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Eyepiece type question.


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Although this concerns the eyepiece I'm using, it's also how the view with that eyepiece is represented in Stellarium.  If I've got this in the wrong section, my apologies.

My Skywatcher telescope is 150mm diameter and 750mm focal length.  I bought a 32mm plossl eyepiece for it.  I didn't pay a lot, it's a generic sort of EP, I figured on the lines of "no amount of fancy tyres on my Astra is going to make it run like a Ferrari", and it's been fine for me.

On the Amazon page it says this EP has a 52 degree FOV, and I've read that Plossls usually are around 50 degrees, so that seems fair.   I put that information into Stellarium so that I can use the 'ocular view' function.

However, what I see in ocular view, is wildly different from what i see through the scope.  Through the scope I see a much bigger field of view, many more stars around the central 'target' than I expect.  I tried a little experiment last night, I altered the FOV setting in Stellarium, increasing it until ocular view matched what I could see through the eyepiece, and i was surprised to find that i had to put in 90 degrees to achieve that, and suspect I could have gone higher.

Any ideas what's going on?  It's obviously not a problem, I can live with it, but if I'm missing something I'd like to know what it is.

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Assuming that your 32mm plossl is a 1.25 inch eyepiece, 52 degrees is the largest apparent field of view that a 1.25" eyepiece of 32mm focal length can have. The field stop (which defines the edge of the field of view) can't physically be any bigger within the constraints of the eyepiece barrel internal diameter.

So I suspect the issue is with the way that Stellarium is representing the view. I don't use that feature though so I'm not sure what the problem could be :icon_scratch:

 

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3 hours ago, bomberbaz said:

Have you got the scope set up correctly and/or not selected a barlow along with it.

Simple I know but easily done.

No, nothing like that.  :)

 

And above that, yes, it's 1.25" eyepiece, and i accept that it can't be magic or breaking the laws of physics.  So I guess I'm really asking what am i putting into Stellarium that makes it wrong.

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3 minutes ago, Capt Slog said:

No, nothing like that.  :)

 

And above that, yes, it's 1.25" eyepiece, and i accept that it can't be magic or breaking the laws of physics.  So I guess I'm really asking what am i putting into Stellarium that makes it wrong.

Sorry what I meant was putting the scope details into stellarium incorrectly.

Add some screen prints maybe if you like!

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I have an F/5 150mm, and a 32mm 52°AFOV eyepiece shows in Stellarium like this:

image.png.ec8d883fac40b4ed884f5030c8486a24.png

The Eyepiece Planner website gives this for your scope/EP combination:

image.thumb.png.e0fc4945727eb870493fe5b5d00ff1c4.png

calculating a 2.2° TFOV based on the 52° AFOV. 
However that's optimistic, as @John says above it's going to be limited by the 1.25" barrel format. The maximum field stop is around 27mm, not the 29mm that the web site has calculated.

Stellarium will presumably do a similar calculation if you set up your EP with the focal length and AFOV, but you can enter the field stop diameter if you know it. When I put a 27mm field stop for a 32mm eyepiece, Stellarium showed this:

image.png.132a1edd7a300a6698ca69046d5b3949.png

which is slightly smaller. If you don't know the actual field stop diameter, the AFOV<>TFOV calculation is imprecise due to distortions at the edge of the field.

But if you really have discovered a 90° AFOV Plossl, you might want to let us all in on it 🙂

 

 

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28 minutes ago, Zermelo said:

But if you really have discovered a 90° AFOV Plossl, you might want to let us all in on it 🙂

It would probably perform even worse than my 29mm 90° AFOV Rini Modified Plossl (possibly a 3 doublet (6 elements) Erfle variant in reality).  It's the one second from the left below and second one down in the FOV images.

1503910180_29mm-30mm.thumb.JPG.beb0e0b0d494a0fb027e38e2a180acef.JPG1270098715_29mm-30mmAFOV.thumb.jpg.b72cf50a97eb28a4217fd5188677c85a.jpg

It measures out to have a legitimate 90° AFOV photographically.  Projection yields a wider and more inaccurate AFOV measurement because the lower barrel forms the field stop, and the projected circle gets quite fuzzy and indistinct near the edges, so I won't list it here.  Usable eye relief is 10mm, but it feels much less in use.  The edge is a blurry mess of astigmatism and other aberrations, so it is of limited utility.  I didn't pay much for it from Paul himself back in the day, so I keep it as a rather unique, hand crafted eyepiece against which to compare other UWA/HWA eyepieces.

It's ironic that it performs about as well or better than the 30mm Kasai Super WideView 90° (which is really only 83° AFOV) and cost over $400 20 years ago.  It was a bad 30mm Leitz 88° copy, was priced at a fraction of it, and was still a terrible deal.

One plus for the Rini is that it somehow has no chromatic aberration across the field while the Kasai has loads starting just off axis.  Check out the neat rainbow effect in the edge view of the Kasai.  I don't know of another eyepiece with that much chromatic aberration.

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13 hours ago, bomberbaz said:

Sorry what I meant was putting the scope details into stellarium incorrectly.

Add some screen prints maybe if you like!

It's with considerable embarrassment that I have to say you've got it!  Well, almost.

I've just had a check and it appears that at some point in the evening, I managed to 'catch' the "Lens" function in the ocular section, and had inadvertently been using it at 2x. 🙄  If you think it should have occured to me that I'd never noticed a problem before, well yes it should, but there's always been a mismatch although not that pronounced.

 

I use the PC by having the screen in the window of my shed, and facing outwards.  The mouse is on a little folding shelf below.  It's not ideal, and obviously I sometimes manage to get it wrong.

Thanks for the input, all. 

 

EDIT  Interesting site, @Zermelo, and I'll use that stop info to adjust the view in future.

Edited by Capt Slog
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