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How necessary is coma corrector at f/5?

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#1 Virtus

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Posted 18 May 2021 - 09:22 AM

I will most likely be purchasing an ES Firstlight 10 (254/1270) dob from a club member. I'm curious of everyone's thoughts if a coma corrector is necessary at f/5?

 

My EPs are Baader Morphei and APM 24 UFF. I'll eventually get an APM 30 UFF, ES 30 82, or similar for low power.


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#2 J A VOLK

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Posted 18 May 2021 - 09:39 AM

I have a 10" f/5 Dob.  You can see some coma, but it doesn't bother me to any great degree.  I tried a coma corrector and it didn't think it was worth the trouble and sold it.  How much the coma bothers you is an individual factor.  I use an ES 30 82 for low power and thinks it works well.  I would just get the scope and see if the coma bothers you.  It is really not that pronounced at f/5 for visual IMHO.


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#3 me42

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Posted 18 May 2021 - 09:40 AM

I think this is very subjective. I personally use Morphei and the APM 30 UFF and find the coma corrector not necessary, but certainly nice to have in my f/5 dob.


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#4 Avgvstvs

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Posted 18 May 2021 - 10:09 AM

I have tried 2 brands of coma correctors for my 10" F5.

Like others I never really found it was needed visually.

And I think that is why quite a few come up on the secondary market


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#5 Jon Isaacs

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Posted 18 May 2021 - 10:10 AM

I use a coma corrector at F/5, I use a coma corrector at F/5.5.

 

One can certainly observe without one but I think it's actually more useful to me at mid to high magnifications because coma reduces the size of the planetary/double star "sweet spot". A planet can be sharp nearly edge to edge with quality eyepieces and a coma corrector.

 

If you've invested in quality eyepieces, what's left is coma... 

 

I will say that there are a variety of coma correctors available, they all work reasonably well at F/5.

 

What sets the Paracorr apart is the ease of use, it's easy.

 

All that said, now is not the time to buy a coma corrector for your F/5 Dobs get to know the scope and find out if you like it.

 

The coma free, diffraction limited field of a 10 inch F/5 is 0.125°, a quarter of the moon. That's 25° AFoV at 200x.. 30 seconds drift time

.

 

Jon


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#6 DAG792

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Posted 18 May 2021 - 10:13 AM

Coma correctors at F/5 are sort of like 'its good to have them, but you can do without them'. I own two F/5 scopes; a 6 inch and a 10 inch, and I have never felt the need for a coma corrector. I can certainly see the coma quite easily, but I do not think that it is severe enough that it subtracts from the beauty of the view.



#7 cst4

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Posted 18 May 2021 - 10:26 AM

I prefer one, especially at low power, but do not find it necessary.  



#8 eyeoftexas

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Posted 18 May 2021 - 10:40 AM

I have 12" f/4.9.  I use one because I saw the improvement when I used one.  The classic case of a little knowledge is a dangerous thing.  My Paracorr 2 now lives in the focuser, no need to think about it.


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#9 Rajput

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Posted 18 May 2021 - 10:52 AM

All the posts I've read on Coma correctors talk about using them with quality eyepieces or you will not see much benefit. Would there be a benefit of using a coma corrector with Plossls at high power? I was thinking specifically about high power views of Saturn and Jupiter. Last year, primarily used a 10mm Plossl with a Barlow to view. Views were great when I kept them in the center of view but deteriorated as they moved off center.

 

I have a limited budget and was trying to see if it made sense to invest in a higher quality eyepiece or a coma corrector. (my scope is F4.7)


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#10 junomike

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Posted 18 May 2021 - 11:21 AM

Also EP dependent.  I find the 31T5 acceptable as low as F4 whereas the 21E benefits from a Coma Corrector at F5



#11 Jon Isaacs

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Posted 18 May 2021 - 11:24 AM

All the posts I've read on Coma correctors talk about using them with quality eyepieces or you will not see much benefit. Would there be a benefit of using a coma corrector with Plossls at high power? I was thinking specifically about high power views of Saturn and Jupiter. Last year, primarily used a 10mm Plossl with a Barlow to view. Views were great when I kept them in the center of view but deteriorated as they moved off center.

 

I have a limited budget and was trying to see if it made sense to invest in a higher quality eyepiece or a coma corrector. (my scope is F4.7)

That's a good question.

 

I see you have an explore scientific 11mm 82 degree.  That is good enough that it would benefit from a coma corrector. 

 

A 10mm Plossl with a 2X Barlow, that's a 5mm Plossl.  I calculate that the field stop of a 5mm plossl is about 4.4mm, the coma free field of your scope is about 2.3mm, I don't a coma corrector would make much of a difference.

 

Jon


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#12 Jon Isaacs

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Posted 18 May 2021 - 11:26 AM

Also EP dependent.  I find the 31T5 acceptable as low as F4 whereas the 21E benefits from a Coma Corrector at F5

 

At F/4, the 31mm Nagler can provide pinpoint stars edge to edge if the coma is corrected.  Without the coma corrector, there's no reason to be using a 31mm Nagler.

