Celstron C8 SCT eyepieces

[rolion]

Hello to All,

Many thanks for the helpful and warm sharing of info here.

I need your help...

Got recently my first telescope, a Celestron C8" SCT that came with standard eye pieces.

Upgraded the diagonal to a much better 2" and got a focuser .

I'm looking to upgrade the EP to something better.

Searched here, online, audio/video reviews and I have stopped to Explorer Scientific 82 degrees series.

I'm planning to buy 30mm,18mm ,11mm and one under 10mm,whichever comes available in the shop.

All are in 2" range.

I understand the concepts of "magnification", low power vs maxim power.

The only think that I can't get in my mind is the AFoV or FoV.

Can you help me understand this concept please ?

This FoV means that at 82 degree I can see a lot more in trough the eye piece ,that been something good or bad ?

It will really help me if I can visualise it as such what's the "visual" difference between 50,82 and 100 degrees FoV pieces.

Thanks in advance !!

[rolion]

Ok,

Please stay with me...

Did some maths here, using the 2000 focal length of the SCT 8" divided by each Eye Piece magnification size,8mm,11mm,18mm,30mm :

 

2000 / 8 = 250
82 afov / 250 = 0.32 FoV

2000 / 11 = 181x
82 afov / 181 = 0.45 FoV

2000 / 18 = 111x
82 afov / 111 = 0.74 FoV

2000 / 30 = 66
82 afov / 66 = 1.24 FoV

How do those "numbers" helps me in real time watching !?

Thanks

PS

Did few typing mistakes in above post, not sure "how-to" edit it.

[rolion]

Ok, changed the aFoV of the Eye Piece:

Again, some maths here, using 50 and 100  degrees aFoV with 8mm and 30mm pieces:

2000 / 8mm = 250x
50d afov / 250x = 0.2 FoV
100d afov / 250x = 0.4 FoV

2000 / 30mm = 66x
50d afov / 66x = 0.75 FoV
100d afov / 66x = 1.5 FoV

Still looking for a "visual" difference...

Thanks.

[bitnick]

The FoV tells you how large a part of the sky you'll see. For example, the size of the moon as seen from the earth is 0.5°, which means you will see all of the moon (with more or less black sky around it) with all eyepieces which give you more than 0.5° of FoV. In an eyepiece which gives 1° FoV, the moon will fill half the field of view.

Some deep sky objects are quite large - larger than the moon by several times (e.g. the Andromeda galaxy is something like 3°, so quite difficult to fit into many telescope/eyepiece combinations!).

[bitnick]

Put another way, the FoV tells you how large an angle of the sky the telescope sees. The AFoV tells you over how large an angle that sky will be displayed to your eyes. Larger magnification and the same visible sky area of course requires a larger image in the eyepiece to fit, i.e. a larger AFoV.

A very small AFoV will look like you're peeking at the sky through a tube. A very large AFoV will be like looking through a (small) window, and will require you to move your eye (and maybe your head) around to see everything that's available to see within the AFoV. With the small AFoV you would intead need to move the whole telescope around.

[SpaceWalker]

Install Stellarium (it's a freeware) on your computer, it has telescope, eyepiece, Barlow simulators, try it and you'll get the idea.  Anyway, you'll need Stellarium for many other things as well.

[knobby]

This isn't perfect but gives you an idea ...

http://www.12dstring.me.uk/fov.htm

[-Aldebaran-]

If you are looking into the Explore Scientific 82° range, the 2" eyepieces are the 18mm,24mm (including maxvision) and 30mm only, all others are 1,25".

100° range eyepieces are sold in 2" format 5.5mm,9mm,14mm,20 and 25mm. They have a 30mm 100° and it's a biggy at 3"!!!

Try your calculations on the focal lengths available to you.

The "visual difference" you mention to be looking for is in you calculation between the 50° afov(unavailable in Explore Scientific) and 100° afov, the viewable sky (fov) doubles.

If you want an ES 82° <10mm I'd suggest their 8,8mm as it came up as "choice" Ep for higher magnification in a well documented review on here.

If someone could help by sticking it to this topic as I don't have the editing/pasting knowledge yet.