Can someone explain the real differences between an f/15 and say an f/5 as far as differences in viewing and/or photographing thru a scope? I've heard some people talking about short or long focal length scopes but no one has really been able to explain the advantages/disadvantages for either one. Your responses would be greatly appreciated......................Thanks!
FL........long/short
Started by oneaudiopro, 10/28/2006 06:23AM
Posted 10/28/2006 06:23AM
Opening Post
Posted 10/28/2006 12:16PM
#1
>>>I've heard some people talking about short or long focal length scopes but no one has really been able to explain the advantages/disadvantages for either one. Your responses would be greatly appreciated......................Thanks!
----
In addition to Jim's comments, heres a couple of thoughts:
1. A 4 inch F/6 scope is about 24 inches long. A 4 inch F/15 scope is about 60 inches long. This makes a big difference in the practical aspects of viewing. One can use a simple mount with the 4 inch F/6 scope and be quite comfortable. The F/15 scope will require serious mount and the eyepiece position will change drastically as the scope is swung from the horizon to the zenith.
2. The maximum possible field of view will be much smaller with the longer focal length scope. With a 2 inch focuser, at F/15 the max TFoV will be about 1.7 degrees while the F/6 (4 inch) will be capable of more than 4 degrees, really quite nice.
3. The F/6 scope will be much more prone to aberrations such as false color. Eyepieces may show edge astigmatism because of the faster focal ratio.
4. Photographically, at Prime focus, the F/15 scope will require exposures that are about 6 times as long as the F/6 scope but it's image scope will be about 2.5 times as large.
Put this all together and what do you get???
The modern, fast APO as envisioned and developed initially by Roland Christen does it all. An APO (Apochromat) that is properly designed and manufactured is free from chromatic and spherical aberrations so it provides better images than a long focal length achromat while keeping the virtues of compactness and a well corrected widefield. On an inch for inch basis, there is no other design that can compete with the modern APO, it essentially does as good a job as can be done in all areas, low power, high power, photography, ease of use, cool down, robustness...
This is really one place where you can indeed "have it all." Of course "having it all" is not cheap so on a dollar for dollar basis, there are many other viable choices.
Jon
----
In addition to Jim's comments, heres a couple of thoughts:
1. A 4 inch F/6 scope is about 24 inches long. A 4 inch F/15 scope is about 60 inches long. This makes a big difference in the practical aspects of viewing. One can use a simple mount with the 4 inch F/6 scope and be quite comfortable. The F/15 scope will require serious mount and the eyepiece position will change drastically as the scope is swung from the horizon to the zenith.
2. The maximum possible field of view will be much smaller with the longer focal length scope. With a 2 inch focuser, at F/15 the max TFoV will be about 1.7 degrees while the F/6 (4 inch) will be capable of more than 4 degrees, really quite nice.
3. The F/6 scope will be much more prone to aberrations such as false color. Eyepieces may show edge astigmatism because of the faster focal ratio.
4. Photographically, at Prime focus, the F/15 scope will require exposures that are about 6 times as long as the F/6 scope but it's image scope will be about 2.5 times as large.
Put this all together and what do you get???
The modern, fast APO as envisioned and developed initially by Roland Christen does it all. An APO (Apochromat) that is properly designed and manufactured is free from chromatic and spherical aberrations so it provides better images than a long focal length achromat while keeping the virtues of compactness and a well corrected widefield. On an inch for inch basis, there is no other design that can compete with the modern APO, it essentially does as good a job as can be done in all areas, low power, high power, photography, ease of use, cool down, robustness...
This is really one place where you can indeed "have it all." Of course "having it all" is not cheap so on a dollar for dollar basis, there are many other viable choices.
Jon
Posted 10/29/2006 12:41AM
#2
Despite the "APO" revolution and trend toward faster refractors made possible by low dispersion modern glass types, a long focal length refractor just seems to look like a "proper" telescope compared to the chunky, stubby apochromats. 
Posted 10/29/2006 03:53AM
#3
Hello all:
The emphasis on the last few posts was on the photographic ability of the apo refractor. However, many of us are more interested in visual study of the sky (although I am beginning to eye CCD technology). Moreover, for those of us cursed with light polluted skies, a small apo is a blessing. It is easy to transport and its short length gives you more leeway in the type of mount you can use. The longer the OTA, the more sensitive it will be to vibration whether from human touch or wind, and will likely require a sturdier mount. The f/15 ratio is very forgiving with regards to lens manufacturing but obviously makes for very long scopes. At a star party last year, someone brought along a home-made f/15 refractor of 6 inches of aperture. It was a beauty, but the thing was seven feet long!
If transporation or mount issues are not important, but cost is, one could argue that high quality medium fl (such as f/9) achromatic refractors are a great deal. At f/9 color correction can still be superb if the lens is made to precise specifications. For example, the 90mm Orion / Vixen versions of these scopes are quickly sold whenever they appear on the Astromart classifieds. And they can be bought for substantially less than apo's of the same aperture.
Any comments on the medium focal length refractors?
Rey
The emphasis on the last few posts was on the photographic ability of the apo refractor. However, many of us are more interested in visual study of the sky (although I am beginning to eye CCD technology). Moreover, for those of us cursed with light polluted skies, a small apo is a blessing. It is easy to transport and its short length gives you more leeway in the type of mount you can use. The longer the OTA, the more sensitive it will be to vibration whether from human touch or wind, and will likely require a sturdier mount. The f/15 ratio is very forgiving with regards to lens manufacturing but obviously makes for very long scopes. At a star party last year, someone brought along a home-made f/15 refractor of 6 inches of aperture. It was a beauty, but the thing was seven feet long!
If transporation or mount issues are not important, but cost is, one could argue that high quality medium fl (such as f/9) achromatic refractors are a great deal. At f/9 color correction can still be superb if the lens is made to precise specifications. For example, the 90mm Orion / Vixen versions of these scopes are quickly sold whenever they appear on the Astromart classifieds. And they can be bought for substantially less than apo's of the same aperture.
Any comments on the medium focal length refractors?
Rey
Funding Member
