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F4 vs F4.5 when does one need to upgrade eyepieces

Eyepieces Reflector
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#1 Jared b

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Posted 06 December 2021 - 01:34 PM

I currently use a combination of Apertura and Paradigm eyepieces  with my f6 8in dob. and they work fine for that. However when it comes to larger scopes with the shorter F ratios when is necessary to upgrade one's eyepieces. I assume F4 and below. can I get away with using my current eyepieces for a while with a f4.5? I assume a paracorr/ coma corrector will be in my future. I have looked through several pages including the one below. having never looked through premium optics, I am blissfully naïve of what I am missing.   

 

https://www.cloudyni...ry-observation/



#2 KerryR

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Posted 06 December 2021 - 02:18 PM

You'll generally find upgrading ep's useful at f5 and faster. Same goes for using a coma corrector, though, personally, I just tolerate coma and rarely use a coma corrector. I also tend to use simple ep's at f5, and in faster scopes that are limited to 1.25" ep's, but move to Panoptics and Naglers at f4.5 in scopes with 2" focusers; even without a coma corrector, the view is much improved owing to those ep's improved tolerance of steep light cones. I'm sure there are less expensive alternatives to going with Televue, but I have no specific familiarity with them, and so can't comment.

Much of this will depend on personal tolerance to field aberrations. I tend to find I get used to them, but they can be very noticeable/obtrusive if you move from a stint of observing with, say, a longer focus refractor or a fast newt with a well-tuned coma corrector, and move to an f5 or f4.5 Newt without a corrector. I generally find I adapt after a bit of time. Planetary and double star observation in a fast non-tracking scope is a different beast, though- in those cases, you really want as much of the field to be free of aberration as possible, so that you can observe the image at best focus as long as possible while it drifts across the field.


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#3 BrentKnight

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Posted 06 December 2021 - 03:30 PM

No eyepiece by itself will correct coma.  And some people say they notice coma at F6 (I wasn't bothered by it though).  At F5 and below it becomes apparent if you use wide apparent field eyepieces (2" or 1.25").

 

Your Paradigm eyepieces should work fine if you go below F5 though.  If you decide to get better eyepieces - and I'm talking better corrected at the edges, then you would be wasting your money if you don't also get a CC.

 

This is an image of M35/NGC 2158.  Viewing this pair of cluster in my F4.6 Dobsonian convinced me that I needed a CC.  Both clusters fit within the field of view of a 17mm Ethos, but NGC 2158 looks more like a diffuse nebula than a tight open cluster without the Paracorr.

 

M35 & NGC 2158 (annotated) 20211201 010518 ASIAIR
 
 

 



#4 Jared b

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Posted 06 December 2021 - 03:44 PM

To clarify, I anticipate the need for a coma corrector. I did not know if that would extend the life of my current eyepieces. I do have two GSO super wide 30 and 42 that I use often.

#5 KerryR

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Posted 06 December 2021 - 05:15 PM

It's perfectly possible to observe at nearly any 'normal' (f4 and slower) focal ratio with just about any (reasonably modern) eyepiece design; they pretty much all work well enough to be functional. Many of us observe down to f4 without eyepieces specifically designed to handle it, like Plossls and Erfles (The GSO super Wides are Erfle-type design-- 5-element, affordable, nice mid-wide field, but don't handle steep light cones that well, but well enough for many of us). The degree to which the field aberrations are prohibitive is a bit personal. Some folks can't stand the view without a coma corrector and a Ethos. Others, myself included, are fine with the view down to f4 with a Plossl, Erfle, Konig, or other more affordable moderately wide-field eyepiece... And, in my case, that's despite owning a full compliment of fast-scope-optimized eyepieces and a coma corrector.

Note that a coma corrector corrects for coma, which makes stars that aren't in the center of the field smear laterally into little comet-shapes. But, when using an eyepiece that's not specifically optimized for a fast scope, the more prevalent aberration will be eyepiece astigmatism, which smears non-central stars into little arks or crescents. Many observers mistake eyepiece astigmatism for coma, because it's so easy to see in any fast scope with non-optimized eyepieces (including refractors). So, pair a coma corrector with an eyepiece that doesn't handle steep light cones well-- which, sadly, includes most of the more affordable eyepieces out there-- and you won't see much of a change, because the eyepiece astigmatism is so much more prevalent. Thus, the first order of business in the quest to reduce field aberrations is to get ep's that can handle the steep light cone, and add the coma corrector later, unless you can afford both at the same time.


Edited by KerryR, 06 December 2021 - 05:17 PM.

