Question regarding focal lengths.

[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.:shrug:

Thanks in advance,
Molly.

[wavelandscott]

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]

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]

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]

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. :shrug:

Regards, Rob

[M54]

Thanks Rob, wavytone and Scott,

It looks like my scope is a long/short/medium focal length:lol:.
I think you're right rob, it's like everything else, everyone has their own opinion.:rolleyes:
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.:P

Ta from Molly:).

[troypiggo]

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]

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]

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]

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]

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

[M54]

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):whistle:.
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.:P
Listening to a discussion like this, understanding begins to seep into my slow brain.:rolleyes:

[Robh]

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]

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]

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]

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]

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]

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]

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 :D

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]

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.

[mswhin63]

Thanks for that info, I was part of a info night at ASWA with a talk on Square Kilometre Radio Telescope.

At the beginning of the talk it was mentioned that if you get light to hit 2 area of a mirror equally you would still receive an image. Hence the design of the sq km radio telescope placing dot of radar dishes over a sq km to give an effective narrow angle or a bigger scope effectively.

I believe this principle is being used with World Biggest telescope where 4 large scope and a few others combine in accurate alignment could provide a much larger effective scope. (still learning)

So it was just curiosity whether 4 very accurate hole in a lid could provide a similar focal length effect. I wasn't important though I would consider a single off axis hole of 4 to 8" on my DOB. Be a cheap on destructive experiment. I tried the single 2" holes but found the magnitude level severely degraded.

[M54]

Am I beginning to suspect that whether a focal length is short or long is entirely dependent on the relationship to its aperture?
So the 5.1M Hale Telescope would have a short focal length when related to its mirror?

[Robh]

Molly,
Great insight. And it's the terminology that's causing the confusion.
A relatively short focal length in comparison to the aperture size is often referred to as a short focal ratio. This is where the confusion comes in. As, for example, the Hale Telescope has a short focal ratio but a long focal length. That's why I like Troy's point of using fast for a short focal ratio to save confusion with focal length. A relatively long focal length in comparison to the mirror size is often referred to as a long focal ratio but the term slow would again save confusion with long focal length. For example, an 80mm aperture, f/10 telescope only has an 800mm focal length.

Regards, Rob

[Robh]

I'm assuming the 4" off-axis "aperture" would require a different telescope collimation. Yes or no?
If so, how would you collimate the scope with respect to the new off-axis "aperture"?

Rob.

[mswhin63]

Not totally sure but from opinions of other I have spoken to re-colimination is not needed. The light does not spread when is enters the 4" hole and it reflects of only the portion of the mirror in line with the hole.
As the mirror is curved within collimination it will arrive at the secondary mirror at the right point.
The only real affect is the amount of light collected.

[Wavytone]

No recollimation.

With four holes, when its out of focus you'll see four sets of out-of-focus images, these will all converge to a single point as you move the eyepiece through focus.

Try it some time, if you haven't.

BTW there was a point with sub-diameter aperture masks if you are looking at close doubles - sometimes the secondary star is invisible if its buried in the second or third bright airy rings around the primary. By reducing your scopes aperture the right amount, you change the spacing of the rings (wider) putting the secondary in the dark space between the rings.

[Robh]

I have no problem at the secondary but what happens at the eyepiece?
A reflection at the primary mirror centre will end up at the eyepiece centre. A reflection near the primary mirror edge will end up towards the eyepiece edge. So, if you have a 4" circle of reflection away from the primary mirror centre, won't this end up as a smaller field away from the eyepiece centre? Which means the scope is off-axis alignment.
Rob.

[mswhin63]

Good point, the cap on the main cover I use for bright moon viewing when the moon is really bright (save time fitting a filter), but I still seem to get a good picture. Never really took much notice but maybe the Field or View is reduced.

I was using my elcheapo scope when I tried that BTW

[Robh]

I'm still intrigued by this idea ... reducing the 12" aperture size to say 4" and increasing the f ratio.
I did some more thinking on this. As the aperture focal length is still the same, the true FOV for whatever eyepiece you're using must still be the same. However, this same field of view will be appear in a smaller exit pupil of lower brightness (as exit pupil depends on aperture size).
As the aperture hole is 4" and off-centre with respect to the primary mirror, the reduced exit pupil will, I think, appear off-centre in the eyepiece. You probably won't notice this as your eye will shift slightly from the eyepiece centre to the centre of the exit pupil circle. This probably won't cause too many optical nasties providing its not too far off the eyepiece centre. I'm not sure how you could re-collimate the exit pupil to the eyepiece centre (star testing?).

Regards, Rob

[Wavytone]

Using a small aperture off axis the scope effectively becomes an unobstructed Herschelian of the same focal length, and reduced aperture.

There's no real gain by doing this unless you really want a light loss, eg to observe the moon which is otherwise painfully bright in a 12".

Robh - you're right re the reduced exit pupil appearing off-centre in the eyepiece, it does, having done this with a 60 mm aperture on my C8. Not that it was noticeable as in my case it becomes 60mm f/30.

[Robh]

Thanks for the confirmation. I was pretty curious about the effect.

Regards, Rob