Plossl vs Others

[DemonicOmelette]

Right, I bought a 32mm Plossl lens for my scope, and it is lovely. Now, my scope (skywatcher 130pm) came with a 10mm and a 25mm. Would it be worthwhile upgrading those to Plossl lenses? Or are they already? What IS the advantage of plossl lenses? And I understand 32mm is about the largest I can use, but would it be worthwhile getting a 4 or 6mm aslwell?

-eli

[Doc]

Your focal length is 650mm so your 32mm will give you x 20 your 25mm x 26, your 10mm x65 and if you get a 6mm it will be x108.

Your highest practical mag according to FLO's site is x260 so in theory you could get a 4mm which will give you x162 and still be within your scopes ability.

But at that mag your seeing will have to be very steady.

[DemonicOmelette]

Ok, cool. But am I better off with a Plossl, or another type?

[scotastro]

I have a feeling that many on SGL think that the supplied EPs particularly Skywatcher are rubbish without comparing them to any other EP.

I would say that the 25mm and 10mm EPs are more than adequate particularly in the smaller scopes like your 130mm. When I was trying out the new AZ GOTO version of the scope I compared the supplied lenses with a number of others including Vixen Lanthanums costing over £100 each. In my dark sky I could see very little difference in the views.

When used with a barlow the SW EPs give a good range of magnifications.

Following on from Doc - a 4mm Plossl has a very short eye relief so if you wear glasses to observe you may well struggle to get your eye close enough to the EP. At higher powers the wide angle EPs usually have better eye relief than Plossls

Mike

[Doc]

Depends how much money you want to spend. I personally say in your scope stick with plossls.

You could get 6mm ortho which would give great sharp views but have very short eye relief and if you wear glasses I would say No.

[Doc]

Sorry Mike posted same time

[johninderby]

Unfortunately the name Plossl is now being put on a lot of eyepieces that aren't true plossls (semi-plossls?). Just take a look at what is available on eBay for example.

A true Plossl is a good eyepiece and doesn't have to have the Televue name on it to be a true Plossl. The eyepieces that come with a lot of Skywatcher scopes are actually not too bad at all.

If in doubt go by price. If it's cheap it isn't going to be a real Plossl.

John

[DemonicOmelette]

I'm still trying to find the difference between Plossl and not plossl. The ones that came with my scope, I am presuming they are not plossl?

[Doc]

Eyepieces come in a variety of designs. Choosing the right one depends on what you plan to observe, how finicky you are about image quality and field of view, and how much you're willing to spend.

Kellner

The 3-element Kellner gives sharp, bright images at low to medium powers. Best used on small to medium-size telescopes, Kellners have apparent fields around 40� and good eye relief, though short at higher powers. Good, low-cost performer, far superior to simpler Ramsden and Huygenian designs.

Orthoscopic

The 4-element "ortho" was once considered the best all-around eyepiece, but has lost some of its luster because of its narrower field compared to newer designs. Excellent sharpness, color correction, and contrast. Longer eye relief than Kellners. Especially good for planetary and lunar observing.

Plossl

Today's most popular design, the 4-element Plossl provides excellent image quality, good eye relief, and an apparent field of view around 50�. High-quality Plossls exhibit high contrast and pinpoint sharpness out to the edge. Ideal for all observing targets.

Erfle

The 5 or 6-element Erfle is optimized for a wide apparent field of 60� to 70�. At low powers, its big "picture window" viewing area provides impressive deep-sky views. At high powers, image sharpness suffers at the edges.

Ultrawide Types

Various improved designs incorporating 6 to 8 lens elements boast apparent fields up to 85� - so wide you have to move your eye around to take in the whole panorama. Light transmission is slightly diminished, but otherwise the image quality in these eyepieces is very high. So too is their price.

[Chris Stacey]

Your highest practical mag according to FLO's site is x260 so in theory you could get a 4mm which will give you x162 and still be within your scopes ability.

Out of interest Doc, where on FLO's site can you work the above out, i'd like to know the highest practical mag for a skywatcher 8" dob!

[Doc]

A general rule of thumb for applying power is no more than x50 of magnification per inch of aperture.

Your scope is 200mm aperture so taking that times 50 gives me a limit of about x400. Generally you don't come close to using that much power.

FLO lists your scope as x406.

[Chris Stacey]

Thanks. Handy to know. :THUMBS UP:

[Merlin66]

I don't know what the design of the Skywatcher eyepieces are...

The Plossl desgn which has become more and more popular over the last 20 years are very good and can be used on all telescopes.

There are other which will work equally well on telescopes which have focal ratio's above f5 or so; the Modified Achromatic ( Kellners) were very popular; the Orthoscopics ( three element) are still acknowledged as very good eyepieces for lunar and planetary.

The real answer; use the ones which you feel do the job for you.

