Wide field or not?

[newbgalore2]

If given between two telescopes of the same apeture, does focal length of primary matter? Since both scopes have the same useful magnification, a wide field telescope should give the same image as another telescope with a longer focal length at the same magnification right? Or will higher magnifications in a wide field break down? Since wide fields are more portable,why do individuals choose scopes with longer focal lengths instead? Or is it because of the short eye relief of shorter eyepieces needed for a wide field? Need help from u 'gosus'! Thnx.

[fizzy123]

Basically f ratio do not affect the view of objects visually, but photographically yes. In photo, high f ratio is good for lunar and planet as the background is darker more contrast, low f ratio for DSO as a wide field needed to capture it entirely, eg.cluster. In achromat-refractors, short f ratio may result in some colour fringes, and some eps or barlow designs are not suitable for low f ratio scopes. 8) :o

[zong]

Whoa hey, slowly la.. one qn at a time..

If given between two telescopes of the same apeture, does focal length of primary matter?

Focal length matters because it's what you need to calculate magnification! Haha
Actually you're talking about same aperture, so different focal length must be due to different focal ratios. 1) If using the same eyepiece focal length, larger focal length of scope gives larger magnification. 2) Small ratios ie. fast scopes give brighter images, but are less tolerant of "lousy" eyepieces. Vice versa for large ratios. Difference in brightness is sometimes unnoticeable, but in dim objects they probably do. Anyone to verify?

Since both scopes have the same useful magnification, a wide field telescope should give the same image as another telescope with a longer focal length at the same magnification right?

I assume you want same aperture, different focal length, same magnification. But that will mean different eyepieces to achieve the same magnification. So assuming (again) your different eyepieces both got same FOV, then the wide field scope (ie. shorter focal length) should give you a brighter image, as said above. But still, it depends on more factors, such as optics quality, etc etc.

Or will higher magnifications in a wide field break down?

What do you mean by "break down"? If I take break down to be anything that doesn't look as nice as before (brightness, contrast, whatever) then many factors count, not just magnifications. Aperture, optics quality, collimation(if applicable), eyepiece quality, number of elements in eyepieces, etc. all contribute to image breakdown.

Since wide fields are more portable,why do individuals choose scopes with longer focal lengths instead?

Well, that's up to consumer choice isn't it Aperture constant, longer focal length means I don't have to buy a super-super-super pro eyepiece, which saves money. That's one reason possible.

Or is it because of the short eye relief of shorter eyepieces needed for a wide field?

Possible.

Need help from u 'gosus'! Thnx.

Eh pardon my slang, always never been good What's gosus?

Sorry if I made any mistakes, haha preparing for exams but feeling sian so come here de-stress myself.. but brain not working very well

[shoelevy]

and some eps or barlow designs are not suitable for low f ratio scopes. 8) :o

this is true.
according to weixing if you imagine the light cone approaching the EP, a shorter focal ratio would mean the light cone has a very steep gradient.
to focus such a light cone properly would require a very good EP.
besides, long focal length isn't so much a problem if you are into SCT or MCT so some individuals choose long focal ratio teles

[weixing]

Hi,

If given between two telescopes of the same apeture, does focal length of primary matter?

Theoretically, it doesn't matter, but reality is that scope with shorter focal length are more difficult to manufacture. So if you compare the quality of two telescope of the same apeture, but different focal length from the same manufacturer, you will find that the telescope with longer focal length usually have better quality.

Since both scopes have the same useful magnification, a wide field telescope should give the same image as another telescope with a longer focal length at the same magnification right? Or will higher magnifications in a wide field break down?

Theoretically, the image produce should be the same if they are at the same magnification, but reality is that since longer focal length have better quality, they can tolerate higher magnification than a shorter focal length telescope(from the same manufacturer).

Since wide fields are more portable,why do individuals choose scopes with longer focal lengths instead? Or is it because of the short eye relief of shorter eyepieces needed for a wide field?

Since wide fields telescope has short focal length, so in-order to have high magnification they require a short focal length eyepiece. Short focal length eyepiece generally have a very short eye relief, unless you are using those very expensive eyepiece. So if your primary interest is planet observation which usually require high magnification, you will go for a longer focal length telescope. Also, a longer focal length telescope are usually cheaper(shipping not included) and better quality than a short focal length telescope (from the same manufacturer).

Have a nice day.

[MooEy]

2) Small ratios ie. fast scopes give brighter images, but are less tolerant of "lousy" eyepieces. Vice versa for large ratios. Difference in brightness is sometimes unnoticeable, but in dim objects they probably do. Anyone to verify?

misconception, given the same magnification and same aperature, a f/1 scope and a f/100 scope will give the same brightness.

Since both scopes have the same useful magnification, a wide field telescope should give the same image as another telescope with a longer focal length at the same magnification right? Or will higher magnifications in a wide field break down?

in theory, if they are both grind to perfection, they should perform the same. but perfection is not attainable. given 2 very gd piece of glass, the one with the longer focal length should perform a tiny bit better. but if given 2 average piece of glass, the difference should be more noticeable. many other factors must be included also, like barlow needed for the faster scope, compared to no barlow for the slower one, etc.

