Question regarding focal lengths.

M54 (Molly) · #1 24-05-2009, 11:39 AM

Hi all,

I've been reading about eyepieces and the author is talking about long focal length telescopes and short focal length telescopes.

My telescope is a 130cm, 900mm focal length, f/7 reflector.
Is this a long or short focal length, I don't know what is considered long and short.

Thanks in advance,
Molly.

wavelandscott (Scott) · #2 24-05-2009, 12:42 PM

Quote:

Originally Posted by M54

Hi all,

I've been reading about eyepieces and the author is talking about long focal length telescopes and short focal length telescopes.

My telescope is a 130cm, 900mm focal length, f/7 reflector.
Is this a long or short focal length, I don't know what is considered long and short.

Thanks in advance,
Molly.

I'm not sure I've ever seen a definition my self but here are my personal guidelines...

Short (Fast) to me is anything less than f/5

Everything else I think of as long (slow)....

Robh (Rob) · #3 24-05-2009, 01:40 PM

Molly,
A short focal length telescope can actually have a longer focal length than a long focal length telecope.
The terms short and long refer to the ratio of the focal length to the aperture diameter.
For example, a refractor with focal length 900mm and aperture lens 90mm has a focal ratio 900/90=10 or f/10.
A reflector with focal length 1500mm and primary mirror 300mm has a focal ratio of 1500/300=5 or f/5.
Focal ratios around f/5 are short while those around f/9 or higher are considered long.
The term fast is also used. A fast telescope implies a short focal length and a wide field of view.
I guess f/7 would be considered short.

Regards, Rob.

Wavytone · #4 24-05-2009, 03:54 PM

Quote:

Originally Posted by M54

Hi all,

I've been reading about eyepieces and the author is talking about long focal length telescopes and short focal length telescopes.

My telescope is a 130cm, 900mm focal length, f/7 reflector.
Is this a long or short focal length, I don't know what is considered long and short.

Thanks in advance,
Molly.

Yours is medium focal ratio, a good all-rounder. Long enough to provide good high power views of the moon and planets, yet also able to give wide fields of view provided you use some pretty big eyepieces like Vixen's LVW42 or LV50mm.

f/3 to f/5 is short, suitable for low power wide field views but nt much good at high power,
f/6 to f/10 are the all-rounders, and
f/12 to f/15 are long focal ratios - high powers suitable for lunar and planetary but not so good for low power wide field stuff.

Robh (Rob) · #5 24-05-2009, 04:25 PM

Because there is no defined scale on this, I guess everyone is going to have a different view on where short and long focal length boundaries are.

Regards, Rob

M54 (Molly) · #6 24-05-2009, 10:08 PM

Thanks Rob, wavytone and Scott,

It looks like my scope is a long/short/medium focal length

.
I think you're right rob, it's like everything else, everyone has their own opinion.

Going by what each has said, I conclude it's probably short to medium, and that's my opinion and I'm sticking to it.

Ta from Molly

.

troypiggo (Troy) · #7 25-05-2009, 10:55 AM

I think that long/short focal lengths is completely different to fast/slow, which usually refers to apertures and f ratios.

Your focal length is 900mm. I'd say that's at the shorter end of focal lengths if it's in the 600-900mm range. Focal length determines the field of view.

Your aperture is 130mm (not 130cm!

), and dividing this into your focal length is about 7 which is where the f/7 f-ratio comes from. This is a measure of the lens/scope's light gathering capability. The wider the aperture, or hence smaller the f-ratio for the same focal length, the quicker it can gather the same amount of light, which is why they call them "faster".

Considering some Newtonians/reflectors are in the f/4.5-f/5 range (without reducers/flatteners), and some SCT's and Maks etc are in the f/10 range (without barlows etc), I'd say your in the medium fast range?

Robert9 (Robert) · #8 25-05-2009, 11:06 AM

Just to add confusion to the chaos, I have also seen reference made to SGTs as being of short focal length even though they might be 1500mm. Seems that because the optical path is folded, it is therefore a short focal length scope.
Robert

troypiggo (Troy) · #9 25-05-2009, 11:12 AM

In the context of the question (regarding eyepieces), wouldn't the focal length be the light path "folded out"? More relevant to magnifications and fields of view.

