EP question - beginner

[Starslayer]

Trying to get my head around this one. Help please. 
Been looking at sun during day and had my first play with the night sky yesterday.  I have 40mm and 10mm stock eps and a baader zoom. I understand why the 40mm is better using a lower powered ep for the sun and moon etc and that the higher powered ones are better for going deeper but, why does higher power mean also that the view is so drastically reduced in size.  Can I not get say 150x magnification but with an eye view the same as using my 40mm with 37.5 mag?   Hope that makes sense.  Does higher power always mean a great reduction in the width of what I see through the ep?

[M40]

What you are describing is the "field of view" which is basically the amount of sky you can see in your eyepiece. The eyepiece itself has its own "apparent field of view" then by sticking it in a telescope the value changes, this is then called the "field of view". The greater the magnification, then the narrower the field of view and vise versa. There are lots of different eyepieces out there that offer different apparent field of views but then life is never that simple as then the telescope itself limits the field of view that the combination of eyepiece and telescope will allow you to see. There you go, clear as mud

There are lots of calculations and formula out there for calculating field of view, magnification, focal ratio etc, may be worth seeing what you can find at the same time the book "Turn left at Orion" which you have probably seen mentioned on here does provide a few sample calculations. Enjoy and HTH

[Starslayer]

Thank you.  I am not embarrassed to ask though -  HTH? 

[Zermelo]

Think of it like this. If you want to see the same patch of sky, but at a larger magnification, then you're going to need more "image real estate" on which to display it - the whole thing needs to be bigger. So the view presented to your eye needs to be wider to achieve this, i.e. a larger apparent field of view (AFOV) to achieve the same true field of view (TFOV) at that higher magnification.

If magnification increases but the AFOV stays the same (typically, different focal length eyepieces within the same range, with the same internal design) then inevitably the TFOV will decrease. But you can increase the magnification and retain the TFOV (or at least, not lose so much) by switching to a different EP design - hence the popularity of wide and super-wide eyepieces. The problem with those is that to achieve the extra AFOV, they need more complicated designs, which can reduce light transmission and contrast, and can cause deterioration of the image at the edges (this also depends on the scope they're used in). The best performing widefields are, therefore, pricey.

Note that this logic needs to be amended for zoom EPs, because the way that they achieve different magnifications usually results in a wider AFOV at the highest magnification, which is the opposite of what you'd expect.

[Dark Vader]

Hope that helps...

Ask away, part of what makes this the best forum is there's no such thing as a silly question and no matter how obscure the query, somebody with years of experience will know an answer to it.

[Starslayer]

Thank you

[Tiny Clanger]

The simplest explanation of apparent vs actual field of view I came across (and I was bemused enough by the subject to need very simple ) was to imagine you are looking through a small window at the view outside , then imagine not moving , but putting a bigger window in the small one's place.  That is a wider apparent field of view.

However, the apparent FOV of an eyepiece is not the end of the story : different focal length eyepieces (e.g. the 10mm or 40mm of yours) in combination with the rest of the telescope optics, magnify by different amounts. So actual field of view is a function of the magnification the eyepiece / telescope combination , the more magnification, the less sky you will see in one view.

As has been said, wide FOV eyepieces need big glass (and maybe 2" diameter barrels too ..) and therefore cost far more than narrower ones.

Heather

[M40]

54 minutes ago, Starslayer said:

Thank you.  I am not embarrassed to ask though -  HTH? 

I had to check but it's in there...

Trust me, I have visited this topic more than a few times

[Tiny Clanger]

Here's a few EPs from the same brand who offer lines with different apparent FOV :

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-62-series-ler-eyepieces.html

62 degree apparent FOV, 20mm £85

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-68-degree-series-eyepieces.html

68  degree apparent FOV , 20mm , £135

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-100-degree-series-2-inch-eyepieces.html

100 degree apparent FOV, 20mm / 2" barrel .... wait for it ...... £479

Those big windows don't come cheap !

[Starslayer]

10 minutes ago, M40 said:

I had to check but it's in there...

