They give no further specifications. I am just guessing that it fits the barrel diameter 1.25", and its AFOV is 43 degrees.
I tried to find what it was rebranded from, but could find nothing. All the Plossls at 40 mm (Celestron, Meade etc) are a rebrand of GSO Plossl. However, the shape of the barrel in this one is different.
Can anyone suggest if this Plossl is a good buy ? I have a refractor 90 mm diameter, 910 mm focal length. My intention is to use the eyepiece for the initial scoping of the sky.
One thing you need to understand about eyepieces is how barrel diameter affects things.
Take two straws, same length but one larger diameter than the other. Looking through each, you'll notice that you see a wider field through the larger diameter than the smaller one. With eyepieces it is the same. There comes a point in eyepiece focal length that you just cannot go any wider in the True Field of View. You can go longer in focal length, but the AFOV just gets smaller, and the true field of view stays the same.
There is another thing (a few more actually, but one at a time), and that is as you increase foacal length you also reduce contrast in the resulting image. This means you start to battle with the background sky glow. Increasing magnification also increases contrast that can help us see fainter things.
Now, barrel size vs AFOV. As we now know that we can only see a limited amount through a straw, it also means that the widest AFOV is also limited. You cannot get a 40mm 1.25" EP with an AFOV of 68°. The longest focal length that gives an AFOV of 68° in 1.25"barrel is 24mm.
Plossls typically have an AFOV of 52°. But there is also a limit to this. The longest focal length that will yield an AFOV of 52° in 1.25" barrel is 32mm. Longer focal lengths only give smaller AFOV.
While at first it may seem like a good deal, just be aware of the limitations. You need to decide if it really suits your requirements. You may be better off with a 32mm plossl.
2" eyepieces have the same limitations, allbeit the TFOV is wider. The longest focal length giving a 68° AFOV is 42mm here.
Note also, that as you increase AFOV you reduce the maximum focal length shortens:
82° AFOV in 1.25" is approx. 16mm
82° in 2" is approx. 31mm
100° in 1.25" is approx. 13mm
100° in 2" is approx. 21mm
Having said all this, there still is a place for longer focal lengths. Certain EP designs in a given scope design can yield an image with fewer aberrations. You also need to be careful here to as there can be other consequences to this too.
Alex.
Last edited by mental4astro; 04-01-2015 at 04:22 PM.
i sometimes find the lower mag helps with surface brightness even though the fov seems a bit restricted
Yep, me too. Thanks for mentioning this Daniel. Part of the things that a longer focal length EP can help with. The restricted FOV is the 'no free lunch' bit though...
Experience and an open mind is ultimately your best friend. Knowing the ins and outs is the starting point. You might be pleasantly surprised when you push the envelope...
The maximum true field of view possible in an eyepiece is set by the inside diameter of the barrel. For polls this will be achieved with a ~25-30mm eyepiece. The effect of using a longer focal length eyepiece (in this case 40mm) is that the true field of view remains limited by the barrel, and its apparent (angular) field of view is limited to ~ 30 degrees. A bit like looking into a toilet roll.
Personally I'd suggest for your scope you should look for eyepieces in the range 7 ~ 28mm in 1.25 " barrels, and there are plenty of bargains to be had under $50. If you can find one secondhand, a set of Edmund RKE's (8, 12, 15, 21 and 28mm) would be perfect for your scope.
If your scope can accommodate a 2" eyepiece barrel, then you can usefully use something like a 40-50mm, but these don't come cheaply.
I've never really understood why anyone makes a 40mm plossl in 1.25", what with a 32 having the same tfov. Your and Daniel's point about surface brightness is a good one though, makes me rethink.
Experience and an open mind is ultimately your best friend. Knowing the ins and outs is the starting point. You might be pleasantly surprised when you push the envelope...
Hm... should I push the envelope by acquiring the Baader Hyperion-Aspheric 36mm, http://agenaastro.com/baader-36mm-hy...-eyepiece.html ? It is specified to have an AFOV of 72 degrees with the 2" barrel, but it unspecifiedly shrinks when a 1.25" appendage is used. The reviews suggest that the AFOV is the same as when using a Plossl with F=32 mm and AFOV=52 degrees, or a Plossl with F=40 mm and AFOV=43 degrees. This seem to be a maximum FOV for the 1.25" barrel.
