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When is a Televue 55mm Plossl no longer a Plossl?


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Here is a humble Televue 55mm Plossl (my most used eyepiece).

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It has its flaws and Televue have been listening to their customers and have been working to improve the eyepiece and also to increase the focal length to the maximum possible and still provide a 40 degree afov.

Here is the new Televue 67mm conversion kit...

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Remove the screw top from the 55mm Plossl...

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Screw in the new 67mm conversion lens (using the inner thread of the top)...

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Replace the screw top of the 55mm Plossl...

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It still looks like a 55mm Plossl, but it is now a 67mm 40 degree afov eyepiece.

I guess we should no longer call it a plossl as it now has an extra lens?

Alan

Edited by alanjgreen
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The 55mm plossl did not perform well when used with a fast telescope, my f3.6 dob for example.

Early adopters are saying this new lens fixes all the issues with the eyepiece for fast scopes.

Night vision devices only show 40 degree afov anyway so why not increase the focal length at the same time and allow a wider exit pupil too. It now lets 20% more light through the eyepiece than the vanilla plossl (as an added bonus).

Alan

Edited by alanjgreen
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5 hours ago, alanjgreen said:

It now lets 20% more light through the eyepiece than the vanilla plossl (as an added bonus).

my understanding of optics is pretty non-existent but I’m keen to learn. How can adding another lens at the eye end of an ep increase the light throughout of that eyepiece?

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27 minutes ago, markse68 said:

my understanding of optics is pretty non-existent but I’m keen to learn. How can adding another lens at the eye end of an ep increase the light throughout of that eyepiece?

By compressing the amount of existing light into a smaller image circle.  The absolute amount of light remains the same, but the amount of light per unit area increases.  Basically, it's a focal reducer that increases the focal length and exit pupil while decreasing the AFOV.  TFOV remains the same (or close to it).

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On 16/07/2020 at 20:01, alanjgreen said:

The 55mm plossl did not perform well when used with a fast telescope, my f3.6 dob for example.

Early adopters are saying this new lens fixes all the issues with the eyepiece for fast scopes.

Night vision devices only show 40 degree afov anyway so why not increase the focal length at the same time and allow a wider exit pupil too. It now lets 20% more light through the eyepiece than the vanilla plossl (as an added bonus).

Alan

Nice one Alan - Gavin has also recommended this nifty little addition to the night vision arsenal. 
I have one winging its way across the Atlantic too.

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I don't own a 55 but reckon it's the best EP I've used in friends Quark 

Plus I note that in an older DayStar brochure they recommend the 55 in conjunction 

with their Quantum Etalon series and the allowance of 20% more illumination 

would be a bonus with the adaptor.

Brian

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