Image Size Question

[Cypher]

Hi I've been reading a lot of your posts over the last few weeks and the Skywatcher Skyliner 200P is now high on my list of choices as a likely first telescope, but I really am going around in circles deciding which way to go.

Today I noticed a link someone had posted to a Field of view calculator - Field of view Calculator<script src="title2.js"></script> which I have been playing around with.

Could someone explain to me why a 150P, 200P and even a 250px all appear to give the same image size. I had expected the 250px to give a larger image than the others.

Here's an example I did using Saturn with 10mm EP and 2x Barlow on both a 150P and 250PX

Is the advantage of the 250PX over the 150P purely down to how much light it is able to capture, therefore making the image clearer rather than bigger ?

In the images above both scopes show a Magnification of 240x, and a field of view of 13' but the 150 shows a resolution of 0.9" and the 250px a resolution of 0.54" . By resolution am I right to take it they are referring to the clarity of the image and being able to see finer detail down to 0.54" on the 250px ?

Sorry for the newbie questions, I'm just trying to get my head around everything at the moment and I've been spending a lot of time going around in circles especially when it comes to choosing a telescope.

[Slim]

The two scopes have the same focal length, at least that is what you have told the program, therefore with the same eyepiece will produce the same mag. The diameter of the mirror is related to how much light the scope can 'gather' a bigger diameter gathers more light.

[Merlin66]

If the scopes all have the same focal length, then the plate scale will be the same.

Larger apertures will gather more light and show fainter stars as well a better resolution.

[hemihaggis]

Let's not get over complicated to start with !!! Buy the biggest out of these scopes you can afford .... bingo ,all good

[John]

Let's not get over complicated to start with !!! Buy the biggest out of these scopes you can afford .... bingo ,all good

Thats probably a sound approach

Also, trying to simulate / anticipate exactly what the views of specific objects will be like with varying apertures of scopes can be a fairly fruitless exercise as what you can actually see depends on a whole load of variables that are outside of the scope such as seeing conditions, eyepiece quality, observer experience etc, etc.

If you go for the largest aperture you can handle and afford at least you can be assured that the scope has the potential to outperform smaller aperture scopes, if all the other factors fall into place

[rowan46]

the difference will be in resolution as you reach optimum magnification although the size of the image will be the same, the larger apparture will show more detail and fainter objects be easier to see.

[Cypher]

Cheers for the reply's. Thats definitely cleared up a few things I wasn't 100% sure about. I had wrongly expected the larger scopes to give a larger image so I think the 200P is still top of my list, but the 150P isn't as much of a step down as I first thought it would be, which just gives me another option to consider.

[Slim]

Cheers for the reply's. Thats definitely cleared up a few things I wasn't 100% sure about. I had wrongly expected the larger scopes to give a larger image so I think the 200P is still top of my list, but the 150P isn't as much of a step down as I first thought it would be, which just gives me another option to consider.

What sort of site will you be observing from? If there is quite a bit of light pollution around then the extra light gathering of the 200 will be invaluable in trying to pick out the DSO's.

[Cypher]

Yes light pollution will probably be an issue for me, although I'm quite sheltered / shielded from the worst of it but I think it will definitely still be an issue.