Eye relief vs eye lens diameter

[jbloggz]

I recently bought my first telescope (second hand 12" dob) and it came with a handful of (pretty average) eyepieces.

I'd like to now put some money into a couple of decent eyepieces and decided to first get a high power one so I can get a good look at the planets (saturn and jupiter especially) as well as the moon.

I was thinking that the 7mm william optics UWAN would be a good choice since it's seems good value (~$200) and has a large FOV (82 degrees) which I imagine will help with manual tracking of the planets.

Then I found the following info about it:
Eye relief = 16mm
Eye lens diameter = ~20mm

After a bit of trigonometry, this doesn't quite add up..... It's not possible to see the entire FOV at the eye relief distance. You can only see about 64 degrees before hitting the edge of the eye lens. So you would have to move your head around to see the entire FOV???

Is this common with large FOV eyepieces? It seems the longer focal length ones have much larger eye lenses, so prehaps it's just an issue with shorter focal lengths. Why can't they have bigger eye lenses? Ideally I'd love to be able to position my head in a single spot and just move my eye to see the entire FOV. This would give a much more immersive feeling.

Thanks to anyone who can shed some light on this.

[janoskiss]

You're right and your maths checks out. Yes for sure the eye relief is overstated. And possibly the FOV is as well.(?) All but the most conscientious manufacturers will have some bogus info in their specs, so sadly you cannot take the specs on most EPs at face value.

[jbloggz]

Thanks for the reply.
So would this mean I could simply move my eye closer to the lens and I would get the full 82 degrees (about 11.5mm would give the right angle)?

Is there any downside to being closer than the eye relief? Is 11.5mm considered to be a comfortable distance for someone without glasses?

[janoskiss]

Yes. It just means that the eye relief is overstated so you have to get your eye closer to the eye lens. If you don't wear glasses ~11mm of eye relief is still okay.

IME Pentax and few other lesser known Japanese manufacturers have been the only ones to deliver what the spec sheet promised. Maybe it's a Japanese thing..? Televue is not bad either but they do stretch the truth a little sometimes.

[jbloggz]

Thanks for your expertise Steve. I think I can live with moving that much closer to the lens.

Another thing I was hoping to do with this eyepiece is add a 2x barlow to give me 3.5mm focal length (~428x magnification), which would be great when the seeing conditions allowed. I read a review on this particular eyepiece that said:

"adding a barlow increased blackout to uncomfortable levels"

Any idea why a barlow would do this? Is it something to do with the barlow increasing the eye relief? I've seem mixed comments about whether barlows do this or not

[janoskiss]

Btw...

:welcome: to IIS!

[janoskiss]

Blackout is not an issue with short focal length EPs. Barlow issues only show up above 20mm. For me personally, I can see the problem, but I can also move my head back, so it's no big deal.

But you'll almost never be able to use a 2x barlowed 7mm in your scope because of turbulence in the atmosphere that almost always limits you to <300x (web search: "seeing"). Also note that most 2x barlows are in fact about 2.1-2.2x.

Re whether you'd be happy with 10mm of eye relief is a matter of personal preference and your physiology. If you're not experienced with different types of EPs, you should try some at a star party or local club gathering (or maybe even hook up with an IIS member in your area).

I assume you have an f/5 12" Dob. I used to have one of those, a GSO solid tube. It was a nice scope: collects lots of light, a bit big and cumbersome to move around, but still fits in a small hatchback for deep sky weekend getaways. The 7mm UWAN is not the most useful EP for it. But if you can afford it, by all means get it.

The way I learnt what works best for me is by buying and selling a lot of EPs (many 10s, maybe ~100+) and going to star parties. The fact that you're thinking about the problem of EP selection mathematically (ie, you're able to calculate max theoretical FOV given eye relief and lens diameter) puts you well ahead of most amateur astronomers. Just keep in mind that the ultimate proof is in the pudding. And trying other people's EPs at star parties or astro club observing nights is the quickest and cheapest way to find out what works best for you.

[wavelandscott]

I would echo Janoskiss's advice...get to a star party/viewing night and look through some different eyepieces to find out what you like/don't like in gear.

One person's gem is sometimes another person' lump of coal...

There is no single "best" eyepiece ...just different designs that have different trade offs and imperfections. Once you find the most important factors to you the selection process gets narrower but not strictly easier.

[Don Pensack]

As for using an eyepiece, the general rule is to approach the eyepiece until you can *just* see the edge of the field stop with your peripheral vision.
Then you stop and do not get closer to the eyepiece than that. If you do, some of the light will be intercepted by the iris of your eye and you will see the edges of the field black out.
So if you start seeing blackouts, back up just a trace and they will go away.

With a really wide field, you'll roll your head over to the side to look directly at the edge of the field. This keeps your eye's pupil in the same location as the exit pupil of the eyepiece, but allows you to look at the edge of the field with direct vision.

[jbloggz]

Thanks for the advice everyone.

I live in Karalee (near ipswich, west of Brisbane). Can anyone recommend a good place to start meeting people and viewing nights in the brisbane/ipswich area? Hopefully there will be some out there that don't mind helping out beginners.

Even if there's someone who lives nearby that's willing to show my how to get the best out of my new toy. I've read plenty of info online, but it's not the same as someone actually showing you with your telescope.

[janoskiss]

Not in your area exactly, but only a short drive away, maybe try one of Astro Pete's viewing nights or monthly getaways http://astropetes.com.au/