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Hi all,

Can anyone explain what a coma corrector is/does and whether it would be useful for a 10inch f4.7 Dob used for visual purposes?

If it is suitable can you recommend a good one?

Thanks

Simon

Edited by cyborg421

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A coma corrector corrects edge of field distortion in short-focus newtonians. This distortion causes stars at the very edge of your field to look like little comas (",") instead of sharp points. This effect, is particularly pronounced at the lowest magnifications; not so much so at higher powers.

Whether you need one or not depends on your personal taste. The standard recommendation is that you should consider a coma corrector at f-ratios below f/5, and you are just barely under this. However, you still might not need one. I often use my f/4.5 travel scope without coma correction and am generally very happy with it. If you're viewing is mostly at higher powers, then you need it even less. Ultimately, it is one more thing to mess with....

But if you are in search of the absolutely most perfect image that your scope can generate under all conditions, then yes, by all means get one.

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Sorry, as a follow up--you asked for a recommendation for a comma corrector.

I think in your position, I'd get a Televue Paracorr Type 1. This is very good, though not quite as good as the current Type 2. However it weighs less, which means that it's easier to use and you won't have balance issues if your scope is already finely balanced.

I don't know if these are made any more, but they were in production till at least a couple years ago, so are often available used.

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there are certainly a few around.i use the skywatcher with my 200 pds and i would have to say it transform the scope when used.pin point sharp stars across the field of view when used.

http://www.firstlightoptics.com/coma-correctors/baader-mpcc.html

http://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html

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Coma in an optical system refers to aberration inherent to certain optical designs which results in off-axis point sources (i.e. stars) appearing distorted, appearing to have a tail like a comet, hence why it is known as coma. Coma is an inherent property of telescopes using parabolic mirrors, which Newtonian telescopes do. In a system using a parabolic mirror, only incoming light rays that strike the mirror exactly parallel to the axis of the parabola will be precisely focused. When looking at a point that is not perfectly aligned with the optical axis, some of the incoming light from that point will strike the mirror at an angle. This results in an image that is not in the centre of the field looking wedge-shaped. The further off-axis (i.e. the further from the centre of the field), the worse this effect of coma is.

The effect tends to worse the faster the mirror is, so F5 and faster, the performance of the scope could benefit from the use of coma corrector. The Televue Paracorr is a well known example of a coma corrector. However it is approx £460 so rather expensive. So before you buy one, you should decide whether the problem with coma in your telescope system is one that you find bothersome and worth the expense to correct.

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In general, coma wavefront aberration occurs either due to the incident wavefront being tilted, or decentered with respect to the optical surface. Hence, it is either an aberration affecting off-axis image points, or the result of axial misalignment of optical surfaces.

When we collimate a telescope we align the mirror, secondary and eyepiece in some manner, the bit missing from all this is the distant objects we intend to look at. If the mirror were centered with the axis going exactly up the tube to the object and the mirror profile were perfectly cut into the glass or whatever substraight then you would get very little (no) coma. If however the mirror axis is just 1 degree out then coma occurs. Not aware of anything that checks this optical axis alignment with the tube axis. Again the accuracy of the built scope comes into play.

The catch is the mirror profile is to an extent simply cut, it will not necessarily be exactly correct, if the initial glass block were not perfectly parallel across rear and front face then the curvature is ground a little wrong and you get coma. I would say that comapnies that are know to produce "good" mirrors with little errors simply take the extra time to set things up better and so reduce the potential of errors being introduced at the early manufacturing stages.

Mirrrors are less expensive because the substraight need not be optically perfect as it does with a refractor, I wonder if this lower requirement leads to a general lessening of the overall quality especially in the mass produced market.

Edited by Capricorn
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Coma is visible off axis in fast Newtonians. It affects all Netwonians of a given speed equally. It is not due to a defect in manufacture, but instead is a limitation of the Newtonian design, with its single figured mirror. The effect is is inversely proportion to the cube of F-number, so it is much more pronounced in fast telescopes. So the coma in an F/4 is (about) double that in an F/5, which is double that in an F/6.3, which in turn is double that in a traditional F/8. An F/4 has 8 times the coma of an F/8.

For a given telescope, the visual effect is proportional to how far a star is apparently off axis, so it is more visisble at the edge of wide angle eyepieces. The abberation is also more visible on longer length eyepieces, both because of the brighter image that these give and because at higher magnifications more of the coma stays within the airy disc (within the maximum resolution of the scope).

