Low Power EP Recommendation

[Naemeth]

I'm looking for a low powered EP around 25mm (25mm gives 26x on my scope) which I can use as a finder and Messier hunting EP. It needs to have a FOV wider than my stock EP (which is 52 degrees) so I have an easier time of finding what I need if possible. Budget is about £50, although less would be good .

One last thing, it needs to be a 1.25" EP, as my scope only has (and probably can only take) a 1.25" focuser.

[John]

A 32mm plossl will show more sky (ie: a larger true field of view) than your current 25mm even though the apparent field of view of the 32mm is the same at 52 degrees. A good one like a GSO would be well within your budget too. A 25mm eyepiece giving the max FoV for the 1.25" size (around 68 degrees) shows no more sky and may exceed your budget.

[Andy_E]

have you considered the BST explorers (now renamed 'starguider' I believe). they really do offer excellent value at the sub £50 mark.

[Naemeth]

have you considered the BST explorers (now renamed 'starguider' I believe). they really do offer excellent value at the sub £50 mark.

I have, but hearing mixed reviews about the 25mm, it being the weakest of the lot gives me more to think about. What is the eye relief like on the GSO? I find my current 25mm is fine mostly, but my 4mm which has much more eye relief is often slightly more comfortable.

How much difference is 25x and 20x in magnification? Is 20x still fine to see Messiers?

I've had a look on Sky's the Limit, and have seen a TMB Planetary I Designed available in 25mm (58 Degrees), I really like these EPs, but haven't heard anything about the longer focal lengths, would these be worth trying (as they are cheaper than the BST's and have a comfortable eye relief)?

[John]

Your 4mm is a TMB planetary - these have more eye relief than most plossls and the eyepieces that are supplied with scopes. The 32mm plossl will have an eye relief of around 20mm or so which should be more than the 4mm TMB planetary and quite comfortable. 20x is good for the larger deep sky objects like M31, the Pleiades etc and as a finder for other DSO's before you apply more magnification to them.

[Mr Moff]

Celestron x-cel LX 25mm is a great eyepiece and not too expensive. 60 degree field of view.

[Eastridge]

What is the eye relief like on the GSO? I find my current 25mm is fine mostly, but my 4mm which has much more eye relief is often slightly more comfortable.

Hi, I have bought the GSO 32mm, I wear quite thick glasses & have found it fine, albeit limited use so far re clouds . I actually went for the photo visiual one at £40 from Pulsar Optical as you can unscrew the rubber eyecap which helps ER (according to video on their site & a member here) but personally found I didn't need to.

Clouds means no serious 'finding' use yet but happy with a brief view around Saturn & moons the other day which definately had something 'more' to it than thru supplied SW 25mm, although I am not expereinced or eloquent enough to say exactly what.

[michael.h.f.wilkinson]

A 32mm Plossl would work, but does give a 6.4mm exit pupil in a scope of F/5. This is OK for many, but a bit too big for some people (those whose pupils will no longer dilate that far). Also you will tend to see a shadow of the secondary more quickly. A 24 or 25mm wide angle is better, but cheap ones will show some aberrations at the edges of the field of view. This one, with 60 deg FOV seems to be suitable:

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3438_HR-Planetenokular---25mm---1-25----60----langer-Augenabstand.html

as is this one (in German, no translation available) with 70 deg FOV.

http://www.teleskop-express.de/shop/product_info.php/language/de/info/p2717_Antares-25mm-Weitwinkel---70--Feld---Max--Gesichtsfeld-mit-1-25-.html

They are cheap, and Erfle designs can show astigmatism in a fast scope.

[Naemeth]

Hm... I did consider a 25mm Planetary design. My maximum dilated pupil should be 7mm, but often I can't get my eyes fully dark adapted due to street lights, so this would probably be an issue.

[michael.h.f.wilkinson]

Hm... I did consider a 25mm Planetary design. My maximum dilated pupil should be 7mm, but often I can't get my eyes fully dark adapted due to street lights, so this would probably be an issue.

Do not forget that even if your eyes are fully dilated, a bit more magnification gives darker backgrounds which makes spotting objects easier (often). The view through a 31mm Nagler in my F/6 scope is better for spotting many things than with the 40mm Paragon.

[Naemeth]

Do not forget that even if your eyes are fully dilated, a bit more magnification gives darker backgrounds which makes spotting objects easier (often). The view through a 31mm Nagler in my F/6 scope is better for spotting many things than with the 40mm Paragon.

What happens if your pupils aren't dilated enough for an exit pupil of 6.4mm, and you try and look through that eyepiece?

[michael.h.f.wilkinson]

What happens if your pupils aren't dilated enough for an exit pupil of 6.4mm, and you try and look through that eyepiece?

Not all the light from the scope enters your eye, and you get an image on the retina equivalent to a smaller scope (with an apparently larger central obstruction). Suppose the exit pupil is 6.4mm and your pupils dilate to 5mm. In that case an 8" scope becomes a 6.24" scope effectively. Otherwise the image is normal.

[Naemeth]

Not all the light from the scope enters your eye, and you get an image on the retina equivalent to a smaller scope (with an apparently larger central obstruction). Suppose the exit pupil is 6.4mm and your pupils dilate to 5mm. In that case an 8" scope becomes a 6.24" scope effectively. Otherwise the image is normal.

Doesn't sound too bad, if the obstruction gets in the way as it were, could I just move the DSO to around the edge of the FOV and be able to avoid the secondary mirror?

For my 5.1" scope, what would my scope become (or what is the calculation you use?)