Robert9 (Robert) · #10 25-05-2009, 11:25 AM

Absolutely, undoubtedly, for sure and absolutely for certain. Its a bit like the tube on a wind instrument eg trumpet. It doesn't matter how much its bent, twisted or other wise, it will have the same resonant length.

dannat (Daniel) · #11 25-05-2009, 02:40 PM

I agree with Troy, f7 is medium.. 5 & below is fast, 7-8 medium, 9 above is slow

M54 (Molly) · #12 25-05-2009, 03:47 PM

Quote:

Your aperture is 130mm (not 130cm!

), and dividing this into your focal length is about 7 which is where the f/7 f-ratio comes from. This is a measure of the lens/scope's light gathering capability. The wider the aperture, or hence smaller the f-ratio for the same focal length, the quicker it can gather the same amount of light, which is why they call them "faster".

Considering some Newtonians/reflectors are in the f/4.5-f/5 range (without reducers/flatteners), and some SCT's and Maks etc are in the f/10 range (without barlows etc), I'd say your in the medium fast range?[/QUOTE]

Oops! I meant 130mm.
Thanks, troypiggo, for that explanation of the relationship between the aperture and the focal length making it fast or slow.
I'm beginning to see light at the end of the tunnel (or is that tube)

.
It makes sense that on a scale of around f/4.5 to f/10 that f/7 is medium/fast.
Thanks everyone for your input, this technical stuff sure is a steep learning curve, at least for me.

Listening to a discussion like this, understanding begins to seep into my slow brain.

Robh (Rob) · #13 25-05-2009, 04:44 PM

Quote:

Originally Posted by troypiggo

I think that long/short focal lengths is completely different to fast/slow, which usually refers to apertures and f ratios.

Thanks Troy,

You've clarified a few points for me as well.
From what I've seen,there is a lot of confusion in the usage of the terms fast/slow and short/long focal length. Many sources simply refer to a slow scope as one with a long focal length, a fast scope as one with a short focal length. space.com considers long focal lengths to be f/9 or greater. This means an 80mm aperture lens with focal ratio f/9 would have a "long focal length" of 720mm.

The concept of fast and slow is also used in photography where focal ratio determines the photographic speed. A lower f ratio requires a shorter exposure.

Regards, Rob.

troypiggo (Troy) · #14 25-05-2009, 05:02 PM

Well I must admit that my background is from the photography point of view before the astronomy point of view. So "fast" for me means shorter exposure lengths to capture the same data.

Alchemy (Clive) · #15 25-05-2009, 06:50 PM

400-750mm short
750-1250mm medium
1250-2000 plus long

f5 and below fast
f5 to f8 midrange
f8 to f15 plus slow

both have different applications, depending on what you are doing, imaging , visual.

generally for visual the lower f ratio you have the deeper you see independant of the focal length to a certain degree, eyepeices etc come second

for imaging, the lower the f ratio the quicker the photons pile up, the longer FL is an issue for guiding and seeing conditions (scintillation)

what about aperture - aperture is king, bigger often equals better,

Liz · #16 25-05-2009, 07:10 PM

Molly ... i hope you have a clear understanding of this topic now!!!!
Alas, it has left me confused again. I will re read tomorrow, and hopefully will sink in again.

Wavytone · #17 25-05-2009, 07:32 PM

Liz,

Ignore focal length - bear in mind there are many scopes far bigger than amateur ones.

What matters is the focal ratio f = F/D where F = focal length, D = aperture.

f/3-f/5 are short, suitable for wide fields, low to medium magnification, but not suited to high magnification.

f/6 to f/9 are medium focal ratios, these scopes can give high power up to the diffraction limit with "reasonable" eyepieces, yet also give fine wide field views. IMHO f/7 is optimal for visual observing, and I'll explain why, below*.

f/10 is the beginning of "long", f/15 scopes are definitely best suited for high powered observations only (lunar and planetary) because a low power eyepiece is impracticably large.