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#6 BrentKnight

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Posted 06 December 2021 - 05:31 PM

I have always owned well corrected eyepieces (Baader Morpheus, Tele Vue Delos, Nagler and now Ethos), so I can't comment on less corrected on-axis issues with a CC.  But my thought for you in your situation is to get a good quality CC and give it a try with your current eyepieces (since you are already open to getting one anyway).  I don't think you will see any real benefit with an 8" F6, but certainly with a faster scope.  The aberrations are there, it really depends on how much it bothers you though...


Edited by brentknight, 06 December 2021 - 05:31 PM.


#7 Jon Isaacs

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Posted 08 December 2021 - 03:21 AM

I currently use a combination of Apertura and Paradigm eyepieces  with my f6 8in dob. and they work fine for that. However when it comes to larger scopes with the shorter F ratios when is necessary to upgrade one's eyepieces. I assume F4 and below. can I get away with using my current eyepieces for a while with a f4.5? I assume a paracorr/ coma corrector will be in my future. I have looked through several pages including the one below. having never looked through premium optics, I am blissfully naïve of what I am missing.   

 

https://www.cloudyni...ry-observation/

This is the way I see it:

 

My "Varsity" set of eyepieces is made of Panoptics, Naglers and Ethos's.  I have a Paracorr 2 and have been using one Paracorr or another for more than 20 years.   These are very good eyepieces in a fast scope.  My Dobs range from F/4.06 to F/5.5.

 

My "Junior Varsity" set consists of all the Paradigms except the 15mm and a Svbony 34mm 72 degree.  I also have the 30mm GSO Superview but it's loaned out. I am explaining all this so you will know where I am coming from.

 

The 5mm, 8mm and 12mm Paradigms are quite sharp across the field even in the F/4.06 used with the Paracorr. I haven't tried them without the Paracorr. Coma is not as bothersome with the 60 degree AFoV as it is with wider AFoVs. I don't feel like I am giving up much other than field of view with these three. These would be your primary planetary eyepieces.  The 25mm and the 18mm are not so sharp off-axis but I still enjoy the views.  The 3.2 mm is a bit wonky even in an F/7 refractor.

 

The GSO SuperViews are pretty much as Kerry described, functional but not so pretty.  

 

Jon


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#8 Markovich

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Posted 08 December 2021 - 09:13 AM

No eyepiece by itself will correct coma.  And some people say they notice coma at F6 (I wasn't bothered by it though).  At F5 and below it becomes apparent if you use wide apparent field eyepieces (2" or 1.25").

 

Your Paradigm eyepieces should work fine if you go below F5 though.  If you decide to get better eyepieces - and I'm talking better corrected at the edges, then you would be wasting your money if you don't also get a CC.

 

This is an image of M35/NGC 2158.  Viewing this pair of cluster in my F4.6 Dobsonian convinced me that I needed a CC.  Both clusters fit within the field of view of a 17mm Ethos, but NGC 2158 looks more like a diffuse nebula than a tight open cluster without the Paracorr.

 

Well, there was the Pretoria eyepiece :)


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#9 BrentKnight

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Posted 08 December 2021 - 10:32 AM

Well, there was the Pretoria eyepiece smile.gif

I had to look that one up.  Always an exception...



#10 Jon Isaacs

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Posted 08 December 2021 - 12:08 PM

I had to look that one up.  Always an exception...

 

I think the Pretoria was around before Al Nagler decided he needed to design a coma corrector.  If I remember right, the field of view of the Pretoria was rather narrow.

 

Jon



#11 MitchAlsup

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Posted 08 December 2021 - 01:06 PM

 If I remember right, the field of view of the Pretoria was rather narrow.

Let us just say that its FoV was competitive for the generation of EPs it was from (1960s).



#12 kathyastro

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Posted 08 December 2021 - 01:45 PM

The time to upgrade eyepieces (or any equipment, for that matter) is when you can identify exactly why you are upgrading.  If you cannot see any coma, you do not need to upgrade.  If you can see it, but can live with it, you don't need to upgrade yet.  If it is obvious and you can't stand it, then it is time to upgrade.

 

To learn what it looks like and how much better an upgrade can make things, borrow eyepieces from others and try them out.


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#13 MeridianStarGazer

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Posted 08 December 2021 - 02:54 PM

Coma looks like spikes at 60 degrees, whereas astigmatism looks like spikes at 180 degrees. Mix the 2 and you get seagulls. A coma corrector is needed to clean it up perfectly. I found goldline eyepieces at f5 look as good at the edge as 82 deg eyepieces do. The 82 are just working harder and have a larger absolute clean view. They are also bigger and weigh more.


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