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A Comma Corrector


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Is it really worth buying a Comma Corrector for my f4 reflector for strictly visual observing ?  Hopefully others will get their similar thoughts answered.

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It's probably more worthwhile when imaging than in visual use.

Some, like the Baader Mk111, will induce some under-correction and soften the image in the fastest Newts. I think by about 0.85 waves in an F/4, inversely proportional to the 4th power of the F ratio. So, dubious improvement. Maybe fine at low power.

The more expensive Paracorr corrects for spherical aberration as well as coma. There are a number of coma correctors and I'm not sure if sharpness and lower price have been combined.

David

Edited by davidc135
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It's personal choice but the faster the scope the more potential benefit there is. I use a Paracorr 2 in a f4.6 scope and I never observe in  cyclops mode without it.

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On 17/02/2022 at 16:39, LDW1 said:

Is it really worth buying a Comma Corrector for my f4 reflector for strictly visual observing ?  Hopefully others will get their similar thoughts answered.

I would say it depends on how bad you are at punctuation.  ;)

If on the other hand you mean coma corrector then I think its more worth while for imaging than visual. It really depends on how much its bothering you, so only you can really say. 

Adam

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Agree, visually you are most likely to be looking at an object in the centre of the view only, where coma is never an issue- this is why people don’t really consider it necessary for visual stuff. Different story for imaging where the corners of the image have to look perfect as well.

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2 hours ago, Adam J said:

I would say it depends on how bad you are at punctuation.  ;)

If on the other hand you mean coma corrector then I think its more worth while for imaging than visual. It really depends on how much its bothering you, so only you can really say. 

Adam

In Canada thats how we spell Comma (Coma), lol.  Just kidding !   PS.   Punctuation ?

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It depends on how wide and well corrected your eyepieces are.  Once in a while I pop in one of my ES-92s without putting the GSO CC in first and get apprehensive about why they're performing so poorly at the edge even at f/6.  Then I realize I have forgotten to put the CC in the focuser first.  Put it in, and my stress levels decrease markedly.  As mentioned above, the GSO CC will add SA at high powers, so I remove it for high power observing.  I do the same with my TSFLAT2 in my refractors to get the sharpest on axis images at high powers for the same reason.

If you're just using 40 to 50 degree orthos and plossls at f/4, other edge aberrations such as astigmatism will completely overwhelm any coma from the mirror because these simple eyepieces were simply not designed for fast scopes.  A CC will help flatten the field and remove coma, but eyepiece aberrations will remain.

If you're using Nagler and Ethos eyepieces, you would naturally want them to perform at their best and add a TV Paracorr T2, so it depends on your eyepiece collection.

BTW, at f/4, the coma free section on axis is only 1.4mm (0.022mm x f-ratio³) in diameter for the comatic aberration to fit within the Airy disc.  A TV Paracorr T2 will extend that to at least 40mm.

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53 minutes ago, Louis D said:

It depends on how wide and well corrected your eyepieces are.  Once in a while I pop in one of my ES-92s without putting the GSO CC in first and get apprehensive about why they're performing so poorly at the edge even at f/6.  Then I realize I have forgotten to put the CC in the focuser first.  Put it in, and my stress levels decrease markedly.  As mentioned above, the GSO CC will add SA at high powers, so I remove it for high power observing.  I do the same with my TSFLAT2 in my refractors to get the sharpest on axis images at high powers for the same reason.

If you're just using 40 to 50 degree orthos and plossls at f/4, other edge aberrations such as astigmatism will completely overwhelm any coma from the mirror because these simple eyepieces were simply not designed for fast scopes.  A CC will help flatten the field and remove coma, but eyepiece aberrations will remain.

If you're using Nagler and Ethos eyepieces, you would naturally want them to perform at their best and add a TV Paracorr T2, so it depends on your eyepiece collection.

BTW, at f/4, the coma free section on axis is only 1.4mm (0.022mm x f-ratio³) in diameter for the comatic aberration to fit within the Airy disc.  A TV Paracorr T2 will extend that to at least 40mm.

I will be using a half dozen Morpheus and a 24mm Panoptic with the cc / scope. And 3 Svbony zooms as a test.