[michael.h.f.wilkinson]

Doesn't sound too bad, if the obstruction gets in the way as it were, could I just move the DSO to around the edge of the FOV and be able to avoid the secondary mirror?

Not really. The shadow of the secondary tends to move as you move your eye through the exit pupil. In the dark, the effect is not that large, in daytime, it can be a pain.

For my 5.1" scope, what would my scope become (or what is the calculation you use?)

It depends. If you stick to 25mm, it will stay a 5.1" scope, even as you grow really old. Assuming a 32mm EP giving a 6.4 mm exit pupil and 5mm dilated pupil diameter, the scope gets an effective diameter of 5.1" x 5 / 6.4 = 3.985" Though this does not sound bad, you are losing 39% of the light assuming no central obstruction. Assuming a 20% central obstruction that figure becomes 41% loss or 0.57 magnitude.

Even if your eyes still dilate to 6.4 mm (most likely) the few through a 25mm will be a bit more contrasty, due to a darker background.

[Naemeth]

I think it's safe to say I'll go for a 25mm EP then. Presumably some can get away with the much longer focal length EPs because they have a higher focal ratio.

[John]

If you can find a wide field 24mm / 25mm eyepiece, something offering around 68 / 70 degrees apparent field of view, you will be seeing as much sky as a 32mm plossl could show, with a smaller exit pupil and a darker background sky due to the higher magnification - thats a "win-win-win" situation really although finding such an eyepiece for your budget might be a challenge.

My original steer towards a 32mm plossl was due to it being easily accessible within your budget. Another option might be the Vixen NPL 30mm which would give you an exit pupil of 6mm - I doubt you would have any problems with that and the Vixen is a nice quality eyepiece for its £40 price.

[Naemeth]

If you can find a wide field 24mm / 25mm eyepiece, something offering around 68 / 70 degrees apparent field of view, you will be seeing as much sky as a 32mm plossl could show, with a smaller exit pupil and a darker background sky due to the higher magnification - thats a "win-win-win" situation really although finding such an eyepiece for your budget might be a challenge.

My original steer towards a 32mm plossl was due to it being easily accessible within your budget. Another option might be the Vixen NPL 30mm which would give you an exit pupil of 6mm - I doubt you would have any problems with that and the Vixen is a nice quality eyepiece for its £40 price.

Could I get a 68 degrees 2nd hand for about that?

Perhaps I should ask another question, how much would a preferable 24/25mm EP be (2nd Hand/New)?

[Carbon]

Since getting my 24mm 68° 1.25" Explore scientific EP I hardly use my 26, 32, & 40mm narrow field 2" EP's.

Granted, these 2" Celestron EP's were part of a kit so they weren't that great to begin with.

[DirkSteele]

A Televue 24mm Panoptic has a 1.25" barrel, 68 degree AFOV, and on your telescope would yield a 2.4 degree field, 27x mag and a nice comfortable exit pupil of 4.8mm. I own one of its big brothers and have been very impressed. The downside of course, is a premium eyepiece comes with a premium price.

[Naemeth]

A Televue 24mm Panoptic has a 1.25" barrel, 68 degree AFOV, and on your telescope would yield a 2.4 degree field, 27x mag and a nice comfortable exit pupil of 4.8mm. I own one of its big brothers and have been very impressed. The downside of course, is a premium eyepiece comes with a premium price.

Ouch at the price... I think that's definitely out of my budget... it may be too heavy for my scope too.

[Eastridge]

Being aged 46 / 51 with measured 5mm EP's we did a lot of thinking about Exit Pupil effects before deciding on the 32mm GSO Plossl. The long and short was that we wanted a wide FOV for large areas, finding etc. and didn't have anywhere near the budget for a 24/ 25mm wide field ( 68' ish) EP, even through that would clearly have been the optimmum solution.

New things that would have been good for us / our scope were all over £200 new & so reckoned that even secondhand would be over £100. And we could have gone the 2" route that you don't have which narrows your options considerably.

So, yes it's a comprimise and there is some 'lost light' & the background sky may not be as dark but happy so far. Depends how restricted your budget for what is ultimately only a 'finding' EP truely is I guess.

[michael.h.f.wilkinson]

The "finder EP" remark is important. Starting from a 5.1" scope, having effectively a 20x100 "finder" is not really a problem, is it (I must "make do" with a 16x70 ). You find something with the 32mm, and then switch to a 25 to have a closer look. Where the wide-field EPs shine is when you want to look at truly wide-field objects, like M31, M33, the Veil, the North America Nebula, or groups of galaxies in Virgo. These are actually few and far between.

[JamesF]

For what it's worth, I use my Revelation/GSO 32mm ep for DSO hunting in my ST120 more than any other eyepiece. I find it very comfortable to use and it's great for star-hopping. Last week when I was DSO hunting on the southern horizon I happened to have a Messier object (M28, possibly?) bang in the middle of the field of view when I first set the scope up and from there I found about a dozen more entirely by star-hopping using the scope rather than going back to the finder.

James

[Naemeth]

I think I'll try out the GSO, and switch to my 25mm EP if I find something.

[Moonshane]

I used to use a 35mm eyepiece in my f5.3 scope and this created an exit pupil of 6.6mm which in my light polluted skies was a little much. I then sold this and put the money toward a 26mm wide field eyepiece which created a far more impressive image and works well with my f4 scope, far better than the 35mm would have ever done.

another option for you is to save your money at the current time and put what you can towards a bigger scope.

in my experience, more aperture (e.g. an 8" dob) wil provide a far greater enhancement than replacement eyepieces.