What is the range of "fast" scope

JJLoke · Post by **JJLoke** » Sun Apr 15, 2007 11:44 pm

i would like to know what is the range of fast scope. F/5 scopes are considered fast scopes, how about F/6 ?

thanks

Post by **acc** » Sun Apr 15, 2007 11:51 pm

Don't think there is a definite cut-off but I think most consider F5 and below to be fast.

yanyewkay · Post by **yanyewkay** » Mon Apr 16, 2007 9:50 am

there's only a very slight difference between f/5 and f/6. f/6 is only ~1/3 stop slower than f/5. Taking photos with it won't pose much of a diff and i believe can be easily fixed in software.

For visual wise, I'm sure most cannot tell the difference.

Canopus Lim · Post by **Canopus Lim** » Mon Apr 16, 2007 2:22 pm

Mainly on a fast scope (f/5 or faster), it is easy to tell the difference if the eyepiece used is not able to correct for such fast speeds. The stars at the edge will look like 'seagulls' instead of points or the planets/moon does not look sharp when placed towards the edge of the field stop. Even for f/6 scopes, some eyepieces are not sharp to the edge. However for slow scopes like Maks/ SCTs, most eyepieces (cheaper ones especially) will be sharper to the edge compared to when they are used in a fast scope. Hence, there is always a trade off. If one needs a faster scope to get wider field, he needs to buy a better corrected (usually more expensive) eyepiece. If one is ok with the smaller FOV of a Mak/SCT, he can get away with cheaper eyepieces.

JJLoke · Post by **JJLoke** » Tue Apr 17, 2007 8:48 pm

i have ordered a dielectric mirror diagonal, im afraid that the quality will be impaired since mirrors are generally used for "fast" scope

Post by **weixing** » Tue Apr 17, 2007 9:11 pm

Hi,

i have ordered a dielectric mirror diagonal, im afraid that the quality will be impaired since mirrors are generally used for "fast" scope

Any decent dielectric mirror diagonal will work well for both "fast" and "slow" scope.

Anyway, optical accessories design for "fast" scope will work very well (if not better) if used on a "slower" scope, but optical accessories design for a "slow" scope may not work well on a "fast" scope. Basically, "fast" scope is more demanding on the quality of the optical accessories (eyepiece, diagonal and etc).

Have a nice day.

JJLoke · Post by **JJLoke** » Tue Apr 17, 2007 9:30 pm

i see .. thank you very much !

Post by **rcj** » Wed Apr 18, 2007 4:05 pm

congratulations on your dielectric diagonal purchase, JJLoke! besides more "resistant" to focal ratios particularly faster telescopes (F5 and below), the mirror should maintain its high reflectivity for a decade without recoating. An equivalent enhanced al version would have dropped below 90% reflectivity by then. dielectrics are also showing better resistant to repeated cleaning without developing much streeks. enhanced al would would have a softer surface and less resistant to repeated cleaning. You should enjoy better planetary contrast on the dielectric as well.