What is long focal distance scope good for and why?

[turboscrew]

I've been wondering, what are the merits of long focal distance scopes?
Big aperture is needed for dim objects, and planets are usually bright enough so that big aperture means "dead weight"?
Short focal distance is needed for bigger magnifications, but why long focal distance?
Smaller optical errors?

[Lady Fraktor]

Actually short focal length telescopes are made more for low to mid powers not high power viewing.

Fast telescopes of f/4 to f/6.5 are widefield sweeper, DSO telescopes requiring better corrected eyepieces also good for faster AP times.
Mid range telescopes f/7 to f/9 give a nice balance of widefield viewing with higher magnifications obtainable. A general purpose telescope
Long focal length telescopes of f/10 and longer are great for planetary, lunar, double and carbon star viewing. Capable of much higher magnification using longer focal length eyepieces.
The design also allows for use of simple eyepieces with very good results.
Generally not good for AP as the longer focal length adds to much time.

[turboscrew]

Ah, I must have been messed up due to the heat... magnification = F object / F eyepiece.
Then again, the idea became clear: long F for big magnification, short F for big field of view?

[Lady Fraktor]

Each telescope style has slightly different requirements and uses.
Your 300mm will let you dig quite deep for faint objects a refractor or SCT may not be able to see.
Imaging times will be 4x faster than a same sized f/8 reflector
If you are going to be doing visual you are going to require eyepieces that are more expensive due to the extra work that goes into creating them.
The Explore Scientific 82° line is a good indicator of what you will pay for eyepieces.
At f/4 you may also require a device called a Parracor to fix the coma that is inherit in these systems when used visually.

[turboscrew]

Yes. I've been thinking of getting a coma corrector. They are just quite expensive, and at least some of them are not very good. They mentioned on a Finnish site that Sky-Watcher's coma corrector (only about 200 eur) does good job in coma correction, but creates reflections all over.

[Lady Fraktor]

I believe Explore Scientific makes one as well and possibly Teleskop-Service and APM Telescopes.
If the elements are not well executed they can cause ghosting in the eyepiece.

The TeleVue is most likely the best as all others are based off of it but it is also the most expensive version.

[turboscrew]

Thanks for the suggestions.

[John Baars]

For refractors:

Smaller optical aberrations is one of the reasons too.
- If you compare a 80mm f/12 APO and a 80mm f/7.5 APO, both perfectly formed, the f/12 would have 4X less spherical aberration, 2,5X less coma and 1,6X less astigmatism, field curvature , distortion.
- Moreover a f/12 has 2,5X more focus depth than a f/7,5, which means 2,5 less vulnerable for a small distortion in the wave-front of light that reaches the objective, in other words less sensitive to seeing. ( less defocus aberration)

Furthermore the objective is higher up and further away from the bodyheat of observer and ground, which means less vulnerability to warmth radiation from these sources.
These are reasons, apart from less CA in an achromat, why long focal refractors still exist and are being made.

Source: Neil English, Choosing and using a refracting telescope, Springer, New York 2011. ISBN 978-1-4419- 6402-1
pp. 29, 30, 254, 266.

[Lady Fraktor]

All excellent points John and are why I love my long focal length refractors