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AFOV of 32mm Celestron Omni Plossl?

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#1 HellsKitchen

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Posted 14 July 2019 - 03:51 AM

I'm in the market for a 32mm plossl and have considered picking up a Celestron Omni. I've been seeing 44* as the quoted AFOV on Celestron's website, but I've also read forum reports suggesting it is 50*. I've even seen a picture of the box from an ebay seller clearly marked as 50*. What is the actual AFOV of this eyepiece? 



#2 beggarly

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Posted 14 July 2019 - 04:35 AM

48° according to the 2019 Buyers Guide.

https://www.cloudyni...de#entry9488663


Edited by beggarly, 14 July 2019 - 04:35 AM.


#3 HellsKitchen

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Posted 14 July 2019 - 06:04 AM

Awesome link, thanks! Looks like I'll be going for the GSO 32mm. 



#4 M11Mike

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Posted 14 July 2019 - 06:19 AM

That's an error on the Celestron site.  

 

Eyepiece is 52 degrees.  

 

See High Point / Agena / etc., etc. websites.

 

I personally have side by sided this against a Televue 32mm PL and the field of view is exactly the same and the TV which is 52 degrees.

 

BTW - I sold the my TV 32 as the views in the Celestron Omni 32 were as good or better for less than 1/2 the $$$ (least in my F10 SE8).

 

It's a excellent EP (IMHO - a "sleeper") - I think many pass it up as it's a lower priced Celestron EP thinking it can't be all that good.  Well, (at least in my F10 SCT) it's excellent.  MATCHES the actual FOV put up in my scope as a TV 24mm Panoptic - which is well known to provide the largest actual FOV possible in a 1.25" EP.

 

While I have owned at least 25-50 EP's in the last 20 years - MOST costing considerably more - MOST sold - not this.  IMO worth MORE than the $45 NEW that they sell for.  Try it - I bet you will be impressed.

 

M11Mike

 

I suggest 


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#5 Jon Isaacs

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Posted 14 July 2019 - 06:59 AM

Awesome link, thanks! Looks like I'll be going for the GSO 32mm. 

The Omni Plossl and the GSO Plossl are very likely the same eyepiece.  

 

Regarding the 32mm TV Plossl, it is spec'd at 50 degrees.

 

http://www.televue.c...page.asp?id=214

 

Over the years, I have learned to suspect the manufacturers AFoV ratings and this is particularly true with 32mm Plossls, there's not a lot of room in the barrel for the field stop.  A 27.0mm field stop probably won't provide a 52 degree AFoV in a Plossl.

 

I have a 32mm Celestron "Halloween" Plossl that I purchased in 1996, it was made in Taiwan and I assume it was made by GSO.  It has a 27.5mm field stop, that's barely anything.. 

 

Jon



#6 HellsKitchen

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Posted 14 July 2019 - 07:01 AM

Thanks for the feedback Mike. Good to hear a first hand account from someone who's actually used and compared one to other eyepieces. 

 

Ordered one on ebay just now, with a 20% discount coupon which expires in about 2 hours and with free shipping! Just under A$39 total. Got a 15mm aswell with the discount, could not resist. Might even grab the other focal lengths on offer from ebay, they are going for good prices, and build up a low cost grab n go set. 



#7 sg6

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Posted 14 July 2019 - 07:40 AM

Like Jon I have always been a little suspect of the claims on assorted plossl's, especially as so many are the same items from likely the same source with a different name on them.

 

I have eventually come to the simple approach that I treat both 30mm and 32mm focal lengths as 30mm and any field of view I assume to be 50 degrees. Nice(ish) round numbers so makes the maths easier. Especially as any maths I do is outside at night, in my head and trying to deciding what to try next.



#8 Allanbarth1

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Posted 14 July 2019 - 07:46 AM

I own the Omni 32mm plossl. My box says 50° fov. I have not compared it to any other 32mm ep because I don't have on to compare it to. What I can say is it a keeper for me. I have a ES 24mm 68° that gets as much use as the Omni 32  and thought of replacing the Omni with an ES but for me, the view that I get out of my Omni 32 plossl is quite nice and I have no intention of replacing it. A definite sleeper ep in my opinion. When I picked it up a 3 years age on Amazon it was $39. No way I can replace the views that I get out of that Omni 32 mm plossl for that price. I love mine.


Edited by Allanbarth1, 14 July 2019 - 07:47 AM.

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#9 M11Mike

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Posted 14 July 2019 - 08:58 AM

Jon - 50 it is....just dug out my box and it IS marked "50 Degree FOV".  

 

Jon - I compared the 32 Omni to the TV 24/68 (field stop 27mm) --- and found exactly the same actual FOV in my SE8.  

 

Both crystal clear / sharp to the edge / high contrast images.  The TV obviously providing some additional power and a slight darkening the sky for fainter DSO's.  Did also find the T/V 24/68 little easier on the eye for full view eye placement due to the shorter eye relief. 

 

Jon - FWIW, I am a T/V believer (IMHO the very best there are in telescope EP's) - I love T/V EP and products - but kid you not - when I side by side compared the T/V 32 PL to the Celestron Omni 32 PL the views were pretty much identical - with a possible slight edge to the Omni (which I found hard to believe) but THAT is MY EXPERIENCE.   Immediately sold the T/V.  BTW that T/V 32 PL was bought NEW from High Point as well as the Celestron Omni. So I was comparing NEW to NEW.   

 

Mike 



#10 Starman1

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Posted 14 July 2019 - 09:30 AM

A 27mm field stop translates to a 49.5° afov with the normal amount of distortion found in a 4-element 32mm "Plössl"

If there were Zero distortion, the afov would be 45.75°

One thing you can rely on, though, is that you cannot achieve >50° at 32mm unless there is no field stop and then the outer edge would not be sharp.

