Paracorr

[janoskiss (Steve H)]

Originally Posted by bratislav

Again, brightness doesn't factor in untill you start to see the diffraction structure as extended image. That is pretty much at the same point (well below 5mm eyepieces for f/4 mirror). Extended comatic blur (that subtends say 5 arc minutes) will have same flux (# photons per retinal area) so it will look the same regardless of eyepiece's focal length (as long as diffraction structure isn't resolved - say 1mm exit pupil or thereabouts).

I see your point. I have no direct experience of how that looks in practice. I certainly cannot see anything wrong with star images at the edge of a 70 degree FOV of 10mm XW eyepiece at f/5, or even at f/4. Looking at Al Nagler's graphs (above), I should. I remain curious to try a Paracorr.

Again optics 101.

Let's leave that out of it. Beginning optics courses concern themselves with ideal lenses and mirrors with perfect focus for all colours, and perfectly flat infinite focal planes. Only if real-world optics worked like that...

[bratislav (Bratislav)]

Originally Posted by janoskiss

I see your point. I have no direct experience of how that looks in practice. I certainly cannot see anything wrong with star images at the edge of a 70 degree FOV of 10mm XW eyepiece at f/5, or even at f/4. Looking at Al Nagler's graphs (above), I should. I remain curious to try a Paracorr.

Depends on many things, mainly on how critical the observer is. Some eyepieces may in fact reduce visible aberrations (some well known examples are Pretoria + paraboloid or Plossl + Jones-Bird). Unfortunately, I don't have any experiences with 10mm XW, but I'll keep an eye on it.
Naglers definitely show coma of an f/4 paraboloid, compared to a much better corrected scope like my homemade f/4 Wright. But difference is indeed rather small compared to say astigmatism of 'ordinary' eyepieces.

Bratislav

PS '101' as in elementary principles, not some ideal cases that don't apply to real life

[iceman (Mike)]

Bratislav, you're obviously very experienced with optics, and as such your opinions and experience are valued in discussions such as this.

Just please try and stay pleasant, and as Steve said please leave sarcastic or rude remarks out of it.

This discussion has brought up a lot of great points and i'd hate to see it degenerate.

[norm]

So is a Paracorr a goer for Rod ?

[janoskiss (Steve H)]

Originally Posted by norm

So is a Paracorr a goer for Rod ?

I think Rod's rods would love it!

[Merlin66 (Ken)]

I agree with Don Pensack....
In my experience as an amateur the coma is related to the fast cone of the f ratio. "faster" systems ie f 4 produce more coma than "slow" systems independent of the focal length. I agree you need to consider the FL for the overall sizing of the optics but coma is f ratio. My understanding is that the Hale 200" at f3.3 was coma limited hence the use of the Wright etc correctors.
My 2c.

[janoskiss (Steve H)]

Merlin, I think everyone who posted here agrees about faster scopes producing more coma, and the amount of coma being dependent on the f-ratio.

[bratislav (Bratislav)]

Originally Posted by iceman

Bratislav, you're obviously very experienced with optics, and as such your opinions and experience are valued in discussions such as this.

Just please try and stay pleasant, and as Steve said please leave sarcastic or rude remarks out of it.

This discussion has brought up a lot of great points and i'd hate to see it degenerate.

Apologies Mike, I'm a bit thick when it comes to niceties and group hugs.
But rest assured, I won't do it again in your forum.
Cheers !

[Satchmo]

Originally Posted by AstroJunk

I borrowed a Parracor for an evening and was very impressed using my 20" f5. My 17mm nagler was greatly improved (though the 13mm made only a little difference). Most surprising to me was a marked increase in contrast that I wasn't expecting - the Homunculus of Eta Carina took a fabulous mottled appearance (confirmed by the others) that had never been so prominent and the coma was beutifully corrected.

Aside from all the theory, this was my impression too. Even at F5 , coma blurs the star images bigger than the Airy Disc , quite close to the centre of the field , so contrast and sharpness is improved even near the centre with the Paracor.

This also means that if you are looking at a subtle small object like the Humunculus, you don't have to have it sitting exactly in the field centre to get the best detail. As one of the US reveiwers posted here said, the Paracor is a device that people gradually decide to buy as they have more exposure to it and increase their observing skills.

Its nice to know there is still one bit of kit left that can improve the view in most of your eyepieces, beyond having a good mirror.

