Coma is produced by the primary mirror, not the eyepiece.
I have read of this. The spot diagrams would appear to indicate that this is so. I have also read that coma is not produced by the mirror, rather by the inability of the ocular to handle the fast light cone.
Quote:
Originally Posted by Don Pensack
I do see that individual perception plays a role in the visibility of coma. Some people see it at f/6, others not even at f/4. Why, I can't say.
Don
individual variation plays a big factor in what we preceive.
The aberrations most likely to be produced by an eyepiece's inability to handle the lateral rays of a wide, shallow, light cone of a short f/ratio scope are astigmatism, lateral magnification distortion, and lateral chromatic aberration, not to mention spherical aberration. Coma CAN be produced by a simple lens (say, a one-element eyepiece (not common since the 1600s). Coma, however, is solely an aberration, with today's eyepieces, of the mirror.
I agree that individual variation , such as the genetic abilities to see in the dark, plays a role in the visibility of coma, but the size of the star image at the edge of a 40mm focal plane in an f/5 scope is literally 9X the width of the Airy disc. It's hard to believe that anyone could be unable to see it. I suspect that everyone can SEE it, but not everyone is BOTHERED by what they see. That's a psychological response, not a physical one.
On the subject of perception.. Since I've gotten into astronomy and started fussing over all the possible aberrations in scopes and EPs, I learnt to see chromatic aberration in my own eyes! Streetlights with mercury lamps have a purple halo around them when I look at them with averted vision. I was quite surprised the first time I noticed. Normally the brain would remove/ignore such aberrations.
I hear you. I demanded better glasses to correct my distance vision from 20/20 to better. Why? Because the images in my telescope where sharp, while my daytime view of distant objects was a little blurry. I wanted the distance to be as sharp as my nearer vision and as sharp as an image through my telescope. I ended up correcting my distance vision to 20/10, and achieved a sharp long-distance vision.
Telescope viewing definitely warps your perceptions.
Now, if I could just get rid of the floaters!
Don
I've researched coma more fully. Here are the two best sites I found that explain the concepts: http://www.opticalmechanics.com/about_coma.htm
and http://www.astromart.com/articles/ar...?article_id=50
The best quote is:
"Using this method to specify field position, all telescopes of a given focal ratio have the same amount of off-axis coma, regardless of aperture. For example, a 10” F/4 has the same amount of coma as a 20” F/4 at the edge of a 30-mm diameter fov."
The formula is:
Linear diameter of coma is 3r/(4N)^2 where r=off axis distance in mm, and N=focal ratio. No mention of focal length is made.
Just thought people would like to know.
Don
Interesting inputs from all involved. The physics (geometric optics) demands the coma be the same for optics of the same f ratio. And the paracorr doesn't know how big the mirror is or its focal length, it just deals with the convergence angle of the incoming focused light. So while Bratislav's arguments are thought provoking (not a bad idea) I couldn't see how they would apply. I'll re-read them as the arguments are subtle.
The moral of the story (based on people's reported experience and the various arguments here) seems to be that for any Newtonian of f/8 or less, and especially f/4-f/5, with first class eyepieces, a paracorr is a damn good gadget to have available.
Thanks to all for the enlightening discussion.
One question arises: since astigmatism seems to swamp coma for eyepieces below a certin quality, at what point on the quality curve does a paracorr start to make a visible difference? Anything better than el-cheapo Chinese eyepieces? Something like the Meade 5000 series? Or only for things like Naglers and top Pentaxes? Comments aprpeciated.
2. Floaters: Don, if you find a way to deal with them, please post here!! My eyes are a zoo
... Linear diameter of coma is 3r/(4N)^2 where r=off axis distance in mm, and N=focal ratio. No mention of focal length is made.
Don, I think there is a bit of confusion about what Bratislav was trying to get across. He was not arguing that the amount of coma will be different for different focal lengths, but that the optics needed to correct it will need to be designed for a particular focal length, and independent of f-ratio (once you have chosen the fastest f-ratio you want the corrector to still work in).
Thinking about the problem in my simple-minded way (having no experience with designing optics), this seems to be correct, for two reasons.
