F ratios and observing

[MartinB]

This is an observing rather than imaging question - I'm a bit confused. It follows on from this thread http://stargazerslounge.com/index.php/topic,23066.msg235633.html#msg235633 in which I reported on an observing session comparing an F5 300mm Skyliner Newtonian on a Dob mount with an F10 LX250R.

The conclusion was that even for bright objects at high power in moderate seeing aperture is more important than focal length.

So what I now want to know is why do planetary observers and double star splitters prefer longer focal length scopes? These get very unwieldy with increasing aperture.

I always thought higher F ratio scopes put less strain on the optics at high powers allowing them to deliver cleaner views but now I just don't know!

[FLO]

It's not that a low f-ratio telescope cannot be made to deliver high performance, it's just more difficult.

Low f-ratios have shorter, more steep light cones which are more difficult for the objective or mirror to create without aberrations. The eyepiece, whose job it is to straighten the cone before it meets the eye, must also work harder. (Martin, I suspect that you would notice more difference between the two scopes if your eyepieces were not high-quality 'exotics' 8)).

By comparison, the longer, more narrow light cones produced by high f-ratio telescopes are much easier to create and correct. They also deliver a larger 'sweet spot' (area of optimum sharpness) and are less fussy about collimation.

Long focal lengths also generate more magnification from an eyepiece (magnification= scope focal length / ep focal length) making it possible to achieve high magnifications with more 'comfortable' longer focal length eyepieces.

With a Newtonian, the longer focal length light cone also requires a smaller secondary mirror which has a less detrimental effect on contrast.

Hope that helps,

Steve

[cygnusx1]

Thanks Steve, v helpful explanation for those of us to whom eyepiece quality + observing is still a bit of a mystery

[MartinB]

Thanks Steve. Interestingly my 9mm and 5mm orthoscopics performed very well with the newt but aren't really practical on a dob mount. I'm sure simple plossls would also have done well.

It would be interesting to compare a pair of Orion newts with identical mirrors but different focal lengths.

[FLO]

A simpler and more affordable comparison would be the Skywatcher Explorer 150p and Explorer 150PL

Perhaps we could arrange it for a Star Party sometime.

[MartinB]

That would be interesting!

[GazOC]

As Steve has said, the use of high focal lengths is mainly due to the convienience they offer in achieving high magnifications, the smaller CA they allow is also a consideration.

"Aperture isn't everything but it sure beats whatevers in second place"....

The "double star" question is more complicated as it depends of what type of stars are being observed as to what type of scope is prefered. Refractors for bright doubles that are close together and larger Newts for the fainter, wider doubles, Maks and SCTs tend to be a compromise (cost and size wise) between these two options.

[MartinB]

Thanks Gaz

[Macavity]

I think it's been an interesting ahem... "journey" (personally) to compare my "fast" refractor and the "slow" MAK. And, in part, that was my intent. Whereas it's probably conventional (sensible) to recommend a general purpose scope i.e. Newt for beginners, I begin to understand WHY (aside from [initially assumed] "show"!), folks do like to explore the possibilities of multiple scopes. And, of course there is a VISUAL paradox - At the same magnification (with bright subjects?) there is surprisingly little brightness difference between F5 and F12.

[Andrew*]

THANK YOU for this thread :hello1: :notworthy: This has long caused me confusion.

Just to get this straight, I have 3 questions -

1. assuming perfect collimation, perfect focus, and half-decent eyepieces (orthos, even), can say an f/5 newt can perform equally to an f/10 newt at the same magnification?

2. If so, are the advantages of a longer f/ratio purely a matter of convenience (collimation, focus, eyepiece quality etc. less critical)?

3. I had the idea (from photography) that large f/numbers make for easier focusing, due to a larger depth of field. Is this equally true in astronomy?

Andrew

[James]

THANK YOU for this thread :hello1: :notworthy: This has long caused me confusion.

Just to get this straight, I have 3 questions -

1. assuming perfect collimation, perfect focus, and half-decent eyepieces (orthos, even), can say an f/5 newt can perform equally to an f/10 newt at the same magnification?

Not exactly as Coma would be much more visible in the f/5 Newtonian.

2. If so, are the advantages of a longer f/ratio purely a matter of convenience (collimation, focus, eyepiece quality etc. less critical)?

Yeeees... I think so, unless I've missed something!

3. I had the idea (from photography) that large f/numbers make for easier focusing, due to a larger depth of field. Is this equally true in astronomy?

I've seen that in longer focal length fractors. I don't know if it applies in all circumstances.

James

[GazOC]

1) There is a lot more coma as you move out from the centre in a f5 scope, hence why f10 scopes are more forgiving of budget eyepieces and f5 scopes need more precise collimation.

2) Largely

3) I think 'end' magnification plays a more important part than the focal length of the scope used but thats just my 'gut' feeling.

[MikeP]

Now I'm confused.

Andrew asked if longer f ratio was more for a matter of convenience. I think the answer is "yes" for tolerance of cheaper eyepieces i.e. the light cone is less steep and hence easier to bring to focus (per Steve's comments). I think the answer is "no" for collimation i.e. more care must be taken to collimate accurately.

Could you correct my (mis)understanding please.

Mike

[GazOC]

Thats pretty much it Mike. You also have the size factor, a 12" f10 Newt would be a bit of a beast!!!

EDIT: Sorry, just re-read my post!! It should read "and f5 scopes need more precise...."

[MikeP]

Thanks Gaz. I'd taken your pre-edited post literally, should have realised you had a typo.

I found my 10" Newt too big for me to handle on an equatorial mount - I shudder to think what a 12" would be like regardless of whether it was f5 or f10.

Mike

[Whippy]

A f10 12" newt is going to be about 10 feet long is it not? :shock: :shock: :shock:

Tony..

[Andrew*]

I think the answer is "no" for collimation i.e. more care must be taken to collimate accurately.

eh?? That's not right, is it? I thought slow scopes were much less fussy about collimation than fast scopes?

Andrew

[GazOC]

Theres a lot of typos in this thread....

[FLO]

I think the answer is "no" for collimation i.e. more care must be taken to collimate accurately.

eh?? That's not right, is it? I thought slow scopes were much less fussy about collimation than fast scopes?

Slow scopes are less fussy when it comes to collimation and have greater depth of focus. Fast scopes need quite careful collimation and will most likely need re-collimating more often. They also have a more narrow depth of focus which gives the impression of the object 'snapping' in/out of focus.