TELESCOPE SPEED?

[bcfcciderhead]

My scope (SW130P) has a focal length of 800mm dia is 130mm, divide the two = 6.15, so if that is correct the speed of the scope is F/6?

What does this mean? is it a fast or slow scope? and whats the difference?.........more questions to follow:iamwithstupid:

[KevG]

The smaller the f ration, the faster the scope , e.g. an f/4 will provide a wide field of view suitable for DSO work, whereas a higher f ratio of say f/10 will provide a narrow field of view and better suited to high power planetary use.

[Doc]

The faster the scope (smaller F number) the more light enters, the slower the scope ( higher F number) the less light enters.

[bcfcciderhead]

cheers guys, thats quite straight forward, what would you say mine is and best suited for?

[Moonshane]

most of the mass market scopes seem to be aimed at the 'good all rounder' market and f6 - f8 are definitely that so yours will be OK for most targets that can be seen with the aperture you have. I would say that f6 is on the cusp of fast and slow with anything lower being fast and anything higher being slow.

slower scopes handle slight collimation errors and lesser quality eyepieces better than faster scopes where collimation accuracy becomes increasingly critical and cheaper eyepieces show more of their design faults but you can still get great views!

[Moonshane]

ps in answer to your next question by 'targets that can be seen with the aperture you have' I mean that some targets need larger apertures and or better/darker conditions to be seen.

[Moonshane]

some calculations you might find useful:

magnification = telescope focal length divided by eyepiece focal length (eg 800 / 10 = 80x)

focal ratio = telescope focal length divided by aperture

exit pupil = eyepiece focal length divided by focal ratio

a larger exit pupil than about 6mm can create loss of contrast and or make the secondary visible.

[bcfcciderhead]

Thanks Moonshane that answers the questions for tonight, i was guessing it was a kinda inbetween model, which will suit me for a while till the call of more aperture gets me

[Moonshane]

oh, and it will

[Scopemaster]

Thanks guys this helped me too, was looking at this Skywatcher scope: http://www.amazon.co.uk/SkyWatcher-Explorer-130-900-EQ2-Telescope/dp/B0017GSLT8/ref=sr_1_2?ie=UTF8&qid=1368389243&sr=8-2&keywords=orion+telescope where the f value tapered towards slowness, tbh I'd like to see Saturn, Jupiter and some nebulae and galaxies as the moon is a given best look for lower f values as I realize for my budget and being a novice in need of a portable scope 5 inches (130mm) is all I'll pretty much get as 8inches (200mm) are huge!

[ronin]

The f number is a photographic term and if you did visual only it may be considered to be of little relevance, not quite irrelevant but less of a concern.

A 150mm scope of f/10 will collect the same amount of light as a 150mm scope of f/5. The aperture of both is the same 150mm.

A 150mm scope of f/10 has to have a longer focal length then a 150mm scope of f/5.

The focal length determines the size of the image formed at the focal plane, basically (but not quite) double the focal length and the image size doubles etc. So the 150mm f/10 scope produces an image that is twice as big as the one formed by a f/5 scope.

However the scopes have the same diameter so that image of the f/10 scope is dimmer then that of the f/5.

In photographic terms that means the exposure time has to alter.

If you stick an eyepiece in the way then it goes out the window.

A 150mm f/10 scope set up to give 100x will give an image the same size and brightness as a 150mm f/5 scope set up to give 100x.

There are characteristics associated with a fast scope and a slow scope, but for visual no real hard and fast rules. For imaging then it is more relevant.

f/6 is a little on the fast side, but there are faster. Fast scopes tend to need a bit more time spent on keeping them set up, but they are shorter (aperture being the same) so a little easier to move round.

My personel preference is for something around f/7 in either refractor or reflector. Although practicality has to come into it - an f/7 200P reflector puts the focal plane at a reasonable position, however if 300mm it is too far if you had to observe near the zenith. So for a 300mm reflector you very likely have to conside a scope no slower then f/5 - otherwise you cannot look through it at times.

I will say be a little wary of f number, it is bounced around as if it is the most imortant part of a scope, it's not. Have been to exhibitions where I was told several times the f number, when I asked a bit more the guy knew the diameter and the f number and just about nothing else. It does however sound good.

[laser_jock99]

If you are planning on doing any deep sky photography of faint objects then this is where the fast F numbers come into play. An F4 Newtonian is a reasonable fast/cheap instrument, although as has been said precise collimation and focussing become more of an issue at lower F munbers. If you're really mad then even faser scopes are availble (eg F2.8 astrograph Newtonians) which if they are anything like mine require shot-to-shot tinkering. The payoff is you can get a picture in minutes that would otherwise take a slower scope hours.

