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The Wollensak 51mm 2" Plossl... not bad!

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#1 magic612

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Posted 18 November 2010 - 11:42 PM

So I was ordering a few telescope making components from Surplus Shed, and on a whim, went ahead and ordered this Wollensak 51mm Plossl eyepiece. I suppose it wasn't all that much of a whim, as I'd been wanting a long focal length 2" Plossl - it's just got a longer focal length than I was looking for. It's a 51mm f/l, 2" barrel Plossl EP, and Surplus Shed lists the specs as a 60 degree AFOV, 35mm exit aperture and 35mm eye relief.

Okay, sure, so light is "wasted" because the exit aperture is enormous. And yes, the eye relief is almost uncomfortably long. Almost.

But here's the thing: It gives HUGE wide field views, and tack-sharp images in a short f-ratio scope. I got the eyepiece yesterday, and although the Moon is bright and there were some high, hazy clouds tonight, I brought out my 72mm Megrez William Optics apo to try out this EP, and compare it to the 42mm GSO Kellner I also own.

It was NO contest - the Plossl clearly won. Sharper image across the field, slightly wider TFOV, and although the sky background was darker in the 42mm, I felt like I could see more in the 51mm because the image was sharper across far more of the field.

Some of you are thinking, "Why get such a long F/L eyepiece?" Well, in the WO scope, I could have hand-held it if I'd wanted (I had it on a sturdy photo tripd). At 432mm F/L, my magnification was about 8.5, with a FOV of, I'd guess about 5 degrees. That's binoculars-range FOV! I could easily get all three Orion belt stars with plenty of room on either side. The entire Hyades AND Aldebaran. The Alpha Perseus region was a lovely view, and the Pleiades looked strangely small, but with a more stars visible than I normally see with hand-held binocs (perhaps due to the steady image thanks to the tripod).

Now I've got some other plans for this EP: I have a 127mm f/4.4 refractor. I haven't take that out yet, but at 11x and with a FOV that will likely rival the William Optics scope, I'm betting that even more stars will pop out in those same regions. Now put that same EP in a 10" f4.7 Dob - better-corrected image, and I've now got a 23x magnification with a (likely) 2 degree or so field - full Pleiades in a 10" scope anyone? I think that would be amazing - especially for an EP that only costs $52!

So for those of you on a budget (like me), here's a long F/L eyepiece that's worth considering.

#2 Arizona-Ken  Happy Birthday!

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Posted 19 November 2010 - 12:20 AM

Never heard of it ... thanks for the post!

Arizona Ken

#3 magic612

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Posted 19 November 2010 - 10:17 AM

You're welcome Ken. I was not terribly keen on such a long focal length eyepiece previously, but so far, I'm glad I got it.

Should be really interesting when I get the 2" focuser put in my 90mm f/10 refractor. 18x magnification with a 3+ degree field. That'll make a nice grab-n-go set up... :watching:

#4 Pasiukov

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Posted 19 November 2010 - 01:05 PM

My experience with such long focal lenghts in a fast reflector are bad. Not because there is lights loss, but because the secondairy mirror IN the exit pupil becomes so large, it shows up as a black spot in your view. I've tried a 50mm eyepiece in an F5 dob, so an exitpupil of 10mm. Even with a 40mm paragon wich gives an exitpupil of ''only'' 8mm the effect was still disturbing.

#5 magic612

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Posted 19 November 2010 - 01:28 PM

Pasiukov - that's a good point I neglected to consider for my reflectors. When I read your post, I remembered that using a 32mm Plossl on my little Celestron Firstscope (a 76mm f/4) produces a black spot to some degree. So I'll have to check on my 10" Dob and see what happens.

Fortunately my refractors won't have that issue. :)

#6 obin robinson

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Posted 30 January 2014 - 07:07 AM

I just wanted to say thanks for this review! I picked up this eyepiece during the last Surplus Shed sale. I was eager to try it out in my telescopes. Last night after imaging I dropped it into the focuser of my 4.5" and was amazed! The eyepiece performed quite well. The views of the Orion Nebula were among the best I've ever seen through that telescope. The only problem was that bright objects like Jupiter overwhelmed the eyepiece off axis. My solution: stick to dimmer objects.

This eyepiece is one you use for wide field views, not planets. It works great and I am glad I bought it.

obin :jump:

#7 Jon Isaacs

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Posted 30 January 2014 - 09:20 AM

Pasiukov - that's a good point I neglected to consider for my reflectors. When I read your post, I remembered that using a 32mm Plossl on my little Celestron Firstscope (a 76mm f/4) produces a black spot to some degree. So I'll have to check on my 10" Dob and see what happens.