Nevertheless, it will probably possible to tease out a few more percent of field of view when using the Baader. Are there other reasons to get Baader Aspheric over Plossl ? Like better image quality, less distortions etc ?
writing looks similar to the old meade [chinese made] as opposed to the GS [taiwan made]
see the 26mm here
Thanks for the link. I notice that the 26 mm Meade Plossl looks like the 40 mm Australian Geographic (AG) Plossl.
However, the said 26 mm Meade Plossl is a part of the current line up of their 4000 series Plossls, http://www.meade.com/products/access...es.html?cat=45
Their current 40 mm Plossl looks differently to the AG 40 mm Plossl.
It appears that the Meade ("enhanced" 5-element) Plossls were made in Japan until 1998. After that, Meade moved the manufacturing to Taiwan, and downgraded the eyepieces to the "real" 4-element Plossls. In 2002-04, Meade moved manufacturing of this series of Plossls from Taiwan to China. The Plossls were downgraded even further. The article author did the testing and comparison of those Plossls... it appears that the performance decreased with each move.
So... I wonder if the AG has the "old", Taiwan-made Plossls in stock. Anyway, I must confess of my foolishness -- I have placed an order for the AG 40 mm Plossl for $25. Now I am thinking that this is a waste of money.
There will be nothing wrong with the Plossl apart from the aforementioned slight tunnel vision all 40mm inch and a quarter eye pieces suffer , you will get great views of star cluster,s etc in your scope. Enjoy the views and don't worry , my first low power eyepiece was much worse , trust me.
Brian.
Quote:
Originally Posted by va1erian
Thanks for the link. I notice that the 26 mm Meade Plossl looks like the 40 mm Australian Geographic (AG) Plossl.
However, the said 26 mm Meade Plossl is a part of the current line up of their 4000 series Plossls, http://www.meade.com/products/access...es.html?cat=45
Their current 40 mm Plossl looks differently to the AG 40 mm Plossl.
It appears that the Meade ("enhanced" 5-element) Plossls were made in Japan until 1998. After that, Meade moved the manufacturing to Taiwan, and downgraded the eyepieces to the "real" 4-element Plossls. In 2002-04, Meade moved manufacturing of this series of Plossls from Taiwan to China. The Plossls were downgraded even further. The article author did the testing and comparison of those Plossls... it appears that the performance decreased with each move.
So... I wonder if the AG has the "old", Taiwan-made Plossls in stock. Anyway, I must confess of my foolishness -- I have placed an order for the AG 40 mm Plossl for $25. Now I am thinking that this is a waste of money.
Anyway, I must confess of my foolishness -- I have placed an order for the AG 40 mm Plossl for $25. Now I am thinking that this is a waste of money.
Not foolish. Compulsive maybe,
Like Brian said, you won't be disappointed. You will learn a lot from having it. I for one am now very curious on how you find using it!!! I am interested knowing how the eye relief on it is - eye relief is the distance you need to put your eye from the eyepiece to see comfortably through it. I have a 55mm plossl, and to see into it you need to put your eye some 2.5cm from the eye lens! Normally with plossls, the longer the focal length, the further away you put your eye. Conversely, the shorter the focal length, the closer you need to put your eye. With a 6mm plossl, you practically need to put your eyeball on the eye lens to look through the pin hole sized lens. If you are interested in shorter focal length eyepieces, there are other eyepiece designs that are newer than plossls and have a large eye lens and do not require you to put your eyeball on the eyelens to look into them, have a little wider AFOV and only cost a little more than plossls, but they are a far superior eyepiece. I'll post info about them if you like. I won't do so now as it is not what the thread is concerned with.
It could well be that you love this 40mm plossl. It could also be one eyepiece in your kit that novice telescope users find very easy to use. I've got a small set of eyepieces that I keep just for novices to use, say at an outreach night at a school. While these are modest pieces, such eyepieces are much more valuable to me for novices to use than hyper-expensive, whizz-bang-wow eyepieces that can require a bit of getting used to. I also don't mind these eyepieces getting soiled by finger prints, eyelash goo, and whatever muck is going around...
