Getting started on stars

[miguel87]

Or perhaps you mean how do you make the actual physical size of the image larger to look at (i.e. the eye has to roam farther to see the whole scene).

In that case you are talking about AFOV (actual field of view). Again this depends on the eyepiece far more than the telescope. Angles of more than 60 or 70 degrees can get pretty expensive.

You could argue that faster (f5 and below) scopes cant display wide fields with as much accuracy (sharpness of image around the edges is worse) they can still show a super wide field of view.

Its all about the eyepiece.

A 70° field of view eyepiece will give you 70° regardless of the telescope it is in.

Edited April 18, 2020 by miguel87

[Sprint]

ahh ok so with a 70deg eyepiece as a constant an F6 scope will show the same area as an F10 but the latter will remain sharper to the edge of the area??

[John]

13 minutes ago, Sprint said:

ahh ok so with a 70deg eyepiece as a constant an F6 scope will show the same area as an F10 but the latter will remain sharper to the edge of the area??

No. The eyepiece will show more sky (a larger true field of view) with the F/6 scope. If it is a low cost eyepiece then there may well be some distortion of star images towards the edges of the field of view at F/6 though. 

 

 

[miguel87]

8 minutes ago, Sprint said:

ahh ok so with a 70deg eyepiece as a constant an F6 scope will show the same area as an F10 but the latter will remain sharper to the edge of the area??

Sort of, the image will be 70 degrees across, to your eyes, in both scopes.

But the area of sky seen could vary, depends on the focal length of the telescopes, not the F number (speed).

So if I asked "what area of sky will I see with a 70° eyepiece in an F6 scope?" It is impossible to answer without knowing the focal length of the scope.

[miguel87]

A generally true simplification is just to say that F number makes no difference to visual observing.

It is much more important to photography.

The only thing I would consider is that lower numbers like F4 or F3 are less forgiving of imperfections in optical systems. 

[John]

From my experience, eyepieces optics start to be challenged (ie: show aberrations if they have them) from around F/7 and faster.

From F/5 things can get really messy in the outer parts of the field of view if the eyepiece is not well corrected.

[miguel87]

Just now, John said:

From my experience, eyepieces optics start to be challenged (ie: show aberrations if they have them) from around F/7 and faster.

From F/5 things can get really messy in the outer parts of the field of view if the eyepiece is not well corrected.

Coming from a more experienced observer this sounds accurate.

I have an f5 scope and with my widest eyepiece of 70°, there is fairly significant stretching of stars right at the edge of the view. I would guess this would worsen with 90 or 100 degree eyepieces. An f8 scope would cope much better if you want crisp, wide angle views. Although many amateur astronomers are more than happy with f5 or faster scopes.

[miguel87]

I just found out that the Hubble space telescope is f24 😯

My 8 inch would need to be 192 inches long, that's about 16 feet long?!

My 6mm eyepiece would give a nice 812x magnification 😁

Edited April 19, 2020 by miguel87

[John]

15 minutes ago, miguel87 said:

I have an f5 scope and with my widest eyepiece of 70°, there is fairly significant stretching of stars right at the edge of the view. I would guess this would worsen with 90 or 100 degree eyepieces. An f8 scope would cope much better if you want crisp, wide angle views. Although many amateur astronomers are more than happy with f5 or faster scopes.

Actually, the 100 degree eyepieces that I have used have been well corrected even at F/5. Look at the prices of such eyepieces though  . With such eyepieces another aberration called coma becomes more obvious but that is produced by the primary mirror of the scope, not the eyepiece.

Astigmatism (eyepiece produced) looks like this:

Coma (scope mirror produced) looks like this:

You can have a combination of both of course !

[Padraic M]

Hi Sprint, welcome to the astro community.

This is a very complicated question as I guess you're figuring out already.  I'm still fairly new to this, after a little less than two years, and just about starting to get my head around it but it is very rewarding. I'd repeat what some of the other posters have already said about tempering your expectations about observing galaxies and some nebulae - they're very faint! However for observing, the fun is in the hunt, and in finding the needle in that haystack that you set out to find. Then, in realising what exactly you're looking at, how far away and how big it is. Sometimes it's meticulous planning that gets you there, other times it's just fooling around and stumbling on something interesting. I spent nights trying to find the Andromeda galaxy in my scope. Couldn't figure out where it was. Didn't find it until I was in a dark sky reserve, and just looked up and saw it with the naked eye - too big to see through the scope!

Visual and photo astronomy are very different beasts, both in terms of the approach taken, and the kit to buy. It's not that easy, as I'm finding out, to start with visual and then expand into photography.

A few suggestions about how to dip the toe in the water. You have an advantage if you travel and camp out etc. as you're likely to encounter dark sky locations.

1 - Install Stellarium planetarium software on your PC. It's free, and a fantastic way to learn your way around the night sky. It will also show you exactly what field of view you will get with specific scopes and eyepieces . Used by beginners and pros alike for planning nights of observing and photography.

2 - a decent pair of binoculars can be every bit as rewarding as a telescope, especially for galaxies, nebulae and clusters. Generally a lot cheaper and easier to lug around.

3 - I started with a Celestron Astromaster 130EQ. Not by any means a great scope or mount, but for €250 for a full starter kit you're not likely to get a better value to begin with.  Give yourself a year with that, and you'll either realise astronomy is not your thing, or you'll know what you want to do and what kit you need to get you there. Either way you will probably write off the €250 but it's well worth it for the learning journey. In my case, I was so entranced by seeing things like the Orion nebula, Jupiter and its moons, Saturn's rings, and the Leo trio through it that I've moved on to some more expensive kit but I am getting to know what direction I want to take.

hth

Padraic.