Wide Field EP for Dob

[NorthernLight]

Hi There,

after a long time with my Celestron eyepice kit (Ploessls) I feel that I want a wide field eypiece to scan the sky in my 12" Dob without:



seeing false colour
enlarged stars
my eyelashes constantly oiling the eyepiece


but with



good contrast,
sharpness
and a field wide enough to see the planets at 300X long enough in the field for the vibration induced by moving the scope to subside and then a bit longer.

I´d like to use it alone and couple it to a barlow or powermate so the ideal fl would be around 17mm (2"barrel).

I was looking at various threads on the net and here on IIS but consensus seems to state that viewing pleasure starts above $500 AUD in initial expenditure.

Is there a good eyepiece to be had that comes close to what I am after for $200-$300 (new)? If the difference to my Ploessls is only marginal in this price range, then I might have to save a bit longer. I would like to be blown away when changing - at least a little.

Cheers

[omegacrux]

Celestron Ultima LX 17mm 70' fov
they can be picked up for around $120 second hand
or from the USA cheaper than here

David

[dannat]

18mm ES [82deg], or 17mm LVW [65 deg but better eye relief]

you might also get the 16mm ES [68 deg fov but 1.25"]

[mental4astro]

The Celestron Ultima LX line is designed for SCT's and Maks. They are not a good optical match with Newtonians. These in SCT's and Maks are excellent - I have the 8mm, and until recently the 13mm, and love/loved them both in my C8. BUT, put them in a Newtonian, and the image is terrible. Nothing wrong with the eyepieces. Nothing at all.

One aspect of scopes and eyepieces that is not understood well enough is that scopes and eyepieces actually need to be matched. This matching comes firstly from the 'focal plane' that different scope designs ( Newtonian, refractor, SCT, Mak, etc) is not a flat one. Newtonains produce a concave focal plane. Refractors, SCT's, Mak's, produce a convex focal plane. Eyepieces are then designed primarily to perform best for a given scope focal plane shape. Add further complications of big variations in f/ratio, radius of focal length, etc, in scopes, and then the built in corrections in different eyepiece designs, and the matching of eyepiece to scope becomes a very complex and difficult one.

There are very few entire lines of eyepieces that perform really well across all scope designs. Even in these lines, there are small variations in performance quality within the individual focal lengths. It is not plausible to expect all focal lengths to perform equally well - there are just too many lens variations to give different focal lengths, so it is a given that each focal length will handle different focal plane shapes to different quality capacities. Even within some very high end eyepiece lines there can be huge performance differences within the different focal lengths. It is unfortunate though that it is left upto individual consumer to inform themselves as to what eyepiece best suits their individual scopes. And keep in mind that within a whole line, there may be only ONE individual focal length that may actually suit a specific scope design!

For this reason, in my own eyepiece kit you will find a variety of brands, models and focal lengths that suit the scopes that I have.

Eyepieces that DON'T suit Newtonians:
Baader Hyperion, Orion Stratus, Celeston Duo, Saxon Superwide - exact same eyepiece design! The one exception here is the 5mm. The 5mm is actually a very good optical match. It is the only Hyperion that is in my kit. The Hyperion and its clones are designed for SCTs and Maks.

Celestron Ulitma LX. Similar to the above, but there are some optical design differences. Great in SCT's and Maks. Disaster in Newts.

Celestron Luminos.

Eyepieces that ARE a good match with Newts:
Plossls - very old eyepiece design that is very forgiving in most scope designs. They perform best in slower Newts, but still throw up a good image in fast Newts. Main problem though with plossls is the shorter the focal length, the smaller the eye lens, and the closer and closer you need to put your eye to the eye lens to see into it. These are very cheap to produce which is why they are the most common eyepiece design you will find included with half decent scopes. There are newer designs that also utilise newer glass types that easily out perform plossls, have a wider AFOV, and are only a little more expensive than plossls, :)

Explore Scientific 82 deg and 100 deg - These two lines are designed for fast Newtonians, and have a considerable degree of built in coma correction in them. This is essential as the faster the focal ratio, the stronger the coma, so these eyepieces need to be able to deal with this. A coma corrector performs very well with these to give sharp stars across the whole FOV. Coma correctors are not essential to use - I don't use one as the way I use my eyepieces I just don't get a benefit from them.

TMB Planetary type II - FANTASTIC short focal length line, and cheap. The 6mm is the weakest and should be avoided. This line is cloned by just about every brand of eyepiece, but they are all essentially the same eyepiece as they are inexpensive to produce so there is no advantage in changing optical components. These all have the same nice big eye lens, from the longest to the shortest, and all have the same long eye relief.

Vixen LVW - flaming brilliant eyepieces. These are one of the very few entire lineup of eyepieces that perform really well in all scope designs. Yes there are some individual variations within the individual focal lengths, but these are not too significant.

