mm vs degrees

[Raga]

Hi guys.

which would give a better view/image of larger dso's, 30mm 70degree fov or 40mm 65degree fov? Or are both eps worth buying?

cheers guys

[Moonshane]

it depends on your focal length and focal ratio. what scope are you using?

[John]

The 40mm would show a wider field of view but, as Shane suggests, a 40mm eyepiece might not a be a great idea if the focal ratio of the scope it's being used in is faster than around F/6.5 as the exit pupil gets too large to be efficient.

[RikM]

Why not have a play with this FOV simulator http://www.12dstring.me.uk/fov.htm You can select your telescope and eyepieces and then choose objects to see how they fit into the field of view. Do remember that the images used are all long exposure photographs, rather than what you will actually see, but the relative size of the objects in the field is about right.

[Raga]

Thanks for the replies. My scope is SW 150pds so it's f/5

[RikM]

I have a 150P and I can't use a 40mm ep with it. I get a big dark blob in the centre of my vision, the shadow of the secondary mirror I think. I don't know if this just to do with my eyes but that has been my experience. My widest EP is a 30mm 60° Vixen LV and this works very nicely.

[smoothound54]

this calcualtor is brilliant since it allows you to compare ( side-by-side ) a number of eyepieces in a telescope that you specify - and you can add a barlow too

http://www.davidpaulgreen.com/tec.html

cheers

alan

[Astrobits]

If you have got STELLARIUM ( free download ) then you can put your telescope and eyepiece details in and see the effect on the objects you are interested in viewing. I think that this is much better than a number from a table.

Nigel

[michael.h.f.wilkinson]

In an F/5 scope I would not go beyond about 30 to 35mm EPs as the exit pupil gets rather large. having both a 30 and a 35 is probably not ideal. I found when I got my 31mm Nagler (82 deg) I no longer used my Paragon 40 (68 degrees). Very good EP, but the slight increase in FOV was not worth it. Given two EP with the same true FOV, I would favour the shorter focal length any day, as it has the smaller exit pupil and therefore darker sky background.

[crazyjedi]

If you have got STELLARIUM ( free download ) then you can put your telescope and eyepiece details in and see the effect on the objects you are interested in viewing. I think that this is much better than a number from a table.

Nigel

How clever is that??