Why favour a circa 20mm?

[Alan White]

My eyepiece collection has just had a major churn, sold almost all TeleVue to raise funds towards a new mount, well maybe....but that is another story.

So pondering my light polluted skies and past favourites, all have been 22-19mm range.

Vixen 22mm LVW, why did I ever let that one go!

Panoptic 19mm, my precious as I called it.

18.2mm DeLite.

Plossl 20mm, Tele Vue.

RK 20mm, came with my first ever Bresser Scope and remains with me.

and more that are not popping to mind at the minute.

So I was pondering why, it must be that it works ok with my astigmatism, sky brightness and I like how it presents things to my eye. Clearly I favour this focal length, my scope have mainly been 750 - 900mm focal length.

 

Any thoughts on this?   And any thoughts on options going forward for this one special slot?

 

 

[Pixies]

Could it be an exit pupil thing? I guess if your focal ratios have all been similar, the same EP focal length would mean a similar exit pupil. This would affect the sky brightness and, if the exit pupil, wasn't too large, would help with astigmatism?

[chiltonstar]

Maybe also your comfortable field of view. When my eyes were tested for high power laser working a few years back, the optometrist commented that I shouldn't drive a fork lift truck (I never had that ambition anyway) and that I had slight tunnel vision - that maybe explains why I've always found 50 to 60 degree eyepieces a lot more comfortable to use than anything wider. That was a major consideration when I upgraded from a 25mm plossl a month ago, as this FL is one I use a lot with my 180 Mak.

Chris

[Louis D]

What's your budget?  The 70 degree 22mm eyepieces (from Long Perng?) (Olivon 70, AstroTech AF70, Omegon Redline, Arcturus Ebony, Telescope Service Expanse ED, Astromania SWA, Celestron Ultima LX and SkyWatcher SWA) is the best of that line and very reasonably priced.  The Nagler T4 22mm is also quite nice, but tighter on eye relief.  If you want to stick with 1.25" only, the LVW 22mm is probably your best bet again.  If you don't need the eye relief, either 20mm Nagler (T2 or T5) is supposed to be quite nice, but again, 2" only.  The APM HDC XWA 20mm would also fall in this category.

[Alan White]

On 18/06/2020 at 08:59, Pixies said:

Could it be an exit pupil thing? I guess if your focal ratios have all been similar, the same EP focal length would mean a similar exit pupil. This would affect the sky brightness and, if the exit pupil, wasn't too large, would help with astigmatism?

Yes, I think that is the true reason behind my favoured focal length.

A moderate exit pupil does indeed make this focal length in my scopes work better with my astigmatism.

I do also seriously wonder if that original 20mm RK that was used for many years has almost worked in a muscle memory kind of way with my head. So my default finder view is 20 mm based?

Oh and it plays well with the local sky conditions and light pollution, so darkens the sky enough.

Edited June 20, 2020 by Alan White

[mikeDnight]

It could be the darkness of the sky background that balances so nicely with the magnification and field of view that makes it a magical focal length. The darkness of the sky background has a massive impact on contrast of DSO's, and peppered star fields. I loved my 20mm Nagler when used in a 4" or 5" refractor  of F8 or less. The Orion Nebula was at its most spectacular around the focal length in such scopes, with different levels of black nebulosity giving a real 3D impression as brighter nebulosity explodes from behind the blackness. The 17.5mm Morpheus is well worth adding to the list of potential contenders,

[F15Rules]

That's interesting Mike.

My best ever views of M42 (note: not the Trapezium, but the whole nebula) were through a Vixen LVW 22mm on a very good night.

The contrast was so amazing, jet black sky background, pinpoint stars all over and the detail within the Batwings and Fishes Mouth were just wonderful.

Also, the LVW barlowed very well, so with a good x2.25 Barlow I could turn the LVW into a very nice x100+ medium power unit but still with good eye relief.

I also have really liked other eyepieces of around this focal length: my Pentax 20mm XW was also very nice, but came a clear second vs the LVW22 in a prolonged head to head on M42. Also, I had several Ultima LX's a few years back and for me the 22mm was the pick of the range.