Edited by LDW1
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You'll definitely notice an improvement with both the Morpheus and Panoptic eyepieces.

For the GSO CC, I add a 25mm spacer ring between the eyepiece holder and the optical nose piece to get to the 75mm separation sweet spot with a 1.25" adapter typically adding another 5mm of separation.  The CC designer has stated elsewhere that 5mm on either side of the design separation is almost indistinguishable visually.  I've found that to be true.  Eyepieces that focus within 5mm of their shoulder (reference plane) don't show enough improvement to bother with parfocalization.  However, I had to parfocalize my 12mm NT4 with five 4mm thick O-rings because it focuses 20mm below the shoulder which led to noticeable uncorrected coma.  So little 2" barrel was left that I had to add a 20mm M48 spacer ring.

Televue Nager T4 12mm Eyepiece.jpg

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Ernest over on CN posited in this post that the following apparent fields of view (AFOVs) in degrees will not show coma at the following f-ratios (simply square the f-ratio):

  • F4 - 16
  • F5 - 25
  • F6 - 36 (classic ortho will have whole thir FOV free from coma)
  • F7 - 49 (super plossl are coma free in F7 Newton)
  • F8 - 64 (WA eyepieces are coma-free in F8 Newton)
  • F9 - 81 (even Naglers are coma free in F9 Newton!)

Thus, at f/4, you're looking at a coma free field with a 16 degree AFOV eyepiece.  Those Morpheus and Panoptics, being well corrected to the edge at f/4, would show coma at f/4.

I know I observed for about 15 years without using a CC because I didn't notice it, it having always been there.  However, I started upgrading and widening my eyepieces about a decade ago and started noticing the coma and decided to get a used GSO CC for $75 to try out.  It was an amazing difference.  I didn't think I'd notice, but it was quite obvious to my eye at least when the CC is in or not in use even at f/6.

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My tu’pence worth

Most telescopes are designed to focus light on a curved surface - the back of MK1 eyeball.

Coma is generally caused by projecting light on to a flat surface, camera sensor.

I suspect that unless you have have a particularly bad optical path, you wouldn’t experience a bread deal of difference at the edges of the image.

you could see an increase in apparent light levels 9brighter images) as most correctors are focal reducers, for example the corrector I have on 8" SCT reduces focal length from c2000mm to 1280mm, and focal ration from f10 to f6.3

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1 hour ago, iapa said:

My tu’pence worth

Most telescopes are designed to focus light on a curved surface - the back of MK1 eyeball.

Most folks use an eyepiece to view the image from the telescope rather than view the aerial image directly.  Eyepieces themselves may have curved or flat focal planes that may or may not match the curvature of the telescope's focal plane.

Coma is generally caused by projecting light on to a flat surface, camera sensor.

Coma is inherent with a Newtonian's paraboloidal figure.  It has nothing to due with the curvature of its focal plane.

I suspect that unless you have have a particularly bad optical path, you wouldn’t experience a bread deal of difference at the edges of the image.

Have you ever looked through an f/4 Newtonian using an ultrawide eyepiece?  Coma is blatantly obvious in them if you're used to a coma-free field.

you could see an increase in apparent light levels 9brighter images) as most correctors are focal reducers, for example the corrector I have on 8" SCT reduces focal length from c2000mm to 1280mm, and focal ration from f10 to f6.3

Perhaps in an SCT, but the OP specifically asked about Newtonian coma correctors.  Visual Newtonian coma correctors usually are mild focal extenders.

See inline comments above.

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In late 2020 I was about to sell my Panoptic 35mm eyepiece because the distorted "seagull" stars at around 50x magnification in my 12" newt when looking at Open Clusters were really bothering me. I attributed the distortions to the eyepiece. Then on reading some more I realized that perhaps I needed a coma corrector, so I bought a Paracorr2. What a difference! It completely transformed the stars from objectionable to entrancing.

Magnus

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  • 2 weeks later...
On 17/02/2022 at 11:39, LDW1 said:

Is it really worth buying a Comma Corrector for my f4 reflector for strictly visual observing ?  Hopefully others will get their similar thoughts answered.

Mandatory at F/4 if you want sharp stars across most of the field. I use one at F/4.7 and without it, my 100 degree eyepieces are swamped with coma.

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