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Is the 9mm Plossl really that bad?

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#26 sg6

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Posted 25 January 2021 - 12:46 PM

A 9mm plossl isn't too bad if you at least get a reasonable one. The other factor is people go for a fast scope and fast and short plossls are not a great mix.

 

The supplied items tend to be at the budget end which does nothing to help.

 

So it is unfortunately biased towrads a 9mm just not working too well.

 

Eye relief is down, around 6mm. And if you wear glasses then too short, and if you dislike getting up close and personal with an eyepiece it doesn't help.

 

I just found it best to accept there will be limitations. But as I sort of have too many eyepiece to ever use then whatever is supplied is often simply stored away.

 

9mm is "odd", Meade used to be the ones who supplied a 9mm eyepiece. That and the other "oddity" of a 26mm.

 

Easy answer is try it and see. If it works for you then fine. If it doesn't you will have lots of other options suggested. Many not even close to a replacement 9mm, +/-1mm say.



#27 Naja keravnos

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Posted 25 January 2021 - 02:13 PM

Thank you for the replies! In light of your advice, I think I'll try out the eyepiece first. Just wondering; will it yield pleasing results with an average barlow? (with it I'll probably use the 1.5x lens section most of the time, for around 200x power)



#28 aeajr

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Posted 25 January 2021 - 02:53 PM

Yes it will work fine with a 1.5x/2x barlow.

If you are itching to buy something, consider an 8-24 mm zoom, about $60-$300 depending on which one.

When teamed with your Barlow it will give you 50x to 300x and everything in between.

Edited by aeajr, 25 January 2021 - 02:58 PM.


#29 aeajr

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Posted 25 January 2021 - 04:09 PM

My XT8i is similar to your AD8. Based on my sky conditions I often topped out at 180x. I had my zoom eyepiece fit with the 1.5x Barlow for 75 to 225x. This was my most used eyepiece.

#30 rowdy388

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Posted 25 January 2021 - 04:10 PM

Plossls were highly in demand three & four decades ago and  were considered upgrades. The free eyepieces that came with most telescopes were usually,

and still often are, quite poor. You have a good quality start for your collection. I wouldn't even replace either before filling in the gaps first, but no hurry to do

that. A 12mm Paradigm would make a fine gap filler for a 2mm exit pupil.


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#31 Tony Flanders

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Posted 25 January 2021 - 05:47 PM

A 9mm plossl isn't too bad if you at least get a reasonable one. The other factor is people go for a fast scope and fast and short plossls are not a great mix.


That could be an issue in a scope with a very short focal length, say f/4 or shorter. But Plossls work beautifully at f/5, and the Original Poster's scope is f/6.

The Plossl's focal length is not relevant in this particular context. A 9-mm Plossl exhibits exactly the same aberrations as a 32-mm Plossl in any given telescope. And the eye relief is unaffected by the telescope's focal ratio.


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#32 Tony Flanders

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Posted 25 January 2021 - 05:52 PM

After doing a bit more digging into it I saw a Celestron zoom eyepiece pop up for 65$ on the classifieds and went for that. It covers from 8-24 mm and according to everyone's reports, has decent view quality.


The Celestron zoom is an outstandingly good value. It has much longer eye relief than a 9-mm Plossl, perhaps slightly bigger apparent field of view (at that focal length), and comparable optical quality. And as a side benefit, you can also use it at 8 mm, or any other focal length up to 24 mm, though the field of view at the last of those does leave something to be desired.

In fact my very best 8-mm eyepiece is a Baader 8-24 zoom at the 8-mm setting. Almost worth the price even ignoring the zoom capability.


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#33 JorgeDeAroGarcia

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Posted 25 January 2021 - 08:36 PM

Perhaps the plössl of focal lengths smaller than 15mm have very narrow exit pupils, so I do not choose them, remember that if you do not like it you can put it on sale, +1 in 68 ° and 82 ° eyepieces as they said above, although to Some of us like the somewhat more defined field stop, 52, 68? ... others will seek immersion. +1 on the purchase of a 2x Barlow and +1 on a zoOM eyepiece the 8 \ 24mm Meade / Celestron I love them. The focal length of the telescope and whether or not you use a flattener is also important. Health and clear skies.

#34 Tony Flanders

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Posted 26 January 2021 - 05:27 AM

Perhaps the plössl of focal lengths smaller than 15mm have very narrow exit pupils ..


Just to clarify: the exit pupil of an eyepiece in any given telescope depends entirely on the eyepiece's focal length, and not at all on the eyepiece's design. A 9-mm Plossl has the same exit pupil as a 9-mm Nagler.

