Equipment for planetary viewing

[ColHut (Colin)]

Would I be right in supposing that if I like large detailed planetary views...

Other things being equal....

1 - magnification is dependent upon light path length a 4.5" f8 gives the same magnification for a given eyepiece as a 9" f4. (of course the 9" object will not dim quite so quickly...)
2 - short newtonians are not the best really here but do enable you to look at large dim objects with large fields of view for when you tire of planets...
3 - Brightness not withstanding, exit pupil puts a downward limit on eyepiece size (aperture in mm/magnification) - 0.5mm being about the usual limit until floaters etc start being a real problem
4 - larger apertures for a given length mean larger exit pupils and more likelihood of keeping it into the normal range of 2-5mm for a given eyepiece.
5- If the mount cannot track Saturn whilst viewing it at x300 there is no point anyway, likewise if it blows in the wind, you cannot see it.
6 Is it practical to actually view planets at high magnification without motor drives and using slow motion controls - it does not seem likely to me ?

cheers

[skies2clear]

On some of your points respectively:

1. True. Mag = focal length of primary divided by focal length of eyepiece. The larger primary will give better resolution/contrast and brighter images.

2. The shorter focal ratio (assuming a reflector), will give a larger secondary obstruction, reducing contrast performance a bit. That's why planetary scopes are usually longer F ratio (8 or higher). If you can keep the central obstruction to 20% or less is good/ 15% is better! F ratios of F5 to F7 are good allround I think for different types of objects, whereas F10 or F15 scopes are really suited more to planets, high magnification, narrower fields.

4. Exit pupil is a function of F ratio and eyepiece focal length and not objective diameter. Exit pupil = focal length (eyepiece) divided by F ratio of primary/objective mirror/lens. Also other ways to calculate it.
Yes, with a given length, increasing aperture will increase exit pupil. Calculate the exit pupil to give you the useful range you mentioned. 5mm is often considered around the max for people whos eyes are aging. As you go larger in exit pupil, aberations of the eye become more prominent too.

5. No mount can be too steady. A good Dob mount with smooth action can be used to track at near 300X manually, although being driven is better. Some say having a good tracking drive can increase the overall performance of your scope by seeing fainter magnitudes (increase of 0.5 to 1 magnitude).
If your mount is affected by wind easily, it can be an absolute pain at high mags. A great scope deserves a great mount and will make a huge difference in you actually enjoying using the scope.

6. Again, depending on the mount, it can be just fine. I regularly look at planets at 300X or higher without motor drives. As long as the mount is stable and moves smoothly, you can get away with it. I'm not saying it's as good as a motor drive though.

My 2 cents anyway...

Others will add more info I think.

Clear skies.

[ColHut (Colin)]

Many thanks

[OneOfOne (Trevor)]

Another factor for point 6 to consider is do you expect to be showing things to other people regularly? We often have neighbours or friends over for some star gazing and I think they would find it difficult to track a planet at high magnification. Of course, you could use a lower magnification in these cases as they would be happy enough just to SEE Saturn's rings and Jupiter's moons! A mount with a drive in these cases would be an advantage. Mine has a motor driven EQ mount so I can't really comment on this subject with much authority, I am sure someone else can give a better evaluation on how "outsiders" find using a non driven mount.