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John Kuraoka
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Reged: 12/12/12

Loc: Sunny San Diego, CA
Reality check requested re: 32mm eyepiece
      #5582593 - 12/21/12 02:33 PM

What I have: An Orion 90mm Mak-Cas (fl 1250mm, f/13.9), with the stock Orion Sirius 25 and 10mm Ploessls.

What I'd like to see: Enough of the Pleaides to make out the curlicue bits.

What else I have: Two growing kids, a dog, a mortgage, and compact cars that get stuffed to the rafters for outings and camping trips as it is (so taking that 90mm will pose a not-unsurmountable-but-significant packing challenge when we head to the Sierras).

Anyway, we've binoc'd it, but I think we all (meaning I) would love to see it bigger.

I've plugged the telescope specs into the online telescope simulator, and it indicates that I'll just be able to see the curlicues with a 32mm eyepiece assuming a FOV of 52 degrees (don't know how to make the "degree" symbol). So, before I start shopping for a 32mm stocking stuffer ... is that view realistic? Or should we just continue exploring with the 7- and 8-power binocs we have?


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csrlice12
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5582622 - 12/21/12 02:50 PM

The 32mm should do, but you could go to like a 35 Pan as well (would cost more). Truth is, a 90mm Mak is not a wide'field scope

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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: csrlice12]
      #5582670 - 12/21/12 03:20 PM

Thanks!

Quote:

Truth is, a 90mm Mak is not a wide'field scope




Yeah ... we chose it for its packable size and the wow factor for the kids was, at first, the moon and planets. Now we're pushing it in directions that it's not ideally suited for, but I'm hoping to keep whetting their appetites. I just didn't want to get the eyepiece only to discover that it didn't give quite enough as an appetizer.

Is the 35 Pan available as a 1.25" eyepiece?


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csrlice12
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5582680 - 12/21/12 03:27 PM

If all you have is a 1.25" focuser, the 24pan or ES68* 24mm would give you the widest FOV (same as the 32mm plossel, but with more mag). The ES68* 24mm is on backorder though, I've been waiting since May on the one I ordered (and still waiting). You might find a used 24 Panoptic though.

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REC
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Re: Reality check requested re: 32mm eyepiece new [Re: csrlice12]
      #5582706 - 12/21/12 03:49 PM

I use a Meade 32mm SP in my ETX-90 and works out well. They are about $50 new and sometimes come up used.

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Jon Isaacs
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5582747 - 12/21/12 04:11 PM

Quote:

Thanks!

Quote:

Truth is, a 90mm Mak is not a wide'field scope




Yeah ... we chose it for its packable size and the wow factor for the kids was, at first, the moon and planets. Now we're pushing it in directions that it's not ideally suited for, but I'm hoping to keep whetting their appetites. I just didn't want to get the eyepiece only to discover that it didn't give quite enough as an appetizer.

Is the 35 Pan available as a 1.25" eyepiece?




The 35mm Panoptic is a very nice 2 inch made by TeleVue. They offer a 68 degree AFoV that is very sharp even at the edge even in large, fast focal ratio telescopes. Wonderful eyepiece but they cost $375 new and about $250 used. You could buy a nice short tube refractor and mount for that...

As far as a 32mm Plossl and your ETX-90, I think it is a good choice. With it's long focal and slow focal ratio, as you have found, the maximum possible field of view is limited, it's only about 1.25 degrees, not enough to take in the all the Pleiades but maybe enough. But regardless of how it performs on the Pleiades, it will provide you with the widest possible field of view with some increased brightness over the 25mm PLossl.

When choosing a 32mm Plossl, it is good to pay attention to the AFoV. There are some 32mm Plossls that only offer a ~44 degree AFoV, you want one that offers an AFoV of about 50 degrees. AgenaAstro sells the GSO Plossls. GSO is a large manufacturer in Taiwan. In 1996, I purchased a Celestron 32mm Plossl, it was manufactured by GSO, I still have it, it's a good eyepiece.

GSO 32mm Plossl with Free Shipping

Jon


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: REC]
      #5582751 - 12/21/12 04:14 PM

Thank you all for the guidance, alternatives, and reassurance! Next ... into the classifieds I go to look for an inexpensive 32mm. Those ultra-wide 24s, as tempting as they are, look pretty rough on a wallet not yet recovered from Christmas!

