Caveats with Wide AFOV eyepieces in "fast" telescope

[amaury]

Hi again,

I'm still learning a lot about this, but every time I take my Dob out I wish I had a lower magnification with wider field of view.

I think this is the very first thing you realize as a beginner that is shockingly counter-intuitive at first. 

Now I am totally decided to get a low power wide AFOV for my 8" f/6 Skywatcher.    

I was doing my research and read in several places that fast scopes struggle with wide field of views, showing aberration at the edge of the field of view. Only premium eye pieces seem to have good edge performance with fast telescopes.

I will quote what I read

"If you have a fast focal ratio scope, you basically have three choices: (1) pay the price for premium wide-field eyepieces; (2) limit yourself to Plössls and similar older designs, which have narrower apparent fields but are sharp to the edge even in fast scopes; or (3) buy inexpensive wide-field eyepieces and resign yourself to very poor edge performance"

Article where I read this: https://www.oreilly.com/library/view/astronomy-hacks/0596100604/ch04s07.html

If that is true then, option 3 is really a waste of FOV and I much rather go with option 2.

My question is, how bad edge performance would I have with an affordable low power wide AFOV like the one below (30mm 68 AFOV)

https://www.firstlightoptics.com/stellalyra-eyepieces/stellalyra-30mm-2-superview-eyepiece.html

would an equivalent Explore Scientific (34mm 68 AFOV) have much better edge performance considering that it's 3x the price of the stellalyra for around the same specs on paper?

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-68-degree-series-eyepieces.html

Thanks in advance,

Amaury

[KP82]

The stellalyra 30mm is a modified Erfle design I believe. So yes they have poor edge performance. If you had f/5 or faster scope, you would probably see a major difference between it and a more expensive ep like the ES68. But at f/6 maybe not a lot.

[vlaiv]

You don't have to break a bank to get half way there.

Get one of these (in 35mm version other two are not available):

https://www.firstlightoptics.com/ovl-eyepieces/aero-ed-swa-2-eyepieces.html

In F/6 scope - maybe outer 10% will suffer somewhat so you won't get that "to the edge" sharp performance - but if you concentrate on the target in the center of the field and use outer part just for framing / context - well you don't see that sharp with peripheral vision anyway.

Other than that - ES68 is very nice performer. I have 28mm version and use it in my 8" F/6 and it is comfortable and sharp and excellent performer.

[Pixies]

36 minutes ago, vlaiv said:

You don't have to break a bank to get half way there.

Get one of these (in 35mm version other two are not available):

https://www.firstlightoptics.com/ovl-eyepieces/aero-ed-swa-2-eyepieces.html

In F/6 scope - maybe outer 10% will suffer somewhat so you won't get that "to the edge" sharp performance - but if you concentrate on the target in the center of the field and use outer part just for framing / context - well you don't see that sharp with peripheral vision anyway.

Other than that - ES68 is very nice performer. I have 28mm version and use it in my 8" F/6 and it is comfortable and sharp and excellent performer.

I have the 30mm version and the edge is pretty poor in my F/6 8" dob. I'd say > outer 10% affected. The 35mm version is better, apparently.

[amaury]

@vlaiv Thanks for that, It looks like a good middle ground. I'll keep my eyes open for a second hand ES68 too.

Interesting @Pixies, is the 35mm better?, or just because it has lower magnification there's more FOV close to the centre relative to the 30mm? If that is the case I much rather have a high quality Plössl that is sharp edge to edge

[John]

41 minutes ago, Pixies said:

I have the 30mm version and the edge is pretty poor in my F/6 8" dob. I'd say > outer 10% affected. The 35mm version is better, apparently.

I've owned the 30mm and 40mm. The 30mm edge correction is not that great. I believe that the 35mm is better corrected and the 40mm better again proving sharp across the field even in my F/5.3 12 inch dob. The exit pupil diameter is rather over-large using the 40mm though so the 35mm is probably the best compromise in an F/6 scope.

 

 

 

 

 

Edited March 31, 2021 by John

[michael.h.f.wilkinson]

I have used the TMB Paragon 40 mm (of which the Aero is a clone), and that was fine in an F/6 APO triplet. The 40 mm is reported to be the best of the bunch, but sadly no longer available. If you can get your hands on the MaxVision 34 mm 68 deg, that would be a good option (same optics as ES)

[Louis D]

4 hours ago, amaury said:

My question is, how bad edge performance would I have with an affordable low power wide AFOV like the one below (30mm 68 AFOV)

Well, I've taken lots of images through my eyepiece collection at f/6 (albeit a field flattened 72ED refractor).  Here's the ~30mm group and the 32mm and up group.  You can judge for yourself how much edge falloff there is for simpler designs.