Tel Vue Ethos, Panoptics and Nagler. Excellent high end pieces.



There are others that are good and poor in Newts, and I'm sure other folks will add their experiences to this.

Cheers,

Mental.

[N1]

A litte blown away, eh? :eyepop::zzz:

Some good suggestions here. I have the 16mm ES 68° and while it's indeed a great EP, its eye relief will probably not meet your requirements. Same for a 19mm Panoptic. The TMBs & their clones are really good also but don't come longer than 9mm. Anything that does is not based on the original TMB design.

Consider a 17.3mm Delos or 17mm T4 Nagler second hand. Probably the best value for what you are looking for. I've bought a few used EPs (including here on IIS classifieds), and they had all been well looked-after. Failing that, 17mm Vixen LVW.

[mental4astro]

Yes! Forgot about the Delos line.

I didn't mention the ES 68deg line as for a few reasons for me they fall into the second tear line of ok eyepieces for Newts. The ones Mirko & I mentioned I say would be the top shelf stuff - a couple of others too that I haven't mentioned too. But most of these are $$$.

There is another group that are good, save a lot of $$$, just have a few little aberrations. These are by no means fatal, and you certainly won't be trying to look through a brick. They are a compromise between $$ & performance. In many instances you may be happy just with these :). Particularly if you are $$$ conscious, these are excellent value.

GSO Superviews - based around the Erfle design. Nothing touches these for value for money. I still have my 30mm & 15mm Superviews, though I use them with outreach nights ss they are very easy for novices to use. Bintel also carries thrse under their own brand name.

Explore Scientific & Meade 68 deg. Essentially the same eyepiece - come from same factory. Nice eyepieces, though show some astigmatism in very fast Newts at the very edge of the FOV, and eye relief is tight with 20 & 16mm, as Mirko mentioned.

Mental

[rrussell1962]

Alex, that was a very interesting post you made above regarding matching eyepiece design to telescope types. Do you have any original sources you could point me towards as it is an area I would like to investigate further. I didn't realise the superviews were basically an Erfle (which autocorrect seems determined to change to earful) Many years ago I had a 30mm Erfle, a big brass thing with the old RAS thread fitting, I think I picked it up second hand at Fullerscopes in London. I used it on my home made 6 inch F8 Newt and it gave very nice views. I also had an adapter and a selection of 0.965 Orthos which I used. Probably unthinkable now, but back then Orthos were the bee's knees!

[NorthernLight]

Hi Guys,

thanks a lot for your comprehensive overview of the ins and outs.
Alexander, your essay on eyepieces is a real eye-opener. It should be up in the stickies. I could imagine that I am not the only one wondering which piece of glass works best.

Just one more question:
ES and others come in 1.25" and 2" barrel sizes and others have both in the same eyepiece. Does the barrel size matter then? I was reading about vignetting with 1.25" barrel sizes for wide field eyepieces and yet, the Ethos line can be inserted into a 1.25" focuser. Is it then still achieving ist claimed 100° AFOV or will there be losses whenever one chooses 1.25" over 2"?
I am leaning a lot towards the ES 82° line of EP´s but struggle to make up my mind which one to get. The 2" barrel ones appeal but I would have to get a 2" Barlow or Powermate. All of my other EP´s, filters and Barlows incl a 5x Powermate are 1.25". It would mean creating a new collection, incompatible with the others. But if the visual gain by 2" barrel size would be a big difference, then I guess I would go for it.

I have a GSO superview 30mm as well. It came with the Dob. It has a nice wide field but my suburban skies are pretty light poluted and whenever I use the 30mm I see a washed out, grey sky. The stars also seem to become comets away from the dead centre of view. I haven´t tried it with a barlow, though. Perhaps that makes it better.

[mental4astro]

Russell,

A good mate of mine turned my attention to this aspect of matching optics. I was having a poor time with some eyepieces, and in speaking with Wavytone (here on IIS), he explained the importance of matching eyepieces with scopes.

Eyepieces have been designed for particular scopes since scopes started being described mathematically. Eyepieces are still designed this way. Unfortunately eyepiece producers fail to tell us this, and it is left up to us to figure out what is going on. The biggest problem this creates is when an eyepiece is reviewed, someone with eyepiece 'X' and puts it into scope 'Y', and the image is crap, then the reviewer dumps on eyepiece 'X'. The reality is eyepiece 'X' should have been used in scope 'Z'. Or, consumer 'A' puts eyepiece 'X' into scope 'Y' after reading a view where the eyepiece was used in scope 'Z', and ends up very disappointed and none the wiser as to why - this happened to me with the Hyperions.