I have found that the GSO coma corrector (designed by Roger Ceragioli, also known as Astro-Tech and formerly known as Altair Astro) makes a very worthwhile improvement to the image in my Newtonians. It is currently available from Agena and Telescope Services for a fraction of the the price of a Televue Paracorr. It does not come ready for use though and you need spacers (empty 2" filters or Baader fine tuning rings) to get the eyepiece focal point to 75mm (+/-10mm) from the last lens of the corrector. It adds about 10mm in-focus and needs to sink into the focuser tube by about 70mm. It lengthens the focal length of the scope by about 10%.

Edited by cs1cjc

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You've already got very thorough descriptions of what the correctors are for. I just want to echo that you will notice coma only at lower powers with your scope. The corrector is nice to have at your focal ratio but not "necessary." They're very easy to use (basically "set and forget", they are adjustable but you get 90% of the performance even without adjusting them). If you have eyepieces that show astigmatism (masquerades as coma) then the corrector will not clean that up. So don't necessarily expect sharp edges with a corrector. Here's more reading: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4266737/Main/4208983

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From my experience I use mine visually only for under 50 times magnification, for which I'm using 2" Eps. Above that I use 1.25" Eps and don't bother with the corrector, as the views are perfectly live-able with.

Of course, if I get the camera out, then it is a definite 'must have'...

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As has been said, unless you use premium eyepieces, the coma may not even be noticeable because the astigmatism caused by the eyepiece will largely mask the coma.

I do use premium eyepieces and notice coma sometimes when using my 10" F/4.8 newtonian. Somehow coma does not bother me as much as astigmatism though so I don't use a coma corrector. Thats probably a bit perverse but there it is ! :rolleyes2:

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I have an f4 16" dob and at f4 I think you need at CC. I don't need one at f5 and you are closer to that then f4 so may be fine. I use a Paracorr and got it used for about £170. I just leave it in the dob all the time as it's effectively part of the scope now.

I could just live without it but it's far better with.

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It might be worth putting some figures on this. Coma correction and collimation are two reactions to one problem.

Even a perfect newtonian mirror suffers from coma which, off the optical axis, softens the image. Collimation is the business of putting the sharp coma free area (the area where the coma is less than size of the airy disc) into the centre of view. The diameter of this area at the focal plane is 0.022*F*F*F mm (where F is the focal ratio of the mirror), which for some popular f/ ratios gives the following diameters:

f/4: 1.4mm

f/4.5: 2mm

f/5: 2.75mm

f/6: 4.75mm

f/8: 11mm

This can be compared with the field stop of a high(ish) power 9mm orthoscopic (40 degree AFOV) eyepiece, of about 6mm. In such an eyepiece, a perfectly collimated f/4.5 telescope will only show the central 1/3 of the view as sharply as it could be for the size of scope. If the collimation is out by 4mm or more then the sharp area will actually be outside the field stop of the eyepiece, so not visible. It is reasonable to work to about 1/4 of the diameter as a tolerance for collimation so that the coma free area will overlap with the centre of the field of view, so for example for an f/4.5, 0.5mm. However in a given telescope coma only increases linearly with the distance off the optical axis, so precise collimation is much less important at low powers when paradoxically coma is at its most visible. So the odd few millimetres of misalignement do not matter much when the field stop is 42mm in diameter, as it is in a 31mm Nagler. See also the Oldham Optical page on astrophotography:

http://www.oldham-op...co.uk/Photo.htm

A coma corrector, in a properly collimated telescope, will reduce coma over the whole field as is shown by the spot diagrams for the Paracorr II (which is probably the best available coma corrector, but it certainly ought to be, given the price):

http://www.televue.c...orr_2_chart.jpg

Precise collimation is more critical with a coma corrector. For analysis of some simple correctors see:

http://www.telescope...e_corrector.htm

Edited by cs1cjc
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Great post, Chris!

Hm. It might have been a great post if it had been right but I have got it wrong...

Apologies

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Second attempt. It might be worth putting some figures on this. Coma correction and collimation are two reactions to one problem. Even a perfect newtonian mirror suffers from coma which, off the optical axis, softens the image. Collimation is the business of putting the sharp coma free area into the centre of view. This table (from Nils Olof Carlin) gives the diameter of the "sweet spot" where coma is less than 1/14 wavelengths RMS, and the Strehl ratio is lowered by no more than 0.2.