Mark

[Don Pensack]

I just spoke at length with David Nagler (Al's son) and he confirmed what Al had told me some time ago:
1.Coma correction is a function of f/ratio only, and focal length has nothing to do with it. The Paracorr applies exactly the same correction to a 4" f/4.5 that it does to a 20" f/4.5. It is the angle of the lateral rays that are the determinant of the coma in the instrument, not the focal length.
2. David also confirmed my second statement that coma becomes less visible in eyepieces with shorter focal lengths (and smaller field stops), even if AFoV is the same, and that the reason is the drop in the brightness of the lateral parts of the comatic star image. I have seen coma in all my eyepieces all the way up to a 5mm, but it is/was nowhere near as intense to my eye as it is/was at lower magnifications. Since coma is linear, the argument that doubling the power and having the field halve in width should show exactly equal coma at the edge of the field SOUNDS right, but my experience says that's not true, and that there's some other factor at work. That factor is the simple fact that the extended star image is 1/4 as bright while the magnification has only doubled. This is the reason many observers (and I mean many) stop using their coma correctors at high powers. The lateral edge of the comatic image is simply too faint to see, so the star image doesn't appear as bloated. A lab instrument or CCD image (compensating for brightness by a longer exposure) would show the star image just as wide, but the eye can't see it.

Sorry for any confusion on the issue. I stand by my earlier comments.

[bratislav (Bratislav)]

Originally Posted by Don Pensack

I just spoke at length with David Nagler (Al's son) and he confirmed what Al had told me some time ago:

Can you please get Al to confirm this quote of yours ?
"Al told me directly he designed the Paracorr for a focal ratio of f/4.5, but that it works better on longer f/ratios. At some length of f/ratio, the Paracorr would be applying reverse coma, but it is above f/8 and no one will be using one above that f/ratio. "

I'd like to hear it from Al himself, not your paraphasing, please.

1.Coma correction is a function of f/ratio only, and focal length has nothing to do with it. The Paracorr applies exactly the same correction to a 4" f/4.5 that it does to a 20" f/4.5. It is the angle of the lateral rays that are the determinant of the coma in the instrument, not the focal length.

Except coma correctors actually correct two things - coma and field curvature. And field curvature is proportional to mirror's focal length. Hence, when uncle Al set to design Paracorr, he picked one focal length (as there will be one and only one f.l. where field is 100% flat), and NOT one f/ratio (as coma correctors will correct for all f/ratios, not only f/4.5).

In simple terms,

1) Paracorr will work best with only one focal length paraboloid (once you move from prescribed f.l. field curvature creeps in)
2) Paracor will work with ALL f/ratio paraboloids (up to the point where high order aberrations including spherochromatism start to bloat images - around f/4 for Paracorr) and
3) there is no one "optimal" f/ratio and "applying reverse coma" only exists in one's head. You can put Paracorr in f/10 Newtonian and coma will be reduced by exactly the same amount as in 4.5 one.

Hence, statement that coma correctors are designed around focal lenght is indeed correct.

2. David also confirmed my second statement that coma becomes less visible in eyepieces with shorter focal lengths (and smaller field stops), even if AFoV is the same, and that the reason is the drop in the brightness of the lateral parts of the comatic star image. I have seen coma in all my eyepieces all the way up to a 5mm, but it is/was nowhere near as intense to my eye as it is/was at lower magnifications. Since coma is linear, the argument that doubling the power and having the field halve in width should show exactly equal coma at the edge of the field SOUNDS right, but my experience says that's not true, and that there's some other factor at work. That factor is the simple fact that the extended star image is 1/4 as bright while the magnification has only doubled. This is the reason many observers (and I mean many) stop using their coma correctors at high powers. The lateral edge of the comatic image is simply too faint to see, so the star image doesn't appear as bloated. A lab instrument or CCD image (compensating for brightness by a longer exposure) would show the star image just as wide, but the eye can't see it.

Sorry for any confusion on the issue. I stand by my earlier comments.

The confusion is indeed rather evident.
Your main point of confusion (apart from not understanding that f/4 paraboloid contains all slower beams) is failing to understand that comatic image at the edge of the field of view of a shorter (20mm) eyepiece is smaller by factor of two, so even if it is magnified twice more, its angular size (as seen by eye) is exactly the same as in 40mm one.
Same angular size, same number of photons = same flux reaching the retina == same brightenss.

If you or David Nagler could explain the mechanism to make the same angular size aberrated image appear brighter in longer f.l. eyepiece I'm all ears.

As this is my last post in this forum, if anyone (including Don, David Nagler or Uncle Al himself) wants to continue discussion, please use my bratislav3162 at hotmail dot com address.

[janoskiss (Steve H)]

That factor is the simple fact that the extended star image is 1/4 as bright while the magnification has only doubled.