Fortunately my refractors won't have that issue. :)


Your XT-10 has a CO of 25%. If your eye opens to 7mm, then the 25% CO becomes a 39% CO. The effective aperture is reduced from 10 inches to about 6.5 inches.. there will be some slight dimming, about 10% due to the CO and it is very likely you will see a dark spot.

The issue of TFoV versus magnification and reduced aperture can be looked at in a number of ways. In this situation, what it means is that your view will be the same as 6.5 inch F/7.3 Newtonian with a 39% CO, 24x with 2.2 degree TFoV. It will show the Pleiades but it won't really be a 10 inch scope. It won't be a lot different than your 127mm F/4.4 with a different eyepiece and a 6 mm exit pupil except that the 127mm can provide a much wider TFoV.

It's a $50 eyepiece and it buys you a wide field. An expensive 30-31mm Ultrawide field could provide a 2.0 degree TFoV at 40x with a reasonable 6.4mm exit pupil. That view would be the Pleiades in a 10 inch scope.

Just sayin'

Jon

#8 starcanoe

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Posted 30 January 2014 - 10:57 AM


Nice review.

I too saw that at Surplus shed. 50 bucks for a so so plossl is good. For a very good plossl thats a steal.

You'll love the view with the 90mm F10 refractor. Back in the day I was pretty impressed using my department store Jason 60mm with a surplus 50mm eyepiece.

Good find and good for ya.

#9 JustaBoy

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Posted 30 January 2014 - 12:01 PM

Does this eyepiece have a field stop?

Thanks,

#10 youngamateur42

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Posted 30 January 2014 - 12:42 PM

If I'm not mistaken, Russell optics has 1.25" and 2" 50mm Plossls, including eyepieces in the 80mm range.

#11 Vondragonnoggin

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Posted 30 January 2014 - 01:22 PM

Thanks for the heads up Dave. just ordered one for my F/12 MCT

That's about a 1.7 degree TFOV and 35x for my scope. 4.3mm exit pupil. Cool.

#12 buddyjesus

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Posted 30 January 2014 - 05:10 PM

I have been curious for quite some time about this eyepiece. good to hear it is a keeper. I plan on using it for the galileo program and for sketching finder fields.

#13 careysub

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Posted 30 January 2014 - 06:15 PM

I just wanted to say thanks for this review! I picked up this eyepiece during the last Surplus Shed sale. I was eager to try it out in my telescopes. Last night after imaging I dropped it into the focuser of my 4.5" and was amazed! The eyepiece performed quite well. The views of the Orion Nebula were among the best I've ever seen through that telescope. The only problem was that bright objects like Jupiter overwhelmed the eyepiece off axis. My solution: stick to dimmer objects.

This eyepiece is one you use for wide field views, not planets. It works great and I am glad I bought it.

obin :jump:


I have one of these - I picked it up to max out my TFOV with my Celestron C8 (F/10) at a low price, and it was a great bargain. I was very pleased with its performance, and it does the same for my inexpensive Celestron 102GT (also F/10).

It is a great complement to both instruments.

It also works great with other inexpensive small F/7 - F/15 achromatic lenses available from Surplus Shed or Sheldon Faworski (on the CN Classifieds), which require little more than 2" PVC pipe to make a functioning telescope.

#14 Scanning4Comets

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Posted 30 January 2014 - 10:11 PM

Here's M-42 in a 38mm eyepiece in a 10" F/4.7 scope. A lot of people over-emphasize how useless large exit pupils are, but hey, if you like what you see, then use it !!! :waytogo:

My 38mm is already pushing it, but here is what I see, (and this is cropped). In this eyepiece, there is a lot of room to spare. The 38mm Orion Q70 shows off axis astigmatism, but it also has easy eye placement, can be used with glasses and there are no blackouts AT ALL. It plays extremely well with filters too. And it is cheap!

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#15 Scanning4Comets

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Posted 30 January 2014 - 10:12 PM

Here is the moon with the same eyepiece using afocal photography:

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#16 BillP

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Posted 30 January 2014 - 11:00 PM

Pasiukov - that's a good point I neglected to consider for my reflectors. When I read your post, I remembered that using a 32mm Plossl on my little Celestron Firstscope (a 76mm f/4) produces a black spot to some degree. So I'll have to check on my 10" Dob and see what happens.

Fortunately my refractors won't have that issue. :)


FWIW, I often use a 40mm EP in my XT10 and enjoy its view quite a bit even though the exit pupil is over 8mm. When I had a 56mm Plossl it was fine in my XT10 also and I never saw the CO in the view unless observing the full moon. So was just fine on star fields and great wat to get wide views. Things dim a little withe the large exit pupils but that is to be expected due to light loss since your eye's pupil does not open that wide. But bottom line is that it works and can be quite enjoyable observing, especially for M45 which is bright.

