Focal length advantages

[Adz]

I feel like a proper noob but,

Why does a longer focal length benefit planetary visual and imaging?

I am currently convincing myself its about a longer light path to get each part of light properly aligned...but its probably wrong

Thanks

[mdstuart]

A longer focal length is certainly better for visual high magnification work on planets. The magnification is the focal length of the mirror / the focal length of the eyepiece..So a long focal length mirror will give a higher magnification than if the same eyepiece is used on a short focal length mirror..

Its actually probably more complicated than that but thats my humble understanding.

Mark

[Adz]

I see...I think!

But if 2 mirror sets of long and short focal ratios have the same aperture, isn't the ability to push to a higher magnification irrelevant if the limiting magnification is the safe?

Thanks for replying

[John]

Another factor is that with a long focal ratio (slow) newtonian you can have a smaller secondary mirror. This reduces the diffraction caused by the secondary and helps deliver more contrast - it gets more "refractor like".

Slow newtonians have a much larger collimation "sweet spot" as well so are easier to get into good collimation and keep there.

[mdstuart]

I agree that if they have the same aperture they pick up the same light and the limiting magnification would be the same.....

What I do know is that an F5 refractor is considered wide field and even with a 6mm eyepiece it struggles to give a high mag view...Switch to a f12 refractor and put the 6mm in you get a high power view and if the seeing is good excellent planetary views..

So even if I do not know the exact physics the longer focal length must be best for plantary viewing..

Mark

[Adz]

I see now thanks guys. It is more of a combination of things than a spoken rule or maths as such.

[perrin6]

Also mirrors of shorter focal ratio take more skill to make because the curve has to be more accurate thus these are more expensive to buy. An amateur can easily make an F8 6 inch mirror but an F5 is more tricky.

There is more coma distortion in low F ratio newtonians which means an object will be slightly distorted the further it is from the centre of the eyepiece field.

There's more to this astronomy business than meets the eye ain't there hee hee ?

[umadog]

A barlow lens can always be used to multiply a telescope's effective focal length if the focal ratio is very low. In practice a telescope's highest useful magnification is limited by the skies, the aperture, or the optical quality. I can't see a situation where it will be limited by the focal length. So there's no particular reason why long focal length scopes should be good for planets. One can hit a <20% obstruction in a Newtonian even at f/4 (although not all manufacturers achieve this) and obstructions of this size are indistinguishable from unobstructed (barring diffraction from the spider, of course).

It's true that shorter focal lengths make wider fields easier to achieve and this produces prettier of views of many DSOs. Then again, those big Dobs with 2m or 3m focal lengths produce wonderful DSO views, you just have to accept the narrow field.

So yes, focal length matters. But you have to take the whole telescope into account: no one single parameter of the optical train defines a telescope. Simply knowing the focal length of a telescope won't tell you much about what it may be best suited to doing.

[ollypenrice]

Yes, you can get a long effective focal length (the combined focal length of EP and scope) either by using a shorter FL EP with shorter FL scope or a longer FL EP with longer FL scope. If the aperture is the same and the effective focal length is the same they will give the same magnification and more or less the same brightness.

As stated above, it is cheaper to make long FL optics and Newts of long FL have smaller obstructing mirrors. However, a premium apo refractor at F7 will take some beating on the planets, though a long FL Newt might do so at a tenth the cost. (Might. I'm an apo refractor enthusiast!!)

Olly

[Adz]

However, a premium apo refractor at F7 will take some beating on the planets, though a long FL Newt might do so at a tenth the cost. (Might. I'm an apo refractor enthusiast!!

Yes this is exactly what I have found thanks Olly!

[Moonshane]

I don't know why but my 16" f4 when using an aperture mask at 6.7" (f11) creates a superb image of planets, doubles and moon. is it something to do with the light cone being slimmer in a slower ratio scope and therefore putting more of the available light and detail into the airy disk??

[Adz]

Better than your planetary newtonian Shane?

[Stu]

I works expect it to be, slightly larger aperture, same mirror quality and unobstructed optics so should perform better?

Stu

[tony4563]

Heres an interesting one.

I have a 6" mak which, using jupiter as an example, gave a brighter image through the eyepiece than a 5" mak I had. Fine. Both had roughly the same focal lengh (1500/1800) so the 6" was expected to be a little brighter given the extra inch.

I've just bought a new 7" mak. I've not used it yet as there is an issue with my new mount. The focal length of the scope is 2700mm.

Should I get a brighter image given the extra inch over the 6" or will the extra long focal length of the 7" mak mean the light has to travel further to the eyepiece, and remove any advantage of a bigger aperture?

[Stu]

Assuming you use an ep giving a similar magnification in each, you should get an image of the same size but slightly brighter. Distance to ep doesn't have any effect.

With my 4000 f/l you get good image scale whilst using longer f/l ep's which are more comfortable to use. Also the central obstruction is relatively smaller so the contrast is better.

Stu

[Moonshane]

Better than your planetary newtonian Shane?

Hi Adz - definitely http://stargazerslounge.com/diy-astronomer/183426-aperture-mask-16-f4-dobsonian.html

that said, I doubt I'll ever sell the 6" f11 as this tracks and has the slight edge on doubles for some reason.

[tony4563]

Thanks for clearing that up Stu.

Tony