Celestron Omni Plossl 40mm

[russ.will]

My father in law bought a Celestron 40mm Omni Plossl in the mistaken belief that a bigger number makes for higher magnification. Why he didn't ask first, god only knows...

Anyway, he bought it over tonight and as the 200p was out on the lawn, I tried it. Ignoring everything else, the one thing that struck me was the tiny FOV. Strange I thought, it's a Plossl and so 50deg shouldn't look THAT much smaller than the 60deg of my BSTs, but this thing is like looking through a keyhole.

The box and indeed website state 43deg FOV - Is this limitation a product of the field stop allowed in a 1.25" EP? If so, what's the point?

Russell

[Moonshane]

you are quite right that it's all about the field stop. broadly the widest field in a 1.25" eyepiece is achieved with a 40mm 43 degree, a 32mm 50 degree or a 24mm 68 degree eyepiece. I agree that there seems no point in a 40mm 1.25" eyepiece.

[John]

A 40mm plossl does indeed have a max AFoV of 43 degrees so it looks rather "tube like" compared to, say, a 32mm which often has 52 degrees.

And you are right, it's the internal diameter of the 1.25" barrel that sets the limit - the field stop can only be a certain size.

It's why 2" eyepieces are favoured for low power and wide views.

Edit: Shane and I typing at the same time there !

[Moonshane]

ha ha

at least we said the same thing!

[John]

There is a 40mm eyepiece in the .965" format on sale on E.Bay - imagine how narrow the FoV of that one is going to be !.

[twotter]

Blimey, when two James Bond villains are saying the same thing then everyone needs to take notice....hello MI6....

[Moonshane]

if you don't then we'll be coming at you with our "laser" (collimator).

[John]

The villains are so much more interesting than the good guys in the Bond films

[russ.will]

There is a 40mm eyepiece in the .965" format on sale on E.Bay - imagine how narrow the FoV of that one is going to be !.

If you can manage to do astronomy through that sort of FOV, you should probably be a surgeon.

I think I'll sling it on eBay and ping the father in law the proceeds.

Russell

[russ.will]

Okay, so if I may tax your fiendish cat strokers a little further!

My ST80 is piggybacked on the 200p for imaging. Last night, whilst doing the bit of planetary observing the seeing permitted, I had the 25mm (60 deg FOV) BST Explorer in the 45deg diagonal that the ST80 comes with. It was handy for zeroing in the 200p, when the GOTO had missed by enough to place the target outside of the 8mm EP FOV.

it struck me that it therefore worked quite well as a rather extravagant finder, but for those times when I prefer to manually, just have a look around rather than use the GOTO, a lower magnification for this purpose would be nice.

The 40mm example in this thread is already clearly a no-no. Is the example of the 24mm/68deg mentioned above the sweetspot in terms of minimum magnification coupled with a large FOV for clarity?

Thanks for your patience and now you can get back to world domination through nefarious means.

Russell

[John]

The 24mm / 68 degree eyepieces are a nice combination of as much sky as the 1.25" format can show with a decent amount of magnification to darken the background sky a bit to help spot the fainter deep sky objects.

The 45 degree diagonal will probably vignette (cut off) some of the field of view of the wider angle eyepieces though so you may want to consider swapping that for a 90 degree mirror diagonal.

The BST is 60 degrees but you can go as far as 70 degrees with a 24mm if you just that little bit more.

[russ.will]

Thanks John. I was just using the BST as an example, simply because I have one. I'll use that for finder/step back a bit views, but I'm reluctant to spend much on an additional EP for this purpose - It's very much a secondary use for the ST80. If I want proper wide field refractor views, I'd wheel out the ZS80 with it's 2" WO dialectric diagonal instead.

Cheers again,

Russell

[LukeSkywatcher]

I bought a 2nd hand 40mm Celestron E-Lux (1.25"). It had the same 43 degree FOV as you mentioned. It was like looking through the insert of a toilet roll. It was a horrible EP.

[russ.will]

That's the one! Thing is, I'm not sure what do with it. If I sling it on eBay, which let's face it, is the only place it will sell, I'll end up with guilt trips.

Russell

[LukeSkywatcher]

Took months for mine to sell here. Not much demand for 40mm.

[John]

The Omni 40mm plossl is not a bad eyepiece but it just suffers from the same issues that all 40mm plossls in the 1.25" fitting suffer from, even the Tele Vue 40mm plossl.

If it's on e.bay with it's spec in the advert then I don't think you need to feel any guilt about selling it - apart from the fact that it was a gift

[russ.will]

Indeed, back in my original post I did say "Ignoring everything else". It is beautifully made, easy to look through and actually nice and sharp across much of it's field. It's just that the field is very small.

I would of course list it accurately, which would elevate me above some websites I've seen, that list it as having the 'standard' Plossl 50* FOV.

Russell