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Hungrymark

Planetary nebulae - what's the knack?

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On 1/18/2018 at 13:42, Merlin66 said:

No, that would be the wrong prism....you need a 60 deg equilateral prism.

http://www.ealingcatalog.com/30-mm-equilateral-prism-bk7-glass.html

The prices are pretty high... I'd recommend trying Surplus Shed; they always have loads of neat stuff cheap.

https://www.surplusshed.com/pages/item/L1851D.html

There are also some flint (F2) prisms which are even better.

The last image shows the prism sitting on a TV eyepiece with the "viewing port" towards you.

 

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Sorry Merlin, about to mount our prism but noticed a hole in the back of your film canister mount in the last pic (atop your TV) that doesn't seem to be there in the other pics. Is this necessary and if so, why?

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The end on the eyepiece is the open end of the plastic cassette, slips onto the outside of the eyepiece when in use. This allows the exit beam from the eyepiece to be centred onto the prism. The refracted dispersed spectrum beam leaves the prism at an angle and viewed from the rear.

The viewing slot is cut across the base of the cassette....

Hope that helps.

 

Edited by Merlin66

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I see a small round hole w/ green background popping in through the back of your cassette in the last pic--but my eyes must be confusing that for the open bottom. Sorry. Thanks

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p.s. Because we're virtually at the field stop of our 31mm Hyperions w/ the 30mm prism, and the cup size is identical for our ES 24mm 68*, we'll be fitting our small pill bottle prism mount into a larger pill bottle that fits snugly over both, and if we want the prism on the 11mm Nagler we'll just swap out the large bottle and attach the cap for the smaller mounting bottle wh/ just happens to fit the 11mm eyecup perfectly. Getting a little excited. Oh, and w/ care to get the prism as close to the EP as possible w/ the viewing window as near as possible to the prism flat.

Edited by laowhoo

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4 hours ago, laowhoo said:

I see a small round hole w/ green background popping in through the back of your cassette in the last pic--but my eyes must be confusing that for the open bottom. Sorry. Thanks

Ahhh,

That's the view of the exit pupil of the eyepiece. It just confirms that the viewing "port" is pretty well placed.

 

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Just finished. Wonderful. Can't wait to try it out..Borders on computer screen are green and red--no white. This will be interesting. We'll have the scope back together in a few days and will report back. I also think that buying a decent piece of glass wasn't a total waste. Thanks.

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Sounds good!

If there's sufficient interest in the viewing prism - I'll prepare a more detailed sketch of the necessary arrangement.

 

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My only question would be  the angle of the mount--whether it's straight up/down or slightly tilted toward the viewing port. Couldn't help but notice how round is the exit pupil in your pic above, but when I placed it on my keypad, the letters wanna stretch upward at all but the flattest viewing angle. Maybe I wanna cut my viewing port a little lower, closer to the plane of the flat. But I'll see when I can get it on an EP. I also made the mistake of visiting your site. Holy cow. Truly impressive, and "the soul of a star." Very kool. Glad you jumped in, and thanks again.

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The prism sits an an angle relative to the eyepiece axis.

Pretty poor illustration, but I hope it helps.

 

100_3436.JPG

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LOL Yeah, that might make a difference. LOL But there it is then. You've got all you need here for others to try. Wonder how it'd be for BVing, say on Orion? 

Okay Merlin, we'll report back in a coupla three days. Just have some fresh Teflon to add. Maybe drink less coffee (jittery lines :icon_biggrin:)

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When you're over 70 you cut some slack for steady hands ;-)

 

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Quite interested in giving this a go. Need see if I can to find a cheap 60 degree Prism locally though, not sure it is worth the hassle of ordering from the US.

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eBay or your local Nat Geo sell cheap prisms....

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A drawing, taken from the "Webb Society Deep Sky Observer's Handbook", Vol. 2, Planetary and Gaseous Nebulae, p. 17 (an almost antique source, edited 1979, the pre-narrowband filter era):

DSC_0701.thumb.JPG.541ade683b2261d2193871e7528ec6e5.JPG

To detect a small planetary, using a UHC or O III filter, you may also flip the filter, holding it with your thumb and index finger, between the eyepiece and your eye in and out of the light path. The filter will darken all the other stars markedly, while the star-like planetary's brightness remains almost unchanged, causing a "blinking" effect. This helps with finding  very tiny, starlike planetarys of only a few arc sec apparent diameter.

Hth

Stephan

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Excellent! Re: distortion, once I tilted it in the cartridge the images are erect, enough so that I can't discern any distortion in the newsprint. Also interesting that they show a prism polished on only 2 sides, w/ one matte finish. I'd almost asked about this before buying b/c it would have saved us half the $$ but I didn't wanna embarrass myself a second time. Great find. Thanks

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Here's the finished product before blacking. Just shows how it can work for both 2" and 1.25" EPs (36mm and 31mm Hyperions have same eye cup size as our ES 24mm 68, and the Nagler T6 11mm and ES 8.8mm and 4.7mm also have the same cup sizes, so this will work for virtually all our EPs. Forgot to try it on the Barlow, and BGO and Edmund are too small) Cheers

 

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