Author Topic: Focal ratio and Coma.  (Read 172 times)

Lasaro Tourabi

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Focal ratio and Coma.
« on: December 27, 2017, 10:02:49 PM »
I had a F4.9 Z10 that seemed a little large for me. I would like to hear from people who have had  8" F6s and 6" F8s. I am trying to decide between the two dobs or a C8 Edge. Of those three which will have the "refractor like" stars. I understand the aperture effect. Would a Coma Corrector make the dobs as good or better than a C8 Edge? I want something with a little more aperture to compliment my C102.



Eric Ayyagari

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Re: Focal ratio and Coma.
« Reply #1 on: December 28, 2017, 09:55:22 AM »
Paracorr will make stars in a fast reflector as sharp as in an EdgeHD. These will both give excellent visual performance.

As to which one you should get, I have zero to offer. They are very different to use and only you can really decide what pros and cons suit your particular situation.

Anthony Cejudo

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Re: Focal ratio and Coma.
« Reply #2 on: December 31, 2017, 06:50:00 AM »
Thank you Eddgie. The star presentation is the important part. I know what the F4.9 stars look like with out a Parracorr. Could you make a statement about the relative view in the F6 and F8 without a Parracor? If refractor stars are 10s and F4.9s are 6, what would F8 and F6 be? I know this is a very subjective question.

unllamerblood

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Re: Focal ratio and Coma.
« Reply #3 on: January 09, 2018, 02:15:00 PM »
You could also get a Baader Planetarium 2" Mark III Multi Purpose Coma Corrector - MPCC. for half the price. I have no experience of it yet, having just got started in imaging with my F5 Newt, so I am researching the issue too.

http://www.highpoint...bfXaRoCltfw_wcB

Edited to include link.

vicareeti

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Re: Focal ratio and Coma.
« Reply #4 on: January 10, 2018, 03:44:36 AM »
I have 6"f8 and 8"f6 newts. The 6"f8s give a slightly sharper views of star fields across the full field of view, center and edges. Stars tend to be more like pinpoints in the f8 scope.

I believe improved edge sharpness is the result of reduced comma and improved center sharpness at low power (50x) is the result of smaller exit pupils making the astigmatism in my observing eye less apparent.

ertafsurpnant

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Re: Focal ratio and Coma.
« Reply #5 on: January 10, 2018, 05:58:49 AM »
I am very picky about coma. Anything faster than F/6 and i will use a Paracorr. F/8 is even better. For pure planet detail the Newts will beat out a Edge if they have great optics.

nisatourpo

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Re: Focal ratio and Coma.
« Reply #6 on: January 10, 2018, 06:53:43 AM »
An f/8 parabola does NOT warrant a coma corrector. Even at f/6 it's not really necessary. Indeed, coma correctors are optimized for a particular f/ratio, typically around f/4.5. If you install such a corrector on a Newt notably slower than than optimal, you might actually make things worse.

Be aware that the MPCC is known to introduce spherical aberration as the price for the reduced coma, which limits to low to moderate magnification. The Paracorr is, I understand, better corrected axially, permitting higher magnification.

tmasnilypho

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Re: Focal ratio and Coma.
« Reply #7 on: January 12, 2018, 09:07:55 PM »
Quote
An f/8 parabola does NOT warrant a coma corrector. Even at f/6 it's not really necessary. Indeed, coma correctors are optimized for a particular f/ratio, typically around f/4.5. If you install such a corrector on a Newt notably slower than than optimal, you might actually make things worse.

Be aware that the MPCC is known to introduce spherical aberration as the price for the reduced coma, which limits to low to moderate magnification. The Paracorr is, I understand, better corrected axially, permitting higher magnification.


Here's the correction curves for the Paracorr II:

Paracorr Type II Chart

The chart only covers up to F/5.

Here is the chart for the original Paracorr:

Paracorr Type I Chart

It shows improvement at F/8.

I have a Paracorr Type I and I use it at F/5.5. When I got the scope I figured I would probably be happy using it without the Paracorr but I only took a few minutes to decide that I would be using the Paracorr pretty much full time. Without the Paracorr, my 12.5 inch F/4.06 was better corrected than the F/5.5 without it..

----------

If one is concerned about the sharpness at the edge of the field, besides coma correction, there's field curvature and off-axis eyepiece astigmatism to be concerned with. The Edge corrects for field curvature and coma, a Newtonian has very little field curvature and the Paracorr corrects the coma.

That leaves the eyepiece astigmatism. Star that are essentially perfect across the field are possible with fast (F/4-F/5 even F/3) Newtonians but it requires eyepieces that are free of off-axis astigmatism. To get the ultimate, this is TeleVue territory.

The big difference in the Edge is that is an F/10 design so the True Field of View is limited.

-----------

Discussions like these can be quite academic.. one can also discuss the flaws in field correction of a 4 inch F/10 achromat.. Sometimes "refractor like views" are not so refractor like.. For a 4 inch F/10 refractor, I calculate that at the edge of a widest possible field of view that the stars are out of focus by about 0.8 mm.. that's the field curvature. It results in stars that are about 0.08mm = 80 um in diameter.

At F/10, the Airy Disk is about 7um in diameter so the out of focus blur for stars at the edge of the field is about 12 times the diameter of the Airy disk..

Can you see it? Probably if you look closely. Can you see coma at F/6, probably if you look closely.

jon

barlaliblo

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Re: Focal ratio and Coma.
« Reply #8 on: January 13, 2018, 09:01:21 AM »
Quote
Thank you Eddgie. The star presentation is the important part. I know what the F4.9 stars look like with out a Parracorr. Could you make a statement about the relative view in the F6 and F8 without a Parracor? If refractor stars are 10s and F4.9s are 6, what would F8 and F6 be? I know this is a very subjective question.

