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TV 55mm Plossl and exit pupil

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#1 ggalilei

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Posted 07 July 2014 - 10:32 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, .... ?

#2 Eddgie

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Posted 07 July 2014 - 12:02 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.

#3 Aquarist

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Posted 07 July 2014 - 12:54 PM

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

#4 Jon Isaacs

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Posted 08 July 2014 - 04:49 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

#5 ggalilei

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Posted 08 July 2014 - 08:58 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.

#6 areyoukiddingme

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Posted 09 July 2014 - 01:38 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.

#7 ggalilei

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Posted 09 July 2014 - 01:49 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.

#8 faackanders2

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Posted 12 August 2014 - 08:44 PM

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

+1  41mm 68AFOV Panoptic or 40mm 70AFOV UO MK-70 Koenig would be better with same TFOV and higher power/darker background, than 55mm 55 AFOV Possil.



#9 gnowellsct

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Posted 13 August 2014 - 07:27 PM

I would venture to add that the 55mm plossl is not *the worst* eyepiece I've ever used, but it is getting down into the bottom 10% for sure.  GN



#10 hfjacinto

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Posted 13 August 2014 - 09:11 PM

In my opinion the 55mm plossl is only good to keep a 2" focuser covered. The only worse eyepiece is a 4mm plossl. Get your self a 31mm Nagler or ES82, they are better for the focal ranges of your scope. Also a 55 mm eyepiece on a obstructed f5 scope will show you the secondary shadow.

#11 Sasa

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Posted 18 August 2014 - 01:48 PM

I bought 55mm TV Plossl for my former ED100. I did not use it too much, as 36mm Siebert was providing for my lighter skies 4mm exit pupil. I considered it as a special purpose eyepiece. On some objects (not a lot, I admit), the 16x magnification in 100mm scope was very useful. Some very wide clusters, like Cr72 with nebula NGC1980, were very charming and finally started to look like open clusters.

Now, i have refractors from f/13.3 to f/20 and 55mm eyepiece would be more usefull but I rarely use it as I got lazy and I'm not using my 2" eyepieces anymore. Instead, I'm using Baader/Zeiss zenith prism with 32mm clearing And for that I have two M44 threaded eyepieces, ATC 53mm Plossl, and ATC 40mm Kellner.

#12 faackanders2

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Posted 05 September 2014 - 08:03 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.

If you can increase TFOV there is an advantage of going lower power; but for a 2" focuser 41mm Panoptic or 40mm 70 AFOV UO MK-70 Koenig will provide the widest TFOV possible.  Hence ther is no advantage of going any lower power than this unless you have a larger focuser for even wider TFOV.








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