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32mm or 40mm


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Both those are threaded for 1.25" filters but the 40mm does not show you any more sky than the 32mm, despite it's longer focal length, because the 1.25" format limits the size of the field you can get - so go for the 32mm. Also, if your scope is around F/5, the exit pupil (thats the round disk of light produced by an eyepiece / scope combined) will be oversize with a 40mm eyepiece which can cause problems such as seeing the shadow of your secondary mirror.

Edited by John
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Thanks for the fast reply :)

I have been looking on eBay and I have found a couple of suitable eyepieces...

NEW! MEADE 32mm Series 4000 SUPER PLOSSL Eyepiece Lens on eBay (end time 19-Oct-10 01:40:33 BST)

and

NEW CELESTRON 32mm PLOSSL 1.25" Telescope Eyepiece Lens on eBay (end time 26-Sep-10 05:53:32 BST)

Do you reckon it's worth the extra £10 for the Meade eyepiece?

Thanks again

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TBH I bought one of those Meade 4000 32mm SP's, and the contrast is rubbish with it (I'll have to see about taking it apart and blackening all the internals and lens edges at some point). May just have got a duff one though.

What is the focal length of your 'scope?

I got a bit caught out with my ST120, as EP's over 20mm (which gives it 30 x mag with the 600mm fl), generally didn't work satisfactorily with it (magnification was too low, even with the supplied 25mm - an 'ok' EP - at 24 x mag). eta: should add, that a 21mm Hyperion, for 28.5 x mag, was fine for contrast as well.

Personally I'd be really hesitant about using an EP that gives much below 30 x mag in a telescope, and would want to try it out first before parting with the cash.

Edited by Ogri
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The focal length is 650mm

I already have a 20mm eyepiece...

Do you not reckon its worth getting a 32mm in that case then?

I use a 31mm eyepiece in my 4" refractor which has a focal length ofr 663mm. Some nights the light pollution makes the background sky a bit too light for good views of deep sky objects so I switch to a 20mm eyepiece. On dark nights though the views are very enjoyable - especially of large objects such as M31, M42, the double cluster etc.

I think it's always useful to have a 30mm-ish focal length in your eyepiece case :)

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Well, it'll only give you 20 x Magnification.

If the contrast isn't good, the view will be awful, imho, and the EP won't get used, so wasting money. Lots and lots of content can so easily swamp the view, and that's where contrast really gets to be critical. More magnification, gives better contrast, and an EP with a wider field of view at higher magnification, can give essentially the same afov, but with far superior contrast.

It would certainly help if you could try one before you buy, to see how you get on with it?

How about a 25mm BST Explorer ED with a 60 degree afov? That'd give 26 x magnification, definitely better contrast, and about the same or better field of view to the 32mm plossl? It wouldn't break the Bank either?

1.25" Eyepieces

I've not tried one personally (yet), but even though the 25mm isn't supposed to be the very best of the range, it's supposed to still be a pretty darned good EP? Maybe someone with one here can comment on it?

I know it's pretty tough getting to grips with EP's for the shorter focal length 'scopes, and with my ST120, the 21mm and shorter Hyperions work very well (not that cheap even secondhand though), and I also get on amazingly well with the 20mm erfle design Adler Optik 20mm WA eyepiece .

If the eye relief on the shorter versions of that erfle design hadn't decreased so much, ruling them out for me, I think I'd have got the set (Sky's the Limit do them too I think).

Best of luck finding yourself something really enjoyable to use though. Despite the headaches, it is worth it when you find something that really suits you. :)

eta: The Tal plossls are supposed to be excellent, and the price of this one isn't bad http://www.warehouseexpress.com/buy-tal-25mm-super-plossl-eyepiece-1-25in-31-7mm-/p10648 ?

Edited by Ogri
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Getting good enough contrast with 21 x magnification is good going John. What eyepiece is it?

{eta: for perspective, I get very nice views with a 38mm eyepiece - in the 200p Skyliner - 1200mm fl - at 31 x magnification. I haven't had the chance to try it in the ST120 yet due to the weather, sad to say, but that should be an interesting insight at 15.7 x magnification - I may just be having my opinion affected by a couple of EP's that are giving low magnifications, one of which is doing so, particularly badly}

Llamanaut: "Skywatcher Skyliner 300P Dobsonian - giving 46 x mag; Skywatcher Evostar 120P - giving 31 x mag"

See the issue Llama?

It's the actual magnifications in a given 'scope, that got me into a right pickle when trying to decide on EP's when I first started. :)

Edited by Ogri
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I agree with all the comments suggesting the 32mm.

I have both a 40mm Celestron E-Lux and a 30mm Vixen NPL.

I had a 32mm Celestron which was my favourite EP for searching for DSO but i broke it by accident.

I now use the Vixen 30mm EP for searching for DSO in my scope which is also F/L 650. The contrast is FANTASTIC so DSO really jump out at you.

I know comparing both EP's is like comparing chalk and cheese. But basically in a 5" scope, DSO will be lost in a 40mm EP.

P.S.~~~Is your scope the SW Heritage? or the Explorer?

