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Lowest power eyepiece

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#1 Sweep

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Posted 15 January 2020 - 02:53 AM

Beginners question here need some advice?
Is there any real benefit in having a 45 to 60 mm eyepiece for a 5” refractor f7

Fov will not increase dramatically from largest eyepiece owned ( mag 31x)

 

Magnification in the 40 to 60mm range 15 to 20x

 

Cant see eyepiece in this range being used enough, as within binoculars territory, opinions appreciated?

 

 

 



#2 Barlowbill

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Posted 15 January 2020 - 03:18 AM

Definitely bino territory.  Next question is, what are exit pupils at those magnifications?  8.5 and 6.35.  Not that good!  



#3 ButterFly

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Posted 15 January 2020 - 03:30 AM

If one's pupils dilate to 7 and 8.5mm, the exit pupils of the 45 and 60mm would help brighten the image.  That's useful with narrowband filters.  Of course, it also reduces magnification, so there are diminishing returns for smaller objects.

 

Field of view is determined by the eyepiece field stop - not focal length.  A 20mm plossl has a very different field of view than a 21mm Ethos.



#4 TOMDEY

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Posted 15 January 2020 - 03:43 AM

For a refractor, that's OK, even a good thing! The 6.4mm and 8.6mm pupils may and will underfill your eye's, but that's not necessarily a bad thing, especially when using a refractor... provided you would gain field by doing that (some refractors have huge available field). The TeleVue Panoptic 41mm most certainly comes to mind. Indeed, that is an eyepiece that you should have!  If you don't --- buy it now; will love the immersive experience!  Tom


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#5 25585

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Posted 15 January 2020 - 04:15 AM

As finder eyepieces 35 to 55 are good FLs. For OIII & UHC filters 35 & 40 are best.


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#6 OldManTaco70

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Posted 15 January 2020 - 04:30 AM

I second 25585's "finder eyepieces," comment.

I have used a Meade 56mm 4000 Super Plossl as my finder ever since I made the jump to a 12" SCT.



#7 luxo II

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Posted 15 January 2020 - 04:43 AM

Sweep there are two practical limits:

 

1. The barrel diameter defines the maximum actual field of view possible.

 

2. The exit pupil from the eyepiece should be a tad smaller than the entrance pupils your eye - otherwise the edge rays strike your iris.

 

This is typically 5-7mm diameter and a few factors to consider - age, and the ambient light level - an urban light-polluted sky will not allow your iris to fully open.

 

In this respect 5mm is a safe value.

 

5mm x f/7 suggests a 35mm eyepiece, though a 38mm SWA (70 degree apparent field of view) would be pretty much ideal. Longer than 38mm will be somewhat pointless.


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#8 Sweep

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Posted 15 January 2020 - 04:49 AM

I have a 2” OIII filter.

Sky magnitude 21.39 rural area at least 10 miles from nearest town.

Is there any targets worth using this range of eyepiece for or is it just a finders eyepiece for my 

I was interested in a high quality Masuyama eyepiece in the 45 to 60mm range too expensive if there is not enough targets that would justify its purchase?



#9 Shorty Barlow

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Posted 15 January 2020 - 05:12 AM

I sometimes use a 55mm TV Plossl in an f/7.5 refractor with an OIII filter for the Cygnus Loop and similar.

 

gallery_249298_10131_1578.jpg

 

It gives me about 11x for a 7.3mm exit pupil. 


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#10 luxo II

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Posted 15 January 2020 - 05:40 AM

Masuyama eyepiece in the 45 to 60mm range

That will produce an exit pupil approaching 9mm. Nobody has eyes that big -  not even my wife.

Conversely it implies you're only using 100 mm of your scopes aperture, the rest of the light is not entering your iris.

 

Better stick to 30-something mm.


Edited by luxo II, 15 January 2020 - 05:42 AM.


#11 Jon Isaacs

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Posted 15 January 2020 - 05:41 AM

I have a 2” OIII filter.

Sky magnitude 21.39 rural area at least 10 miles from nearest town.