The 35mm Aero ED is in the second group.  It's pretty decent out to 75% of the way to the edge.  The 40mm Meade 5000 SWA is optically identical to the 40mm ES-68.

Edited March 31, 2021 by Louis D

[amaury]

Thanks @michael.h.f.wilkinson and @Louis D

So Meade 5000 and MaxVision have the same optics as the Explore Scientific. That's good to know in case the show up in the second hand market.

2 hours ago, Louis D said:

You can judge for yourself how much edge falloff there is for simpler designs

I can certainly see the Meade 5000 SWA is objectively superior. Based on those images I would still choose any of the 32mm Plossl (the Orion or the GSO, pretty sharp from edge to edge) over a simple under-performing wide FOV. 

2 hours ago, Louis D said:

(albeit a field flattened 72ED refractor)

@Louis D maybe it's a silly question, what's the implication of using a f/6 refractor compared to my 8" f/6 reflector? Should I expect similar relative performance?  Does it change the analysis in any way? 

[Louis D]

5 hours ago, amaury said:

@Louis D maybe it's a silly question, what's the implication of using a f/6 refractor compared to my 8" f/6 reflector? Should I expect similar relative performance?  Does it change the analysis in any way? 

Not really as long as you're using a coma corrector in the Newt.  Even at f/6, I see noticeable improvement in wide field eyepieces when using a coma corrector.  It doesn't even have to be expensive.  I use a GSO/Revelation/TPO coma corrector with a 25mm spacer ring between the eyepiece holder and the optics section and don't even both with parfocalizing most eyepieces.  The lone exception is my 12mm Nagler T4 which focuses 20mm below its shoulder.  I put five 4mm O-rings around the 2" skirt along with a 20mm extension on that skirt to give the focuser something to tighten onto as seen below.

Pretty much, focal ratios behave much the same across design types as far as eyepiece edge correction.  Differences arise due to other aberrations like coma, chromatic aberration, and field curvature.

[amaury]

Thanks @Louis D. I haven't really used a wide field eyepiece yet. I am hopping for things to normalize so I can interact more with my local astronomy club and test a couple of things I have in mind. Like how bad the coma is in my f/6 with a wide afov eyepiece. Most people don't bother with coma correction until they get to f/5 focal ratios. I believe there is a strong matter of personal preference there too. I am still in the process of forming my own opinions when it comes down to preferences in my telescope. 

[vlaiv]

24 minutes ago, amaury said:

. Like how bad the coma is in my f/6 with a wide afov eyepiece.

I have not noticed coma in my F/6 8" newtonian with 28mm 68° eyepiece (to be fair - I was not looking for it either).

Thing with coma is that it is magnified same as everything other in focal plane when using eyepiece. What could be perceived as large coma in image - could well end up being unresolved by our eye when using  small magnification.

If you look at this diagram, you'll notice that geometric shape of coma at F/6 is equal to airy disk size at 3mm away from optical axis. Airy disk size for say 8" F/6 is 1.28" and if you are using x40 magnification - it will be 51".

Observer with sharp eyesight can't resolve anything below 1 arc minute so you need coma blur to be at least few arc minutes across when magnified in order to be able to tell that it is coma.

Note that above is geometric radius, but light will be concentrated away from above circle into "neck".

[Louis D]

Note that coma grows linearly with angular field of view, so it's twice as large at 100 degrees as at 50 degrees.  I really notice it in my ES-92 eyepieces without a coma corrector even at f/6.

[vlaiv]

22 minutes ago, Louis D said:

Note that coma grows linearly with angular field of view, so it's twice as large at 100 degrees as at 50 degrees.  I really notice it in my ES-92 eyepieces without a coma corrector even at f/6.

Indeed, on top of that 92° 17mm eyepiece will give larger magnification versus say 52° eyepiece with the same field stop so even if star at the edge will have same coma due to same field stop size - 92° eyepiece will magnify image much more and make coma larger and easier to se.

[amaury]

6 hours ago, vlaiv said:

I have not noticed coma in my F/6 8" newtonian with 28mm 68° eyepiece (to be fair - I was not looking for it either).

