f#,s

[kevin321]

Hi all

Now this may be a stupid question but hear goes.

Looking at the spec's of different telescopes and one thing I do not understand is the f# after the focal lenth number. I think I read once that the higer the f number the better the scope is for planetary observing and the lower the f number the better for DSO'S. Is this true and if not can some one please explain whot thay mean.

Thankyou all

Kev

[brantuk]

Focal ratio is the focal length divided by aperture. Generally for astro imaging - lower f-ratios of f6 and down are better suited to gathering light from dso's in long exposure photography.

For imaging planets/moon higher f-ratios (f7+) are better (f10 is common) and longer focal lengths for sharper focussing.

Check out this excellent primer for a full explanation: http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html

[kevin321]

brantuk thank's a lot matey that has been buging me.

So I wos part right lol.

[JamesF]

That's been the impression I've got so far, I think partly because there's greater chromatic aberration in refractors with low focal ratios.

I don't know if the same applies to reflectors though.

I'm struggling to get my head around some of the optical effects relating to telescopes, so I'll be following this with interest. I did find some useful stuff here:

F-number - Wikipedia, the free encyclopedia

Focal length - Wikipedia, the free encyclopedia

James

[brantuk]

Yup - part right Kev lol

F-ratio is very important for imaging - I'm not sure of the difference for just observing though. I've used both types of scope (SCT and Appo) for observing and get good views both ways (with a bit of ep manipulation).

The better quality appos usually correct the CA very well with a mixture of 2 or 3 lens elements. CA is the colour fringing you see around the object in lesser quality scopes.

[kevin321]

Good links thear James very handy

[CGolder]

Focal lengh is more important than focal ratio for planets if you are using budget eyepieces then a longer focal length will offer the same magnification with a larger focal length eyepiece which should means its nicer to use.

Example

150mm F5 Reflector to reach 250x mag requires a 3mm

150mm F10 SCT to reach 250x mag requires 6mm

the 6mm will be a lot more pleasant to use in the cheaper ranges like plossl's

the problem is that the f10 SCT will have half the maximum field of view as the f5 reflector which means those larger targets might be out of reach from being framed correctly.

[kevin321]

brantuk do you know if you get CA/colour fringing with the skywatcher skyliner 250px Dob as I am ordering mine on tuesday. thank's

Just finished reading the link you put up thank's a lot matey.

[Mr Spock]

No - the Dob is a Newtonian reflector and uses a mirror, no CA. CA only affects 'achromatic' refractors.

[brantuk]

I've not observed or imaged with that scope Kev but I can say that higher quality eyepieces are going to be more useful observing with lower f-ratio newts (like my SW 300p) in which I see very little if any CA.

I'm still learning some of this stuff myself but I understand a barlow can help to reduce CA in fast Newts - not sure how that works though - someone will elaborate I'm sure.

I tend to accept the advice of the guys who take great images (you'll soon get familiar with who they are if you check the imaging sections) and then work out the maths after lol. They're usually right

The SW dobs though have great optics for observing so rest assured you've made a great choice (even if I can't qualify it lol)

[brantuk]

Thanks Spock - now why didn't I remember that? lol

[kevin321]

Thank's guys and I must say that is one thing I have allways got of this forum is good solid sound advice. Thank's

[sixela]

Hi all

Now this may be a stupid question but hear goes.

Looking at the spec's of different telescopes and one thing I do not understand is the f# after the focal lenth number. I think I read once that the higer the f number the better the scope is for planetary observing and the lower the f number the better for DSO'S. Is this true

In general, for visual observation, you will compensate f/ratio differences by selecting other eyepieces (or by using a barlow).

But scopes with low f/ratio and 2" focusers are going to be able to show you more sky with a large 2" eyepiece tha scopes with high f/ratio.

f/ratio also indirectly affects a lot of things:

-For one particular design, long f/ratio scopes are easier to focus using a single speed focuser, unless the focuser is something that moves a mirror (in an SCT, the focuser is often a mirror mover for the primary and the primary is an f/2 mirror even though the system is f/10!).

-Edge of field aberrations, particularly in cheap wide field eyepieces, are worse on fast scopes.

-For high magnification, fast f/ratio scopes require short focal length eyepieces. If they are of simple design, they have very short eye relief and you may feel them to be less comfortable, so you may prefer mor expensive long eye relief eyepieces or using a barlow.

[JamesF]

So what we're really saying here is that if you want to get good views of both solar objects and DSOs, two scopes would be useful?

[brantuk]

Well James - I have one for imaging planets (and or guiding), one for imaging dso's. and a nice big dob for looking at everything. Of course you can use any scope for observing, and you'll get a wide range of views with it. It's just that for imaging, some scopes are better than others at a particular type of object

By the time you realise that of course - you're totally hooked and you're pockets are half empty lol

[kevin321]

Hi James from what I can gather you will get great views with one scope of planets and DSO's but if you are imaging two scopes are better one for planets with a higher f number and one for DSO's with a lower f number.

[JamesF]

Well James - I have one for imaging planets (and or guiding), one for imaging dso's. and a nice big dob for looking at everything. Of course you can use any scope for observing, and you'll get a wide range of views with it. It's just that for imaging, some scopes are better than others at a particular type of object

By the time you realise that of course - you're totally hooked and you're pockets are half empty lol

Oh, don't start. Even after having my scope for a month I'm already getting frustrated with having to drag it all out of the house and up into the field and wondering how I might justify building an observatory Though actually that's as much to keep the scope out of the way of small children charging around the house as not having to move it.

James

[brantuk]

Lol James - pretend it's going to be a shed for storing dangerous gardening tools to protect the kids and to get the scope out of the house. Then when it's built put the tools in the garage and bingo - one observatory and everyones happy lol