Mind you, that 55mm plossl I have, that one is NOT in the novice kit!!!! That one is a very special piece.
Anyway, please post your thoughts on that 40mm plossl when you finally get to use it!!!
I agree with Alex, there is indeed a lot to learn from owning a "fringe" EP such as a 40 Plössl. Mine taught me that apparent field of view is less important to me than I had thought, which enabled me to forget about converting to 2" and get some orthoscopics instead. One year later I own 3 of them and I've become a huge fan of the compact, no-BS design of a good ortho. Not to mention the ease of use and razor sharp views. For wider views, I'm quite content with my shorter FL pieces of 24mm or less.
I also learned that a 40mm Plössl is not just an inferior version of a 32mm Plössl because they might have the same TFOV. I have both, and while the 32 is probably the better all round unit, the 40 frequently comes out when viewing with an OIII filter or any other time when sheer exit pupil acreage is needed while mag can be low. True it virtually never gets used under light polluted skies, but I can't imaginge a dark-sky observing night without it.
I find my 24mm Ortho gets the most use in the 4" travel scope I use for community viewing nights. Good Eye relief, tac sharp to the edge of FOV and easy for novice users too. They must be OK, I keep selling out of them !
I took the advantage of the AG sale to pick up one of the 32 mm Plossls to fill in a gap in my collection - or so I thought. (It's labelled "Super Plossl 32mm Multi-Coated".)
I started with a pair of 25 mm and 10 mm Plossls that came with a York Optical telescope about 10 years ago - I don't know their make and brand, but they're quite adequate "starter" eyepieces for my needs.
I then recently bought an Orion Sirius 40 mm Plossl from Bintel when it was on special, as a wide angle eyepiece. I knew about the issue of the narrower apparent field of view of 40 mm Plossls compared to 32 mm Plossls, but the 32 mm wasn't being discounted at the time. The supposed narrower apparent field of view never bothered me - in fact, I never even noticed it as I swapped eyepieces around. But I thought a 32 mm Plossl would be a good addition, giving me pretty much the same actual field of view at higher magnification, due to its supposed wider apparent field of view.
I was surprised then when I fitted the 32 mm to my telescopes, to find that it gave a significantly narrower actual field of view than the 40 mm, and it was even a bit narrower than the 25 mm!
The reason seems to be that the AG 32 mm is significantly "stopped down", and you get a real sense of "tunnel vision" as you look through it, which is not there with either the 25 mm or the 40 mm - it's definitely a lot narrower FoV than 55 degrees. The 25 mm has a minimal stop, but it does not impede the FoV - you can just see the bottom edge of the 1 1/2" barrel, so I guess it is giving the full theoretical 55 degree FoV. The 40 mm has no stop at all that I can see, but the apparent FoV is just a little bit narrower than the 25 mm - if the 25 mm is 55 degrees, I would guess the 40 mm is about 50 degrees, and the 32 mm (with the stop) is probably about 45 degrees - significantly smaller than 25 mm, and even a bit smaller than the 40 mm!
I have tried to show these details in the attached shots. The eyepiece shots of a printed imaging resolution chart were taken at a range of about 10 metres through the eyepieces with my camera phone - it was very tricky to get decent un-vignetted views, but they fairly represent the effective Field of View of each eyepiece.
If I unscrew the barrel and remove the stop of the 32 mm eyepiece, the unstopped "native" apparent field of view of the eyepiece seems to pretty much match that of the 25 mm - that is, about 55 degrees. I've popped the stop out of the barrel and reassembled the 32 mm, and it has indeed opened up the view, as expected. It seems to have slightly wider apparent FoV compared to the 40 mm, with slightly smaller actual Field of View, and slightly higher magnification.
I assume I will pick up a bit of aberration / distortion at the edge of the FoV if I do this, but I can't say that I have noticed it in my indoor tests. (And it's cloudy tonight, so I can't do any real star tests tonight.) The reduced field of view with the stop in place makes it pretty much redundant as it was supplied - are there any other issues with using / removing the stop on the 32 mm?
