|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Barking up the wrong tree
Can someone answer a question for me? I want to see if I've understood a
few things. As I understand it, the higher the F number of a 'scope, the more contrast it gives and it will be correspondingly better for planetary viewing. Hence a 'scope of F11 will give better contrast than a same sized 'scope that's F4. Since the F number is a ratio of the focal length to the diameter of the primary, would using a barlow lens effectively increase a 'scopes F number and hence contrast? Or in other words, would a 20mm barlowed e/p be better for planetary work than a 10mm non-barlowed lens? Sorry if this is me going waaaaaaay off kilter but it's something that's puzzled me over the last few days. Thanks, Jim -- AIM/iChat:JCAndrew2 "We deal in the moral equivalent of black holes, where the normal laws of right and wrong break down; beyond those metaphysical event horizons there exist ... special circumstances" - Use Of Weapons |
#2
|
|||
|
|||
Barking up the wrong tree
"Jim" wrote in message m... Can someone answer a question for me? I want to see if I've understood a few things. As I understand it, the higher the F number of a 'scope, the more contrast it gives and it will be correspondingly better for planetary viewing. Hence a 'scope of F11 will give better contrast than a same sized 'scope that's F4. Since the F number is a ratio of the focal length to the diameter of the primary, would using a barlow lens effectively increase a 'scopes F number and hence contrast? Or in other words, would a 20mm barlowed e/p be better for planetary work than a 10mm non-barlowed lens? Sorry if this is me going waaaaaaay off kilter but it's something that's puzzled me over the last few days. I would recommend using a scope designed for what you want to do, if only because adding extra optics to the light path adds risk of optical aberrations. Also, a long-focus telescope is much easier to design and make to high image quality specs than a short-focus telescope (esp. off-axis). In such situations as you describe, I would use the shorter FL eyepiece rather than use a Barlow. My experience of most Barlows is that they degrade the image slightly, add a bit of chromatic aberration, etc. -- Mike Dworetsky (Remove "pants" spamblock to send e-mail) |
#3
|
|||
|
|||
Barking up the wrong tree
"Jim" wrote in message m... Can someone answer a question for me? I want to see if I've understood a few things. As I understand it, the higher the F number of a 'scope, the more contrast it gives and it will be correspondingly better for planetary viewing. Hence a 'scope of F11 will give better contrast than a same sized 'scope that's F4. Since the F number is a ratio of the focal length to the diameter of the primary, would using a barlow lens effectively increase a 'scopes F number and hence contrast? Or in other words, would a 20mm barlowed e/p be better for planetary work than a 10mm non-barlowed lens? Sorry if this is me going waaaaaaay off kilter but it's something that's puzzled me over the last few days. I would recommend using a scope designed for what you want to do, if only because adding extra optics to the light path adds risk of optical aberrations. Also, a long-focus telescope is much easier to design and make to high image quality specs than a short-focus telescope (esp. off-axis). In such situations as you describe, I would use the shorter FL eyepiece rather than use a Barlow. My experience of most Barlows is that they degrade the image slightly, add a bit of chromatic aberration, etc. -- Mike Dworetsky (Remove "pants" spamblock to send e-mail) |
#4
|
|||
|
|||
Barking up the wrong tree
Mike Dworetsky wrote:
I would recommend using a scope designed for what you want to do, if only because adding extra optics to the light path adds risk of optical aberrations. Also, a long-focus telescope is much easier to design and make to high image quality specs than a short-focus telescope (esp. off-axis). In such situations as you describe, I would use the shorter FL eyepiece rather than use a Barlow. My experience of most Barlows is that they degrade the image slightly, add a bit of chromatic aberration, etc. Also, having given it a wee bit more thought, a Barlow doesn't double the focal length of the 'scope - it doubles the focal length of the e/p, yes? Hence it's not going to alter the F number at all. I'd need to whip up an aperture mask to do that. Thanks for the reply. Jim -- AIM/iChat:JCAndrew2 "We deal in the moral equivalent of black holes, where the normal laws of right and wrong break down; beyond those metaphysical event horizons there exist ... special circumstances" - Use Of Weapons |
#5
|
|||
|
|||
Barking up the wrong tree
Mike Dworetsky wrote:
I would recommend using a scope designed for what you want to do, if only because adding extra optics to the light path adds risk of optical aberrations. Also, a long-focus telescope is much easier to design and make to high image quality specs than a short-focus telescope (esp. off-axis). In such situations as you describe, I would use the shorter FL eyepiece rather than use a Barlow. My experience of most Barlows is that they degrade the image slightly, add a bit of chromatic aberration, etc. Also, having given it a wee bit more thought, a Barlow doesn't double the focal length of the 'scope - it doubles the focal length of the e/p, yes? Hence it's not going to alter the F number at all. I'd need to whip up an aperture mask to do that. Thanks for the reply. Jim -- AIM/iChat:JCAndrew2 "We deal in the moral equivalent of black holes, where the normal laws of right and wrong break down; beyond those metaphysical event horizons there exist ... special circumstances" - Use Of Weapons |
#7
|
|||
|
|||
Barking up the wrong tree
On Sun, 10 Aug 2003 13:53:17 +0100, (Jim)