A good reference to this is 'Telescope Optics'. It goes into depth on scope and eyepiece design:

http://www.willbell.com/tm/tm6.htm

Max,

The best way I can describe the reason for 1.25", 2" even 3", 4" and 4.5" eyepieces is with an example:

You have two straws, same length but one is wider than the other. When you look through each you see a wider field through the larger diameter straw. This is what a larger diameter barrel will give. The maximum true field of view will be greater with a 2" barrel than a 1.25".

You would have noticed that there are different Apparent Field of View (AFOV) values with eyepieces. These can be 52deg, 58deg, 68deg, 82deg, etc. Keeping in mind that there is a maximum amount of sky that a given size barrel will show, there is a maximum value of focal length that can give a given AFOV. The longest focal length that will give a 68deg AFOV in a 1.25" barrel is 24mm. In a 2" barrel this is 42mm. You just cannot get a 40mm 1.25" barrel with an AFOV of 82deg - it just can't be done. But there are 40mm eyepieces in 1.25" barrel. The thing is the AFOV of such eyepieces will be small, around 33deg.

That aberration that you've noticed in your Superview is seagull shaped, more than 'comet' shaped. Seagulls spread out in the same direction as the edge of the FOV. This is actually astigmatism in the eyepiece being shown by the optical design of the fast Newtonian. There will be a little 'comet' tail showing coming from the centre of the star between the 'seagull'. This is coma and is inherent to fast Newtonians. Astigmatism is one aberration that is shown with an eyepiece/scope optical mismatch.

The eyepieces I mentioned in my first post here have astigmatism much better controlled. Those noted as second tear start showing astigmatism - not a grotesque, but it is there along the edge of the FOV. This can be acceptable, but it ends up being a personal determination.

I can understand that the aberration that you see in your Superview is now challenging. If you are considering a better eyepiece, then those listed in my first post will better meet your requirement. Note that they will display coma if used neat in a Newtonian. Remember, coma is not an aberration, but a phenomenon inherent to Newtonians. This can be controlled by using a coma corrector. I'm happy to deal with a little coma as this is seen really only along the very edge of the FOV, and the way I use my gear, this is not a concern for me.

Note that what I've written here is not everything about eyepieces and scopes. Not by a long shot. It is just intended to start giving you some understanding that as "oils ain't oils", eyepieces ain't eyepieces and scopes ain't scopes.

[MattT]

Max,
If you want nice stars to the edge of field do yourself a big favour and get a GSO coma corrector. Set it up with info from this CN thread.

http://www.cloudynights.com/topic/462985-setting-up-the-gso-coma-corrector/

A coma corrector is a game changer. I have all the ES 68's and most of the 82's which I use in my 6" f12 Refractor…perfecto! and 10" f4.8 Newt…not so perfecto except when my coma corrector is in the focuser.

My CC is an ES HR with the helical top making it easy to adjust.
Like the GSO on CN I found 19mm out the best setting for the 68's and 15mm out the best for the 82's.

Sadly ES have put their prices up of late, but they are still half the price of TV.

Back to your question…ES 18mm 82º + GSO coma corrector :thumbsup:

Matt

[NorthernLight]

Hi Matt,

thanks for the tip with the Coma Corrector. I will try the new eyepiece and then see if I want more correction. I had a chat to my local Astro Club-Shop and it seems as if they can import ES eyepieces unbranded directly from China. They expect shipment in a month or so. I´ll hold my breath until then and see how it goes.

[ZeroID]

I'll go with Mental's comment about the GSO Superviews. I have the 15mm and 20mm and they are just brilliant for the price. My most used EPs by far. TMB 6mm, haven't found a use for that yet, the 9mm is ok.
I have a 25mm plossl I acquired somewhere. It matches the GSO 20 in field and is excellent in the Newt. Also I have 2 x 2" EPS, a 30 and a 32. Again acquired as part of bought lots. They are both excellent wide view pieces, the 30 is almost panoramic. Sorry I can't quite remember the make but nothing spectacular. Both simple lenses, minimal glass (Kellner ?) and nice and clear, no colour cast.

[NorthernLight]

Hi Brent,

I`ve seen the 15mm and 20mm 1.25" GSO EP`s at AstroNZ for $55 each. I think I get one and see how it compares to my set of Ploessels. 15mm seems to be the best.

Cheers

[ZeroID]

Good price, get both. With a Barlow 2 x gives you some good options.

Do you belong to Auckland Astronomical Society ? I joined middle of last year. At $25 subs well worth while.

[NorthernLight]

Yes, I am a member since 2012 but I pay $40 p.a.???
Have you been to the talk on Monday about Comets? Might have seen you unknowingly.

[ZeroID]

$40 is family I think. No, wasn't at comet night, nor will I be at the Astrophotog night tomorrow as it happens to be a special occasion and I better stay home for it.
I'll let you know when I'm next going and try and catch up.

[NorthernLight]

Sounds good, mate! See you next time.