Low power EP suggestions...

[wulfrun]

I have a Skywatcher 150PL (1200mm focal length and f/8) for which I have a selection of EPs but I think I'm maybe lacking a low-power one. The longest I have (not counting a zoom) is an ES20mm/68-degree, which gives x60 and a TFOV of 1.13 degrees. If I bought (say) a 32mm plossl, I'd gain a bigger exit-pupil, slightly more TFOV (1.21 degrees) and a lower magnification of x38. Ultimately, I believe the greatest TFOV is limited by having a long F/L and a 1.25" focuser, which I do not plan to fiddle with.

Is this a sensible and desirable purchase? Considering a 32mm plossl isn't big money (£25-30)...or are there better ideas from the knowledgeable?

I do have another scope if I need wide FOV but I tend not to have two scopes out at the same time, for various reasons.

[Louis D]

A 32mm Plossl at f/8 will provide the largest TFOV in your scope and be pretty well corrected.  The only downside is the 50 degree field of view.

You could get a 24mm wide field like a Panoptic, ES-68, or APM UFF for about the same TFOV at a higher power and wider AFOV.  However, the difference between it and your 20mm ES-68 wouldn't be all that much.  There's also the 25mm BST Starguider that would come close to maxing out the TFOV as well, and might perform well at f/8.

[jonathan]

You could "try out" the various eyepiece specs using Stellarium, there's a function where you can enter different eyepieces along with your scope (many known / common scopes and eyepieces are already in there).  Just depends how wide you want your low power eyepieces to show, for example wide clusters or asterisms.  I usually start with my old (but good) Celestron 25mm Plossl for finding targets, it's definitely far better than the standard eyepieces that Skywatcher include with their scopes, and is still my go-to for most of the popular open clusters.

[wulfrun]

Thanks for the input, I just wanted to know if a long FL plossl was a poor idea, to be avoided. I'm aware that low magnifications are limited by the 1.25" focuser, in terms of TFOV. I'm more interested in what the gain in exit pupil might bring.

@jonathan
thanks, yes I know I can "try" it in Stellarium but it doesn't provide a realistic view, just shows me FOV and what is, potentially, in it.

[Pixies]

A 32mm plossl will give the maximum TFOV and an exit pupil of 4mm

An equivalent TFOV will come from a 24mm TV Panoptic - with an exit pupil of 3mm:

As far as the views go, you will see the same area of sky, but with the TV, you will be "sitting much closer to the cinema screen"

But the difference in exit pupil will mean with the plossl, the view will be brighter. This might adversely effect the contrast without filters, as the sky will appear brighter. However, if you were observing a nebulous target using a filter like an Oiii or UHC, the extra light should help make a target brighter. That's my limited understanding and experience with a Oiii filter.

 

Edited March 28, 2021 by Pixies

[Pixies]

Here is an excellent recent thread on this very subject. @Don Pensack's comment on exit pupil vs filters is gold: 

 

[wulfrun]

Thanks to all. I've pressed the button and a 32mm plossl should be appearing at my doorstep before too long...

[Louis D]

1 hour ago, wulfrun said:

Thanks to all. I've pressed the button and a 32mm plossl should be appearing at my doorstep before too long...

Good choice.  When I went to Nebraska for the 2017 eclipse, I brought along a 32mm Plossl and an 8-24mm zoom along with my ST80, solar filter, and Bogen tripod.  I didn't feel like I needed anything else.