As you gain more and more experience with observing, you'll become more conscious of the benefits of the "high corrected" eyepieces ie Wide angle/ monocentrics etc etc.

Walk before you run.

[Astro_Baby]

The Sky-Watcher site lists them as modified achromats so I assume they are closer to Kellners than anything else.

To be honest I never really found much to moan about with them. Both performed ok as far as I was concerned. The 25mm never really had the contrast that the Celestron Plossls had but to be honest I found them pretty good myself for budget EPs.

[DemonicOmelette]

Kellner

Kellners have apparent fields around 40�

Plossl

Today's most popular design, the 4-element Plossl provides excellent image quality, good eye relief, and an apparent field of view around 50�.

Erfle

The 5 or 6-element Erfle is optimized for a wide apparent field of 60� to 70�.

Ultrawide Types

Various improved designs incorporating 6 to 8 lens elements boast apparent fields up to 85�

I thought that your FOV was a factor of the aperture? Like a 10mm automatically has a much smaller FOV than a 32? Or am I mis-understanding something?

I will keep the EP's I've got for now, anyway. I think I'm just bored with the cloudy evenings, so have been browsing Ebay and such...

Thank you all for the 411 though, I learn by asking questions, and you are all brilliant at answering them!

-eli

[ZOG]

As far as I was aware, the EP's that came with my 130pm were Kellners. The ones that came with my dob were plossls, and guess what ? I far preferred the view through the Kellners. this was only true among the SW ep's though. My Meade 32mm plossl and my Televue 40mm blow all of the SW eyepieces out of the water.

I'm afraid that, like so many other things, you only get what you pay for.

[John]

....I thought that your FOV was a factor of the aperture? Like a 10mm automatically has a much smaller FOV than a 32? Or am I mis-understanding something? ...

True Field of View (TFoV) is the amount of sky you see through a particular eyepiece / scope combination and is a factor of the diameter of the field stop of the eyepiece and the focal length of the scope it's being used in. The formula thats usually given is:

TFOV (degrees) = (180/Pi) X field stop diameter in mm / Scope focal length in mm

So it is possible for an ultra wide angle eyepiece (eg: Tele Vue Ethos) to offer more magnification AND a wider TFoV than a longer focal length non-wide angle design.

The aperture of the scope does not come into the equation.

John

[Merlin66]

The Field of View ( as seen in the sky) can also be approximated by:

Apparent Field of eyepiece/ Magnification.

ie if you have a 25mm eyepiece with a FOV 52 degrees and it gives you x100 magnification, then the field would be 52/100 =0.52 degrees - about the diameter of the Moon.

If the same eyepiece had an apparent FOV of say 80degrees ( wide angle type), you would get the same magnification, but the field would be 0.8degrees; almost twice the diameter of the moon!!

Hope this helps.

[FLO]

FLO lists your scope as x406.

Others have said this but it is worth repeating: You can reckon on around 40-60x magnification per inch of telescope aperture, hence an 8" telescope supporting around 400x. However, on all but the very best nights our atmosphere (dust, thermals, moisture, etc) reduces that to around 200-250x.

HTH

[The Warthog]

The eps that came with your scope may well be Plossls. If they are, they will usually say so, or have a letter 'P' stamped near the focal length. 'K', 'MA' or 'SMA' designates a Kellner.

Plossls have a AFOV (apparent field of view) of 52º. This is respectable. Most 'brand name' Plossls really are what they say, and are decent eps. I have kept my Celestron 32mm as a back up to my 24mm SWA on dewy nights.

The biggest drawback of the Plossls I had was that once you got below 6mm, they were uncomfortable to look through. You can solve this, to an extent by using a Barlow and a longer ep, which retains the eye relief of the longer ep but gives you the magnification of the shorter ep. By all means, get yourself a 6mm Plossl and a 2x Barlow. It doesn't have to be an expensive Barlow, but a good mid-priced Barlow will do fine. (My Barlow happens to be an Apo, but for all the difference I can see, I could have saved half my money.)

You want to avoid the trap of spending far too much on accessories for this scope, but it's an easy pit to fall into. It is possible to buy an ep which costs more than your entire kit, but at that point you would benefit more by buying a new, bigger, scope.

However, you may want to consider a short focus ep with a higher eye relief and wider angle than a Plossl. In my ep kit, listed in my sig, I have an Antares W70, 4.3mm, which fits the bill, and cost only twice what a Plossl would. The SWAs listed cost me twice as much again, but still my basic ep kit cost about what each of my scopes cost me, and less than they would cost new. My basic ep kit is only 3 eps and a Barlow, notice.

So, Plossls are good eps, less so in the shorter lengths. You can get good views without breaking the bank, and if you have to 'make-do' with Plossls and Kellners for a bit, that's still pretty good.