 

Jon


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#13 Virtus

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Posted 18 May 2021 - 11:28 AM

Also EP dependent.  I find the 31T5 acceptable as low as F4 whereas the 21E benefits from a Coma Corrector at F5

Is this due to AFOV differences or do other design differences have an effect? 



#14 Jon Isaacs

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Posted 18 May 2021 - 11:51 AM

Is this due to AFOV differences or do other design differences have an effect? 

 

Coma is a linear function of the true field of view.  If the AFoV is constant, then the coma seen at the edge will be constant because while the greater magnification has a narrower field of view, the shorter focal length increases the magnification so the coma ends up being constant.

 

The coma from the mirror at F/4 as seen with an eyepiece like the 31mm Nagler is major. 

 

The 31mm Nagler has a field stop of 42mm, a half field of 21mm. 

 

This chart shows the uncorrected coma as well as the coma correction of the Paracorr 2. 

 

https://www.televue....orr_2_chart.jpg

 

At F/4, the radius of a star is about 3 microns.  Without a coma corrector, the rms radius of a star is about 90 microns at the edge of the 31mm Nagler, 30 times the diameter of a star in the center of the field, that's right, 30 times.. 

 

With the Paracorr 2 at F/4, the coma at the edge of the 31mm Nagler is smaller than the diameter of a star.  

 

I can say that these numbers, they support what I see at the eyepiece with my 12.5 inch F/4.06.  The analysis, it's just there to support what I see.  

 

At F/5, the radius of the coma at the edge of the 31mm's naglers field is 60 microns, the radius of the Star is about 4 microns, that means the coma is only 15 times the star and it's much less visible. 

 

Jon


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#15 Guydive

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Posted 18 May 2021 - 12:25 PM

pass



#16 SeattleScott

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Posted 18 May 2021 - 01:35 PM

So the simple version of what Jon is saying is the wider the AFOV, the worse the coma will be. Plossl is narrow so not a big deal. 65-70 AFOV is getting up to where coma is noticeable but not intrusive at F5. 82 AFOV and coma is getting distracting for some. Most people don’t like 100 AFOV without coma corrector at F5.

I used a 31 Nagler with and without coma corrector at F4.2. Without coma corrector, I would say about 70% of the field was usable (not saying all pinpoint stars, but usable as in able to typically identify a DSO somewhere in the field). But overall I wouldn’t say the view was that great. At a certain point it makes sense to save money and get a 68 AFOV Panoptic if the outer portion isn’t going to be usable anyway.

Scott

#17 SteveG

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Posted 18 May 2021 - 01:36 PM

If you want refractor-like views, yes, get a coma corrector at f5.


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#18 areyoukiddingme

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Posted 18 May 2021 - 01:55 PM

At F5 the coma corrector is not a need, but it sure is nice.

 

I use 100 degree eyepieces often in my F5 scope, and find that the views are much improved with the coma corrector.

 

I also use Pentax XWs, and the coma corrector removes most of the field curvature from the 14 and 20mm eyepieces, as well as cleans up coma.

 

As for the question about quality of eyepieces, I find that the 31 Nagler (for example) puts up better views without a coma corrector than the 30 ES/82. The astigmatism in the ES seems to be additive with the coma.

 

So on average, I think eyepieces with less astimatism will tend to perform a bit better with F5.


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#19 junomike

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Posted 18 May 2021 - 02:26 PM

At F/4, the 31mm Nagler can provide pinpoint stars edge to edge if the coma is corrected.  Without the coma corrector, there's no reason to be using a 31mm Nagler.

 

Jon

The 31T5 is not pinpoint at F4 but acceptable IMO.  Still much better than the 21E sans CC.



#20 Starman1

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Posted 18 May 2021 - 02:43 PM

The visibility of coma depends on:

1) the darkness of your sky.  The faint outer portion of the comatic star image may not be visible against a bright sky, but will be against a dark sky.

2) the apparent field of the eyepiece.  The wider the apparent field, the more visible coma is.  In a sense, coma correctors made 100° eyepieces in dobs possible.

3) the presence/absence of astigmatism in the eyepiece.  If the eyepiece has lateral astigmatism, it will mask coma, a smaller aberration.

4) the field curvature of the system.  I saw horrible coma in a 35mm Panoptic at f/5--really bad--but after the coma was removed, I could see the edge of field stars were slightly defocused.

That made coma appear far worse by defocusing.

5) the presence/absence of astigmatism in your eye.  If you have an eyepiece whose astigmatism goes away when wearing glasses, then using glasses will reduce the aberrations seen to

only FC. coma, and lateral chromaticism in an eyepiece that has no inherent astigmatism.

6) your expectations.  If, like me, you want star images to be tiny little pinpoints from edge to edge in the field, then you will use a coma corrector in a newtonian as long as f/8 to accomplish it.

