Focal length vs magnification confusion.

[Starlord19]

I am having trouble understanding something which I hope someone can help me with. Imagine I have 2 telescopes...

Scope A: 6" reflector, fl 750mm, with a 5mm eyepiece. 

Scope B: 6" sct, fl 1500mm, with a 10mm eyepiece. 

Assuming both EPs are the same quality I know that both will have 150x mag so the image will be the same size, but will it be generally similar otherwise? I'm thinking about brightness, contrast etc. Does the focal length matter or do the different EPs cancel that out?

[Mak the Night]

Are the EP's standard Plossls, orthoscopics or wide angle? Because that can be a factor. Generally, a 5mm Plossl would have a smaller field stop than a 10mm, plus the eye relief would most probably be shorter. Reflecting telescopes display inverted images, SCT's show upright images (mirror reversed if an Amici prism diagonal isn't used).

[Starlord19]

Thanks, I was thinking along the lines of the X-cel lx or BSTs, something with a uniform eye relief and fov. I'm only asking with regards to image quality, I don't mind about orientation really. 

[bingevader]

As long as they are both well made 'scopes, the image quality and brightness will be the same.

You'll sacrifice higher magnification with the shorter 'scope, but conversely, get a wider field of view at lower magnification.

Both the X-Cel LX and the BST should be good in either and be a great deal more comfortable to use than a plossl.

The shorter 'scope will be a bit "faster" at f5, but both makes of EP should manage that.

[Starlord19]

Will the f-ratio make a difference to the image or is it not relevant? 

[bingevader]

Not relevant in this instance, it's just another way of expressing the length and diameter of the 'scope.

[Chris]

17 minutes ago, Starlord19 said:

Will the f-ratio make a difference to the image or is it not relevant? 

f-ratio will make a difference to the image in anything other than premium eyepieces. In the f/5 scope your Plossls will be soft at the edges, but in the f/10 scope they should be pretty sharp right up close to the edge. 

Slow scopes are kind to cheap eyepieces in other words

[Starlord19]

Hi Chris. I've got a set of BSTs to go with my new scope, thank you very much! I already use the 5mm in my newt but wanted to know if it was worth investing in a 10mm eyepiece of similar quality for the C6 or would any difference be to minor to justify the expense? 

[Chris]

18 minutes ago, Starlord19 said:

Hi Chris. I've got a set of BSTs to go with my new scope, thank you very much! I already use the 5mm in my newt but wanted to know if it was worth investing in a 10mm eyepiece of similar quality for the C6 or would any difference be to minor to justify the expense? 

Hi Mark  A similar quality eyepiece will have better edge correction in your new slower scope. Your new SCT also won't have the diffraction artifact that the f/5 Newt has. This bothers some and not others. Personally I wasn't keen on the faint diffraction spikes on Jupiter with my last 150mm f5 Newt (it was fantastic value for money though, so swings and roundabouts).

I'd say that if you have a full set of BST's maybe see of you get on with the 8mm and 12mm either side of possible 10mm purchase. If you find in use that you wish you had the 10mm/150x handy, then that' kind of says it all.

In that case the next question would be...which 10mm eyepiece, and we all like chipping in our 2 pence worth on those threads        

[YKSE]

The image should be of the same size and brightness, maybe tiny (if ever detectable) constrast advantage to the Newt because of smaller central obstruction.

Closer to edge, there'll be more differences, the f5 newt will show more astigmatism from the eyepiece, and coma from the mirror, mostly likely larger area of unsharp stars than the SCT; while f10 SCT will show less astigmatism and coma, it will show field curvature from the SCT, you can refocus to get the stars in edge sharper, but on-axis will be less sharp, a compromise is to focus about half way off center.

[Starlord19]

Thanks everyone, loads of help as usual. My intended use for the C6 was planetary and lunar so I was thinking of moving on the 5mm Bst and getting a decent quality 10mm as my main use planetary ep with the C6. Thats the way I'm leaning at the moment because, well cos thats what I bought it for! I'll stick with what I have for now and will probably decide for definite in time for Christmas!