Fast Focus Short Tube Newtonian Telescopes

[philip d]

I would just like to inform everyone that if you want a scope for planetary viewing it's best to avoid the Fast Focus Short Tube Newtonians.But for deep space object viewing the short tubes do a fine job, I was at a star party recently and people kept wanting to view thru my 6 inch Short tube Chinese Newt that I bought off E-Bay, and there were a lot of much larger and more expensive scopes around me, so I got curious and went and viewed thru some of the larger scopes and to my surprise my 6 inch Newt was doing as good as some of the larger and much more expensive scopes on some of the deep space objects and I was only using a 25mm low power EP.

let me know if i am misguided.
cheers

[star drop]

Any focal ratio reflector will work for viewing either planets or deep sky objects. The large focal ratio reflectors, say around F/8, have a smaller central obstruction and hence provide a little more contrast than a low focal ratio reflector, say around F/4.5. Also they can use fairly inexpensive eyepieces to provide a sharp view. But to get a very wide field of view, required for many deep sky objects, one has to use a reflector with a low focal ratio. This is because of limitations on commonly available eyepiece focal lengths and also eyepiece barrel diameter. Eyepieces that work well with low focal ratio reflectors will cost much more than the ones that work with large focal ratio telescopes. One should use whatever he or she has available and not worry about focal ratios too much.
As the aperture increases the size of a large focal length reflector becomes cumbersome. Hence the popularity of the low focal ratio dobsonians. Also most people prefer to observe with their feet on the ground, not up on a ladder.
Assuming that all telescopes had acclimated to the temperature of their surroundings at the star party you had participated in. And assuming that the air temperature was not dropping rapidly the similar views that you noticed between the larger telescopes and yours is entirely due to atmospheric turbulence. As the aperature increases a telescope becomes increasingly sensitive to the seeing. On a night where the stars scintillate, or twinkle, slightly a ten inch telescope might present a pleasing view at 200x. That same night a four inch might get by with using 175x and still show an image of the same steadiness. The image in the four inch will be much dimmer and not have the fine detail hidden within but it will look similar. Perhaps many of the large telescope users were using a higher magnification in the hope of getting split second detailed views if the seeing steadied periodically. I have found that when the seeing is poor and one compares a four, thirteen, and twenty five inch scope at their lowest power that the largest scope will still provide more detail and a brighter image than the smaller telescopes. All the above applies to either dark or light polluted skies. Bigger always reveals more.