1) If a corrector is matched to a fast mirror of some focal length, then it also must be a match for all slower mirrors of the same focal length, because the fast mirror can be split into a slow inner mirror and a fast periphery. The corrector must work on both slow inner and fast outer halves of the mirror if it is to work on the mirror as a whole. So you cannot have a corrector designed for fast mirror that will not work just as well on an arbirtrarily slow mirror of the same focal length.
2) If you think about scaling up a fixed f-ratio mirror (increase f.l. but keep f-ratio constant), the amount of coma at some point in the focal plane will increase linearly with size of the mirror. But so will the distance r of that point from the optical axis. Combine this with the fact that for any given mirror, the amount of coma is a linear function of r, it follows that the amount coma will indeed be the same for any given r independent of the focal length, as we all seem to agree. However, when you scale up the optics, the field curvature will change. A scaled up field will have less curvature, i.e., for the same r, you are closer to the focal plane in a long focal length mirror. (e.g., think of circular arc of fixed length as you blow up the circle.) So a small fast mirror will have a more strongly curved field than a large mirror of the same f-ratio.
Because of #1, the coma corrector cannot be specific to any f-ratio.
Because of #2, it makes sense that the corrector must be specific to focal length, because while the amount of coma at a given distance from the optical axis is independent of FL, its distance from to the ideal flat focal plane is not, with field curvature getting stronger as the FL gets shorter.
From this I would conclude that there will be an ideal focal length where Paracorr works best, and there is a range of focal lengths above and below this where it works well. I expect there would probably only be an issue with small mirrors, as the field is practically flat once mirrors get large enough, so a corrector designed for a flat or nearly flat field will work well with arbitrarily large mirrors. However, I would not be surprised if the Paracorr did not work with a 4" f/4, for example. Anyone care to check?
...A scaled up field will have less curvature, i.e., for the same r, you are closer to the focal plane in a long focal length mirror. (e.g., think of circular arc of fixed length as you blow up the circle.) So a small fast mirror will have a more strongly curved field than a large mirror of the same f-ratio.
A question: what effect is a paracorr designed to have on field curvature, as distinct from coma?
A question: what effect is a paracorr designed to have on field curvature, as distinct from coma?
I don't know if the Paracorr was designed to have any effect on FC (but I have read somewhere that it flattens the field somewhat). Whether it increases, decreases the FC, or leaves in the same, is beside the point within the context of the present discussion.
My intuition tells me that even if a coma corrector leaves the FC unaltered, it can only properly cancel the coma if it "knows" where that coma occurs, which will depend on FC. So the designer must consider the FC of the system the corrector will be used in, even if there is no desire to change the FC. And as FC of a mirror does change with FL, the design will need to be tailored to specific FL.
My intuition tells me that even if a coma corrector leaves the FC unaltered, it can only properly cancel the coma if it "knows" where that coma occurs, which will depend on FC. So the designer must consider the FC of the system the corrector will be used in, even if there is no desire to change the FC. And as FC of a mirror does change with FL, the design will need to be tailored to specific FL.
Interesting point. I'd be interested to see what Don has to say. My earlier post made the implicit (and wrong) assumption that the paracorr scales up with the mirror! (Whoops).
1. The Paracorr's correction of field curvature is slight, and is related to having the focal length lengthen 15%. this makes field curvature in the telescope less, and since field curvature in the eyepiece and telescope are additive, there is some correction of field curvature.
2.Bratislav's idea that a short f/ratio mirror contains rays of all f/ratios is dependent on the idea that some rays strike and use only a part of the mirror. Unfortunately, and I'll explain what I say in a minute, EVERY RAY HITTING THE MIRROR USES THE ENTIRE MIRROR BECAUSE EVERY RAY STRIKING THE MIRROR IS A WAVE THAT HITS THE WHOLE EARTH. That means that, wherever the star is in the field of view, the f/ratio is the same. What does happen is that the front of the tube and the secondary size do vignette that wave, reducing its brightness from an axial wave, but the wave itself is focused on a point at exactly the same distance from the mirror. If you'd like an illustration, stand 1' from a wall with a bright flashlight. Shine the flashlight on the wall straight ahead. The light on the wall is circular. Without moving the flashlight toward or away from the wall, angle the flashlight 45 degrees to the side. What shape is the light beam hitting the wall? Oval. Now, this is an oversimplification of what occurs on the focal plane of a paraboloidal mirror, but it is somewhat illustrative of why coma is worse farther off axis.
