40mm 1.25" EP

[John]

1 hour ago, Dave In Vermont said:

I have a very good 40mm Plossl from Orion-USA from 2003. My only suggestion is not to spend a fortune on one. Gosky is something I've never encountered, but likely is made by something like Synta or GSO. But I'll drop some good information you may find of use below:

40mm University Optics 43° E.R. 28mm Plossl Eyepiece.pdf

I'm not sure when this was published, but it's a good template of what a 40mm Plossl is likely to be - regarding specifications.

Have fun -

Dave

Looks like promotional info for University Optics. They import stuff from Asia and re-brand it just like so many others. Some of their recent lines have been rather odd choices IMHO. I guess they were targetting the Tele Vue 40mm plossl with that - it's the only 40mm plossl that I can think of that might have been priced at $160 at some point. The Meade 4000 40mm might have been getting on for that though.

Al Nagler said that the reason Tele Vue introduced their 40mm 1.25" plossl was to cater for the person who wore glasses when observing. Otherwise he recommended their 32mm plossl instead.

I did have the 40mm TV plossl for a while. To facilitate the eyerelief it's not par-focal with the rest of the TV plossls. It's well made of course but I found I hardly used it - I think it was an OCD driven purchase to complete my set of TV plossls to be honest !

As you can pick up regular decent 40mm plossls for £peanuts you can have them in the eyepiece case easily enough. I think one of the better ones is the Japanese made Meade 3000 series 40mm plossl and you can get those for around £30 on the used market.

 

[John]

2 minutes ago, YKSE said:

John,

I think you've kept 31T just for Veil in vixen?

How's the Veil looks like in 13E compare to 31T in Vixen?

When we get a clear, dark night again I'll give it a try. It's been over a year since I observed the Veil so I don't want to rely on memories.

 

[Don Pensack]

If you're familiar with apparent field in an eyepiece, the maximum apparent field in a 1.25" eyepieces is about

40° at 40mm

50° at 32mm

68° at 24mm

82° at 18mm (though the longest 1.25" 82° is currently 16mm)

100° at 14mm (though the longest 1.25" 100° eyepiece is currently 13mm)

What is the appropriate maximum focal length for a widest field?  I'd say multiply the f/ratio of the scope by 6 and that's about it.  The OP said his scope is f/4.7, so the longest focal length would be a 28mm.

Given that and other characteristics, the 1.25" 24mm 68° becomes a good choice for a low power if 1.25" eyepieces are used.

On a refractor, there is no minimum because there is no secondary shadow that becomes obtrusive when the exit pupil gets too large.

For 2" eyepieces, there are some 28mm 82° eyepieces out there that would be superb as a low power.

[Stu]

On 8 June 2016 at 12:02, John said:

I can't argue with the logic Yong but I've just not found that it works that way, for me, in practice

99% of the time I view from my back yard though and I do have some light polution to contend with.

 

I know you have used SCTs in the past, but I'm less aware of your experience with Maks. Most of your scopes have been faster than f10, not f15 or so. It makes a huge difference and I'm afraid to say, in line with Yong's comments, it is down to the smaller exit pupils achieved with longer focal length scopes. I used to use a 55mm TV Plossl in my f20 Mak but wouldn't dream of using one in scopes less than f12 or so. I think it's important to acknowledge these differences even if they are outside your experience.

As an example, a 55mm Plossl in my old f20 Mak gave an exit pupil of 2.75mm, good for a number of applications and it kept a nice dark sky background. The same eyepiece in your f5.3 newt would give a 10.4mm exit pupil. It's obvious why you wouldn't use one, but it does not mean they are not useful in circumstances you are not familiar with.

[Stu]

Don's formula basically calculates the focal length which gives you an exit pupil of 6mm. On that basis, an f15 scope could use a 90mm f/l ep and an f20 scope could use a 120mm f/l ep. Of course the vignetting would be horrible and the whole experience would be expensive and unsuccessful, but it does show that if you want a slightly larger exit pupil in a Mak or long focal ratio refractor, some of these longer focal length eye pieces do have a place. Given the choice I would take a 41mm Panoptic, but if price is an issue, the 40mm Plossl is a much cheaper solution.

[John]

8 hours ago, Stu said:

I know you have used SCTs in the past, but I'm less aware of your experience with Maks. Most of your scopes have been faster than f10, not f15 or so. It makes a huge difference and I'm afraid to say, in line with Yong's comments, it is down to the smaller exit pupils achieved with longer focal length scopes. I used to use a 55mm TV Plossl in my f20 Mak but wouldn't dream of using one in scopes less than f12 or so. I think it's important to acknowledge these differences even if they are outside your experience.

As an example, a 55mm Plossl in my old f20 Mak gave an exit pupil of 2.75mm, good for a number of applications and it kept a nice dark sky background. The same eyepiece in your f5.3 newt would give a 10.4mm exit pupil. It's obvious why you wouldn't use one, but it does not mean they are not useful in circumstances you are not familiar with.

Thanks Stu

It's difficult realising what you don't know sometimes.

Apologies to the original poster (curtis) if my posts in this thread have confused or misled

[Mak the Night]

A 40mm Celestron (GSO) EP is one of the standard giveaways with big Celestron SCT's. It's the only real way to get an exit pupil of 4mm.

I've used a 40mm TeleVue Plossl to achieve a 3.1mm exit pupil on a 102mm Mak. FWIW I can get a 3.5mm exit pupil on a 90mm Mak with a 40mm 1.25" Plossl and almost 5.8mm on an f/6.9 Newtonian. 40mm Plossls can take some getting used to, with their long eye relief and limited FOV, but they are inherently useful.

[capncurt86]

Thanks all!  It seems that y'all are converging on an answer.

I really had no intention of buying a 40mm 1.25" EP, I just could not understand why they are made.  I was thinking that it was a scam to get newbies like me to buy a 40mm because it is cheaper than a 40mm 2" or because someone's scope doesn't accept 2" EPs.  But it seems to have its place when exit pupil is the name of the game.  I'm glad I understand  now if I decide that I need a better view of a DSO the big thing is exit pupil which I can get with long focal length EPs with smaller FOVs.

[Mak the Night]

Yes, 40mm f/l, (31.75mm/1.25") Plossls can be quite useful on slow (high focal ratio) scopes. They can enable the use of UHC filters to observe diffuse nebulas (like M42) more effectively.