coma

[stephend]

I read recently that short focal length mirrors always display coma except at the very centre of the field of view.
This would explain why stars in my scope mostly display tails, which no amount of collimation, even with a laser collimator, will fix.
Has anyone any experience of this, and for the future, when I build my 24" reflector, what focal length should I be grinding to avoid this?
Thanx gise.

[Ian Robinson]

f/4.66 newtonian (10")

I hate coma so much I have 3 coma correctors :

- a Lumicon 48mm CCFF to use when I replace my knicked Lumicon 2" newt easyguider
- a Baader MFCC for use at prime focus (w/out an OAG)
- a Tele Vue Paracorr Visual.

If you have a newtonian , a coma corrector is a MUST HAVE.

[Satchmo]

Originally Posted by stephend

This would explain why stars in my scope mostly display tails, which no amount of collimation, even with a laser collimator, will fix.
Has anyone any experience of this, and for the future, when I build my 24" reflector, what focal length should I be grinding to avoid this?

Most of what you are seeing is astigmatism with some coma and field curvature thrown in. The stars look like open umbrellas with their handles pointing to th edge of the field.

If you use a Nagler eypiece it will flatten the field and correct the astigmatism. then you actually see the coma, which you'll see is a much more minor aberration. If you team up a Televue Paracor with a Televue Nagler eyepiece you'll generally excellent tight round star images to the edge of the field even down to F 3.5 focal ratio.

if you are planning a 24" scope the cost of a Paracor is minor. You would be best to make it around F4 to F4.5 , which keeps the wind profile down and try and minimise the number of steps to climb. It is possible these days to get mirrors down to F3.7 these days.

If you have not ground a mirror and you are planning to make a good 24" then you'd better get cracking , you have a long journey ahead of you

[stephend]

So a Televue Paracor .... what, fits over the eyepieces I have?
And a Televue Nagler eyepiece is ... an eyepiece?

Too cold in my shed to get cracking on any project but hopefully it will warm up a bit soon. Point taken that it isn't going to be easy.

[rmcpb (Rob)]

You will be warm if you are grinding a 24"er

[Starkler (Geoff)]

Although deviating from the OP's question I have a question relating to the thread title of "coma".

It is said that coma is inherent to the newtonian design, but nobody ever mentions it in relation to refractors. I remember in my younger days playing with a magnifying glass concentrating the suns rays to a point. Now when the lens is tilted so as to be no longer perpendicular to the suns rays, they no longer focus to a sharp point but display a tail which looks something like coma.

Is it the same thing or no?

[stephend]

Nope, will not be lubricating the process with sweat of brow, first thing is to make a lens grinding machine.

Starkler's comment is interesting, I read about coma in the context of someone saying it was a sign that the light wasn't hitting the mirror square. Which would be fixable. But then it was suggested it was native to all parabaloid mirrors.
Light from a star at the edge of the field of view isn't striking the mirror squarely, I suppose.
Funny that I've never heard of it before.

[erick (Eric)]

Originally Posted by stephend

So a Televue Paracor .... what, fits over the eyepieces I have?
And a Televue Nagler eyepiece is ... an eyepiece?

An obvious bit of research is to go here:- http://www.televue.com

[erick (Eric)]

Originally Posted by stephend

......I read about coma in the context of someone saying it was a sign that the light wasn't hitting the mirror square. Which would be fixable. But then it was suggested it was native to all parabaloid mirrors.
Light from a star at the edge of the field of view isn't striking the mirror squarely, I suppose.
Funny that I've never heard of it before.

A bit more reading, perhaps:-

http://www.astrosurf.com/luxorion/reports-coma.htm

http://www.atmsite.org/contrib/Carlin/collimation/ (search coma)

Coma: an aberration which results in a point object being turned into a pear-shape or comet shape at the focal plane, most commonly off-axis. It is caused by unequal magnification in different zones of a lens for obliquely incident rays from an off-axis object. An easier way of putting this may be to say that coma is caused when light enters a lens or mirror from the side, and rays from different parts of the lens intersect the axis of those rays at different distances. In a Newtonian with a paraboloidal primary mirror, coma is an inherent property.


Coma Correctors
In a reflecting telescope, coma makes stars at the edge of the field look like blurry comets whose tails point out radially from the center of the field. All parabolic mirrors used in Newtonian reflecting telescopes are limited in field sharpness due to coma. The faster your telescope's focal ratio, the worse the effect. Even a perfectly made 13" f/4.5 parabola has a diffraction limited field that’s only about 0.1° across. Coma correctors are auxiliary lens systems that reduce the coma at the edges of the field to provide images that are sharp across the entire eyepiece or photographic field. You no longer have to constantly shift your scope to keep objects centered for sharp viewing.

