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Old 19-06-2018, 06:05 PM
Saturnine (Jeff)
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Join Date: Jul 2008
Location: Wollongong
Posts: 722
Just to be picky Alex . There is the Televue 27mm Panoptic that's been around for a while now and a fine eyepiece. Suited to newts too, kicking myself for selling mine.



Quote:
Originally Posted by mental4astro View Post
As far as a kit of the whole set of these Superviews, no, they are available just individually.

If you use the 42mm and 50mm under urban skies with your SCT, you will find that the shadow of the secondary mirror will become problematic. The issue is not the EP or scope, but the amount of light pollution. Has nothing to do with the design of the EPs either. A 50mm plossl will show the same shadow. Use these long EPs in your SCT under a dark sky, and the shadow won't be noticeable.

However, the 50mm won't show you more of the sky than the 42mm. Both will show you the same amount of sky, but the image of in the 50mm will be smaller in both magnification and in the diameter of the image. This is due to the limitation of the 2" barrel - it just can't show anymore sky than a certain amount. The 50mm may seem redundant then, but it does have a place with concentrating the image that little more which helps bring out some objects.

In a Newtonian, going longer in EP focal length than 30mm won't help you very much. As most Newt's are around f/5 and faster, the "exit pupil", or the size of the light beam coming out of the EP, when you use an EP longer than 30mm will be too large for all the light to enter our human eyes. So effectively you will be reducing the aperture of your scope as a result. This is not to say you can't use an EP longer than 30mm with your Newtonian, and a 42mm Superview will show you as much of the sky as a 2" barrel can give, just be aware that it you won't be maximizing the aperture capacity of your scope.

Exit pupil???

Think about the pupil of your eye. When it is open to its widest, in a young person this will be around 7mm in diameter. As we age, this reduces to around 6mm. So, to maximise the amount of light coming from your scope, you would want to get a beam of light coming out of the EP to be around the 7mm to 6mm range. The term "exit pupil" is a ratio given between the EP focal length and the scope's f/ratio. While the exit pupil generated by a scope can certainly be smaller than 6mm (as you increase magnification the exit pupil gets smaller, so all the light beam is entering your eye), normally you would want to max out the EP focal length FOR A GIVEN SCOPE, to best suit the scope's focal ratio.

Here's how it works:

We want a max exit pupil of 6mm - I'm older than 35years after all, so I need to be realistic about my eyes.

So, for the longest EP focal length, the equation is:

f/ratio of scope X 6mm = longest EP focal length

The scope is an f/10 SCT.

The longest focal length EP that will give me a 6mm exit pupil will be:

10 X 6 = 60mm

So with any EP shorter than 60mm, IN AN f/10 SCT, you know you will be getting all the light going into your eye.

If your scope is f/4.5 (and it doesn't matter if it's a Newt, SCT, refractor, etc, it's all the same here), then the longest focal length EP would be:

4.5 X 6 = 27mm

But there's no 27mm EPs!!!!

Exit pupil is just a guide. You need to be practical about these things, and the % loss between a 27mm and 30mm is not really that significant.

There is a way to counter-act this eyepiece focal length discrepancy. You could get an EP with a wider apparent field of view than 68 (Superviews have an AFOV of 68 - yes the 50mm is smaller, and I explained why). With an 82 eyepiece, the longest 2" eyepiece that will give you this is 30mm - you won't find a 40mm 2" EP with an 82 AFOV, that 2" barrel just can't do it. But you may feel that 30mm is still too long (and that's fine), so you can step things up by going to 100 eyepieces, in which case the longest focal length that will give you 100 AFOV is 21mm. In this way you know that you are both maximising the amount of light coming into your eyes, and maximizing the amount of sky that you scope is capable of showing.

HOWEVER, there's no free lunch here either...

100 eyepieces are not a panacea. There's pros and cons to all eyepieces, and these include $$$, ease of use, optical matching, personal preferences, your own eyeballs, and the such.

There is no "mandatory" reason as to why you somehow need to stick to a strict maximum exit pupil. If you understand what is going on, you may feel just fine with a scope/EP combination that gives a larger exit pupil. I have an 8" f/4 Newt. The longest EP for a 6mm exit pupil would be 24mm. I have a 24mm 82 EP, but I know I won't be getting all the sky this scope is capable to showing me. I could use a 21mm 100, but I don't find this size AFOV practical. However, I am quite happy to use my 30mm 82 eyepiece with this f/4 scope. Sure the exit pupil is "too large", I know this, but I am also getting the largest amount of sky I can get with this scope, and I still very much enjoy the image this EP/scope combo gives me. And there are people who would never entertain the idea of doing this, and that's their personal preference, and that is ok too. Everyone is different.

And I bet you thought that "this is an eyepiece, and this is a scope, so it will work..."

That's how I thought about things too when I started in astro... 35 years ago...

Alex.
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