Edited by LukeSkywatcher
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That's good news Paul. Looks like I just struck unlucky with a couple of EP's then. :)

eta: and I am now drooling at the prospect of trying the 38mm 2" in the ST120! hehe!

Not sure i follow Ogri. Which EP's did you get unlucky with?

Most of mine are the kit Plossl Celestrons and are not exactly top of the range but they were cheap. The 32mm EP in the kit is/was FANTASTIC.

I bought both the 40mm and the 30mm to replace the 32mm because i couldnt decide which was better. It didnt take me long to find out that the Vixen 30mm is QUALITY. The 40mm E-Lux is still good but it has a very narrow AFOV (43*)........so it never gets used.

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....Getting good enough contrast with 21 x magnification is good going John. What eyepiece is it?....

Tele Vue Nagler 31mm :)

I agree with you that it's not effective on every night though - but on good nights, when I can see M31 and the double cluster with my naked eyes, the low power, 3.8 degree FoV is very enjoyable.

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"Tele Vue Nagler 31mm :) "

Xplains much! *grins*

Paul, the 32mm Meade 4000 SP is absolutely pants in the ST120, and only slightly better in the 200p dob. :p

The supplied LER 25mm Plossl (generally acknowledged as being 'ok'), at 24 x mag, isn't exactly hot on the contrast side of things for me either, sad to say.

Once I hit the 28 x to 30 x magnifications with the Hyperion and the WO SWAN clone, erfle type, everything became 'chalk and cheese' and even 'drop dead gorgeous' - even without the contrast boost from the Skywatcher LPF (which is a lovely help with contrast for me).

John:"but on good nights, when I can see M31 and the double cluster with my naked eyes, the low power, 3.8 degree FoV is very enjoyable."

This is why I am now drooling at the possibilities with the 38mm 2" in the ST120 on a good night. ;)

Edited by Ogri
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The SW LPF really is contrasty, aint it?

Besides my 2X TAL barlow, the SW LPF is the best 30 quid i have ever spent.

Couldn't agree more mate, best observing aid for the money going, imho, even with having almost no Light Pollution. ;)

I like the Castell UHC just as much for the difference in contrast, even if it doesn't get used quite so much as the Skywatcher LPF (there's a distinct possibility that might switch around with the extra light grasp of the 200p dob though).

I might not be an optical perfectionist given the state of my eyes, but given the state of my eyes, I freely admit I am verging on the fanatical about contrast. :)

Edited by Ogri
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Somebody must have got a one off bad eyepiece, my Meade 4000 32mm Plössl is very sharp and contrasty.

Yeah I agree it's looking that way, and it was especially compounded by my 25mm Plössl (the one supplied with the 'scope) being no great shakes with the contrast side of things as well.

I know contrast is improved with magnification, but the extreme differences these two particular EP's demonstrated, looks to have inferred wrongly to me that the difference is far more extreme than it would be with a well sorted EP.

I have no complaints with the sharpness of the Meade, by the way. I think it must have missed out on lens edge blackening and other things though. I might try and fix that with a tin of matt black hammerite or blackboard paint.

John sent me the original EP's that came with his 200p Skyliner when I bought it off him, and I'll have to dig out the 25mm and compare it with the one I got with the ST120.

The 25mm isn't supposed to be too bad an EP, after all.

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  • 4 years later...

I have a Skywatcher 250px and used the 25mm EP that came with the scope and was considering getting a 32mm skywatcher super Plossl EP, but was also wondering if a BST 25mm would be better than a 32mm skywatcher?

I think the 25mm BST would be a better bet in the Skywatcher 250PX. It's 60 degree apparent field will show almost as much sky as a 32mm plossl does but the shorter focal length and higher magnfication will give you the benefits of a slightly darker background sky and an exit pupil (as discussed earlier in this older thread) of 5.3mm which is an efficient size.

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I think the 25mm BST would be a better bet in the Skywatcher 250PX. It's 60 degree apparent field will show almost as much sky as a 32mm plossl does but the shorter focal length and higher magnfication will give you the benefits of a slightly darker background sky and an exit pupil (as discussed earlier in this older thread) of 5.3mm which is an efficient size.

Thanks.

I think I will go with the 25 mm BST.

I got a good view of M44 last night (my first Messier since getting my scope a couple of weeks ago) and was impressed with the view that I got through the stock piece that came with the scope, which I believe is a wide angle 25 mm MA.

On a seperate note, I was out until 2am with very nice dark skies where I Iive and I noticed that although I had a lovely view of the stars, that Jupiter and Saturn didn't look very clear? There was a bit of cloud around, but in the main it was clear.

I collimated the scope when I set it up and when I was packing away, I noticed that the secondary mirror was misty. It was a dewy night and my scope was damp etc. The primary mirror looked ok, but would a misty secondary mirror make objects look duller and is it ok to wipe it dry with a glasses lens cloth?

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There are a number of eyepiece types under the BST branding. The BST Explorer or Starguider (both the same) are the ones you are looking for I think. Thay are the best ones with that branding I believe.

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