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What is the better out of the two?

Daz Type-R

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Looking at either this....

William Optics SWAN eyepiece, 2" 33 or 40mm.


William Optics UWAN eyepiece, 2" 28mm.

Apart from the price and the size, what is the difference?

For getting a really good wide FOV for say, M31 or M45, which would be best?

Also, is there any alternatives out there?


Edited by Daz type-r
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I have both Swans - they were my first wide field low power eyepieces and I love 'em to bits. The 40 mm is superb with M31 - little details popping out all over the place on a good clear night - could look at it for hours in this ep without changing. The 33mm is fine too with globs and nebs. But you have to remember they are medium range and not to expect perfect flat fields to the edge of the fov.

The 28mm Uwan is a fabulous piece - I've used Docs many times in his 16" Lightbridge when he had it. It's probably the nearest you'll get to TV performance at a more reasonable price.

One alternative I'd suggest is the Panoptic 35mm - mines a beaut and I got it for £165 second hand when they just didn't seem to be selling. One of the best purchases I made - the clarity is several notches above the Swans as you'd expect. But I'm sure Mick will have more to say on the Uwan :)

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My thoughts on this are as follows...

I don't have first hand observing experience with these particular eps, but can analyse the numbers for you. I'm sure others will fill you in with good advice.

Here are the specs for use in your 200p...


Focal length mm 33 40 28

Magnification x 30 25 36

Apparent field degrees 72 70 82

Real field degrees 2.38 2.80 2.30

Exit pupil mm 6.6 8 5.6

Weight grams 330 570 1000

Price approx. £ 84 89 300

Your dark adapted eye pupil should be about 6.6mm at your age. So to fully utilise it, the 33mm SWAN with an exit pupil of 6.6mm should fully illuminate it. The 40mm SWAN has an exit pupil of 8mm, so a large proportion of the light focussed by that eyepiece would be wasted. The 40mm would not be suitable.

Both the SWAN 33mm and UWAN 28mm would give similarly wide field views of 2.38 and 2.30 degrees respectively. The UWAN 28mm should have give a better quality view with less edge distortion than the SWAN 33, but consider the cost and the weight. The 1 kilo UWAN will only work with a suitable counterweight.

Other eyepieces are available :)

Hope this helps.

I spent ages making that table of numbers look neat, but it's all over the place after posting :wacko:

Edited by petersull
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Thanks for the replys guys.

I was leaning more towards the 33mm SWAN anyway, discounting the UWAN on its weight but thought I may as well ask while I was at it.

I'll add it to the "list".


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Great analysis Pete - I'm glad someone can be arsed to do the numbers - I used to but now I just look through an ep and if I like the view I buy it. Allways glad someone like your good self pops the maths up though :)

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Thanks Kim.

I just love playing with the numbers, I have created spreadsheets to analyse it all to the nth degree. It helps me spend wisely and get the right tool for the job. Still haven't got the right tool yet though!!!

I like wide views. When I win the lottery, I'll have the full set of Ethos eps, plus the soon to be released Explore Scientific 25mm 100 degree ep. And I'd smuggle in an illegal Collins i3 image intensified ep from the US. Probably a £5 grand eyepiece set :o .

You know where I'm coming from :D .



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  • 2 years later...
Guest Live_Steam_Mad

Has the WO SWAN 40mm 72 dergree AFOV eyepiece got a sharp, black field stop (edge to the circle of light that you see when viewing through it) when used "standalone" (just holding it up to the light) ?. Or blurred edge (no field stop, uses the barrel as a field stop)?

Can I remove the rubber eyeguard completely from this WO SWAN 40mm eyepiece fairly easily? And replace it again easily? I tend to remove the rubber eye guards from my other ep's as I wear glasses for severe Astigmatism and I don't like smudging finger grease from the eye guard onto my glasses every time I bump into the rubber eye guard with my glasses.


I like the sound of the 23mm effective Eye Relief (28mm design Eye Relief) that I hear about for the 40mm SWAN, I am always bumping into my TeleVue 8mm Radian with it's 20mm ER when trying to see the whole field at once, and find the TV 32mm Plossl better in that regard with it's longer ER at 22mm and as a result I almost never bump into when seeing the whole FOV, and my SkyWatcher 28mm Kellner is perfect for it's ER and easy to use with glasses and I never bump into it at all and it's easy to see the whole 56 degree FOV at once. I seem to be unlucky as not only have a got a full dose of the Astronomer's Curse (Astigmatism, 3.5 Diopters of it) but I have somewhat recessed eyes and protruding nose compared to some other people who find the ER on the TV 8mm Radian to be just fine with glasses on, with the outer ring of the ep even raised by 2 clicks!


Best Regards,


Alistair G.

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I've never used a WO 40mm 72 degree, but a quick calculation gives a field stop of 50.2mm. This would tell me that in a 50.8mm OD barrel, then the barrel is the field stop and therefore it would be a bit fuzzy.

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