Help understanding eyepieces

[Eddster]

Hi there,

During my astronomy hiatus, I've lost all the knowledge (fractional though it was) I held about optics. Can't remember a damn thing.

Having been scopeless for a great number of years, I've just been practically gifted a Skywatcher 130m (£50 for this with the motor drive I consider a SERIOUS bargain). So, I'm running at focal length 900mm, F6.9 (or thereabouts).

With it, I got a 25mm super wide angle Plossl and a 10mm Plossl with a REALLY dirty 2 x Barlow. I've bought a cheapo 30mm Plossl and 5mm Plossl to go with it. I bought these cheaper ones as I wanted to make sure I had a range to start with before I start splashing out (and in case the kids break them) as funds are low. I also bought a moon filter which I thought gave really good results (especially considering it was £6 on eBay).

So, the questions....

1) With my old scope I had Kellner and Orthoscopic eyepieces. I have no idea what the differences are between them and also between them and the Plossls. Can anyone give me a quick overview please?

This is with a view to forming an opinion about what I should get next. I'd probably be looking to spend up to around £40-£50.

I'm more interested in deep sky objects and the kids like planets. I'd rather spend more money on myself first (  ). I was planning on a 15mm eyepiece, would that be sensible? Or am I better off getting something better for my deeper sky observing?

2) How the heck do I clear up that Barlow (see images) or, more importantly, what's the best replacement without breaking the bank (up to £50).

Thanks in advance, I hope that's not too much at once.

Link to more photos: https://goo.gl/photos/7o91WZFXnxGLE2bC7

[michael.h.f.wilkinson]

Kelner, Plossl and orthoscopic EPs are similar in construction in many ways, in particular because they consist of two groups of lenses each, with convex sides facing each other. The Kelner is the simplest: a single plano-convex lens followed by a plan-convex doublet for better colour correction than the even simpler Ramsden design (just two plano-convex lenses). The Plossl as two achromatic doublets with the convex sides facing eachother. The Abbe orthoscopic is a triplet followed by a plano-convex single lens. Kelners are cheaper, and OK at longer focal lengths, but inferior to Plossls in terms of colour correction. Orthoscopics are better still in terms of correction, but these have a smaller apparent field of view (AFOV). 

 

I would not buy any new kit until you have played around with the stuff you have. For a long time I made do with a threesome of Plossls and a focal reducer on my C8. Only much later did I add more, and replace stuff, once I knew the strengths and weaknesses of the EPs I had, and my own observing preferences

[beka]

I am with Michael. h.f.wilkison on this. You have enough eyepieces to get started on, even without the Barlow.

[Eddster]

6 minutes ago, michael.h.f.wilkinson said:

Kelner, Plossl and orthoscopic EPs are similar in construction in many ways, in particular because they consist of two groups of lenses each, with convex sides facing each other. The Kelner is the simplest: a single plano-convex lens followed by a plan-convex doublet for better colour correction than the even simpler Ramsden design (just two plano-convex lenses). The Plossl as two achromatic doublets with the convex sides facing eachother. The Abbe orthoscopic is a triplet followed by a plano-convex single lens. Kelners are cheaper, and OK at longer focal lengths, but inferior to Plossls in terms of colour correction. Orthoscopics are better still in terms of correction, but these have a smaller apparent field of view (AFOV). 

 

I would not buy any new kit until you have played around with the stuff you have. For a long time I made do with a threesome of Plossls and a focal reducer on my C8. Only much later did I add more, and replace stuff, once I knew the strengths and weaknesses of the EPs I had, and my own observing preferences

Thanks Michael, that's really helpful. Are any of the types of eyepieces considered better for any particular type of viewing though? (I'm thinking deep sky vs. planetary here)

[michael.h.f.wilkinson]

Not as much as is sometimes claimed. Plossls are pretty good all-round performers. Orthos are said to have an edge on planets. All types discussed here are so-called scaled designs: different focal lengths are achieved by simply changing the sizes and spacings of all elements equally. A long focal length Plossl is simply a larger version of the shorter ones. This means that the eye relief (distance between rear element and optimal viewing position) also scales. Very short Plossls and orthos have very short eye relief, making them unusable for people with glasses. Very long ones have uncomfortably long eye relief, requiring the user to let his eye "hover" at quite a distance from the optics. So-called planetary EPs (in different forms) have a different design, often with a more or less fixed rear part, and putting negative lenses (called Smyth lenses) in front of the rear group at different distances. the Smyth lenses act as Barlows of different powers. In this way, eye relief can be kept constant across a large range of focal lengths (16-20mm). This comes at the expense of more elements (and therefore more expense), which can add scatter and internal reflections, but with modern coatings this can be minimal. My Pentax XW and Tele-Vue Delos eyepieces are in this category, and they perform very similarly to orthoscopics on planets, with the dded bonuses of a much larger field of view, and much more comfortable eye relief.