#17 starcanoe

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Posted 31 January 2014 - 09:49 AM

Magic.

If it turns out you really like the views in your 90 f10, consider the classic 6" F8 dob.

That will give you 1.2 times the magnification as the 90mm but 2.5 times as much light. You can use a 1.75" diagonal without much trouble and have little to no light loss at the edge of the field . And if you are careful you can use a 1.5.

But lets not get bogged down in the exact details of that. Just know its quite workable.

So pros to doing this.

More light.
Eyepiece on the right end of the scope.
Perfect size for grab and go or public stargaze.
Basic dob easy and quick to build.
If you dedicate this scope for low power viewing you don't need to mess with fiddly fine adjustments, just build the alignment in.
Optics are cheap. 6" F8s on ebay can be found for $70 give or take.
Later you can work on making the basic scope a high performance model that kicks butt on the moon and planets.

And with a 6.4 exit pupil and a 1.75 diagonal the "shadow" of the diagonal shouldn't be a problem visually and it is small enough to not be blocking a large fraction of the available light.
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#18 EuropaWill

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Posted 03 February 2014 - 08:04 PM

Does this eyepiece have a field stop?

Thanks,


Hi Chuck, No I don't believe it has a field stop. When I had one of these a little while back, it was barrel limited. It also seemed like a 50* AFOV in the unit I had, not the claimed "around 60*". I was however, impressed with the small size and light weight of the unit.

#19 careysub

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Posted 04 February 2014 - 09:37 AM

Does this eyepiece have a field stop?

Thanks,


Hi Chuck, No I don't believe it has a field stop. When I had one of these a little while back, it was barrel limited. It also seemed like a 50* AFOV in the unit I had, not the claimed "around 60*". I was however, impressed with the small size and light weight of the unit.


That is correct - the "field stop" is the barrel. Using the simple FS formula from the quoted AFOV of 50 degrees you get a "field stop" of 44.5mm, whereas the maximum achievable in the 2" format is said to be 46mm.

#20 Sarkikos

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Posted 04 February 2014 - 10:25 AM

The Wollensak 51mm is out of stock now at Surplus Shed. How many CNers bought one?

Mike

#21 Vondragonnoggin

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Posted 07 February 2014 - 02:26 PM

haha - This has to be the cheapest looking construction and lightest weight of any of my eyepieces. Looks to be primarily PVC (I could be wrong as it's already back in the box because I'm at work). Glass looks good though. It was cheap and I'll make no permanent judgement until I use it....

#22 Dave Mitsky

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Posted 07 February 2014 - 07:54 PM

This is a bit off topic I suppose but I can attest after many years of observing that the 40mm University Optics MK-70 (162x, 2.7mm exit pupil) that produces essentially the same true field of view as a 55mm University Optics Plössl (118x, 3.7mm exit pupil) is far more pleasurable and useful to employ with the Astronomical Society of Harrisburg's 17" f/15 classical Cassegrain. The 70 degree apparent field of view is more engaging, the "contrast" is better, and the eye relief is much more comfortable. The same applies with the club's two 14" f/10 SCTs and even more so with its 12.5" f/6.5 Cave Astrola Newtonian.

Dave Mitsky

#23 Vondragonnoggin

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Posted 08 February 2014 - 04:47 AM

I just checked the Surplus Shed pictures of this and it doesn't look like mine. The pics at Surplus Shed show a flat top and eyelens is barely recessed. Mine has about 3/4 " of tube above eyelens.

Think I'll be calling them tomorrow and probably sending this back.

Bunch of glue on the lens too around the edge.

:confused:


Just called them and they said sometimes they change the housing and a lot of people preferred the recessed eyelens because of the long eye relief.

They said to try it out and if I don't like it, I can return it.

At least I can give it a whirl before shipping back.

#24 edwinh

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Posted 09 February 2014 - 09:32 PM

I have one of these from awhile back just for fun. It does have what seems like a mile of ER which makes it harder to hold the image with, and hard to use if there is ambient light nearby. That recessed lens one might be better...

#25 EuropaWill

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Posted 10 February 2014 - 02:56 PM

I just ordered the newer "Low Cost" 51mm Plossl that claims 20mm Eye Relief and has a volcano top design with smaller eyelens. I don't have high expectations but as a lightweight largest TFOV EP, it might find some use. I was going to try the 6-element 48mm but it was out of stock. Wish me luck!


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