Since discovering the world of difference between viewing with Low and High f/numbers my question is...... Given that scope makers have always been a bright bunch, why did it take until somewhere about the 60's-70's for the amateur community to evolve from high f/number scopes to low f/number scopes?
The world of difference between viewing with an expansive wide field low f/number scope and a cluttered and dim high f/number scope, why did the change take so long?
Was it because we had invested so much into our scopes and we were loathed to discount the perceived value of high f/numbered scopes? Why was it that the photographic community, a community so closely aligned with our own understood from the get-go the value of wide field lens? Drawing a ven diagram of the two communities photographic/astronomy many of those members are within the set describing shared membership why was it that astronomy battled with narrow-dark scopes for so very long, much longer than can be reasonably justified via techno lag?
For viewing, the focal ratio doesn't really mean too much as everything is determined by the relationship between the focal length and the eye piece that is being used. Visually a properly collimated F/4.5 dob with a 9mm eye piece will be pretty much identical to a F/9 dob with a 18mm eye piece.
One of the reasons long focal ratio telescopes were around for so long is due to them being less demanding. Trying to collimate a F/3 telescope is a nightmare where as collimating a F/9 telescope is a LOT more forgiving.
In refractors, field curvature and chromatic aberration is a product of not only larger aperture but also shorter focal length.
Ultimately, the f/ratio makes a big difference in photography but not much at the end of the eye piece. If you double your f/ratio (4.5-9) and you double your eye piece focal length (9-18) then there really is not much difference. The F/9 will give more corrected views around the edge of field but thats about it really.
Long focal length Achromat refractors were The primary scope solution for many decades because the technology had not evolved to where other options were available in the retail environment. As has been pointed out, the f ratio is not really significant for visual use, and a f12 to f15 achromat refractor can give you wonderful visual performance with almost no visual CA. True colour rendition is a major advantage of reflector, which do not need expensive exotic glass formulations. Not surprisingly the science community migrated to reflector designs early, as they are the most cost effective apeture you can buy, and can be built in large sizes required for exploration. I believe the change has taken place relatively quickly.
Constant
A very good question, but one that has a wide ranging answer. The obvious answer is that a low f ratio telescope is not necessarily "better" any more than a fast lens on a camera is automatically better. It depends what it is being used for.
Secondly, just like camera lenses, fast scopes are more expensive, often prohibitively so, so for makers to focus on making fast scopes would mean that they would cater to a small market segments. Again relating back to the photographic community, there are a lot more bog standard kit lenses sold that fast telephotos!
Thirdly, until fairly recently, say the early 90s, most scopes were sold for visual use. While that is probably still the case, the market for dedicated imaging scopes would be much larger now than it was 20 or 30 years ago. This is as a result of the availability of CCDs and DSLRs, computer processing etc rather than film. For visual, the main impact of f ratio is on the size of the scope. The popularity of SCTs since the late 60s attests to the value of having a decent aperture in a compact package.
Gentlemen, many thanks I believe I have a new kernel of knowledge! I have been puzzled by the older Unitron's and their design. The explainations delivered here describe the effect of limiting technology and not just a desire for long and bulky high f/number scopes making sense of the Unitron style refractors.
For viewing, the focal ratio doesn't really mean too much as everything is determined by the relationship between the focal length and the eye piece that is being used. Visually a properly collimated F/4.5 dob with a 9mm eye piece will be pretty much identical to a F/9 dob with a 18mm eye piece.
One of the reasons long focal ratio telescopes were around for so long is due to them being less demanding. Trying to collimate a F/3 telescope is a nightmare where as collimating a F/9 telescope is a LOT more forgiving.
In refractors, field curvature and chromatic aberration is a product of not only larger aperture but also shorter focal length.
Ultimately, the f/ratio makes a big difference in photography but not much at the end of the eye piece. If you double your f/ratio (4.5-9) and you double your eye piece focal length (9-18) then there really is not much difference. The F/9 will give more corrected views around the edge of field but thats about it really.
I would love to observe the difference between low and high f/n balanced out against eye piece selection. In particular I would really like to observe the difference in corrected view sound the edges of the visual field.
Clearly it's time for me to find my way to a IIS star party!
Great question for sure and as already said there is no real one answer here as all tho telescopes only gather light and focus it to a point for viewing and magnifying by the eyepiece so we can see a magnified object and on this its a common wrong idea that telescope's/ eyepieces magnify a distant object , they don't , they actually bring an object to the angular size it would look when much closer .
