A "better" field of view???

[vanislandmike]

As an early beginner astronomer, I equated lower magnification with wider field of view in a rather absolute sense.

I assumed a 32mm plossel would give a wider view than a 28mm (period, whether a plossl or a 68degree) because it gave a lower magnification.

I understand now that it is the stated field of view divided by the magnification that ep gives in your scope that determines the field of view at the eyepiece (excluding diameter limitations for now).

This would then imply, to me, that given equal fields of view, the higher mag Ep would give the better visual view. You see all the same stuff, but bigger. The contrast would be higher.


I'm having doubts about this now...


At higher mag, the same amount of light is 'spread out' more, is it not? Sort of like using less pixels, or larger scan lines in a CRT?

The question then, is whether the lower mag EP would still give better wide field views because of sharpness and things like that ... or if it is indeed generally better to choose the higher magnification given the same chunk of the sky?

I'm really quite stuck on this...

Cheers,
Vim

[OleCuss]

I'm hoping someone better than I will chime in on this. So consider this sort of a bump.

If conditions are good enough to support the higher magnification and the object of interest will fit well into the FOV of the higher magnification eyepiece, then I like the idea of the higher magnification.

I keep reading where some say that with the higher magnification eyepieces you'll have less light exiting the eyepiece and entering your eye. I really don't see why that should be so (unless there are design or coating problems).

But sometimes I really enjoy having an entire starfield available even if I'm concentrating on a small piece of it.

[Phil Leigh]

Eyepieces alone don't have a "magnification". They have a focal length and an apparent field of view (AFOV). Magnification (when mounted in a scope) = Scope FL / Eyepiece FL.

The magnification will determine how big individual things appear to be, the AFOV determines how many things will fit in the view.

So, a smaller FL eyepiece gives you bigger things, a wider AFOV gives you more of them. These are independant parameters. You can have short or long FL and wide/narrow AFOV eyepieces. There's no empirical difference in absolute optical quality based on the focal length or AFOV - it's purely down to to how well made each eyepiece is... Contrast in practice turns out to be much more about the lens quality, optical construction/design - ortho's are very good - and the coatings and flocking of the eyepiece, rather than the "numbers"

Sooooo the answer to your question (ignoring anything to do with your scope optics and focussing, mount and seeing conditions) is simply:
how big do you want things to be and how many of them do you want to see?

The BEST way is to try various eyepieces yourself in your scope and SEE.

[vanislandmike]

Thanks very much for the help so far.

I'm comfortable in general with understanding all the basics of focal length, magnification, afov and tfov, and so on. It falls apart for me on the issue of 2 EP's that give the same true field of view at the eyepiece, but one at higher mag, or the higher mag one giving wider tfov.

I just had it in my head that for wide-field scanning, you buy a long FL EP over a shorter one, period. Then came the surprise (to me) that a 28mm 68degree ES gives more of a 'wide-field' view than a 32mm 50degree in my 560mm FL scope.

It seems a no-brainer to get the 28mm, but as I said, I had so subscribed to the mistaken belief that lower mag=best for wide-field immersion, absolutely, that it is somewhat odd to realize there's no need to arbitrarily choose a lower magnification ... unless there is for issues of brightness, sharpness and so on??


Perhaps it's better to ask it this way: Televue 32mm plossl (17.5 mag, 2.85 tfov) or 28mm ES (20x mag; 3.4 tfov)?


Cheers,
Vim

[MitchAlsup]

I understand now that it is the stated field of view divided by the magnification that ep gives in your scope that determines the field of view at the eyepiece (excluding diameter limitations for now).

This would then imply, to me, that given equal fields of view, the higher mag Ep would give the better visual view. You see all the same stuff, but bigger. The contrast would be higher.

You forgot about the part where greater magnification increases the darkness fo the sky blackground.

At higher mag, the same amount of light is 'spread out' more, is it not? Sort of like using less pixels, or larger scan lines in a CRT?

Extended objects such as Neblae andd galaxies are spread out and dimmer. However stars remain tiny pinpricks of light and don't follow this dimming until one gets to absurdly high powers.

The question then, is whether the lower mag EP would still give better wide field views because of sharpness and things like that ... or if it is indeed generally better to choose the higher magnification given the same chunk of the sky?

I prefer the 31mm Nagler over a 40mm Panoptic because the higher magnification gives a darker blackground and the bigger FoV allows me to see the same FoV wwith bigger details. I am hoping the soon to be released 25mm ES 100 does similarly. This EP should have the same absolute FoV as the 31 NT5 but with more magnification and blacker backgrounds.

If you have a fast telescope, a low power EP may produce a bigger exit pupil than your eye can admit. The extra light illuminates your cornea but does not contribute to the image formed on the retna. Tthe cornea illumination washes out sublte detail and makes the sky background appear even brighter than it is.

[vanislandmike]

Great help Mitch, thanks so much. I got what I needed from your answer.

I am aware of the increase in sky background darkness ("The contrast would be higher") as that is the key thing that made me look at a 28mm Explore Scientific over a 32mm plossl.

What you said about DSO's being a bit dimmer in the higher mag eyepiece but not the stars cinched it for me. It's too small a scope for DSO's (what the dob is for), and it was the immersive star fields I didn't want to dim.

Like you, I feel positive now I'll prefer the shorter focal length EP over the longer one - higher magnification, better contrast, same FoV with bigger image scale.

Thanks again,
Cheers,
Vim