[Louis D]

Orthoscopics are generally regarded as the best of those for planetary viewing.  For deep sky, erfles and konigs provide wider apparent views.  You won't find those names on modern eyepieces, though.  They'll likely be labeled Super Wide Angle (SWA) or some such.  There are many newer designs that cost much more and mostly provide better correction over a wide apparent field of view (AFOV) at fast focal ratios (think shorter, stubbier telescopes) and often have green lettering on them (think Tele Vue Optics).  You don't need to splurge on them while starting out.  Get out under the skies with what you've got and observe.

As far as barlows go, if you can find a good used Meade 140 2x from the 80s/90s, you can't go wrong with it for the money.  You can even screw the optics unit onto the front of your diagonal for additional power.  Avoid most of the cheap shorty type barlows.  They aren't worth using in my estimation.

[pipnina]

I had thought the standard 25mm supplied was a plossl, someone else had said it was a modified achromat. I think the standard 10mm is an Ortho.

With the 130/900, you probably do want a 32-30mm EP simply because you'll get the biggest FOV and exit pupil possible from that scope. The slowness of it makes it difficult to get wide FOV with the 1.25" focuser.

I think you're right to have gotten the 5mm & 30mm plossl, the supplied barlow isn't that great to begin with and the one you've ended up with looks manky!... Mind barlows are meant to improve the eye relief and I've been told before that getting, say, a 12mm ortho and a 2x barlow would basically give you a 6mm ortho with more eye relief than a 12mm.

[michael.h.f.wilkinson]

I have heard of modified achromats as standard EPs, although Plössls are more common nowadays. I have never heard of scopes in this class coming orthoscopic EPs. Triplets are expensive to make.

[John]

Some scopes are supplied with a plossl or two now. Some even with 2" a 5 element wide angle such as the Meade 26mm QX. Higher quality scopes usually don't come with any accessories on the basis that the purchaser would already have their preferred items. Tele Vue do supply one of their 20mm plossls and an Everbright diagonal with their refractors though.

Many slightly lower end scopes are still supplied with these 3 element modified achromat types:

[beka]

However, it really is best to use the set you have now and to start observing different objects. You will get to discover the objects you are interested in following which you will be able to make more informed upgrading decisions. You may find that you are more limited by the scope itself rather than the eyepieces if you want to observe wide fields or deep sky. On the othet hand, differences in planetary views between different types of eyepiece can be very subtle and only noticeable by very experienced observers. There is a chance that you  may be disappointed if you purchase an accessory that does not give a result you expect.

[Mak the Night]

The Sky-Watcher 10 & 25mm EP's supplied with many of their scopes are modified achromats (MA). There is even a 3.6mm but I don't think it is an orthoscopic.

http://www.scopesnskies.com/prod/skywatcher/eyepieces/super-maserieseyepieces.html

The 25mm MA isn't that bad at all really.

The supplied 2x Barlow with Explorer 130 scopes is quite poor quality. Even the draw tube is plastic covered with a metal band. It doesn't even belong inside a Xmas cracker.

This Celestron Barlow from FLO is quite good: http://www.firstlightoptics.com/celestron-eyepieces/celestron-2x-universal-125-barlow.html

I'm pretty sure it's manufactured by GSO, like a lot of Celestron accessories.

[John]

37 minutes ago, Mak the Night said:

This Celestron Barlow from FLO is quite good: http://www.firstlightoptics.com/celestron-eyepieces/celestron-2x-universal-125-barlow.html

I'm pretty sure it's manufactured by GSO, like a lot of Celestron accessories.

Those are decent barlows for the price. I think Synta make that one. The Celestron Omni 2x barlow is a GSO product I reckon. Not quite so good as the Synta one when I tried it.

 

[John]

37 minutes ago, Mak the Night said:

This Celestron Barlow from FLO is quite good: http://www.firstlightoptics.com/celestron-eyepieces/celestron-2x-universal-125-barlow.html

I'm pretty sure it's manufactured by GSO, like a lot of Celestron accessories.

Those are decent barlows for the price. I think Synta make that one. The Celestron Omni 2x barlow is a GSO product I reckon. Not quite so good as the Synta one when I tried it.

 

[Mak the Night]

1 minute ago, John said:

Those are decent barlows for the price. I think Synta make that one. The Celestron Omni 2x barlow is a GSO product I reckon. Not quite so good as the Synta one when I tried it.

 

Yes, you're probably right. Although, apart from the weight I couldn't detect much difference, if any between the two.

[Mak the Night]

After some experimenting with a lot of my older, lighter eyepieces, I have found that I only really use these regularly with my 90mm grab'n'go Mak recently. I did have a little box to go into my grab'n'go case along with the Mak containing a load of old Plossls and a WO SWAN.

These five all fit into a compartment of the Omegon telescope carrying bag itself! I can span 40x to 181x with these. Admittedly, the Kellner doesn't see much action lol. The 18mm AH ortho' seems to be permanently in the diagonal.

[Mak the Night]

Four 2x Barlows. The TeleVue (extreme left) is undoubtedly the best out of these. Next to the TV is the Celestron 'Universal' Barlow, John's probably correct about this being manufactured by Synta, as I'm pretty sure these are sometimes supplied with Sky-Watcher scopes as 'De-luxe' Barlows. The two on the right are basically the same and are Celestron. The silver coloured one is the 'Omni' and the black one is from the Celestron Eyeopener Kit. These must be the Barlows manufactured by GSO. The Celestron Barlows are all around £20-25 each. The TeleVue is over £100. Only the TV and the Universal have detachable Barlow elements. The TeleVue is physically heavier than all three of the Celestron Barlows put together!