Single 300s sub of Sadr region, Cygnus with an F2.9 12" scope & DSLR camera.

[AlexB67]

I found this page to be quite handy.

http://www.stargazing.net/naa/scopemath.htm

Various formulae for working out properties are floating around, but this page summarises it quite nicely with added explanations of each item, though note that some of the calculations are approximate of course and act as a guide.

If you are so inclined to collect all the formula, right click on the page and view source and you'll have a nice collection of equations, though they can be found elsewhere in more readable form instead of javascript

[ollypenrice]

The faster the scope (smaller F number) the more light enters, the slower the scope ( higher F number) the less light enters.

Sorry but no. Exactly the same amount of light enters two scopes of the same aperture. The F ratio has no bearing on this.

Olly

[AlexB67]

Sorry but no. Exactly the same amount of light enters two scopes of the same aperture. The F ratio has no bearing on this.

Olly

Thanks for clearing that up. I thought I had misunderstood something but thanks for confirming, I don't think I have. Perhaps the confusion comes in that for a given eyepiece and a scope of same aperture the brightness factor will be different due to the magnification difference. Perhaps this is what Doc was trying to say ?

[ollypenrice]

Thanks for clearing that up. I thought I had misunderstood something but thanks for confirming, I don't think I have. Perhaps the confusion comes in that for a given eyepiece and a scope of same aperture the brightness factor will be different due to the magnification difference. Perhaps this is what Doc was trying to say ?

I don't know. There's a lot of confusion around F ratio and some disagreement among expert imagers. I don't pretend to have any arcane knowledge.

Olly

[Philmarriott]

Thanks Ronin. That helped alot

[ardbeg74]

I like to think of it like this...

Given an equal flux of photons, two scopes with the same aperture but different focal lengths will collect the same number of photons. 

However the scope with the longer focal length will distribute them over a larger area. As the intensity ( brightness if you like) is determined by the number of photons in a given area then the intensity has to drop for longer focal lengths... thus dimmer image.

Course its not that simple as the intensity is a combination of the photon flux and the energy level of each photon and your eye isnt sensitive to all energy levels.  So your not going to be viewing IR and if you catch a close by gamma ray burst, focal length is going to be the least of your worries  But you get the idea.

For imaging this means that longer focal lengths need longer exposures as less photons will fall on each ccd pixel ( bigger pixels / binning may help by catching more photons ) whereas shorter focal lengths will require shorter exposures because more photons are being concentrated into a smaller area.

Not a physicist so may not be 100% accurate, but its the mental picture i have of it.

Cheers

Mark

[HumptyMoo]

This is very interesting to me, the term fast and slow has had me somewhat perplexed. After a flurry of "fast, slow scopes" in Google I settle upon the below statement:

So, Fast means AP needs less exposure (generally) and slow means AP needs more (generally)?

[wxsatuser]

This is very interesting to me, the term fast and slow has had me somewhat perplexed. After a flurry of "fast, slow scopes" in Google I settle upon the below statement:

So, Fast means AP needs less exposure (generally) and slow means AP needs more (generally)?

IMO it does'nt mean you need less exposure for a particular object, this depends on what your trying to get out of an image.

The difference is how fast the exposures are taken.

Lets say you taking 4minute sub exposures of M31 with an F8 scope.

If you had a second scope that was F4 you can in theory set your sub exposures to 1minute.

Every Fstop is a halving or doubling of light, so for the above there are two stops difference, in this case

we can reduce the exposure by 4.

Bare in mind that a lot of things affect exposure times, not just f ratio.

[rwilkey]

Thanks guys this helped me too, was looking at this Skywatcher scope: http://www.amazon.co.uk/SkyWatcher-Explorer-130-900-EQ2-Telescope/dp/B0017GSLT8/ref=sr_1_2?ie=UTF8&qid=1368389243&sr=8-2&keywords=orion+telescope where the f value tapered towards slowness, tbh I'd like to see Saturn, Jupiter and some nebulae and galaxies as the moon is a given best look for lower f values as I realize for my budget and being a novice in need of a portable scope 5 inches (130mm) is all I'll pretty much get as 8inches (200mm) are huge!

Hi Scopemaster, always get your astronomy products from a dedicated astronomy supplier, such as FLO, the forum's sponsers, you'll get a much better service and good advice.