For example looking at say Jupiter at 200x is not seeing the planet 200x bigger but it is what Jupiter would look like if it was 200x closer than it physically is so instead of an average distance of 440 million km's at 200x it would look like it was only 2.2 million km away if using the naked eye , that's 1x .
All telescopes do this in their respective design peramiters to acheave the image .
Another mis-conception is that a long fl will show a dimmer image to a short fl telescope of the same size ,, wrong , for example a 4 inch refractor at f15 would be exactly as bright on all objects as a 4 inch f6 , 4 inches of light gathering is 4 inches of light gathering , law's of physics cant be broken .
But with the awesome quality of todays multi and anti-reflection coatings there is difference to be seen as the older f15 scopes had nothing or at best a single coating of MgFl on the outside but that's another story .
Gentlemen, many thanks I believe I have a new kernel of knowledge! I have been puzzled by the older Unitron's and their design. The explainations delivered here describe the effect of limiting technology and not just a desire for long and bulky high f/number scopes making sense of the Unitron style refractors.
Availability of suitable glass types for short ratio refractors at an affordable price would probably have been the major contributor.
Another mis-conception is that a long fl will show a dimmer image to a short fl telescope of the same size ,, wrong , for example a 4 inch refractor at f15 would be exactly as bright on all objects as a 4 inch f6 , 4 inches of light gathering is 4 inches of light gathering , law's of physics cant be broken .
.
In practise that's not quite true.
A typical old F15 refractor probably came with 1" fittings , and might have a 40mm eyepiece if you were lucky . The user - seeking the lowest power and brightest image will reach for the 1" 40mm eyepiece which will give a narrow true field with a lousy 35 degree apparent field and a rather dim 2.8mm exit pupil . The owner of the modern F6 apo rafractor may reach in for a 2" 40 mm eyepeice and get a much brighter nearly 7mm pupil and a tasty 70 degree apparent field .
I used to own a 4" F15 Polarex rafracter and I am certainly not going to waste any time defending its qualities
Yes , but that's another story , but I do have a very old .965 orthoscopic that is like looking down a straw but it gives views of the planets like no other eyepiece ( modern ? ) I own , the eyepieces really have no say in the OP's original question and I think its more about quality of coatings than most other things in scopes of the same design ( fraks , newts , Mak's and CAT's ) and aperture .
I have looked thru some awesome scopes that are not expensive and a few stinkers that are considered 'Top of the drawer' , but all in all the better the figure /polish/design and coatings makes a better view , eyepieces /focusers and diagonals and the what not are secondary to what the objective/mirror's will produce .
Good point tho .
Brian.
Quote:
Originally Posted by Satchmo
In practise that's not quite true.
A typical old F15 refractor probably came with 1" fittings , and might have a 40mm eyepiece if you were lucky . The user - seeking the lowest power and brightest image will reach for the 1" 40mm eyepiece which will give a narrow true field with a lousy 35 degree apparent field and a rather dim 2.8mm exit pupil . The owner of the modern F6 apo rafractor may reach in for a 2" 40 mm eyepeice and get a much brighter nearly 7mm pupil and a tasty 70 degree apparent field .
I used to own a 4" F15 Polarex rafracter and I am certainly not going to waste any time defending its qualities
[...] its a common wrong idea that telescope's/ eyepieces magnify a distant object , they don't , they actually bring an object to the angular size it would look when much closer .
For example looking at say Jupiter at 200x is not seeing the planet 200x bigger but it is what Jupiter would look like if it was 200x closer than it physically is so instead of an average distance of 440 million km's at 200x it would look like it was only 2.2 million km away if using the naked eye , that's 1x .
I'm not sure that's right. If it made Jupiter look as it would if it was 200 x closer, wouldn't it therefore have a larger angular size relative to background objects? This would affect the timings of moon transits and star occultations, surely? If you take a photo and blow it up x 2, or halve your eyepiece focal length you get the same result, right? Or am I misunderstanding what you meant?
No and No , remember that the back ground objects ( stars ) are basically at infinity and even our 12 month orbit of the sun wont register as moving stars at about 20 light years ( zero movement seen ) I love parallax , works to a degree , now remember Jupiter is 120,000 km across ( it's actually large enough at 440 million km's and 120 thousand km across to be seen at 1x magnification ( our eyes ) as a tiny disc , on a good night ,,,, of course ) that's why the planets don't twinkle like stars ( look it up ) .
It's all about angles (a long isosceles 440 000 000 million km on both sides triangle by the visual angular diameter of Jupiter , at 1x ) , its a very slender 2 lines that at first glance would look parallel to the un-educated , Hypotenuse was a Genius .