[Eddster]

Excellent folks. Thank you all so much for the responses. I'm feeling better educated already.

Thursday night is looking clear for me so if the moon gfets out of the way in time, I'm going to go galaxy hunting and see how out performs. I'm way more interested in deep sky so will be interested to see how far I can push what I've got (especially as I need a new battery for the red dot finder!).

[Louis D]

I have the TV 2x barlow, and the optics unit is not threaded with the standard 1.25" filter thread, so it isn't all the useful separately.  That's why I recommended the Meade 140 3 element "APO" barlow.  I use the optics unit on the front of my binoviewer as an OCA yielding around 3x or so.  I also put it on the front of my 1.25 diagonal and put a 0.5x focal reducer on the front of my binoviewer to yield about a 0.25x OCA.  It vignettes quite a bit, but it is surprisingly sharp in the middle with a wide field of view.  I also have the Orion (USA) 2X shorty from the early 2000s, and it is noticeable worse than either the TV or Meade.  I would recommend just about any 1980s/1990s Japanese made long 2X over any of today's shorty 2X barlows.  Orion (USA) had both a 1.25" and a 2" 2x Japanese long barlow in the late 90s.  I have the 2", and it handily beats the 1.25" TV 2x barlow.  The only thing better is the GSO 2" 2x barlow with the TV Panoptic Barlow Interface.  Absolute perfection.  The Orion's focal length is too long to mate properly with the PBI, so it vignettes the edges of max field of view eyepieces like the 40mm Meade 5000 SWA.  The GSO/PBI combination does not.  It is absolutely invisible except for the magnification increase (and the incredibly long moment arm in your focuser/diagonal).

[Mak the Night]

18 hours ago, Louis D said:

I have the TV 2x barlow, and the optics unit is not threaded with the standard 1.25" filter thread, so it isn't all the useful separately.

It's always fascinated me how eyepiece/Barlow/Filter threads can vary so much. My Celestron (Synta) Barlow element threads into all of my Celestron EP's (as you would expect), my Baader BCO's and my William Optics SWAN's, but not my TeleVue Plossls or Asrtro Hutech ortho's. Celestron filters won't thread into anything other than Celestron diagonals yet my Baader filters will thread into any diagonal I possess (I think). I've decided to get a shorty apochromatic Barlow anyway.

[Eddster]

Excellent information folks, thanks to all of you. I'm going to try the Celestron barlow from GSO to replace that manky object I have. Particularly the night before last. I managed to get some really good magnification, the air was still and I saw a fabulously large disc with distinct cloud belts. VERY happy.

Jupiter has been pretty amazing all round with what I've got (until I try to use that manky barlow!). I haven't made it galaxy hunting yet as every time I get the scope out the kids are commandeering it to look at jupiter and the moon again. Hopefully with the longer nights, they'll be sound asleep by the time it's outside and I can get some peace  

[Mak the Night]

4 hours ago, Eddster said:

Excellent information folks, thanks to all of you. I'm going to try the Celestron barlow from GSO to replace that manky object I have. Particularly the night before last. I managed to get some really good magnification, the air was still and I saw a fabulously large disc with distinct cloud belts. VERY happy.

Jupiter has been pretty amazing all round with what I've got (until I try to use that manky barlow!). I haven't made it galaxy hunting yet as every time I get the scope out the kids are commandeering it to look at jupiter and the moon again. Hopefully with the longer nights, they'll be sound asleep by the time it's outside and I can get some peace  

There's also this: http://www.telescopehouse.com/revelation-astro-2-5x-barlow-lens.html

[happy-kat]

This is a great read on understanding eyepieces.

Link here

[Eddster]

On 15/04/2016 at 20:56, happy-kat said:

This is a great read on understanding eyepieces.

Link here

Thanks, I've just got round to reading that. Really useful.

Thanks again everyone for all the advice.

I bought the Celestron Barlow and have not been disappointed. I feel like I can see again. I also gave in to temptation and bought a 15mm eyepiece.

Happy days.

[Mak the Night]

37 minutes ago, Eddster said:

Thanks, I've just got round to reading that. Really useful.

Thanks again everyone for all the advice.

I bought the Celestron Barlow and have not been disappointed. I feel like I can see again. I also gave in to temptation and bought a 15mm eyepiece.

Happy days.

Was that the Celestron Universal Barlow? Because if it was it is incredibly useful for a lot of things.

The Barlow element can be removed and threaded directly onto an eyepiece.

The adaptor part has a T-thread for a camera and can be used as an adaptor in its own right. I can thread a Celestron ND filter into mine and use the filter with any eyepiece as filter threads aren't always universal.

Not only that, the Barlow itself is pretty good quality for an achromatic unit.

[Eddster]

Yes, that's the one. Got it from FLO. Bargain bit of equipment.