F/4: 1.4mm

F/4.5: 2.0mm

F/5: 2.8mm

F/6: 4.8mm

F/8: 11mm

See: http://web.telia.com...olli/kolli.html

This can be compared with the field stop of a high(ish) power 9mm orthoscopic (40 degree AFOV) eyepiece, of about 6mm. In such an eyepiece, a perfectly collimated F/4.5 telescope will only show the central 1/3 of the view as sharply as it could be for the size of scope. If the collimation is out by 4mm or more then the sharp area will actually be outside the field stop of the eyepiece, so not visible. It is reasonable to work to about 1/4 of the diameter as a tolerance for collimation so that the coma free area will overlap with the centre of the field of view, so for example for an F/4.5, 0.5mm. However in a given telescope coma only increases linearly with the distance off the optical axis, so precise collimation is much less important at low powers when paradoxically coma is at its most visible. So the odd few millimetres of misalignement do not matter much when the field stop is 42mm in diameter, as it is in a 31mm Nagler. See also the Oldham Optical page on astrophotography:

http://www.oldham-op...co.uk/Photo.htm

A coma corrector, in a properly collimated telescope, will reduce coma over the whole field as is shown by the spot diagrams for the Paracorr II (which is probably the best available coma corrector, but it certainly ought to be, given the price):

http://www.televue.c...orr_2_chart.jpg

Precise collimation is more critical with a coma corrector. For analysis of some simple correctors see:

http://www.telescope...e_corrector.htm

Edited by cs1cjc
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Second attempt. It might be worth putting some figures on this. Coma correction and collimation are two reactions to one problem. Even a perfect newtonian mirror suffers from coma which, off the optical axis, softens the image. Collimation is the business of putting the sharp coma free area into the centre of view. This table (from Nils Olof Carlin) gives the diameter of the "sweet spot" where coma is less than 1/14 wavelengths RMS, and the Strehl ratio is lowered by no more than 0.2.

F/4: 1.4mm

F/4.5: 2.0mm

F/5: 2.8mm

F/6: 4.8mm

F/8: 11mm

See: http://web.telia.com...olli/kolli.html

This can be compared with the field stop of a high(ish) power 9mm orthoscopic (40 degree AFOV) eyepiece, of about 6mm. In such an eyepiece, a perfectly collimated F/4.5 telescope will only show the central 1/3 of the view as sharply as it could be for the size of scope. If the collimation is out by 4mm or more then the sharp area will actually be outside the field stop of the eyepiece, so not visible. It is reasonable to work to about 1/4 of the diameter as a tolerance for collimation so that the coma free area will overlap with the centre of the field of view, so for example for an F/4.5, 0.5mm. However in a given telescope coma only increases linearly with the distance off the optical axis, so precise collimation is much less important at low powers when paradoxically coma is at its most visible. So the odd few millimetres of misalignement do not matter much when the field stop is 42mm in diameter, as it is in a 31mm Nagler. See also the Oldham Optical page on astrophotography:

http://www.oldham-op...co.uk/Photo.htm

A coma corrector, in a properly collimated telescope, will reduce coma over the whole field as is shown by the spot diagrams for the Paracorr II (which is probably the best available coma corrector, but it certainly ought to be, given the price):

http://www.televue.c...orr_2_chart.jpg

Precise collimation is more critical with a coma corrector. For analysis of some simple correctors see:

http://www.telescope...e_corrector.htm

Much better :huh:

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Hi guys,
I just saw this post and can see that you know about correctors. Can someone tell me if I need a coma corrector and/or a field flattener if I place my CCD camera at the prime focus of my F5 Newton telescope, removing the secondary completely. I haven't understood why one does not necessarily require a corrector when using the 2 mirror setup but it apparently is recommended for the 1 mirror setup, to me the light paths are identical apart from one being diverted 90 degrees by a flat mirror. I don't want to use my scope visually but purely via my CCD.

Thank you,
Markus

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11 hours ago, Markfan said:

Hi guys,
I just saw this post and can see that you know about correctors. Can someone tell me if I need a coma corrector and/or a field flattener if I place my CCD camera at the prime focus of my F5 Newton telescope, removing the secondary completely. I haven't understood why one does not necessarily require a corrector when using the 2 mirror setup but it apparently is recommended for the 1 mirror setup, to me the light paths are identical apart from one being diverted 90 degrees by a flat mirror. I don't want to use my scope visually but purely via my CCD.

Thank you,
Markus

Probably because a lot of people don't notice coma visually, especially when using 50 degree AFOV eyepieces.  Coma is most obvious in ultra wide field eyepieces.

CCs are almost always called for photographically to correct coma into the corners and to flatten the field to bring everything to focus at once.  Reviewing an entire image easily reveals flaws that aren't always obvious visually.

The exception would be planetary imaging where only the exact center of the field is being used.  Obviously, a single mirror setup is photographic only, so CCs would nearly always be desired.

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