Brian.
Quote:
Originally Posted by Stonius
I'm not sure that's right. If it made Jupiter look as it would if it was 200 x closer, wouldn't it therefore have a larger angular size relative to background objects? This would affect the timings of moon transits and star occultations, surely? If you take a photo and blow it up x 2, or halve your eyepiece focal length you get the same result, right? Or am I misunderstanding what you meant?
Last edited by brian nordstrom; 10-07-2016 at 01:42 AM.
No and No , remember Jupiter is 120,000 km across ( it's actually large enough at 440 million km's and 120 thousand km across to be seen at 1x magnification ( our eyes ) as a tiny disc , on a good night ,,,, of course ) .
It's all about angles (a long isosceles 440 000 000 million km on both sides triangle by the visual angular diameter of Jupiter , at 1x ) , Hypotenuse was a Genius .
Brian.
But you're surely not saying that it makes Jupiter look like it does as if it were many times closer? A simple thought experiment rules that idea out. Say I threw on an EP that made it look like Jupiter was so close it filled the entire field of view (seeing and limits of resolution aside). If Jupiter were that close in real life, entire constellations would be hidden by its disc. So I'm not sure how what you're talking about is different from magnification?
Loving this .
To give you an example how's about you draw a ' Ray Trace ' ( us telescope makers here use these to work out the focal length ( fl) of the scope we are building to within 2mm ) and measure the apparent diameter of Jupiter at 2.2 million km's ( Jupiter at 200x as seen with our eyes ) and a scale down might work ,,,,,,,
Trust me , using any telescope at 200x it will measure the same angular diameter , yes eyepieces , fl of the given scope will give a different perspective but the actual angular diameter of Jupiter will be of the same size .
It can be tested mathematically quite easily
Ouch ! , our constellations ??? ,,, ouch man ! Jupiter at 2.2 million km would be the size of Mars at it's best , ouch and I hear about the Bi-yearly Mars flyby and it will be BIGGER than the full moon .....run away ...
100x makes Jupiter look like its only 4.4 million km away , ok .
1000x it looks like at only 440000km away , looking good by now , except our atmosphere wont allow it ( hence HUBBLE )
keep up these mathematics and tell me straight if you can get Jupiter at 1 metre ? and what would you weigh ??? AND WOULD YOU REALLY WANT TO BE THERE ?
Brian.
Quote:
Originally Posted by Stonius
But you're surely not saying that it makes Jupiter look like it does as if it were many times closer? A simple thought experiment rules that idea out. Say I threw on an EP that made it look like Jupiter was so close it filled the entire field of view (seeing and limits of resolution aside). If Jupiter were that close in real life, entire constellations would be hidden by its disc. So I'm not sure how what you're talking about is different from magnification?
Last edited by brian nordstrom; 10-07-2016 at 02:24 AM.
Trust me , using any telescope at 200x it will measure the same angular diameter , yes eyepieces , fl of the given scope will give a different perspective but the actual angular diameter of Jupiter will be of the same size.
Ah, that makes more sense. You're basically saying that same magnification in different scopes yields same angular size to the eye? It was the 'makes it closer' thing' I was questioning because, you know, parallax. Same as zooming in on an object is different from moving the camera closer to that object.
A 16 inch at 200x will show an exactly the same sized ( angular ) Jupiter as a 2.4 inch refractor at 200x , just don't expect the same image .
Here is Saturn ( 150,000,000,000) km away at 197x . Yes Saturn is 1500 million km away .
Brian
Quote:
Originally Posted by Stonius
Ah, that makes more sense. You're basically saying that same magnification in different scopes yields same angular size to the eye? It was the 'makes it closer' thing' I was questioning because, you know, parallax. Same as zooming in on an object is different from moving the camera closer to that object.
Great question for sure and as already said there is no real one answer here as all tho telescopes only gather light and focus it to a point for viewing and magnifying by the eyepiece so we can see a magnified object and on this its a common wrong idea that telescope's/ eyepieces magnify a distant object , they don't , they actually bring an object to the angular size it would look when much closer .
but it is what Jupiter would look like if it was 200x closer than it physically is so instead of an average distance of 440 million km's at 200x it would look like it was only 2.2 million km away if using the naked eye , that's 1x .
, for example a 4 inch refractor at f15 would be exactly as bright on all objects as a 4 inch f6 , 4 inches of light gathering is 4 inches of light gathering , law's of physics cant be broken .
.