 

I think that Jorge is actually thinking of the diameter of the eye lens -- the lens closest to your eye. Simple 3- and 4-element eyepieces with focal lengths shorter than 15 mm do indeed have small eye lenses.

 

The diameter of the eye lens correlates strongly with eye relief -- how close you need to get to the eyepiece to see the entire field of view. The smaller the eye lens, the closer you need to get. With a 7-mm Plossl, my eyelashes brush against the glass every time I blink. And since eyelashes are impregnated with oil and salt, that does not do good things to the optical quality.

 

Eyepieces with long eye relief (around 20 mm) necessarily have fairly large eye lenses. And given two eyepieces with identical eye relief, the one with the larger apparent field of view generally has the bigger eye lens.


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#35 Jon Isaacs

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Posted 26 January 2021 - 05:44 AM

The Celestron zoom is an outstandingly good value. It has much longer eye relief than a 9-mm Plossl, perhaps slightly bigger apparent field of view (at that focal length), and comparable optical quality. And as a side benefit, you can also use it at 8 mm, or any other focal length up to 24 mm, though the field of view at the last of those does leave something to be desired.

In fact my very best 8-mm eyepiece is a Baader 8-24 zoom at the 8-mm setting. Almost worth the price even ignoring the zoom capability.

 

I agree, the Celestron Zoom is a great value and for someone just starting out, it has the advantage that you can dial in the magnification in real time and see the difference is the views.  

 

I measured the AFoV of the Celestron zoom:

 

24mm  38.8 degrees

18mm  46.5 degrees

12mm  54.5 degrees

  8mm  63.3 degrees

 

That and a 2x Barlow covers a wide range, from 50x to 300x though the 50x is field is quite narrow. But from about 16mm to 8mm, it has at least the AFoV of a Plossl so that would be from 75x to 300x and be a good fit with the 30mm SuperView.

 

Jon


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#36 SlyStrat

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Posted 26 January 2021 - 12:33 PM

I bought a Morpheus 9mm to replace it.

Didn't even try it.

Based on using cheaper eyepieces when I was a kid I knew I wanted better optics.



#37 Dave Mitsky

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Posted 26 January 2021 - 01:53 PM

I own several zoom eyepieces but, except for specific purposes, prefer fixed focal length eyepieces.

Even a complete novice will likely prefer the enhanced eye relief and larger 60-degree apparent field of view provided by the relatively inexpensive Astro-Tech Paradigm Dual ED/Agena Starguider Dual ED eyepieces over Plössls so I second ngc7319_20's suggestion of 12 and 18mm eyepieces and a 2x Barlow lens.

 

https://www.astronom...anufacturer=360

https://www.astronom...ml?___SID=U&p=1

 

CN members get a discount from our sponsor Astronomics.

 

https://www.cloudyni...y_discount.html


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#38 aeajr

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Posted 26 January 2021 - 03:25 PM

I agree, the Celestron Zoom is a great value and for someone just starting out, it has the advantage that you can dial in the magnification in real time and see the difference is the views.  

 

I measured the AFoV of the Celestron zoom:

 

24mm  38.8 degrees

18mm  46.5 degrees

12mm  54.5 degrees

  8mm  63.3 degrees

 

That and a 2x Barlow covers a wide range, from 50x to 300x though the 50x is field is quite narrow. But from about 16mm to 8mm, it has at least the AFoV of a Plossl so that would be from 75x to 300x and be a good fit with the 30mm SuperView.

 

Jon

Understanding Telescope Eyepieces
https://opticsaide.c...-for-telescope/

 

Jon's AFOV chart is a great resource. The specs on the eyepiece say 40 degree at 24 mm and 60 degree at 8 mm.

 

So, the apparent field of view of a zoom varies over its zoom range.  Let me put this into perspective for you.

 

Using the Celestron Zoom in your AD8, at 24 mm, the low power end of the zoom, you would get 50X and about .75 degree field of view.  A 24 mm Plossl (50 degree AFOV) would give you the same 50X but about 1 degree field of view.

 

What does that mean? 

 

The Moon is about 1/2 degree across.   So a 24 mm Plossl would give you about two moon spans while the zoom, at 24 mm,, would provide about 1.5 moon spans.  Both would allow you to view the entire moon, with nice framing around it. 

 

To go to higher mag with single FL eyepieces you would change to the eyepiece you want, re-center the Moon and see how you like the view.   Perhaps you want to try a different magnification so you change eyepieces.

 

With the zoom you twist the barrel and zoom in as little or as much as you like while you are viewing the Moon to see the details you would want to see.  It works just like the zoom on a camera. 

