Eyepiece recommendations for ST120 f5 refractor

[kc19]

There are so many ep's to choose from, I don't want to make a mistake and buy something that doesn't work well in an f5 refractor, so what are your recommendations please?

I'm considering the Baader Hyperion Zoom, Vixen npl 6mm and skywatcher panaview 32mm, will these work ok with my scope or should I be looking at something else?

Is the Hyperion Zoom a good option for an f5 and if I wanted to use it with the 2 inch barrel would I also need a 2 inch diagonal?

I can't buy these all at once but would like to add something soon, your help would be most appreciated

[newman]

At 600mm focal length, I'd pretty much forget about the 32mm Panaview. I'd sooner consider something in 12, 6 and 4mm ranges, giving you magnifications of 50x, 100x, and 150x. Alternatively, plan a barlow into it, and definitely pay attention to AFOVs so the collection makes sense (two different mag eyepieces that show you roughly the same amount of sky in TFOV don't make much sense).

[kc19]

Thanks for the reply.

I'm a bit confused regarding AFOV and TFOV what exactly is the difference between the two? I have read the info on eyepieces on here but am worried about buying the wrong ones for an f/5 scope. Sorry I've only ever used stock eyepieces

Due to the cost of the 2inch EPs I think I'll leave that for a while, I was thinking the 32mm Panaview would give me some nice wide field views for clusters etc.

I'm looking for a reasonably priced upgrade on the stock EPs, I did consider an eyepiece set but have read good things about the Hyperion zoom on here so thought that would keep me going for a while, and then maybe add the Vixen for slightly higher magnification, or the Baader barlow for the Hyperion zoom.

[JamesF]

I use my 32mm EPs a fair bit in my ST120 because it's very useful for star-hopping.

I originally bought a set of Revelation Plossl EPs with which I've been fairly happy, but have replaced the shorter focal lengths (and gone even smaller too) with Baader Orthos, but they also see use in my other scopes for planetary viewing.

James

[newman]

Thanks for the reply.

I'm a bit confused regarding AFOV and TFOV what exactly is the difference between the two? I have read the info on eyepieces on here but am worried about buying the wrong ones for an f/5 scope. Sorry I've only ever used stock eyepieces

AFOV stands for apparent field of view, and is a fixed property of an eyepiece (as in, if an eyepiece has 72 degrees AFOV, it will always have 72 degrees of it no matter in which scope is used). It basically defines how wide-field an ep is. Pretty much every manufacturer these days states what the AFOV of their eyepiece is.

TFOV stands for true field of view - it is the actual amount of sky you'll end up seeing through the eyepiece. Unlike AFOV, this isn't a fixed property of an eyepiece, but rather depends on the eyepiece's AFOV, and the magnification it gives in your telescope. Therefore, the same eyepiece will have different true fields of view in telescopes with different focal lengths. You have a short tube achromat, this is actually a pretty good low/medium power wide field instrument, making it pretty decent for DSO's which aren't too faint for that aperture. But you may find that high magnifications don't work all that well - you'd be best off testing that at a star party before you shell out your hard earned cash.

You calculate the true field of view by this simple formula:

TFOV = AFOV / magnification (in your telescope)

For example, my scope has 1000mm focal length. My 17mm Ethos has 100 degrees AFOV and gives about 58.8x magnification. Therefore, it's TFOV in my scope is, roughly, 100/58.8 = 1.7

When buying eyepieces, make sure you work out not just the magnifications they give you, but also the true fields, so you don't end up with two different magnifications that end up showing you roughly the same amount of sky. To clarify, my planned collection is the following:

17mm Ethos, 100 deg AFOV

10mm Ethos, 100 deg AFOV

6mm Delos, 72 deg AFOV

This gives me 59x, 100x, and 166x, but it also gives me 1.7, 1, and 0.43 TFOV.

Now, let's pretend that the 17mm Ethos I have is actually a Hyperion of the same focal length. The difference would be, the Hyperion has 68 degrees AFOV. While it would give me the same mag as the 17mm Ethos, it would only show about 1.15 TFOV. This would make it redundant, as the 10mm Ethos shows only a bit less sky at a greater magnification.

There are other factors to consider. At f/5, you want to make sure you get eyepieces that work well with a fast scope, and you probably want to borrow some at a star party and test, see what magnifications you can actually afford. An eyepiece that gives you 200x but causes all sorts of aberrations, can't focus, and has all clarity eaten by chromatic aberration will be pretty much useless.

[newman]

Thanks for the reply.

Due to the cost of the 2inch EPs I think I'll leave that for a while, I was thinking the 32mm Panaview would give me some nice wide field views for clusters etc.

A 32mm Panaview would give you a bit less than 19x magnification. I'd recommend a good pair of binoculars instead, I doubt it would be worth the purchase for your scope. I'd aim between 40x-60x for low power, as wide a field as you can afford. This will be your finder and faint DSO eyepiece.

[SiriusB]

The so called 5 element plossls work well, eg celestron ultima, antares elite plossl, baader eudioscopic, parks gold series.

My 35mm ultima gives nearly 3deg fov with hardly any edge distortion- a little too low mag most the time,so i usually start with a 20mm in the diag& work upwards.

If you need wider than 50ish afov,then i've not any experience of the premium brands that work well, but cheaper widefields struggle in this scope.

Depends how much distorted stars towards the edge bother you i guess.