The Paracorr will make stars appear perfect at f/4.9 and you get the benefit of a wider true field and a more comfortable observing position.

There is no real benefit to using slower focal ratios.  I don't know why anyone would do that except maybe for a specialized telescope (planetary) that was not intended for general use.

If you want sharp stars, you can go pretty fast with a reflector and still have stars appear as sharp as they will in the EdgeHD SCT.

For general use, there is zero benefit to going long in a reflector and even for planetary use, the difference between a 23% obstruction and a 17% obstruction is pretty much almost impossible to see, so compromising a Newtonian by making it slow with a small diagonal makes the coma point moot because the field will be much narrower and much less well illuminated.

Again, I an not trying to influence what you buy between a Newt and an EdgeHD. Both produce superb star sharpness (when seeing allows) and I see the choice here as a logistics and comfort choice balanced against the contrast and true field advantages of the different designs.

If you are worried about sharp stars, I can assure you that a good coma corrector will produce visually sharp stars even down to f/2.8.

Don't sacrifice focal ratio because a slow focal ratio not necessary and a focal ratio of f/5 or even faster, along with a coma corrector, is a prescription for glorious wide field views using modern ultra-wide eyepieces like the 21mm Ethos or 31mm Nagler.

That is my opinion anyway.  There is not much point in going slower than about f/5 anymore and the trend is to go ever faster. The Coma Corrector makes that possible.

Myron Apostolics

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Re: Focal ratio and Coma.
« Reply #9 on: January 18, 2018, 12:48:51 AM »
Quote
One cannot ignore the role of the eyepiece..

My Newtonians range from F/4.06 to F/5.5 and with the Paracorr, they do provide sharp to the edge views. But part of that is a couple of eyepiece cases filled with Panoptics and Naglers, there's close to $3000 invested in eyepieces and not one was purchased new.

An f/10 scope is much more forgiving when it comes to eyepieces.

Jon

Nassim Zaragoza

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Re: Focal ratio and Coma.
« Reply #10 on: January 18, 2018, 05:53:03 AM »
Jon makes some good points here concerning the roles of eyepieces and focal ratios and how they interact IMO. There is also lateral color to deal with.

Though you might argue as to the exact accuracy of the eyepiece spot plots in this link, the differences in those plotswith respect to focal ratio, I find very educating:

http://www.telescope...berration_2.htm

I wish this chart included Naglers and Pans.

Jeff

Isaac Griffin

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Re: Focal ratio and Coma.
« Reply #11 on: January 21, 2018, 11:02:04 AM »
Quote
Quote

Thank you Eddgie. The star presentation is the important part. I know what the F4.9 stars look like with out a Parracorr. Could you make a statement about the relative view in the F6 and F8 without a Parracor? If refractor stars are 10s and F4.9s are 6, what would F8 and F6 be? I know this is a very subjective question.

There is no real benefit to using slower focal ratios.  I don't know why anyone would do that except maybe for a specialized telescope (planetary) that was not intended for general use.
To save about $1,500, the cost one comma corrector, two naglers, and a panoptic is reason enough for some people, especially for someone with $300-600 buget for a complete telescope and accessories.

radnatipni

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Re: Focal ratio and Coma.
« Reply #12 on: January 25, 2018, 09:58:40 PM »
Quote

 

From my point of view, fast scopes are about keeping ladders short, sitting instead of standing, fitting in cars, all the various practical aspects.. plus wide fields of view..

Optically there is a lot to be said for slower scopes.. People rave about long focal ratio achromats and go to the expense of building and mounting a 6 inch F/12 which will be heavy and still show significant chromatic aberration. A 6 inch F/12 Newtonian.. it will be tall but it could still be short enough to be a Dob..

The coma free circle of a 6 inch F/12... 1.2 degrees..

I think the only reason people rarely build 6 inch F/12 Newtonians is because 6 inch F/8 are, for all practical purposes, just as good.

Jon

Isidro Harrison

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Re: Focal ratio and Coma.
« Reply #13 on: January 30, 2018, 11:50:51 PM »
Quote
Jon makes some good points here concerning the roles of eyepieces and focal ratios and how they interact IMO. There is also lateral color to deal with.

Though you might argue as to the exact accuracy of the eyepiece spot plots in this link, the differences in those plotswith respect to focal ratio, I find very educating:

http://www.telescope...berration_2.htm

I wish this chart included Naglers and Pans.

Jeff

No doubt about it, all eyepiece designs do better slower. Even the expensive fat glass ones.

But rather than dive into the theory, I went back to read the original post in the thread. What he was looking for was convenient medium aperture.

Go with the Edge!

In terms of ergonomics, SCT's always have a favorable eyepiece position compared to Newts. And less range of eyepiece motion. Fewer thermal issues. Easier collimation. Several mounting options. More accessories, including focal reducers. (Yes, you can get a reducer for a Newtonian - but do you have the focus travel to use it?)

acbacema

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Re: Focal ratio and Coma.
« Reply #14 on: January 31, 2018, 11:12:55 AM »
Quote

In terms of ergonomics, that's personal, I prefer the Newtonian. In terms of thermal issues, most call it the other way. SCTs never quite cool, even the Edges. Newtonians.. a fan and it can reach thermal equilibrium quickly.

Focal reducers: It doesn't make sense for a Newtonian, they are already fast. For an SCT, they're slow.. they need it. The 8 inch Edge Focal Reducer only works with the 8 inch Edge and costs $300.. By comparison, a Paracorr II costs $475 and will work with just about any Newtonian with a 2 inch focuser.

Jon