 

Without a Barlow, at full zoom, 8 mm, that would take you to 150X and about a .4 degree field of view, or a little less than the full span of a full moon, based on Jon's AFOV measurements. By comparison, if you used an 8 mm Plossl eyepiece, you would get the same 150X but about .3 degree field of view. 

 

Using the 1.5X Barlow element you can effectively shift the working range of the zoom to 16mm to 5.3mm.  Or use the 2X barlow and make it 12 mm to 4 mm with a corresponding magnification range. 

 

All this tossing around of specs sometimes is hard to visualize.  I like to use the Moon to help people understand the concept of field of view and how it compares as you change eyepiece designs.


Edited by aeajr, 26 January 2021 - 03:32 PM.

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#39 Jon Isaacs

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Posted 27 January 2021 - 01:23 PM

I own several zoom eyepieces but, except for specific purposes, prefer fixed focal length eyepieces.

Even a complete novice will likely prefer the enhanced eye relief and larger 60-degree apparent field of view provided by the relatively inexpensive Astro-Tech Paradigm Dual ED/Agena Starguider Dual ED eyepieces so I second ngc7319_20's suggestion of 12 and 18mm eyepieces and a 2x Barlow lens.

 

https://www.astronom...anufacturer=360

https://www.astronom...ml?___SID=U&p=1

 

CN members get a discount from our sponsor Astronomics.

 

https://www.cloudyni...y_discount.html

 

I think zooms have their place, particularly for beginners, they let you change the magnification and immediately see the effect.

 

But that said, I prefer fixed focal length eyepieces. 

 

Jon


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#40 Tony Flanders

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Posted 28 January 2021 - 09:34 AM

I bought a Morpheus 9mm to replace it.

Didn't even try it.

Based on using cheaper eyepieces when I was a kid I knew I wanted better optics.

Maybe you should have tried it?

 

I'm not sure when you were a kid. But I still have the eyepieces from the scope that I owned as a child, but never truly figured out how to use. Those eyepieces are truly ghastly! Completely uncoated, and therefore full of ghosts. Short eye relief, field of view of 35 degrees or less. The difference between those eyepieces and a modern 9-mm Plossl is far bigger than the difference between a 9-mm Plossl and a 9-mm Morpheus.


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#41 BlueTrane2028

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Posted 28 January 2021 - 02:42 PM

My XT8i is similar to your AD8. Based on my sky conditions I often topped out at 180x. I had my zoom eyepiece fit with the 1.5x Barlow for 75 to 225x. This was my most used eyepiece.

Ugh, zooms.

I bought one because of one of your posts a long time ago, it gets some use occasionally (I presently have a gap between 8.8 and 14mm in my lineup), but the narrow field of view doesn't jive with me.

I use mine more for terrestrial spotting than astronomy.

To be clear, not mad, not everyone has the same tastes, I just prefer dedicated eyepieces with a generous field of view.


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#42 BlueTrane2028

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Posted 28 January 2021 - 02:47 PM

Maybe you should have tried it?

 

I'm not sure when you were a kid. But I still have the eyepieces from the scope that I owned as a child, but never truly figured out how to use. Those eyepieces are truly ghastly! Completely uncoated, and therefore full of ghosts. Short eye relief, field of view of 35 degrees or less. The difference between those eyepieces and a modern 9-mm Plossl is far bigger than the difference between a 9-mm Plossl and a 9-mm Morpheus.

Absolutely, this, again.

A 9mm Plossl is a revelation in my childhood refractor vs the garbage it came with from new.

I'm as gear oriented as they come, but may as well try what you have first.  If I have a dark sky, a telescope and some Plossls, I'm going to have a great night, regardless of if it's my preferred setup or not.


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#43 aeajr

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Posted 28 January 2021 - 04:00 PM

Ugh, zooms.

I bought one because of one of your posts a long time ago, it gets some use occasionally (I presently have a gap between 8.8 and 14mm in my lineup), but the narrow field of view doesn't jive with me.

I use mine more for terrestrial spotting than astronomy.

To be clear, not mad, not everyone has the same tastes, I just prefer dedicated eyepieces with a generous field of view.

You and I use our zooms in the opposite fashion.  Mine is a primary.  My 82s are my alternates.   You use your wide AFOVs as your primaries with the zoom as a utility player.

 

It is all good.   It is wonderful to have options.  



#44 Sheol

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Posted 28 January 2021 - 07:21 PM

                    You guys are spoiled brats! ROFLOL. You should have struggled with Kellner & Ramsden EPs back in the 70s. Awful doesn't even touch a description!

 

        Clear Skies,

                Matt.

            I did have a Zoom with my 70mm Jason Refractor. I actually did love that thing. I could use it as a finder & then increase magnification.


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