Slightly longer was the ES24mm 68 degree, again a lovely eyepiece. And now I have the Morpheus 17.5mm which offers 76deg and I'm so looking forward to M42 with it this winter.

I also found the Celestron Ultima Japan 18mm and equivalent pseudo Masuyamas to give great views.

Although each of the above had different main strengths, thinking about it now, they all offered a combination of comfortable viewing, nice eye relief, GREAT contrast and nice "framing" of the views combining context of the view and detail visible.

So I think Alan is right, the 20mm range + or - c 3 or 4mm, does seem to offer a real sweet spot for visual observers👍🙂

Dave

[John]

21mm is my favourite eyepiece focal length with my 12 inch dob. Add an 8mm to it and I can be happy for a whole session of Ethos magic

 

[Don Pensack]

Figure whatever eyepiece yields an exit pupil of 1.8-3mm is likely to yield the sharpest images in the scope.

It will be a small enough exit pupil to avoid the worst astigmatism in the eye, and a large enough exit pupil to avoid floaters inside the eye and/or poor seeing.

I used 8 different eyepieces on Thursday night of focal lengths from 6 to 30mm.

Not too surprisingly, my best views were through 11-14mm (1.9-2.4mm exit pupils).

[markse68]

Doesn’t it depend what you’re looking at though? I mean a 20mm gives me a lovely full moon disk but if I want to see crater details something shorter is needed? And Jupiter is pretty small in a 20. Perfectly formed and super contrasty but a bit small to see much detail

[Alan White]

5 hours ago, mikeDnight said:

The 17.5mm Morpheus is well worth adding to the list of potential contenders,

Funny enough it is on the list Mike.

[John]

14 minutes ago, markse68 said:

Doesn’t it depend what you’re looking at though?...

And the scope. In a 14 inch SCT the 20mm is a medium power eyepiece. In an ST80 it's a rather low power sweeper.

Best policy is to have lots of favourite eyepieces

 

 

 

[JeremyS]

You've got me thinking now @Alan White. I've recently been using the Nagler 20mm in my Tak TSA-120, with superb results. I've got longer FL eyepieces, but this one does seem to perform particularly well. I've been using it, for example, to observe a variable star and the combination of FOV, magnification and dark background it yields does seem to help in making the observations.

Also, whilst it's a 2-inch barrel, it's not as bulky or heavy as many.

[F15Rules]

2 hours ago, markse68 said:

Doesn’t it depend what you’re looking at though? I mean a 20mm gives me a lovely full moon disk but if I want to see crater details something shorter is needed? And Jupiter is pretty small in a 20. Perfectly formed and super contrasty but a bit small to see much detail

 

On 18/06/2020 at 08:29, Alan White said:

Any thoughts on this?   And any thoughts on options going forward for this one special slot?

 

Of course, you are right Mark. But the OP, Alan, was specifically asking for views/options at or around the 20mm mark 🙂.

It does depend a lot on your scope of choice as well, and which objects you're viewing..

In my own case, I'd say that the 2 eyepiece focal lengths I go for most often are at 10.5mm and 18mm (now 17.5mm having replaced my lovely Tak 18mm MC ortho with my sublime Morpheus 17.5mm), with quite regular use of higher powers (5 to 8mm, mainly doubles and Luna/planets), and low power (28-30mm) for clusters etc.

At this time of year, with very little actual darkness, I'd be happy to use any focal length under a properly dark sky! 🤣

Dave

Edited June 20, 2020 by F15Rules

[John]

1 hour ago, JeremyS said:

You've got me thinking now @Alan White. I've recently been using the Nagler 20mm in my Tak TSA-120, with superb results. I've got longer FL eyepieces, but this one does seem to perform particularly well. I've been using it, for example, to observe a variable star and the combination of FOV, magnification and dark background it yields does seem to help in making the observations.

Also, whilst it's a 2-inch barrel, it's not as bulky or heavy as many.

The Nagler T520mm was one of my favourite 2 inch eyepieces for quite a time. It was a wrench to let it go but I had to fund the 21mm Ethos somehow