And what is the story with my 40 mm - did I just get lucky in getting a 40 mm Plossl with a wider Apparent and Actual Field of View than the "textbook" values for a 40 mm Plossl? Is it possibly not actually a Plossl, but something else altogether? Or is the Orion Sirius 40 mm a better eyepiece than I expected when I got it so cheap from Bintel?
I would be interested to hear of people's experiences with the AG 32 mm and 40 mm Plossls.
My 12.5mm arrived today but with the current weather it could be a while before I get a look through it.
It did come with a bolt like the Meade but it doesn't say Meade.
The gold lettering looks like the Meade.
This one is 12.5mm but the Meade is 12.4mm.
Overall external appearance is very similar to the pictures of the Meade 4000 series.
My 12.5mm arrived today but with the current weather it could be a while before I get a look through it.
It did come with a bolt like the Meade but it doesn't say Meade.
The gold lettering looks like the Meade.
This one is 12.5mm but the Meade is 12.4mm.
Overall external appearance is very similar to the pictures of the Meade 4000 series.
Stay tuned.
My AG 32 mm Super Plossl came packaged just like your 12.5 mm - a nice sturdy cardboard box (much more substantial than the flimsy cardboard that my genuine Meade accessories have previously been supplied in!), and a two-part "bolt" case.
Both my older Orion Sirius 40 mm Plossl and my new AG 32 mm Super Plossl seem to quite closely resemble the Meade Series 4000 Super Plossls in build details, but they're not identical.
The writing on the AG 32 mm seems to match the Meade 32 mm, but there is a greater length of knurling on the body than photos of the Meade show https://www.astronomics.com/meade-32...ice_p4118.aspx . The body shape and knurling of my Orion Sirius 40 mm seems very similar to the Meade 40 mm, but the writing is white (not gold), and is at the bottom of the body instead of at the top https://www.astronomics.com/meade-40...ice_p5899.aspx . Also, the barrel on my 40 mm has a machined rebate to secure the eyepiece against the thumb screws in the focuser, and the Meade photos don't show this. I guess little details like these could change from batch to batch, but I'm not sure whether any of these eyepieces actually come from the same source(s) as the Meade Series 4000s.
What's interesting to me is that, as supplied, the AG 32 mm (with the field stop installed) had an apparent Field of View that is quiet a bit smaller than the normally expected Plossl 50 to 55 degrees, but this opened up to a full 55 degrees (estimated) if I remove the field stop. Meade claim 49 degrees for their 32 mm - maybe it has a smaller field stop than the AG came with?
Conversely, my Sirius 40 mm has no field stop that I can see, and seems to have an apparent FoV of about 55 degrees (again, estimated, by holding up to the light and matching it against my 25 mm Plossl), whereas Meade claim only 43 degrees for their 40 mm Super Plossl, and my Sirius is definitely bigger than that! (Either that, or ALL of my Plossls, including my venerable 25 mm and the unstopped AG 32 mm, have significantly less than 55 degree FoV! )
Anyway, please post your thoughts on that 40mm plossl when you finally get to use it!!!
I was able to cancel my order of the 40 mm Plossl from AG. The person on the other end of the TCP/IP wire was quite pleasant and quick.
The reason was that the Plossl had the same actual FOV as the 32 mm Omni Plossl which I already had. The inscripture on the AG eyepiece said that it was multi-coated, which means at least one side on at least one lens (out of 4) was multicoated. The GSO Plossl (and its Celestron/etc derivatives) are fully-multicoated, that is every lens is multi-coated on each side. The FMC eyepiece should be brighter than the MC one.
I am the person who likes to optimise his life. I would hate to have an unused inferior eyepiece in my collection which I would not want to throw away, even if it is worth only $25.
But I still wanted to explore the limits of the eyepieces with the widest actual FOV. Thus, I sent an order for a GSO 0.5x focal reducer to Andrews Communications. I am going to pair it with the 8-24 mm Celestron zoom. I expect that I'd be able to find the zoom magnification when the vignetting from too much of actual FOV will be no longer acceptable to me.
There will be nothing wrong with the Plossl apart from the aforementioned slight tunnel vision all 40mm inch and a quarter eye pieces suffer , you will get great views of star cluster,s etc in your scope. Enjoy the views and don't worry , my first low power eyepiece was much worse , trust me.
)