There are a few rules of thumb for eyepieces:

a) for high power with reflectors, the highest useful magnification is x1.2 per millimetre of aperture. This is accomplished by an eyepiece of focal length (in mm) equal to F/1.2.
b) for high power with refractors, the highest useful magnification is x1.5 per millimetre of aperture. This is accomplished by an eyepiece of focal length (in mm) equal to F/1.5.
c) the lowest magnification is set by the eye pupil; where the pupil is X in mm the longest eyepiece focal length for low power is f x X.

For an f/7 refractor with good optics the highest power eyepiece is about a 5mm, while the lowest power eyepiece you can usefully use is 42mm (for a 6mm pupil)**. If you look at the commercially available quality eyepieces, 5mm is about the shortest and 42mm is about the longest. This means an f/7 scope is capable of the widest possible range of magnification with good eyepieces.

**I'm deliberately excluding eyepieces in the range 2.5mm (Vixen) to 4mm as in my view they're intolerably uncomfortable, and Barlows, which IMHO degrade the image to the extent you're better off without them.

By way of example I have an f/7 refractor precisely for this reason, it can be used with my entire set of eyepieces which span 5 to 50mm. At 50mm it has a whopping field of view, while at 5mm the Airy disks around stars are easily seen.

I also have an f/15 Maksutov specifically for lunar & planetary; a 2" widefield 30mm eyepiece gives 90X for starters and its really best suited to 150 - 250X which require very comfortable 13 and 8mm eyepieces. But for low power, forget it. For the Mak to deliver low power I'd need a 75mm widefield eyepiece, fitted in a 75mm diameter focusser. No-one makes such a beast, for good reason - an eyepiece this big would weigh half a tonne, and cost the Australian GDP to make.

For imaging the priorities are different - f/7 down to f/1.

Robh (Rob) · #18 25-05-2009, 11:45 PM

Going over these posts again, you can see individuals estimating ranges based on their own experiences. It's not that anyone's right or wrong. Variations occur because there is no defined scale. But getting a consensus, it looks like most agree that a fast scope is around f/5 and a slow scope around f/10. There is still some confusion over the terms fast and slow as against short and long. I prefer Troy's use of the terms fast and slow with focal ratios. An f/5 scope with large aperture can have a longer focal length than a f/10 scope of small aperture.
Focal lengths can be shorter or longer, but the absence of a defined scale makes this is an even more contentious area. Just where does a short focal length range end ... at 750mm or 900mm or ? And where does a long focal length start?... the 200-inch (5.1m, f/3.3) Hale Telescope on Mt Palomar has a 16.8m focal length.

Regards, Rob

mswhin63 (Malcolm) · #19 25-05-2009, 11:47 PM

Just testing a theory, If I was to get my 12" DOB buy another lid and cut out 4 smaller hole 2" diameter. buy some caps (get my drift).

Theoretically this would increase the focal length to collectively 8" diameter. Making the SW 12" to a F7.5. It would be no longer 12" though

The obvious effect would be reduced magnitude but would the change in F level be proporational to an equivilent F7.5 refractor or reflector that is being discussed? ie: performance overall.

For me I would like to consider this option as an alternative to purchacing a 2nd scope until I can get enough money.

Wavytone · #20 26-05-2009, 12:06 AM

Malcolm,

You you could cut a 4" hole in the lid, off-axis, and turn it into a nice 4" f/15 scope (same focal length as the 12" was). Would be even better than a refractor as its achromatic.

BUT you can't make two, three or even four 4" diameter holes and expect it to perform as an 8" - you might match the light grasp but the resolution will still be that of a 4", and the Airy disk is going to look pretty ugly.

A mask with a single 8" diameter hole is another matter, it will made your scope work as an 8" f/7.5, and the result could be surprisingly good since the worst part of most mirror figures is the outer inch - masking it off often helps the resolution.

Years ago I some fun with assorted masks on a C8, mostly made of thin card with holes cut with a sharp blade - and Andrew James used the idea to create some interesting diffraction patterns that assisted with determining the separation of close double stars.