Planetary scope specsPosted By Joplin Motisher-Chittenden |
I've found that an 8" to 10" newtonian is my instrument of choice for planetary viewing. If I had super steady skies I might consider something larger. For a refractor, 6" seems to be the practical size/ price limitation - they start getting really large/ heavy and expensive after that.
The standard rule is the longer the focal ratio, the better. For newtonians, the longer focal ratio has the advantage of a smaller secondary mirror which reduces the effect of the central obstruction. For refractors a longer focal ratio means less chromatic abberation. For an achromatic doublet in larger sizes,
f-12 is generally considered the minimum. A longer focal ratio also has the advantage of being easier on eyepieces and not requiring the use of a barlow in many cases. For newtonians f-6 is a good starting point. For achromatic refractors 4" and smaller f-10 and above. For lager achromatic refractors f-12 and above unless a chromacorr is used.
Joplin
The standard rule is the longer the focal ratio, the better. For newtonians, the longer focal ratio has the advantage of a smaller secondary mirror which reduces the effect of the central obstruction. For refractors a longer focal ratio means less chromatic abberation. For an achromatic doublet in larger sizes,
f-12 is generally considered the minimum. A longer focal ratio also has the advantage of being easier on eyepieces and not requiring the use of a barlow in many cases. For newtonians f-6 is a good starting point. For achromatic refractors 4" and smaller f-10 and above. For lager achromatic refractors f-12 and above unless a chromacorr is used.
Joplin