 

Likewise, a 40mm Plössl with a 27mm feld stop would be 37.3° with no distortion, but most of them are around 40-41° afov.

 

People who argue about whether one Plössl or another has this afov or that afov should understand that it is customary for a manufacturer to claim all the line is

50° or 52° if just one focal length in the line is.  A typical line of 7 to 9 focal lengths will vary from 45° to maybe 51°, but the manufacturer will call them all 50°.

 

The purpose for even knowing an AFOV on this type of eyepiece is for figuring out how much true field (TFOV) you will see.

Just be aware the TFOV = AFOV/M formula can be in error by more than 10%.

 

A far more accurate formula is

TFOV = (EPFS/TFL) x 57.296, where EPFS is eyepiece field stop, and TFL = telescope focal length

 

Here is an illustration:

A 32mm Plössl has a 27mm field stop.

Let's use it in a dob with a 1200mm focal length, so M=37.5x

 

Using TFOV = AFOV/M and the manufacturer's claim of 50°, then TFOV = 1.333°

Using TFOV = (EPFS/TFL) x 57.296 and a 27mm field stop, then TFOV = 1.289°

In this case, the first formula only exaggerates the TFOV by 3.4%

 

If we use 49.5° as the AFOV, the discrepancy drops to 2.4%, a likely figure for distortion in this narrow a field eyepiece.

 

But a recent example of another brand of 32mm Plössl, where the mfr claims 50°, the TFOV was different by >7%

 

I would suggest using field stop when and where possible to compute TFOV, and obtaining that on a Plössl is as simple as flipping the eyepiece over and measuring the I.D. of the iris with calipers.

 

P.S. no 32mm Plössl is 52°.  I haven't seen one in 56 years of observing, and I don't expect I will.  Even 50° is a rounded-off "maybe".

But it doesn't matter.  What matters are the other characteristics of the eyepiece, like internal light scatter, good coatings, superior polish, spot size, good control of exit pupil aberrations, a sharply focused field stop.


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#11 M11Mike

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Posted 14 July 2019 - 02:34 PM

Don - I use the TV formula myself (where the field stop is known) --- EP FS diameter divided by the scope FL x 57.3.

 

Looking at my 32 Omni just now --- it appears that the field stop is just a very bit smaller than the internal EP diameter - less than 1mm smaller.  Given the outer diameter of 31.7 - my guess a field stop diameter of 27mm for the Omni is probably right.  

 

Note as previously stated in a direct side by side comparison one night with a Televue 24mm Panoptic (field stop 27mm) - which T/V insists offers the widest possible actual FOV of ANY 1.25" EP --- I saw exactly the same FOV as the 32 Omni in my F10 Celestron SCT 8.    

 

Doing the math with manufacture specified FOV (in this case 68 degrees for the T/V Panoptic 24mm) - a FOV of 50 degrees should be theoretically possible with a 32mm EP...that being the absolute maximum if you believe TV.

 

And as you say --- all this MATH really doesn't matter all that much IF you're happy with what you're seeing.

 

And in this case - I mentioned that the 32mm Omni and the 32mm T/V PL --- put up the exact same actual FOV in my SE8.  And T/V (as you mentioned) says their 32PL is 50 degree.  So I figured the Omni to be 50 degrees - and that is what is marked in the Omni box as well.  (to answer the original question of this thread)

 

If the true fields are off by 2,3,4% --- it's almost irrelevant as the human brain has difficulty detecting differences that slight.

 

Don - I have been observing 61 years but admit I know a lot less than you when it comes to astro gear - you're the expert!!!  In this case it simply means "I'm probably older than you"!!!  :-)     

 

 

Mike    



#12 Starman1

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Posted 14 July 2019 - 03:20 PM

Mike,

There have been eyepieces in 1.25" with field stops that are larger:

24mm Hyperion--28.5mm

35mm "pseudo-Masuyama", i.e. Orion Ultrascopic/Parks gold Series/Baader Eudiascopic/Celestron Ultima--29mm

32mm University Optics König--28.5mm

etc.

What they all had/have in common is that the field stops are above the 1.25" barrel (1.25" filters are 28.5mm in O.D., which is a hair larger than the inside edge of the filter threads in a 1.25" eyepiece)

and require(d) a lot of in-travel of the focuser to get them to focus.  Also, of course, they all add a little more vignetting to the edge of the field.

The widest field stop I've measured in the 1.25" barrel itself is a couple eyepieces from 40+ years ago with the inside of the barrel acting as the field stop (about 28mm or a tad more),

but today, the Explore Scientific 24mm 68° with a (per ES) 27.2mm field stop is the largest field stop with the field stop inside the barrel.

 

The Celestron Omni is a rebadged GSO, just as the older Parks silver Series were.  GSOs are also available under their own label.

They've always been bargains.


Edited by Starman1, 14 July 2019 - 03:21 PM.


#13 Peter Besenbruch

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Posted Yesterday, 07:42 PM

That's an error on the Celestron site.  

 

Eyepiece is 52 degrees.  

 

See High Point / Agena / etc., etc. websites.

 

I personally have side by sided this against a Televue 32mm PL and the field of view is exactly the same and the TV which is 52 degrees.

OK, I don't believe the 52° claims, either. I DO believe your claim that the Televue and the Omni/GSO/whatever Plössls have the same apparent field. Televue claims 50°. That may be closer to the truth than the 52° nonsense. I'd even believe something like 48°. I own a decent Meade 56mm, 52° Plössl, but it's apparent field is more like 47°.

 

I treat field of view claims much like I treat claims of "all natural," and "more doctors recommend Camels."


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