3. When the wave passes through the Paracorr, the outside of the wave is bent by the edge of the lens. Like any light ray passing through a lens, the angle of incidence determines the angle it exits the lens. All f/4 telescopes, regardless of focal length, have the same angle of incidence entering the Paracorr lens. Accordingly, the lens curvature can be designed to fully correct a particular angle of incidence (f/ratio). But what happens when the f/ratio is longer? The rays that hit the lens do so at angles closer to parallel, which makes the angle of exit warped to a greater degree. So, if the goal is to reduce the size of the comatic star image to no larger than the size of the Airy disc, this will happen over a small range of f/ratios. At very long f/ratios, where the rays hit the lens at nearly parallel angles, the warpage of the lateral rays can bend the ray the other way and cause "reverse coma". At what f/ratio this would occur, I don't know, but I wouldn't try a Paracorr in a Schiefspiegler.
4. There is a range of f/ratios that would be completely corrected. On the Paracorr, that would appear to be f/5.5 to f/8. That says that Al chose to make the curves gentle to avoid spherical aberration. The 15% increase in focal ratio was necessary to avoid any detectable spherical aberration from the lens for on-axis images. This is also why the other commercially available coma correctors DO exhibit increased spherical aberration, though this is not as important photographically due to the larger size of the star images compared to visual use. It also means that if TeleVue had chosen to fully correct an f/4 telescope, spherical aberration would have had to be controlled some other way, such as additional lenses or aspherical curves, both of which would have made the Paracorr an unaffordable luxury for a chosen few, as well as much heavier.
5.The point to get across, here, is that the linear size of a comatic image will be exactly the same size 15mm off axis in any scope of the same f/ratio, whether having a 25" focal length or a 100" focal length. There is a difference of image scale, though, and it depends on whether you look at coma from an angular or linear distance away from center as to how you describe it. I suspect Bratislav and I were talking about coma from opposite points of view.
But when we look through an eyepiece, we are looking through an eyepiece with a set linear field stop, and the coma present in the image at the edge of the field of any eyepiece is dependent on the linear size of the field, and it is dependent on the f/ratio of the scope.
One question arises: since astigmatism seems to swamp coma for eyepieces below a certin quality, at what point on the quality curve does a paracorr start to make a visible difference? Anything better than el-cheapo Chinese eyepieces? Something like the Meade 5000 series? Or only for things like Naglers and top Pentaxes? Comments aprpeciated.
2. Floaters: Don, if you find a way to deal with them, please post here!! My eyes are a zoo
DN
DN
Here's some info on floaters: http://www.eyefloaters.com/
Note that laser fixes for floaters is not yet considered a standard procedure. But, short of replacing the fluid in the eye, this remains the only current cure.
As for eyepieces, if astigmatism swamps all other aberrations (say, using a Chinese widefield at f/4.5, for example), then eliminating coma will only make a minor improvement. But if the eyepiece is decent (like the new Baader Hyperion or the Orion version), then it can be valuable to add a coma corrector. A few posters on Cloudy Nights have done just that and been pleasantly surprised at which eyepieces were improved and which weren't. It's possible an eyepiece that appears unsalvageable without coma correction may become quite tolerable with it. I think you never know until you try it. But, as you say, the better eyepieces that have little or no inherent astigmatism do seem to be the ones that are made nearly perfect by simply correcting coma.
OK,
I talked with an expert in optical design, and he explained coma to me as a paraboloidal aberration and explained why there was so much confusion. The explanation was really quite simple.
The linear size of a comatic image on the focal plane of any scope is dependent on distance from the center of the field.
How it relates to focal length and focal ratio is as follows:
A 10" f/4 exhibits X coma at the edge of a 40mm field of view
A 20" f/4 exhibits .5X coma at the edge of a 40mm field of view, yet, because the magnification is double, the linear size of the comatic image at the edge of the field of view is exactly the same.