wrote: Can someone answer a question for me? I want to see if I've understood a few things. As I understand it, the higher the F number of a 'scope, the more contrast it gives and it will be correspondingly better for planetary viewing. Hence a 'scope of F11 will give better contrast than a same sized 'scope that's F4. Since the F number is a ratio of the focal length to the diameter of the primary, would using a barlow lens effectively increase a 'scopes F number and hence contrast? Or in other words, would a 20mm barlowed e/p be better for planetary work than a 10mm non-barlowed lens? Sorry if this is me going waaaaaaay off kilter but it's something that's puzzled me over the last few days. Thanks, Jim Jim, you are getting a bit confused :-) Contrast is a function of the OTA design and also the quality of the optics, not a function of the f-ratio (although increasing the f-ratio may increase contrast through indirect means). Think of an f/8 APO refractor, an f/8 Newtonian, and an f/8 SCT. Contrast will be highest for the refractor and lowest for the SCT - with the Newt somewhere between. A lot of this has to do with the Newt and SCT having large obstructions in the light path (secondary reflectors). For reflectors, the smoothness of the reflecting surface is also very important, a 'rough' surface (on the micro scale) causes light scatter which decreases contrast. For lenses light scatter can occur at the air/glass interface and anti-reflection coatings here can also improve contrast. As a visual analogy you can use the Star Test where reduced contrast can be seen as the shifting of light energy from the central airey disk into the outer diffraction rings, APO refractors offer high contrast and produce excellent star tests - pin-point stars and very fine diffraction rings. A comparable view through an optically excellent SCT will show brighter diffraction rings. Using the Newt as an example, would increasing the f-ratio from from f/5 to f/8 increase apparent contrast? Well yes it would - because of two factors. Most importantly the size of the secondary required for the f/8 scope could be significantly reduced, thus decreasing the central obstruction. Secondly, in practical terms the f/8 optics are easier to figure to extreme accuracy and so would tend to be of better quality (you can, of course, pay enough money to overcome this limitation and get any quality of optics you like!). Finally, don't forget the role of the eyepiece. You want the least number of air-glass surfaces possible, and the highest light transmission. If you go to extremes the Tolles design has only two air-glass surfaces, but most people tend to go for high-quality Orthoscopics having just four. More complicated eyepieces (Radians for example) have yet more glass elements but try to compensate (some would say they fail) by using the best quality components and the best anti-reflection coatings. Oh, and a barlow really does increase the effective f/l of the primary (objective or primary mirror). However, because it is yet another lens to be inserted in the lightpath there is also an associated reduction of contrast (and total light throughput) due to transmission and reflective losses. Remember though, adding the barlow has *not* reduced the size of the central obstruction (in a reflector) despite increasing the apparent focal length. Again, quality of manufacture can go some way to minimising transmission and reflective losses and a TV Powermate will perform better than a cheaper model. There are a whole bunch of caveats that apply here, you have hit on a question that is argued over continuously. ChrisH UK Astro Ads: http://www.UKAstroAds.co.uk |
#8
|
|||
|
|||
Barking up the wrong tree
ChrisH wrote:
Jim, you are getting a bit confused :-) I suspected as such, hence the subject title :-) Contrast is a function of the OTA design and also the quality of the optics, not a function of the f-ratio (although increasing the f-ratio may increase contrast through indirect means). I'm curious in that case - what difference does the f-ratio actually make then? My understanding (ha!) is that it has something to do with the angle of the light cone, ie the smaller the number the shorter the cone. Is that right? In which case, what effect does that have on the image? Jim -- AIM/iChat:JCAndrew2 "We deal in the moral equivalent of black holes, where the normal laws of right and wrong break down; beyond those metaphysical event horizons there exist ... special circumstances" - Use Of Weapons |
#9
|
|||
|
|||
Barking up the wrong tree
ChrisH wrote:
Jim, you are getting a bit confused :-) I suspected as such, hence the subject title :-) Contrast is a function of the OTA design and also the quality of the optics, not a function of the f-ratio (although increasing the f-ratio may increase contrast through indirect means). I'm curious in that case - what difference does the f-ratio actually make then? My understanding (ha!) is that it has something to do with the angle of the light cone, ie the smaller the number the shorter the cone. Is that right? In which case, what effect does that have on the image? Jim -- AIM/iChat:JCAndrew2 "We deal in the moral equivalent of black holes, where the normal laws of right and wrong break down; beyond those metaphysical event horizons there exist ... special circumstances" - Use Of Weapons |
#10
|
|||
|
|||
Barking up the wrong tree
"Jim" wrote in message ... Mike Dworetsky wrote: I would recommend using a scope designed for what you want to do, if only because adding extra optics to the light path adds risk of optical aberrations. Also, a long-focus telescope is much easier to design and make to high image quality specs than a short-focus telescope (esp. off-axis). In such situations as you describe, I would use the shorter FL eyepiece rather than use a Barlow. My experience of most Barlows is that they degrade the image slightly, add a bit of chromatic aberration, etc. Also, having given it a wee bit more thought, a Barlow doesn't double the focal length of the 'scope - it doubles the focal length of the e/p, yes? Hence it's not going to alter the F number at all. I'd need to whip up an aperture mask to do that. Thanks for the reply. Jim -- Actually, it effectively halves the FL of the eyepiece. (Twice the magnification.) -- Mike Dworetsky (Remove "pants" spamblock to send e-mail) |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Breakthrough in Cosmology | Kazmer Ujvarosy | SETI | 8 | May 26th 04 04:45 PM |
Breakthrough in Cosmology | Kazmer Ujvarosy | Space Shuttle | 3 | May 22nd 04 09:07 AM |
Breakthrough in Cosmology | Kazmer Ujvarosy | Amateur Astronomy | 4 | May 21st 04 11:44 PM |
Breakthrough in Cosmology | Kazmer Ujvarosy | Space Station | 0 | May 21st 04 08:02 AM |