focal length

[alien509]

just a query on focal length the longer the focal length the better for looking at planets I understand? I was looking at the Dobson 150 and 200 the 150 is f8 and the 200 f6 does that mean the 150 is better view of say the moon and more detailed as the reviews would suggest i'm wrong  ? is that because of the aperture being bigger..

[Mr Spock]

You get more detail from a larger aperture. Focal length doesn't really matter. A properly collimated f6 is no different to an f8, though an f8 is easier to collimate.

[Pete Presland]

i am pretty sure that the 200mm dob would give some fine views of the planets and the moon, as well as many other objects.

[Alien 13]

Hi one advantage of a longer focal length is that it a bit kinder to average quality eyepieces another is its much easier (all things being equal) to get higher power magnifications.

Alan

[CumbrianGadgey]

Hi one advantage of a longer focal length is that it a bit kinder to average quality eyepieces another is its much easier (all things being equal) to get higher power magnifications.

Alan

I agree to a point. Since the options are 150 F8 and 200 F6, the eyepiece focal length is the same for a given magnification, so the extra expense of tolerances on smaller EPs does not apply.

Both should perform well. The 200 will get more detailed images because it can resolve more detail. It will be a bit more fussy about collimation but it is not a very fast scope, so it shouldn't be too bad. I'd go for the 200 if i could afford it and handle it...these things are big! Also, you don't say which brand. There's a big variation in quality out there. The size doesn't matter if the optics aren't any good.

[CumbrianGadgey]

Sorry,

I've just had another thought. I think your two choices of size are fine, as I said before. I'd like to give you a practical example, since you are interested in planetary observation...

...it comes down to the point about resolution again. I have a 130 and a 200 Newtonian (on EQ mounts, not Dobsonian but that is irrelevant to the views). With the 130, I could never see the shadow of Jupiter's moons on the planet because it could not resolve that level of detail. Lastnight, using the 200, I saw a beautiful crisp shadow of Ganymede crossing the face of the planet...another argument in favour of the 200! you may make it out with the 150 on a good night but you'd be assured of seeing it with the 200.

[John]

F/8 is easier on eyepieces than F/6 because the light cone is steeper on the latter. The collimation "sweet spot" at F/8 is that much larger than at F/6 too as has been alluded too. 

That said, F/6 is still pretty manageable in these respects and the additional aperture will deliver higher resolving power and more contrast on the moon and planets when the seeing conditions are decent.

[umadog]

I'd add that there's a power law governing the relationship between focal ratio and stuff like coma, eyepiece finickiness, and collimation needing to be bang on. f/6 is still fairly easy to work with. By about f/5, eyepieces like Hyperions become a disaster and small changes in primary mirror collimation error lead to noticeable deterioration in planetary views at about 200x. At f/5 some people are still ok without a coma corrector. By f/4, pretty much everyone wants a coma corrector. At f/6, hardly anyone will bother with a coma corrector.

[cantab]

The actual focal lengths of a 150mm f/8 and a 200mm f/6 scope are the same! It's the focal ratio that differs. Focal ratio is, as discussed, more pertinent to visual observing.

[Stu]

Secondary size is also a function of focal ratio which will have a small impact on contrast for planetary observing.<br />

<br />

Stu

[alien509]

I see thanks for the replies and to the make I was thinking skywatcher

[John]

The actual focal lengths of a 150mm f/8 and a 200mm f/6 scope are the same! It's the focal ratio that differs. Focal ratio is, as discussed, more pertinent to visual observing.

The OP said that it was visual observing that they were interested in - looking at planets in fact. It's focal ratio that affects the astigmatism that eyepieces produce, the coma that newtonians produce and the accuracy of collimation that is needed.

Perhaps we are saying the same thing though, but in different ways ? 

[ollypenrice]

The size of the image ending up on your retina is governed exclusively by the effective focal length of the scope/eyepiece combination. This determines magnification.

How the effective focal length is made up (short FL scope and short FL EP or long FL scope and long FL EP) is not all that important.

A long FL system, as said, is easier on EPs, collimation, central obstruction and optical engineers (hence your wallet!) But it cannot give as wide a field of view in the longeest FL eyepieces.

A short FL system can go wider if that should matter to you, but at the costs above.

Olly