Explore Scientific 20mm 100 degree eyepiece any good?

[emadmoussa]

Nope, but if coma shows across 50% of the field of view in my 68 degree Maxvisions then I doubt 100 degree ES eps will be an improvement. And I've been told to avoid low power, wide angled eyepieces in an f/4.7 telescope because of the coma they create. It seems that only high power, wide angle eyepieces work well in a fast Newtonian. As I said, forking out for 100 degree wide angle eyepieces when potentially half the view is marred by aberration seems self-defeating. Anyway, I've said my bit about it. If people get on with them then that's fine. I just wanted to put an opposing opinion.

LOL dude, you killed the eyepiece literally. 50% coma...?!!! What I'm saying is, until have you tried say the Explore Scientific 20mm 100-deg, there's no point passing judgements. Your logic regarding coma is intact and I agree with you. A well correct 100-deg eyepiece in a relatively fast Newtonian will show some coma ...in my experience under 10% of the field of view. And considering the massive portion of the sky you see, this 10% of coma can easily be ignored. Get a ES 100-deg and you'll change your mind

[emadmoussa]

No harm at all in an opposing opinion, each to their own and all that! I get the feeling however that you find coma particularly bothersome, even with your 68° eyepieces. Some are more bothered by it than others, and once noticed, (an f4.7 scope will generate a fair bit!) it's hard to ignore. If you are detecting it as far in as 50% from the field edge with your Maxvisions, I would highly recommend a coma corrector of some sort to clean the view up? Just my 2 cents!

Couldn't put it better.

[spaceboy]

Do you mean that 40% of the field of view is affected by aberration?

Sorry for my delay in reply Wolfpaw and sorry for causing any confusion. I know in my head what I want to say but don't always do so well to convey this across in to text. (probably why I don't post as often as I once did)  I should really have explained better.

As for your question I would say the views of an 82° and 100° will show the same ratio of coma. What I mean is that the later 40% will often show one aberration or other depending on the quality of the EP and  F ratio of the scope.

May have been better put this way :-

The later 40% will often start to show aberrations or coma but this is depending on the quality of the EP and F ratio of the scope. The views between the ES 82° and 100° will or should show the same ratio of coma. In other words if in your f/4.8 scope the ES 14mm shows coma in the latter 20% then it will show the same ratio of coma in the 100° ES. The only difference being in the 100° EP this latter 20% will seem to cover a larger area because there is a larger FOV, but that means the other 80% of viewing pleasure will seem larger also.  

As already mentioned by other members, it's important to remember the scope is responsible for coma and not the eyepiece. Faster scope, more coma, slower scope, less coma. Other distractions like aberrations depend on the quality of the eyepiece. Although in this case I should have said it doesn't really apply so much as the ES EP's in question are both are well corrected albeit suffering from some lateral colour.

The only thing I would point out from my experience is that with a large FOV showing an abundance of stars I do feel the need to tweak the focus depending on where I'm looking in the FOV. The only eyepieces I didn't feel the need for this was in Televue Naglers but then Televue has the reputation of being the best for a reason.

[bingevader]

Just been messing around with some circles!

There must be a program on line that would do this for you!

But I make it that an 82° FOV would be just slightly smaller than 70% of a 100° FOV (67ish%).

If for the sake of argument we say the outer 50% of the FOV of the 82° has coma, then the crystal clear central 50% will take up roughly 33% of the 100° eyepiece. If the outer 50% of the 100° EP has coma then that gives you an added 17% of crystal clear FOV over the 82° EP, doesn't it? As long as the 100° EP is well corrected.

I'm sure someone else can do the maths a lot more accurately than I can. 

But Nitram100 says he doesn't notice coma in the 82° 14mm EP, so it just depends on how well the 100° EP is corrected and how noticeable he'd find it in that EP!

Isn't this what coma correctors are for?

[wolfpaw]

Just been messing around with some circles!

There must be a program on line that would do this for you!

But I make it that an 82° FOV would be just slightly smaller than 70% of a 100° FOV (67ish%).

If for the sake of argument we say the outer 50% of the FOV of the 82° has coma, then the crystal clear central 50% will take up roughly 33% of the 100° eyepiece. If the outer 50% of the 100° EP has coma then that gives you an added 17% of crystal clear FOV over the 82° EP, doesn't it? As long as the 100° EP is well corrected.

I'm sure someone else can do the maths a lot more accurately than I can. 

But Nitram100 says he doesn't notice coma in the 82° 14mm EP, so it just depends on how well the 100° EP is corrected and how noticeable he'd find it in that EP!

Isn't this what coma correctors are for?

Forgive me if I'm wrong, but if the ES eyepieces are well corrected they will show more coma than a less well-corrected eyepiece as the coma won't be masked by the astigmatism of poorly-corrected eyepieces.

[wolfpaw]

LOL dude, you killed the eyepiece literally. 50% coma...?!!! What I'm saying is, until have you tried say the Explore Scientific 20mm 100-deg, there's no point passing judgements. Your logic regarding coma is intact and I agree with you. A well correct 100-deg eyepiece in a relatively fast Newtonian will show some coma ...in my experience under 10% of the field of view. And considering the massive portion of the sky you see, this 10% of coma can easily be ignored. Get a ES 100-deg and you'll change your mind

50% minimum, yes, but then I've been told it's entirely normal to have that much coma in a wide-angle eyepiece using an f/4.7 telescope.

[John]

I believe the way these aberrations (newtonian coma and eyepiece astigmatism) manifest themselves is a gradual build up as you move away from the optical axis. The central sharp area will vary with focal ratio, being relatively large with a slower scope but exponentially decreasing as the scope becomes faster.

What I think does vary from observer to observer, because of variations in the acuteness in the eye and maybe also tolerance, is the point when these aberrations become 1) detectable and 2) annoying.

And these are just two aberrations. There is also field curvature, lateral colour, pincushion, edge of field brightening, edge of field darkening and no doubt a few more hiding out there waiting to plague our confidence

I also think that awareness of these issues leads to more consciousness of them. Sometimes this can be triggered by trying out a different scope or eyepiece which exhibits more or less aberration and I can think of a number of occasions over the years when performance that I had hitherto thought pretty good was thrown into a less positive light when I viewed though equipment of better optical quality. 

To be fair I've occasionally experienced the realisation that my gear is doing pretty well having compared the views of the same targets through another setup, so it can work both ways.

Star Parties and observing meets can be very interesting experiences in all sorts of ways 

[spaceboy]

It is clear you have your doubts Wolfpaw and astronomy isn't cheap so I can understand your apprehension completely.

The only advice I can think to offer would be to pop along to a star party or local astronomy club and see if anyone has a 100° EP you can have a look through. Failing that I am sure there are other SGL members who may be with in driving distance of you that would offer some assistance. I have often offered this advice to beginners who are unsure if the hobby is for them or they are unsure in which direction they wish to go regards newts or refractors etc. Some of us may have been doing this hobby for a while and may think these kinds of avenues are only for beginners but some of the best advice I have ever got has been from fellow stargazers at star parties simply because there is a vast array of knowledge and astronomy equipment at such events.

[spaceboy]

Star Parties and observing meets can be very interesting experiences in all sorts of ways 

Just beat me to it John

[bingevader]

Forgive me if I'm wrong, but if the ES eyepieces are well corrected they will show more coma than a less well-corrected eyepiece as the coma won't be masked by the astigmatism of poorly-corrected eyepieces.

Yep, may have been fairly ham-fisted, but that was what I was trying to say. As long as it's well corrected, that's what you'll see.