorials online and it only requires tightening / loosing couple of screws)

- It won't allow for any sort of photography - except occasional snap with mobile phone at the eyepiece (and that probably only of the moon and maybe bright planets).

Photography is really another level and there is no simple way of just "snapping few pics" at the telescope. It requires time and dedication and extensive budget. Even very basic photography (which is just taking long exposure images with camera and lens on a star tracker) - will easily eat up the whole budget that you listed.

[Spile]

I recommend the Dobsonian as the ideal telescope for visual (my reasons here https://astro.catshill.com/why-did-i-choose-a-dobsonian/) and I would recommend doing some reading and research in order to get a picture from a range of observers. 

[ollypenrice]

You will also need to learn to use charts and find your way around. Take a look at a free planetarium like Carte du Ciel or Stellarium. Just going outside with a telescope and no plan would not be productive.

There will be an astronomical society near you: there always is! 

Olly