This is why the linear size of a comatic image appears the same at all focal lengths of identical f/ratios at the edge of the same sized field of view, and why a coma corrector can be designed to work with a set f/ratio in mind.
At the same time, the linear size of the comatic star image on the focal plane of the scope, at the same distance from center, will be less with a longer focal length, and it is here that describing coma as lessening with focal length is correct.
A coma corrector that is designed for visual use will take magnification into account, so would be designed to correct for f/ratios.
A coma corrector designed for prime focus photography will be designed for the size of the photographic plane and the focal length of the scope.
Where the confusion lies is that they are one and the same. It depends on your point of view as to which explanation makes the most sense. FOR IDENTICAL FIELDS OF VIEW, THE LINEAR SIZE OF COMA WILL BE IDENTICAL FOR ALL FOCAL LENGTHS OF A PARTICULAR F/RATIO.
So, the linear size of the comatic star image IS made larger by magnification. He had no explanation of why I (and others) see lessened coma at higher powers. I, since (on another forum), have run into some individuals who see the same coma in the field at all magnifications, so it might be a psychological phenomenon on my part.
Don, with the exception of anti-reflection coatings, all telescope optics (mirrors, lenses, eyepieces, barlows, field flatteners, coma correctors etc) are designed to operate in the regime of geometric or ray optics and are based entirely on ray optics principles. Here, wave phenomena such as interference and diffraction can be completely neglected and light can be viewed as consisting of localised rays rather than extended waves that fill all space. Or, equivalently to the ray picture, you can think of light as point particles bouncing off mirrors and bending at air-glass interfaces according to the law of refraction. Ray optics does not contradict wave theory, but it is a special case of it, i.e., the short wavelength limit.
The ray optics approximation is completely valid for telescope optics in normal use, and only starts to break down at the diffraction limit of the scope, i.e., when one uses enough magnification to start seeing diffraction effects (e.g., Airy disk star images). (It does fail to predict some subtleties e.g. diffraction spikes from spider of a Newt but these are irrelevant in the present context.) It is perfectly valid to split the mirror into multiple components (e.g. a slow part and a fast part) and then figure the total light intensity in the focal plane to be the sum of the light intensities from its composite parts. And the statement that a given f-ratio mirror also contains all slower f-ratios is correct also.
Coma is a geometric property of the parabolic mirror. It is entirely a ray optics phenomenon and is corrected by optical devices such as the paracorr within that theoretical framework.
Quote:
The point to get across, here, is that the linear size of a comatic image will be exactly the same size 15mm off axis in any scope of the same f/ratio
...
FOR IDENTICAL FIELDS OF VIEW, THE LINEAR SIZE OF COMA WILL BE IDENTICAL FOR ALL FOCAL LENGTHS OF A PARTICULAR F/RATIO.
Yes, this has been stated a number of times, and I believe everyone who posted about it in this thread agrees on this point.
It looks like we have had lots of "entertain" from Don so far, but not much of "inform" and "educate" despite vailant efforts of Janoskiss.
I guess I'm to blame as well as so far noone apart from Steve actually understood what I was talking about.
I can't really make it any simpler, I'm afraid if anyone wants to know something about optics I can only suggest a good book like Rutten and Van Venrooij or Conrady.
Let me but in one more time, just to try to clear the confusion.
BTW, I don't really need to talk to an "optical designer friend" - I have designed coma correctors (single element - subaperture menisk, two element - Ross family, and three element units - Wynne/Gascoigne type) for various systems (sphere, paraboloid, hyperboloid, Dall-Kirkhams and Schmidt Newtonians). I've been designing optics for decades, and when I say something, I usually know what I'm talking about.
So let me offer something even simpler to understand. I can show you spot diagarams of two optical systems using the same coma corrector.
Although I have in my posession reverse engineered data for Celestron/Baader and Lumicon units, I don't know the prescription for Paracorr (as noone volunteered a unit to be pulled apart, and I'm not going to shed $$$ just to try to convince someone who doesn't really want to learn anyway).
So, here I will use a well known coma corrector published several times - in May '85 Sky and Telescope (under a peer review of late Bob Cox) and Telescope Optics book (again peer reviewed by R & vV). It is designed for 1200mm focal length (author designed it for 12" f/4 scope).