I guess some people never look at the edge.

7) the f/ratio of the scope.  The coma free zone in the middle diminishes VERY rapidly below f/6.  To wit:

f/8  11.26mm

f/6  4.75mm

f/5  2.75mm

f/4  1.41mm

f/3  0.59mm

8) the quality of the optics in a scope.  If you never see tiny point-like star images ever, then coma is the least of your worries.

 

At f/4, the star images at the edge of the field of a 31mm Nagler are close to 200µ in diameter, while the Airy disc is 6.4µ in diameter.

If I had a scope that bad, I'd have the mirror refigured.  Some of the reasons above may be determinative of whether coma is obtrusive.


Edited by Starman1, 18 May 2021 - 02:50 PM.

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#21 Dave Mitsky

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Posted 18 May 2021 - 02:53 PM

I would put a coma corrector for an f/5 telescope in the nice to have but not strictly necessary category.


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#22 Miranda2525

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Posted 18 May 2021 - 09:12 PM

At F/4, the 31mm Nagler can provide pinpoint stars edge to edge if the coma is corrected.  Without the coma corrector, there's no reason to be using a 31mm Nagler.

 

Jon

Yep. I tried it without and there's coma to me seen. I won't use a short focus telescope without one.



#23 Miranda2525

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Posted 18 May 2021 - 09:15 PM

The visibility of coma depends on:

1) the darkness of your sky.  The faint outer portion of the comatic star image may not be visible against a bright sky, but will be against a dark sky.

2) the apparent field of the eyepiece.  The wider the apparent field, the more visible coma is.  In a sense, coma correctors made 100° eyepieces in dobs possible.

3) the presence/absence of astigmatism in the eyepiece.  If the eyepiece has lateral astigmatism, it will mask coma, a smaller aberration.

4) the field curvature of the system.  I saw horrible coma in a 35mm Panoptic at f/5--really bad--but after the coma was removed, I could see the edge of field stars were slightly defocused.

That made coma appear far worse by defocusing.

5) the presence/absence of astigmatism in your eye.  If you have an eyepiece whose astigmatism goes away when wearing glasses, then using glasses will reduce the aberrations seen to

only FC. coma, and lateral chromaticism in an eyepiece that has no inherent astigmatism.

6) your expectations.  If, like me, you want star images to be tiny little pinpoints from edge to edge in the field, then you will use a coma corrector in a newtonian as long as f/8 to accomplish it.

I guess some people never look at the edge.

7) the f/ratio of the scope.  The coma free zone in the middle diminishes VERY rapidly below f/6.  To wit:

f/8  11.26mm

f/6  4.75mm

f/5  2.75mm

f/4  1.41mm

f/3  0.59mm

8) the quality of the optics in a scope.  If you never see tiny point-like star images ever, then coma is the least of your worries.

 

At f/4, the star images at the edge of the field of a 31mm Nagler are close to 200µ in diameter, while the Airy disc is 6.4µ in diameter.

If I had a scope that bad, I'd have the mirror refigured.  Some of the reasons above may be determinative of whether coma is obtrusive.

Wow !!!  Huge difference from F/8 to F/5, and even more so at F/4. shocked.gif


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#24 fred1871

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Posted 18 May 2021 - 11:50 PM

I may have missed it in the comments above, apologies if I did - but at a given f/ratio, the amount of coma - relative to the diffraction limit - depends on aperture as well. So where a coma corrector might be marginally helpful with a 6-inch Newt at f/5, it will be far more visibly an improvement with a 12.5-inch Newt at f/5. You might as well have the greater resolution of the bigger scope, not just the extra light gathering. As Jon Isaacs points out above, the sweet spot of resolution is a lot bigger with a coma corrector, and that's a big help - especially with a non-driven Dob - for planets and double stars.


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#25 Starman1

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Posted 19 May 2021 - 01:02 AM

To quote from the link that follows:

"the wavefront error of coma in the Newtonian is inversely proportional to the square of its f-ratio for given angular field radius, and to the third power of it for given linear field radius. Thus it decreases exponentially with the increase in focal ratio"

i.e. the size of coma is related to f/ratio, not aperture.

https://www.telescop...ons.htm#clarity

 

The essay points out that at f/5, if the eye resolves to 3', one would see a 20° field radius (40° width) as coma free.

 

There are four common formulae for the determination of the linear size of a coma-free field:

Sky & Telescope:  0.01778mm f/ratio³ This is a diameter measurement.

Sidgwick: 0.0036mm x f/r³ (radius)

Sinnott: 0.0088mm x f/r³ (radius)

Everhart: 0.022mm x f/r³ (diameter)

 

At f/5, that means coma-free fields of:

2.22mm

0.9mm

2.2mm

2.75mm

 

But as Sacek points out in the link, the eye cannot resolve coma immediately outside that area.

Coma has to be large enough to be resolved by the eye.  A high-acuity eye can see it when it reaches 3' in size.  An average eye perhaps 5'.


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