ep fov help

[sunshine185]

Can someone please tell me the difference in fov I will get using a 20mm 66 degree ep and a 30mm 52 degree ep? if my calculations are correct I should get 1.1 degrees and 1.3 degrees using the above ep's in a 1200mm 250px, but I'm not 100% sure. Will an extra 0.2 degrees be noticable or not worth the bother?

[Ags]

I think your maths is right.

Be aware the exit pupil of the 30mm will be over 6mm, resulting in poor contrast and a fainter image. I would expect the view to be better through the 20mm.

[NGC 1502]

You are doing the maths correctly (apparent fov divided by mag) . This will give a near enough idea as to how much sky will be seen, as long as the apparent fov of the eyepiece is being quoted correctly by the manufacturer.

I've come across some eyepieces where the fov was being exagerated by the manufacturer, presumably as a selling point.

The other way to check the amount of sky being shown is the 'drift test', this is the most accurate way, and avoids any errors with quoted fov, and if the calculated magnification is not exact, due to the focal length of the scope or eyepiece not as the quoted spec.

Other factors - how sharp are the stars towards the edge of the field ? As long as the view is ok towards the edge of the field of view, I'd sooner have the 20mm EP you mention, the higher mag will give a darker sky background with greater contrast, even if you have a very slightly smaller field.

HTH, Ed.

[sunshine185]

I'll save my money, thnx guys.

[John]

I'm guessing that the 20mm 66 degree eyepiece you are referring to is similar to, or the same as, a Skywatcher Ultrawide. If thats the case the, while they work fine in F/10 scopes, the outer 50% of the field of view will look "messy" with your F/4.7 250PX.

[sunshine185]

They are the WO 66 degree ep's, the 30mm I was looking at were the vixens. Now that I don't need the 1.6x corrector or a Barlow lens on the binoviewer, I was trying to achieve the biggest possible fov.

[John]

In the 1.25" eyepiece format a 32mm plossl will show you as much sky as that size eyepiece can.

When you start to go wide field and ultra wide field, with an F/4.7 scope the eyepiece costs increase rapidly as it's tough to produce wide views that are sharp right across with these fast scopes.

[sunshine185]

I think 32mm ep's would vignette too much, but I read somewhere that the 30mm vixens work fine in bv's and would give 1.3 degrees fov, but at £70 it would be wasted as I get 1.1 degrees with the 20mm.

[sunshine185]

Plus I have to factor in weight, I would imagine 2 ep's like nirvanas or panoptics would be too much and cause serious balancing issues.