Trust me, I have visited this topic more than a few times

Ok, I went there and scrolled. Honest. Could not see it.  Went back again and had to do a ‘find text’ search and git there eventually.   I did try before asking. 😉

[Starslayer]

22 minutes ago, Tiny Clanger said:

The simplest explanation of apparent vs actual field of view I came across (and I was bemused enough by the subject to need very simple ) was to imagine you are looking through a small window at the view outside , then imagine not moving , but putting a bigger window in the small one's place.  That is a wider apparent field of view.

However, the apparent FOV of an eyepiece is not the end of the story : different focal length eyepieces (e.g. the 10mm or 40mm of yours) in combination with the rest of the telescope optics, magnify by different amounts. So actual field of view is a function of the magnification the eyepiece / telescope combination , the more magnification, the less sky you will see in one view.

As has been said, wide FOV eyepieces need big glass (and maybe 2" diameter barrels too ..) and therefore cost far more than narrower ones.

Heather

Useful thank you. 

[inFINNity Deck]

 

13 minutes ago, Tiny Clanger said:

Here's a few EPs from the same brand who offer lines with different apparent FOV :

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-62-series-ler-eyepieces.html

62 degree apparent FOV, 20mm £85

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-68-degree-series-eyepieces.html

68  degree apparent FOV , 20mm , £135

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-100-degree-series-2-inch-eyepieces.html

100 degree apparent FOV, 20mm / 2" barrel .... wait for it ...... £479

Those big windows don't come cheap !

These are my favourite, but start at £563 only: https://www.firstlightoptics.com/tele-vue-eyepieces/tele-vue-ethos-100-110-degree-eyepieces.html

Nicolàs

[Starslayer]

13 minutes ago, inFINNity Deck said:

 

These are my favourite, but start at £563 only: https://www.firstlightoptics.com/tele-vue-eyepieces/tele-vue-ethos-100-110-degree-eyepieces.html

Nicolàs

Wish list for now then. 😉😉😂😂😂

[John]

I think the closest you can get to seeing as wide a true field of view as the (presumably 1.25 inch) 40mm eyepiece shows but at significantly higher magnification, would be the 17mm Tele Vue Ethos which would deliver 88x magnification, assuming (from the magnifications that you give) that the scope concerned has a focal length of 1,500mm. The 17mm Ethos is a 2 inch fitting eyepiece and costs £729.00 currently

 

 

 

 

 

 

 

Edited May 30, 2021 by John

[Starslayer]

1 hour ago, Tiny Clanger said:

 

As has been said, wide FOV eyepieces need big glass (and maybe 2" diameter barrels too ..) and therefore cost far more than narrower ones. 

I do have the celestron 2” xlt diagonal. I have played briefly with it but came up against the l / r inverted image thing so went back to stock until I get used to things. 

[John]

2 minutes ago, Starslayer said:

I do have the celestron 2” xlt diagonal. I have played briefly with it but came up against the l / r inverted image thing so went back to stock until I get used to things. 

So you are back to using the stock 45 degree prism again ?

What scope are we talking about here ?

 

[Starslayer]

4 minutes ago, John said:

I think the closest you can get to seeing as wide a true field of view as the (presumably 1.25 inch) 40mm eyepiece shows but at significantly higher magnification, would be the 17mm Tele Vue Ethos which would deliver 88x magnification, assuming (from the magnifications that you give) that the scope concerned has a focal length of 1,500mm. The 17mm Ethos is a 2 inch fitting eyepiece and costs £729.00 currently

 

 

 

Thank you John. Shall I send the bill for that particular EP to you or my wife?  😇  I do , however, take the point.  Big field of view at big mag = no holiday next year!   😂

 

 

 

 

[Starslayer]

2 minutes ago, John said:

So you are back to using the stock 45 degree prism again ?

What scope are we talking about here ?

 

Hi. I have the Celestron 6” SCT. 

[Louis D]

The first 10 9mm ES-120 eyepieces were sold without defined field stops, so they actually show as a ~140 degrees apparent field of view.  It's been reported by their owners that the true field of view is roughly equivalent to the 13mm Ethos's TFOV.  Since it has a 22.3mm field stop, this means that the original 9mm ES-120 eyepieces display roughly the same TFOV as a 26mm Plossl.  So, not quite the same as a 40mm Plossl with a 27mm field stop, but getting close and at a much higher power.  So yes, it can and has been done.

Edited May 31, 2021 by Louis D