Which low power e/p to use

[daveb]

I have a 12" dob, FL 1600, F5.3. I'm after an eyepiece for general DSO's but unsure of the best magnification to go for. The E/Ps i'm looking at are TV Nagler T5 30mm 51x mag, ES 25mm 100dg 64x mag or ES 20mm 100dg 80x mag. which should i go for?

[John]

Hi,

I have the same scope - Orion Optics 12" F/5.3. My two main low power / wide field eyepieces are the 31mm T5 Nagler and the 21mm Ethos. I find that I use the Ethos more than the 31 Nagler though because I have some light pollution to contend with and the higher magnification gives me a darker background sky and shows faint DSO's a little better. I still use the 31mm Nagler for the largest objects such as the Veil Nebula and M31 though.

I've owned a 20mm ES 100 as well and thats an impressive eyepiece too. I've not used an ES 25mm 100 but I've read more mixed reports on that one when used with newtonians so it might be worth doing more research into it if it seems a strong candidate to you.

 

[daveb]

Hi John, do you think the ES 20mm would be a better option than the 25mm because my sky's aren't perfect either?

[Ruud]

For DSOs an exit pupil of 2.5 to 3 mm is probably best. This will keep the sky background dark and give optimal contrast between the DSO and the sky. Based on this you'd want an eyepiece with a focal length between 2.5 and 3 times your f/number (roughly between 15 and 18 mm).

On the other hand, you don't want too high a magnification for the larger DSOs, so you might opt for an exit pupil around 3.5 mm. That might no longer be optimal for contrast, unless you have a very dark sky. This would mean an eyepiece of a focal length of 3.5 times your f/number, around 21 mm.

You'd need a quite dark sky to optimally study DSOs with an exit pupil of 4 mm (and eyepiece focal length of 4 * 5.9 = 24 mm approximately).

Here's an exit pupil rule of thumb: for planets 0.5 to 1 mm -- for DSO 2.5 to 3 mm -- for rich star fields up to 5 mm.

[John]

7 minutes ago, daveb said:

Hi John, do you think the ES 20mm would be a better option than the 25mm because my sky's aren't perfect either?

I think you will find 20mm more useful but 25mm might be better than 31mm as well, as long as the 25mm does not have other issues.

I've had sessions with my 12" dob where a 21 / 20mm 100 degree eyepiece (exit pupil 3.77mm) was all I used to pick off galaxy after galaxy.

[Bart]

I agree I find more and more that I use less and less eyepieces and that my 20mm 100* EP stays in the focuser for much of the session. That's whether I'm using the 10" or the 16"

Barry

[daveb]

I think it's a toss up between 20mm & 25mm, I'll have a look at the exit pupil on them both and then make a decision.  Thanks for all the replys