Is there any targets worth using this range of eyepiece for or is it just a finders eyepiece for my 

I was interested in a high quality Masuyama eyepiece in the 45 to 60mm range too expensive if there is not enough targets that would justify its purchase?

Sweep:

 

is that 21.39 mpsas from a chart or measured?  Chart values are generally somewhat optimistic about how dark the skies are, generally they'll be somewhat brighter.  

 

This is how I see it:

 

As others have said, matching the exit pupil to your dark adapted pupil diameter is useful, this maximizes the brightness of both objects and stars while retaining as much magnification as possible. If the exit pupil is larger than your eye, it has the same effect as putting a mask over the objective.  It will dim stars and objects (nebulae etc) will be smaller.

 

For a finder, the goal is to maximize the true field of view but maximizing the brightness of objects is second to magnification since you're mostly looking for stars and smaller objects.  For viewing large faint nebulae with a filter, maximizing the brightness and the field of view can be helpful.  

 

As I see it, at F/7, both a ~40mm SWA (68 degree) eyepiece and a 55mm Plossl are possibilities. Both will maximize the TFOV .  The 40mm SWA will provide a 5.8 mm exit pupil, the 55mm Plossl will provide a 7.9mm exit pupil. 

 

For most observers and most situations, the 40mm SWA would probably be the better choice.  Few observers can take advantage of a 7.9 mm exit pupil and for those than can, (I am one), the smaller 5.8 mm exit pupil with the greater magnification will almost always show more, be more useful.  It's only for the largest, faintest objects with a filter that maximizing the exit pupil is an advantage.

 

A 5 inch F/7  has a 900 mm focal length.  That means a 40 mm SWA and a 55 mm Plossl with both provide a 2.93 degree TFoV, the 40mm will provide 22.5x, the 55mm, 16.4x.  

 

My thinking goes like this:  Regardless of whether you decided to buy the 55mm, you'd want the 40mm SWA.  In general, it would be the more useful eyepiece and for most observers would be the only one needed.  Later, a 55mm could be added.  

 

So, what objects?  This time of year, I find the "Heart and Soul" Nebular region, the California Nebulae, the Flaming Star Nebulae are helped by large exit pupils with filters.  The nebulosity in the Pleiades is best seen at a near maximum exit pupil without a filter.  Barnard's loop is another.  

 

Jon


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#12 luxo II

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Posted 15 January 2020 - 05:49 AM

Sweep, where are you ? 10:1 you're not under what I call dark skies, though admittedly I am spoilt here,


Edited by luxo II, 15 January 2020 - 05:50 AM.


#13 ButterFly

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Posted 15 January 2020 - 05:49 AM

I have a 2” OIII filter.

Sky magnitude 21.39 rural area at least 10 miles from nearest town.

Is there any targets worth using this range of eyepiece for or is it just a finders eyepiece for my 

I was interested in a high quality Masuyama eyepiece in the 45 to 60mm range too expensive if there is not enough targets that would justify its purchase?

41 Pans excel in these skies.  Largest well corrected field.  That alone makes it worth it, as Tom mentioned above.  The Milky Way is lovely with big pupils under dark skies.  Once the field is maximized, there is not much to gain when the exit pupil is already that large.  It's almost 6mm with your scope.  When you exceed your eye's pupil, you decrease the effective aperture, but it may be worth it if you can get more field.  The 41 Pans already max it out, so not much to gain.



#14 25585

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Posted 15 January 2020 - 09:17 AM

By what ND factor do UHC and OIII filters reduce the effective exit pupil to?

 

An exit pupil too large for an unfiltered view, may be dimmed down to an effective smaller size by darkening. In that case there will be a calculation showing what eyepiece & filter combination will give the best result for a particular telescope.   



#15 Ronofthedead07

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Posted 15 January 2020 - 10:10 AM

As long as you're okay with an AFOV in in the 45-60 degree range, it shouldn't be a problem. Depending on your dark adapted pupil, you should be able to go up to about 50mm on an f/7 without losing resolution. At that low a power, it probably won't make much of a difference anyway if you exceed that by a bit.

Personally I wouldn't go any longer than a 55mm Plossl.


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