[ngcles]

Hi Stephen & All,

To echo what Satchmo said, coma is inherent in any optical system that contains a paraboloid. That includes classical Cassegrainians and Schmidt Cassegrainians as well. Though it is true that S/Cs use spherical mirrors, the corrector plate on the front of the 'scope makes the primary mirror "look like" a paraboloid to the secondary mirror -- so in effect it is a paraboloid.

Coma is generally more noticeable in a Newtonian because most Newtonians these days are fast systems (ie f/4 to f/6 rather than f/10 - 15 like most Cassegrainan systems). All proper Newtonian telescopes have coma. The faster (shorter f/ratio) the telescope, the more coma it produces. ie the coma-free field becomes smaller and smaller in angular size. The further out from the optical axis, the more coma intrudes.

As Satchmo says, most of the effect you are seeing in your 10" and its non-symetrical out-of-round stars is caused by lateral astigmatism and some field curvature. You will notice it is much more pronounced the further the star is from the centre of the field -- yes? Once you remove those effects (and you can kill most of it with Nagler-type exotic wide-field eyepieces), most of the coma remains and can be seen nearer the edge of the field with non-round star images (now much less pronounced).

A coma-corrector like a Paracorr (short for parabola-corrector) will (essentially) eliminate the coma then producing round small stars all the way to the edge of the field.

Stephen wrote:

"So a Televue Paracor .... what, fits over the eyepieces I have?
And a Televue Nagler eyepiece is ... an eyepiece?"

A paracorr looks like a barlow lens but works differently. Slip it into the focuser and then put the eyepiece in on top. There are several brands of coma correctors. The Paracorr is marketed by Televue.

Yes a Nagler is an eyepeice design marketed by Televue. It is a wide-field exotic (ie expensive) but you do get what you pay for. There are other brands that also have similar properties and produce excellent results like the Pentaxes.

When you finally do get around to doing that 24" mirror, as Satchmo suggested, keep the f/stop short (ie <f/4.5) get some good eyepieces and a coma corrector. It will be so, so much easier and in the end cheaper than building and lugging around a 24" f/5 - f/7 that will need a 10 foot lader, will suffer image degridation due to tube sag and weigh a ton, be harder to design and fabricate etc etc.

I own an 18" f/4.9. I don't find the coma particularly objectionable, though I know many people do find it so. I do own many (older) Nagler eyepieces and I love them for the above reasons. I can't imagine observing with an f/4.9 'scope without them.

Best of luck with it,

Les D

[GrahamL]

Sorry to shoot off topic a little .. but does anyone know how the glass in a parocorr is set up .. I have pulled the upper group apart in my pretoria a couple of times and have wondered how the mechanics of the coma correcting works

[Ian Robinson]

Originally Posted by nightstalker

Sorry to shoot off topic a little .. but does anyone know how the glass in a parocorr is set up .. I have pulled the upper group apart in my pretoria a couple of times and have wondered how the mechanics of the coma correcting works

No - too expensive to tinker with or pull apart.

Try Advanced Telescope Making, there is probably the recipy for a Abbe or Paracorr or Lumicon style coma corrector in there somewhere. Maybe since Lumicon no long market their CCFF lenses, and maybe Dr.Marlin's patent has expired, it might be possible to dig up his patent and get one made.

try :

C. G. Wynne, "Field corrector for large telescope,"Applied Optics 4(9), pp. 1185 1192, sept. 1965.

C. G. Wynne, "Afocal correctors for paraboloidal mirros," Applied Optics 6(7), pp. 1227-1231 jul.1967.

F. E. Ross, "Lens systems for correcting coma of mirrors," Astrophysics J. 81, p. 156, mar 1935.

They can be done for very fast paraboloids "The Subaru prime focus design which acts on a f/1.8 primary corrects a 30 arcmin field with 7 lenses .The f/1.14 prime foci of the LBT will be corrected over a similar field with fewer lenses where one is aspheric" !!

[Starkler (Geoff)]

Nobody has touched my Q regarding coma and refractors
I hope Satchmo returns to this thread.

[Wavytone]

Originally Posted by Starkler

Although deviating from the OP's question I have a question relating to the thread title of "coma"... nobody ever mentions it in relation to refractors...
Is it the same thing or no?

Coma is easily created in a refracting system (hence the Paracorr, which introduces coma of equal magnitude but opposite sign to that of the parabolic primary its designed for). There are some design principles which can be followed for refractor objectives which suggest that in certain cases - not all, and only to first order - the coma should be either zero (for geometric reasons) or insignificant compared to other aberrations over a sizable field but these are just approximations.

In practice if you're seeing "winged hairy shapes" from a lens it's almost certainly off-axis astigmatism mainly, not coma.