Eyepieces for WO Binoviewer

[gfletcher]

Hi Guys

Having just acquired a secondhand WO Binoviewer complete with 2off  20mm WA eyepieces I was wondering whether to purchase a 2nd Skywatcher 32mm SP Plossl eyepiece to give me a larger FOV and less magnification. (I already own one of the 32mm eyepieces)

 

Has anyone used 32mm eyepieces in a binoviewer with  an SCT , if so , what are your opinions ? 

 

thanks

 

Graham

[Tweedledee]

As far as I know, the clear aperture of the WO binoviewer is about 22mm, but the field stop on the 32mm plossl is 29mm. You may see some vignetting on the 32mm eyepieces because their field stop is larger than the clear aperture of the binoviewers. This may be seen as a slight dimming of the view towards the outer edge of the field of view, but usually this is a very minor effect that you may not even notice.

 

The 32mm 52 degree plossls will give you a 26% wider field of view over and above your WO 20mm WA 66 degree eyepieces.

 

Your relatively long focus SCT at F10 with its massive focusing range means that there should be no focussing issues with the combination.

 

I have never used a binoviewer, but have used an SCT, so I am only going by the numbers as per the above. Someone else may have actually tried this combination, and will comment.

 

In the meantime, you could always get the scope out with the binoviewer and your existing single 32mm ep and just make sure the eyepiece works OK and look for any vignetting, before deciding to buy another.

 

The 32mm plossl is not an expensive eyepiece, so I would give it a go and you will probably enjoy the wider view.

 

Hope this helps.

[gfletcher]

Many thanks for that info Tweedledee

It will be very useful to me.

I knew with the wealth of knowledge available on this forum that I would receive good advice

 

Graham

[+Bino-viewer]

Hi Graham,

 

As Pete says, try the single 32mm first in your bino.

My guess is you won't like the view much. Theres going to be a lot of vignetting going on. I may be wrong though.....only going by what i've read.

 

The 20mm WO eyepieces are a great match for that Bino. I'd go for a higher power pair, rather than thinking about wider fields etc.

And this is in the 15mm area.

 

Bottom line is, with an SCT (at F/10) you're tied into around 100x mag low power, with quite a narrow field of view.

I've used an 8" SCT and have had great views, particularly Lunar using a pair of 19mm Panoptics.

Main thing is, with an SCT you need to keep the lightpath as short as you can (2" diagonals are a   )

You'll still reach focus, but you'll get a dim, over magnified view, with maybe an inch or so of actual aperture reduction.

 

Binoviewing is great : i love the views as i'm sure you will. But things can get very pricy if you want premium wide field views.

Let us know how you get on with the WO's.....

[gfletcher]

Thanks for the info Bino-Viewer.

I will let you know how I get on with the WO eyepieces.

[Tweedledee]

Rob, as I'm sure you know, your superb Baader Mk V Großfeld binoviewer has a 29mm clear aperture which would work far better with wide field eyepieces having larger field lenses.

 

These Siebert Optics ones are nice, and will allow the widest possible fields of view with any binoviewer. They take 2" eyepieces and have a clear aperture of 45mm! However, they wouldn't work with some of my fat eyepieces, unless I could move my eyes further apart     ...

 

http://www.siebertoptics.com/SiebertOptics-2inchbinoviewers.html

[gfletcher]

Would a pair of Celestron X-cel LX  25mm eyepieces be a better choice in my WO Binoviewers?

 

 

I do not know the size of the field stop in these eyepieces

 

 

Graham

[Tweedledee]

I calculate the field stop on the 25mm 60 degree X cel to be 26mm and the field stop on the 20mm 66 degree WA to be 23mm.

They should work fine and may or may not show some slight vignetting, but you will only get a 13% increase in real field of view over the 20mm WA.

[Tweedledee]

As per Bino-viewer Rob says above, the WO 20mm WA's are nicely matched to their binoviewer as probably the widest angle (largest field stop) they recommend for it. This is due to the fact that a larger field stop will cause vignetting, which is more noticeable and detrimental to some people than others. Going the other way, as Rob suggests, with shorter focal length eyepieces and higher magnification doesn't necessarily mean a narrower field of view.

 

For instance, the following eyepieces would provide exactly the same field of view (have the same diameter field stop) as your 20mm 66 degree WOs with higher magnification, whilst having a wider apparent field of view. This is a bit like the difference between watching a movie on on your laptop or on a big screen television.

 

16mm 82 degree eyepieces would give a magnification increase of 1.25x.

13mm 100 degree eyepieces would give a magnification increase of 1.54x.

 

The 100 degree eyepieces are rather expensive, but there are a variety of different makes of 82 degree eyepieces going for very reasonable prices secondhand and new.

 

I gather you can get breathtaking three dimensional bino views of globular clusters. These are relatively small objects which need a higher power to make them large enough to get the effect.

 

Be aware though, that if you need to wear your glasses at the eyepiece, this will likely prevent you from taking in the full field of view of 82 degree or larger eyepieces.

 

Lots of pros and cons to weigh up .

Edited January 8, 2015 by Tweedledee

[+Bino-viewer]

I think its worth noting that light pollution makes this vignetting situation a much worse problem.

 

My site here in Derby is so bad (i reckon an 8.5 out of 10 on my 'light pollution scale')  A 10 would be horrific.

So if you were at a dark site you may not even notice a small amount of vignetting, but with my LP its easily noticeable, and for me personally, a deal-breaker.

 

I also agree with the masses that a field of view in the 60-70 degree range it just right (hits the sweetspot) when bino-viewing.

I much prefer my Panoptics with their 68* FOV to my 13 Naglers 82*

The 82* is nice at high power when viewing Planets (especially with an unpowered alt-az mount) but like others have reported, i do get 'black-out' issues.

One of the best things about using both eyes when observing is the viewing comfort it provides. Trying to take too much in with an 82* FOV hampers this comfort.

 

I'm sure the Siebert gear is nice ( Have you ever tried to make sense of their website ?   )

But using 2 inch eyepieces...?  You'd need a hoist with all that weight 

[Tweedledee]

A bright sky background would certainly show up any vignetting more easily.

I suppose some people see the Nagler blackouts more than others. I had several Naglers without being trouble by blackouts.

People will see a lot more if they are comfortable at the eyepiece and with their equipment, so there is really no substitute for trying before buying.

I agree about the Siebert website, it could really do with being brought into the 21st century, I still haven't found all the hidden links to different parts of the site

They do some massive observatory series very long focal length eps in barrels up to 4.3". There is a photo of one on a giant long focal length refractor at Lick obsy, which is what they are designed for. They do a nice alternative to TV Powermates in a bigger range of amplifications at a reasonable price. They are true telecentric amplifiers like Powermates but with slightly larger apertures (less chance of vignetting with big eps) but still in a 2" barrel. I do like the good reports I've read on their amplifiers.