That cannot be it. The 1/4-fold brightness reduction due to 2x magnification increase would be perfectly cancelled out by the 4-fold increase in brightness due to linear 1/2 x decrease in coma angular size at the same angular displacement in the apparent FOV. So the star and coma 'should' look the same. There must be something else going on that we have not considered.

[Don Pensack]

Originally Posted by janoskiss

That cannot be it. The 1/4-fold brightness reduction due to 2x magnification increase would be perfectly cancelled out by the 4-fold increase in brightness due to linear 1/2 x decrease in coma angular size at the same angular displacement in the apparent FOV. So the star and coma 'should' look the same. There must be something else going on that we have not considered.

I'm not going to bother to respond to Bratislav, as it is not my job to question what Al and David Nagler have told me directly on the phone and in person. If he doesn't like their answers, he can call them on the phone and argue with them. Suffice it to say, what I understand about coma correctors did not come from only the Naglers.

As for the comatic star image, the size of that image is linear, so Janoskiss is correct that there must be some other factor at work that we haven't considered. It might be an interaction with the exit pupil, or something else. If the star image is half the size, but the magnification is doubled, the star image should be of equal size. Indeed, the night sky will be 1/4 as bright at twice the magnification (4X the area), so contrast with the star images should be larger, making the coma more visible.
And yet it is not.

But, thinking aloud, if the star image at 10mm from center at the focal plane is, say, 1" in diameter, and 2" at 20mm from center, and if the smaller field becomes the edge at twice the magnification, then the 1" star image at 10mm has become 2" by means of magnification. But let's say both star images have equal brightness to start out with (at the focal plane). Then the 2" star image created by magnifying a 1" image will be dimmer than the 2" star image at 1/2 the magnification. If there's something wrong with that logic. let me know.

[janoskiss (Steve H)]

Tricky thing about star images is that they are points. So you don't have 1" star image or 2" star image. It's effectively a point at all magnifications, at least until diffraction effects become apparent, i.e. when you magnify it enough to see the Airy disk.

[Don Pensack]

Except a comatic image, which is a LOT larger than the Airy disc. At f/5 it's 9X as wide as the Airy disc in the radial direction at 20mm off axis, which gives it a size (it's even larger at f/4). So it should behave more like a planetary image, where magnification and brightness are concerned.

[janoskiss (Steve H)]

but it is the size of the coma which does not change when you up the power say 2-fold: at the same point in the AFOV, you are 2x closer to on axis, so coma is 2x smaller, but you're mangifying 2x, so it would stay the same. It's the same star with the same apparent size so you'd expect it would look the same (as long as Airy disk is unresolved). Don, interesting point about background sky brightness, yet things go the other way. These are all things I have not really thought about before all that much and never had a look at what they really mean in practice. Maybe time to have a critical look at stars at different powers with the f/4 Newt. (although I don't think the mirror is very good in that one.)

[Don Pensack]

That's the issue. Seeing artifacts will impact the evaluation of coma at higher powers.

I can tell you my experience:

My 5 and 7mm Naglers display coma in their outer fields, so I use a Paracorr for every eyepiece. But they exhibited only annoying, and somewhat minor comatic images at the edge.
My 35 Panoptic, on the other hand, exhibited a starfield that looked like I was standing on the deck of the Millenium Falcon going to lightspeed. The star images looked like radial streaks.
After the Paracorr, the star images are perfect to the edge in the 5mm and 7mm, yet still a tiny bit comatic at the edge in the 35mm. [I have 43 years experience looking at star images through hundreds of scopes, so I am compensating for the tiny amount of positive field curvature in the 35mm]
If simple magnification explained the visibility of coma, coma should have been MORE visible in the 5 and 7mm (82 degree fields) than in the 35mm (68 degree field), yet it patently was not even in the same league.

Here's what I think: Star images are not magnified. They appear the same size in all eyepieces until the Airy Disc is visible, and then start appearing larger. The 5mm eyepiece is a 1mm exit pupil in my scope, yet represents my highest magnification (I'm not a planets or double stars observer, so powers above 400X aren't too useful to me, on an undriven scope), but that magnification *just* makes the Airy disc visible.
Accordingly, the only factor having an influence on what coma I see is field stop size in millimeters--a wider field stop will display more coma and magnification has nothing to do with it. Doubling the power doesn't increase the size of the star image because the scope sees it like a point source. It is only the reflection that is comatic. The eyepiece is just a simple magnifier of the focal plane of the telescope. So, doubling the power with an eyepiece of identical apparent field doesn't show the same amount of coma; it shows less because the field stop is smaller.
In other words, you'd have to keep the field stop the same size to see equal amounts of coma at all magnifications.
What do you think? I think that may just be the explanation I was looking for.