Let's see what Zemax says about two systems. 100mm f/12 (reverse coma anyone?) and 100mm f/4 (which should be spot on if we are to believe Don).
Vertical bar on the left gives you a reference size (200 microns). Black circle is Airy disc size. I've used monochromatic light not to confuse people even more with polychromatic aberrations. Five spots are spread as to cover field from dead on axis to approx 14mm off axis (very edge of the 1-1/4" size eyepiece and approx. corner of a typical DSLR chip).
A 100mm f/12 scope (attachment 1)
{{{ I'm hopeless, can't get the attachment to be where I want them }}}
Reverse coma ? Hmmmm .... Sure. Want to see f/16 ? F/20 ? It only gets better, trust me.
100mm f/4 scope, focused for best images on a flat focal surface (attachment 2)
Hmmm. But it should be perfect, it is f/4 after all !!!! And those off axis images look like -er- coma ????
Don't let me show you what a 50mm f/4 looks like.
I was wrong about reverse coma. The inward bending of the curve in the outer part of the field just indicates complete correction of coma at longer f/ratios, not a backward bending of the curve.
I posted an inquiry about the discrepancy between what I was told and read about coma, and remarks by Bratislav, on another Forum.
Here is a link to that thread (you may have to cut and paste the entire link into your browser address bar): http://www.cloudynights.com/ubbthrea...t=1&PHPSESSID=
I think reading the posts will fill you in on where the misunderstanding was, and point out that I believe that what we had here was a "failure to communicate".
This will be my final post on this subject on this Forum. Moderators should be more aware that ungentlemanly behavior on the part of participants should be remonstrated.
Thanks for your contribution on this topic. Much of it is beyond my current level of experience and knowledge, but I have valued the little that I have picked up along the way.
As a reader and contributor to this forum, I know who the gentlefolk are and by your posts, you have shown that you belong to that community on Ice In Space. Although I am always surprised at what clearly appear to be confrontational or combative responses in these threads, I focus on the real data. My respect for the gentlefolk only increases when in the face of such an onslaught, they do not respond to the caustic comments in the same prickly manner.
The intellect can be a powerful tool and when someone has developed a sharp intellect, one hopes that their overall persona has also matured to the extent that this sharp tool is applied not in a cutting manner, but in a generous and helpful manner. Sharp is good; cutting is simply unnecessary and perhaps the sign of an insecure person?
Most of us are here to learn and work towards a common understanding. I appreciated the positive contributions from all who posted in this thread, and the opportunity to learn a bit about coma. OTOH, I would also like to point out that it should be obvious to anyone with half a brain that the condescending attitude displayed in some posts was totally unwarranted and entirely counterproductive. (see what I did there...? )
I am not going to apologise for my nature of commenting things. That is how I am - plain and simple.
I also understand that this forum is a lot more about getting together and sharing love for astronomy - I fully respect that. More on that later.
That said, I really get upset by Don's ability to twist and misquote things (delibeartely or not, I don't really care).
For example, in 'another thread' he now managed to attribute this to my name :
"He stated quite definitely that coma in shorter focal length f/4 scopes was worse than coma in long focal length f/4 scopes".
I never said anything of sorts, as this would be stupid plain and simple. I have no doubts he misquoted Al and David Nagler in a smilar fashion. Gentlefolk or not, this is equivalent to slander.
Now, "unsecure" nonwithstanding, I am going to defend my point of view and my name to the best of my abilities. If that is perceived "sharp" or "cut" so be it. I am dealing with facts - I don't have patience or "maturity" to wrap things in a rosy glow so everyone can be merry and live happily everafter. Science and engineering forums are like that. Put up or shut up.
I do understand that this forum is different and this sort of behaviour is not welcome here - yup, I've got the message - several times. It is entirely my fault for getting into this discussion here in the first place. I really should have known better, and I sincerely regret it. I will do my very best to avoid this in the future.
I'll let you go back to the regular program now.
(I did want to remove my account from IIS, I already asked moderator how to do that. I guess it doesn't matter now, this should expedite things )
)