That's is good to know. 

5 hours ago, Louis D said:

Note that coma grows linearly with angular field of view, so it's twice as large at 100 degrees as at 50 degrees

I think I'd be happy with 68°. A wider field of view means more coma and a more expensive eyepiece too. 

Is there any other eyepiece option of the quality/price ratio of the already mentioned Explore Scientific / Meade 5000 / MaxVision?

The Aero ED looks ok but the ES is noticeable better. William Optics SWAN apparently don't perform nearly as good the ones above either. 

 

 

[Louis D]

23 minutes ago, amaury said:

That's is good to know. 

I think I'd be happy with 68°. A wider field of view means more coma and a more expensive eyepiece too. 

Is there any other eyepiece option of the quality/price ratio of the already mentioned Explore Scientific / Meade 5000 / MaxVision?

The Aero ED looks ok but the ES is noticeable better. William Optics SWAN apparently don't perform nearly as good the ones above either. 

 

I also got the 40mm Pentax XW for Christmas which is almost as good as the 40mm Meade SWA, but considerable lighter.  It's also a bit easier to hold the view in it.  They were just reintroduced, albeit at a price probably above your budget.

[John]

This would be worth a punt if it is still available:

https://www.astrobuysell.com/uk/propview.php?view=172878

I used to own one and found it excellent even in quite fast scopes. Very, very close to the performance of the best of them.

I have no connection at all with the seller or advert.

 

 

[amaury]

5 minutes ago, Louis D said:

albeit at a price probably above your budget

I would love to get a Pentax or a TeleVue Panoptic at some point. No quite yet. Maybe for the next Christmas as well. XD

[amaury]

@John thanks, I am looking for something between 30 and 36mm. 

I came across the one below but haven't got an answer yet.

https://www.astrobuysell.com/uk/propview.php?view=172006

[KP82]

33 minutes ago, amaury said:

@John thanks, I am looking for something between 30 and 36mm. 

I came across the one below but haven't got an answer yet.

https://www.astrobuysell.com/uk/propview.php?view=172006

Unless this is an old eyepiece from ES that has been discontinued, I don't recall there is an ES68 32mm. Better ask the seller to clarify.

The ES68 24mm is almost as good as TV Pan 24mm. This gives the maximum fov in the 1.25" format.

Edited April 2, 2021 by KP82

[amaury]

1 minute ago, KP82 said:

I don't recall there is an ES68 32mm

That's exactly what I said when I contacted the person (haven't received an answer yet), it must be a ES62 32mm or a discontinued model I haven't heard of

[KP82]

1 hour ago, amaury said:

That's is good to know. 

I think I'd be happy with 68°. A wider field of view means more coma and a more expensive eyepiece too. 

Is there any other eyepiece option of the quality/price ratio of the already mentioned Explore Scientific / Meade 5000 / MaxVision?

The Aero ED looks ok but the ES is noticeable better. William Optics SWAN apparently don't perform nearly as good the ones above either. 

 

 

Unlike in short to medium focal length, our choices in 30mm+ are actually quite limited. If you do not like the performance of the Aero EDs (or their clones), then ES is probably the cheapest option left.

[Louis D]

1 hour ago, amaury said:

I am looking for something between 30 and 36mm. 

If you want perfection and are willing to live with a 70 degrees AFOV at 30mm, you can't really go wrong with the 30mm APM UFF for about $250.  It's basically flawless edge to edge and light and compact (relatively speaking).  It's also sold as the Altair Ultraflat, Meade UHD, and Celestron Ultima Edge.  It's the topmost image in the ~30mm group I posted above.

[John]

1 hour ago, amaury said:

@John thanks, I am looking for something between 30 and 36mm. 

I came across the one below but haven't got an answer yet.

https://www.astrobuysell.com/uk/propview.php?view=172006

There isn't a 32mm 68 degree Explore Scientific eyepiece as far as I know.

Above 24mm the range becomes 2 inch format and runs 28mm, 34mm and 40mm.

Edit: looking at the photo with the advert, I think that is a 62 degree ES. There is a 32mm in that range.

 

 

 

 

Edited April 2, 2021 by John

[Paulhinthe808]

It hasn’t been mentioned, but keep the APM 30mm UFF and its clones on your radar as well.   Haven’t used one, but it is reported to be an excellent performer in a fast scope