[Argonavis (William)]

Originally Posted by Don Pensack

My 5 and 7mm Naglers display coma in their outer fields, so I use a Paracorr for every eyepiece. But they exhibited only annoying, and somewhat minor comatic images at the edge.
My 35 Panoptic, on the other hand, exhibited a starfield that looked like I was standing on the deck of the Millenium Falcon going to lightspeed. The star images looked like radial streaks.

The Nagler are designed to handle the fast light cone from an f4 or f5 scope - so of course they will display less coma. That's why you pay so much money for them.

So I am not sure your comparison is valid. I believe you would need to compare different magnifications of the same ocular design to test whether coma is a function of magnification.

Originally Posted by Don Pensack

Here's what I think: Star images are not magnified. They appear the same size in all eyepieces until the Airy Disc is visible, and then start appearing larger. The 5mm eyepiece is a 1mm exit pupil in my scope, yet represents my highest magnification (I'm not a planets or double stars observer, so powers above 400X aren't too useful to me, on an undriven scope), but that magnification *just* makes the Airy disc visible.

I remember one night using x600 on an undriven 20inch f5. The seeing was exceptional and supported this magnification. It is worth keeping a shortish focal length ocular in your kit for such nights.

and it doesn't seem to make a lot of sense that star images are not magnified, then suddenly they are once the Airy disk becomes visible.

Originally Posted by Don Pensack

Accordingly, the only factor having an influence on what coma I see is field stop size in millimeters--a wider field stop will display more coma and magnification has nothing to do with it.

I suspect both have an effect, along with f ratio and eyepiece design. The combined effect, together with the observers individual variation in visual perception and experience, would be difficult to model. Which is why we have this discussion with Bratislav leaving.

I am sure "Optics 101" says one thing, and the observers eye adds another layer of complexity. We all seem to perceive or react differently to the image presented to us through an ocular. My view is that there is a layer of emotional perception that everyone applies to what they see visually.

That what I think.

[janoskiss (Steve H)]

Don, I'm not sure I agree, because I'm not sure I understand. I wish someone could run some objective (no pun intended) experiments with a CCD at different powers. I think that would be interesting.

Originally Posted by Argonavis

The Nagler are designed to handle the fast light cone from an f4 or f5 scope - so of course they will display less coma.

Now I am no expert. I'm just guessing based on what I've read/heard about EPs. I don't think these EPs fix coma. They just work as well as possible within the limitations imposed by coma of a parabolic mirror. I don't think coma figures into it - they are designed for an ideal coma-free fast scope afaik.

Originally Posted by Argonavis

and it doesn't seem to make a lot of sense that star images are not magnified, then suddenly they are once the Airy disk becomes visible.

Well maybe not so suddenly, but there are low-to-medium powers where the Airy disk is so small that the eye cannot resolve it: here the star is effectively a point source and magnification has no effect on the apparent surface brightness.

[Don Pensack]

Originally Posted by Argonavis

The Nagler are designed to handle the fast light cone from an f4 or f5 scope - so of course they will display less coma. That's why you pay so much money for them.

and it doesn't seem to make a lot of sense that star images are not magnified, then suddenly they are once the Airy disk becomes visible.

I suspect both have an effect, along with f ratio and eyepiece design. The combined effect, together with the observers individual variation in visual perception and experience, would be difficult to model. Which is why we have this discussion with Bratislav leaving.

Coma is produced by the primary mirror, not the eyepiece. Having well-corrected eyepieces reduces astigmatism, lateral chromatic aberration, and a host of other f-ratio-induced aberrations, but coma is not reduced by them. However, reducing the other aberrations does make coma MORE visible because it is not mixed with or confused with other aberrations.

The star image has a definite width, but it is a point as far as our eyes are concerned until the Airy disc becomes visible. At that point, the star image will increase in size with more magnification. But no more resolution can happen once the Airy disc is resolved. that occurs around an exit pupil of 1mm. So, at lower magnifications, the star image stays the same size with magnification change up to that point. This is why fainter stars are visible at high powers--the sky gets darker with increasing magnifications, but the star image itself does not get dimmer until magnifying past the point where the Airy disc has become visible.

Again, eyepiece design will only have an effect on the visibility of coma insofar as other aberrations may swamp the visibility of coma. Field curvature and astigmatism, in particular, can have the effect of reducing the visibility of coma if the comatic star image is distorted in other ways.
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