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Equipment Discussions >> Eyepieces

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ggalilei
scholastic sledgehammer
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Reged: 09/01/11

Loc: Kentucky
TV 55mm Plossl and exit pupil new
      #6621255 - 07/07/14 11:32 AM

I'm considering the purchase of the TV 55mm Plossl, and I need to understand better the effect of large exit pupils. My telescopes with 2" focusers span f/5 to f/8. The exit pupil on the f/5 is 11mm, much larger than the 5-7mm human pupil size. What are the negative results? Dimming of the views, eye aberrations showing up, .... ?

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Eddgie
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Reged: 02/01/06

Re: TV 55mm Plossl and exit pupil new [Re: ggalilei]
      #6621483 - 07/07/14 01:02 PM

You can of course often get a wider field of view, but past your maximum exit pupil size, there is no advantage in terms of seeing higher surface brightness on extended targets.

Of coures as the exit pupil gets larger, the sky brightens too, and this can give the sky a wahed out appearance, but it won't appear any more washed out at an 11mm exit pupil than it does using the largest exit pupil your eye can make.

On a few obstructed scopes, there may also be a dark spot in the center of the field caused by the secondary obstruction (Daytime in an SCT, but not night time).

And this. At very low powers, some smaller extended objects become "Stellar". For example, at super low powers in many scopes, the angular magnification of the Ring Nebula is so tiny that it is not enjoyable to look at, and most smaller planetary nebula become point like.

I think that the best scope for the 55mm Plossl is an f/10 SCT under dark sky conditions.

For most other scopes, there are much better choices because after all, a 41mm Panoptic and a 31mm Nagler will give almost the same size true field and in most dobs, the 31mm Nagler will produce a more pleasing dark sky back ground.

So, no reason why it won't work in some scopes, but may not be the best choice for most.

When I was Mono-viewing, I used 55mm/56mm Plossls under dark sky conditions with outstanding results, with these giving brightest views of dim etented objects.

In the same scope from my back yard though, the 41mm Pan provided a much better view because it kept the sky a bit darker.

Depends a lot on you and what is important to you.


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Aquarist
Pooh-Bah


Reged: 08/27/12

Loc: Illinois
Re: TV 55mm Plossl and exit pupil [Re: Eddgie]
      #6621591 - 07/07/14 01:54 PM

It is my recollection that Daystar recommends them for viewing with their solar filters (which use a 4x telecentric barlow)

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: TV 55mm Plossl and exit pupil new [Re: Eddgie]
      #6622802 - 07/08/14 05:49 AM

Just to add to what Ed said, a ~40mm SWA like the 41mm Panoptic will provide the same TFoV as the 55mm Plossl but at a higher magnification. At F/8, the exit pupil will about 7mm so it might provide a brighter image, depending on the Antonio's dark adapted pupil. For an older observer, this is probably too large and a 6mm or 5mm would provide maximum brightness.

If the exit pupil is larger than the eye's entrance pupil, then a shorter focal length eye can provide the same image brightness, the same TFOV or nearly so (31mm Nagler class) and a greater magnification which is almost always an advantage. F/5 and F/6 and in fact most probably F/7, a 41mm or 30mm will show more..

55mm Plossls are best used in slow scopes when a bright image is an advantage...

Jon


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ggalilei
scholastic sledgehammer
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Reged: 09/01/11

Loc: Kentucky
Re: TV 55mm Plossl and exit pupil new [Re: Jon Isaacs]
      #6623075 - 07/08/14 09:58 AM

I think I understand it now, thanks to all!
As I mentioned in another thread, the "revelation" for me has been that the exit pupil is an image of the objective lens and all the light caught by it (I think this is correct). So if the exit pupil of the telescope is larger than my eye's entrance pupil then I'm effectively using a smaller aperture telescope of the same focal length.


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areyoukiddingme
sage
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Reged: 11/18/12

Re: TV 55mm Plossl and exit pupil new [Re: ggalilei]
      #6624598 - 07/09/14 02:38 AM

Even in my F10 SCT I find I use my 55mm TV plossl only rarely. Eye position is finicky, and it is best used for staying on target--sweeping is difficult b/c of the need for optimal eye placement.

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ggalilei
scholastic sledgehammer
*****

Reged: 09/01/11

Loc: Kentucky
Re: TV 55mm Plossl and exit pupil new [Re: areyoukiddingme]
      #6624605 - 07/09/14 02:49 AM

I think I know what you mean: I love to sweep the Milky Way at low power but it gets me a bit dizzy with several eyepieces.
I must confess that I like the "sound" of 55mm, makes me think of old-time observatories. For now, though, I've decided to try the WO 40mm SWAN instead to give my f/8 scope a wider FOV.


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