Since wide fields are more portable,why do individuals choose scopes with longer focal lengths instead? Or is it because of the short eye relief of shorter eyepieces needed for a wide field?

one thing here, what are we actually comparing, a faster scope to a slower scope or a same class of design, or from different designs?

i'm not sure if u have notice this, refractors and reflectors are typically in the range of f6 to f/8. while cassegrains are in range of f10 to f/15. although cassegrains have longer focal lengths, they are often more compact than refractors and reflectors due to their folded light path.

for reflectors and refractors, from 4" and above, every bit of f/ratio makes a different. a 4" f/6 is barely 60cm long, quite portable. a 4" f/8 is 80cm long, not easy to carry into a bus. a f/10 would be 1 meter long, almost quite hard to carry around and using public transport.

~MooEy~

[MooEy]

another thing i forgot to mention, do not be too bothered about the wide field thing, any 3-4" refractor, f/8 and below, together with a 30mm plossl eyepiece should give u reasonable wide field views.

a 4" f/8 with a 30mm is only 26.7x, quite nice for navigating the skies. trying to go too low will just give u white coloured background due to skyglow.

~MooEy~

[newbgalore2]

Hi there, thnx for the answers...

May i know what is EP?

also, quote from Kamikaze:
with regard to the f/ratio and not astrophotography

what we know: focal ratio determines field of view...
and: actual FOV is approx= apparent FOV(of ep) divided by mag
then with any ep, any mag and FOV can be achieved...

then how does f/ratio determine FOV

can someone ans this question too? thnx

[weixing]

Hi,
I don't think focal ratio will determine the True Field of View. True Field of View is detemine by Apparent Field of View of eyepiece and Magnification.

Unless you use for prime focus astrophotography, which the telescope is directly connected to the camera without the eyepiece. This will cause the telescope to become a fixed camera lens. Anyway, I'm not quite sure how they calculate True Field of View for camera lens.

Have a nice day.

[ChaosKnight]

Nothing explains more than math.

Lets use the Celestron 4" refractor series as examples. There are two models: the longtube f/9.8 and the short-tube f/5.

Specs for the longtube is as follows:
Aperture = 102mm
focal length (FL) = 1000mm
Focal Ratio = (Focal Length) / (Aperture) = 1000/102 = 9.8 => f/9.8

Specs for the short-tube is as follows:
Aperture = 102mm (same as longtube)
focal length (FL) = 500mm
Focal Ratio = (Focal Length) / (Aperture) = 500/102 = 5 => f/5 (about there)

Suppose you use a Plossl eyepiece with the following specs:
Focal length = 20mm
Apparent FOV = 52deg, ie you can rotate your eyeball 52 degrees side to side when looking through the eyepiece to get from one end of your field of view to the opposite end.

And you put it into your longtube. What you get is:
Magnification = (FL of telescope) / (FL of eyepiece) = 1000/20 = 50
Apparent FOV = 52deg
Actual FOV = (Apparent FOV) / (Magnification) = 52/50 = 1deg

Now you put the same eyepiece into the short-tube. What you get is:
Magnification = (FL of telescope) / (FL of eyepiece) = 500/20 = 25
Apparent FOV = 52deg
Actual FOV = (Apparent FOV) / (Magnification) = 52/25 = 2deg

Now you can observe the following for two telescopes of similar aperture:
(1) Magnification is lower for the one with lower f/ratio. And so:
(2) Wider FOV for the one with lower f/ratio.

Because of the lower magnification coupled with the larger field of view, the f/5 can be termed a wide-view scope.

Now supposing you want to observe planets at 100X
You can:
(1) Use a 5mm eyepiece with your short tube. Or:
(2) Use a 10mm eyepiece with a longtube

The 5mm usually have a shorter eye relief compared to the 10mm so it is more difficult to look through. But you can use a 2X Barlow on the short-tube with a 10mm, which may help solve your eye-relief problems. But this translates to additional costs and a possible degration of image since the light passes through more lenses.

Now supposing you want to observe deep sky objects at 25X
You can:
(1) Use a 20mm eyepiece with your short-tube. Or:
(2) Use a 40mm with your longtube.

The 20mm is more common, and is cheaper than the 40mm. For the 40mm, the apparent FOV is usually not as wide as the 20mm.

Now, suppose you attach a conventional 35mm SLR to your focuser for prime focus photography.

Magnification = (FL of telescope) / (constant 50)

For the longtube, magnification = 1000/50 = 20
For the short-tube, magnification = 500/50 = 10

Optical system for longtube is a f/9.8,
Optical system for short-tube is f/5

Since the longtube has almost twice the f/ratio as the short-tube, exposure times need to be 4 times longer for the longtube than the short-tube. The reason is simple: For similar aperture, twice the FL => twice the magnification => area of image is 4 times as large => light is spread out over 4 times the original area => exposure time need to increase by 4 times to make up.

Increasing the exposure times means more problems (vibrations from mirror slap, star trails etc...)

A 35mm film negative is a 23 by 34mm rectangle. If you consider the film lengthwise,
Actual FOV for the longtube = 1.94 deg
Actual FOV for the short-tube = 3.90 deg

Again you can see the short-tube offers lower magnification and higher FOV.

As for optical quality between these two scopes, i think there are a couple of objective questions to ask : for the short-tube, is the performance specs (with the exception of focal length) on par with that of the long tube? And for the average specimen of the short-tube, are the design specs to meet performance specs achieved? If the answer to both is yes, then i'd say the short-tube performs as well as the long-tube.

Personally for me, i prefer the short-tube. But cost constrains only permitted me to settle for the longtube.