Newbie - same aperture different focal length

[ManUtdFans (Alan)]

For SkyWatcher 150/750 and 150/1200, I assume the 150/1200 will have bigger object(eg. Moon) shown when you look at eye piece, right?
So I should buy the 150/1200, am I correct?

[Kunama]

Hi Alan, it depends on what you are trying to accomplish.

The 150/750 is an F5 scope (the focal ratio) the 1200 is an F8, for visual use the the F8 is better if you have a mount that is capable of holding the bigger heavier scope steady. The longer focal ratio is caused by the lens elements bending the light less, that is the lenses are less curved, so you will see less of what is called chromatic aberration (CA). It will still be there but less than the F5 scope. CA is in all refractors to some degree, it shows as a coloured halo around the brighter objects such as planets Venus, Jupiter as well as the moon. Some people find it abhorrent, others don't mind having it as to be rid of it involves the investment of many more $$$$ in apochromatic scopes of the highest quality. Even with brands like Takahashi, there is CA on all but the very top models.

The actual size of the image you see is a factor of focal length and eyepiece focal length so in either scope what you see depends on which eyepiece you use. (the shorter the eyepiece fl the more mag it gives)

For any given eyepiece the F8 scope will give more magnification, the F5 scope will show a wider field of view. Everything is a compromise.

As for what to buy? I would sincerely recommend you don't buy either of the 2 until you have had a chance to look through some more scopes, there are plenty of opportunities around Sydney to go to astro gatherings and the people will be glad to show their scopes.

[Allan_L (Allan)]

Originally Posted by ManUtdFans

For SkyWatcher 150/750 and 150/1200, I assume the 150/1200 will have bigger object(eg. Moon) shown when you look at eye piece, right?
So I should buy the 150/1200, am I correct?

...with the same eyepiece ... YES ... (mag = fl of scope / fl of eyepice)

But the Maximum amount of practical magnification is a factor of (up to) twice the aperture (in ideal seeing conditions), and assuming you can obtain appropriate eyepieces, then this will approximate the same on the two scopes.

In that respect, quality of image (as discussed by Kunama ) comes into the equation.

Additionally, the sheer size of the scope may effect yoou decision based on mobility.

[ManUtdFans (Alan)]

If I use both scopes and point at the same location on the moon (specific crater), can I say the 150/1200mm will show more details? Because it is bigger image.
Provided other settings/accessories are same, only the tube is different.

[rogerco (Roger)]

The simple answer is yes, but... simple answers are often misleading. If you are only interested in planetary viewing then you might be right to follow the magnification route but remember its not just the tube, the more magnification you have the more you need a good mount (often more expensive than the scope) to hold it steady.

However many things of interest out there do not need lots of magnification but you do need aperture to see them because they are dim.

You really need to find out more about what you want to look at because there is no one size fits all when it comes to telescope systems.

[Steffen]

I would tend towards a "no" answer. I know astro-newbies are often obsessed with magnification, but higher magnification doesn't necessarily mean better or more detailed views.

The amount of detail a scope can show primarily depends on its aperture. Magnification, being the ratio of scope and eyepiece focal lengths, is a two-edged sword. Higher magnification means, most visibly at first glance:

- dimmer view
- more unsteady view
- more obvious optical aberrations (colour, coma, depending on the type of scope)
- objects moving out of the field of view real fast

The focal length range of eyepieces is limited for practical reasons both at the long and the short end. Any given set of eyepieces will give higher magnification and narrower field views in a long focal length scope, and lower magnification, wider field views in a shorter focal length scope. However, the sweet range of magnifications and fields of view can be achieved with most scopes and commonly available eyepieces.

You won't be missing out at the high-magnification end (I'm guessing about 200x for these scopes) regardless of whether you pick the 750mm or the 1200mm. Most often you will miss out on high magnification because of less than ideal seeing.

So, forget magnification, it's not really a deciding factor. I reckon you need to weigh between the higher visual quality, contrast and lower aberrations of the 1200mm scope and the smaller size, lower weight and easier mounting of the 750mm scope.

If at all possible, try and check out those scopes in the flesh before buying, better yet, try to look through them at a local astro club's observing night.

Cheers
Steffen.

[ManUtdFans (Alan)]

So ignore the portability issue, the 150/1200mm will have better image viewing quality than the 150/750mm?

[Steffen]

That is a safe assumption, yes.

Cheers
Steffen.

[ManUtdFans (Alan)]

Thanks Steffen.