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GlennLeDrew
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5582779 - 12/21/12 04:34 PM

What are the 'curlicue bits' to which you refer? Do you mean the reflection nebulosity? This can be a tough target even on rather larger telescopes.

With your telescope's focal ratio being f/13 or so, even with a 32mm eyepiece your exit pupil is a small-ish 2.4mm. This results in a fairly dark view overall, and can make detecting those low contrast (dim) nebulae harder than if the view were brighter. Especially if the magnification is more than high enough already for the object under scrutiny.

And so might not a 40mm eyepiece be a more useful step up from the 25mm you already have? The field of view will (or should, if the barrel itself is the field stop) be as large as possible for a 1.25" format. And the exit pupil will be a larger 3mm, thus resulting in a brighter image 'friendlier' to the eye, especially if your sky is reasonably dark.

I suggest this as a consideration because the real difference between a 25 and a 32 is kind of small, in all of magnification, field size and image brightness. A 40 will at least provide a notably brighter image, while offering the largest field possible.


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5582810 - 12/21/12 04:48 PM

Thanks for the alternative!

I had discarded - perhaps prematurely - the notion of a 40mm because, in playing with the telescope simulator, I noticed that a 40mm with an AFoV of 43° delivered roughly the same image as a 32mm with an AFoV of 52°, but a bit smaller. And, I had read, in one forum or other, that 32mm was the widest practical eyepiece focal length for the kind of scope I have. I would welcome advice as to whether or not that's true.

The 40s wider exit pupil has definite appeal, though. (I wear glasses, and view with them on because of astigmatism.)


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Jon Isaacs
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5582830 - 12/21/12 04:54 PM

Quote:

Thanks for the alternative!

I had discarded - perhaps prematurely - the notion of a 40mm because, in playing with the telescope simulator, I noticed that a 40mm with an AFoV of 43° delivered roughly the same image as a 32mm with an AFoV of 52°, but a bit smaller. And, I had read, in one forum or other, that 32mm was the widest practical eyepiece focal length for the kind of scope I have. I would welcome advice as to whether or not that's true.

The 40s wider exit pupil has definite appeal, though. (I wear glasses, and view with them on because of astigmatism.)




Glenn's point is a reasonable one. At F/13 image brightness is an issue and the 40mm would provide you with a brighter, if someone smaller image. Compared to the 25mm, the 32mm is 1.6 as bright. The 40mm about is about 2.5x as bright. I have both 32mm and 40mm Celestron Plossls and in my small F/13.3 refractor, I generally prefer the 32mm to the 40mm.

Jon


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izar187
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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5582896 - 12/21/12 05:38 PM

Another vote for the very affordable and well working 32mm GSO plossl. Frequently available used for a modest cost, or new under various brand names. It is a good basic low power option for any future scopes with 1.25" focuser.

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REC
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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5582907 - 12/21/12 05:43 PM

Ever heard of "Uncle Rod" in these forums? He is an expert on Cat's and Maks and highly recommends a 32mm 52* for these types of scopes using a 1.25" EP's.

Bob


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5582924 - 12/21/12 05:51 PM

Oh, and by "curlicue bits," I mean the top and bottom "legs" of the script letter "E," or perhaps it's a numeral "2," formed by the central group of stars.

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WAVT
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5583830 - 12/22/12 10:30 AM Attachment (17 downloads)

I have a 90mm Mak and it is a real treat to use. The 32mm Plossl design is a very good match for the scope. I hear very good things about the Sterling brand of plossls. The GSO plossls are known by me to be quite good as well. My favorite is the Televue. If you can afford just one more EP, get the 32mm GSO Plossl.

I also have been known to drop in a 40mm EP in the MAK. The view is noticeably brighter than the 32mm. Works good on nebulae.

No mater how you rock it, the MAK just doesn't do wide fields.

Quote:

(don't know how to make the "degree" symbol).




Hold the Alt key down, and type 0176 and you get the ° symbol.


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: WAVT]
      #5584043 - 12/22/12 12:49 PM

I see that Orion offers a 24mm 68° in its Stratus line, which will fit in a 1.25" focuser at $144.

So, to summarize my options for jamming most of the Pleaides into a 90mm Mak-Cas:
24mm 68° - biggest image, least brightness
32mm 52° - middle of the pack
40mm 43° - smallest image, most brightness

Will the 24mm wide have about the same brightness as my current 25mm Ploessl? If so, then the price difference and brightness have me leaning strongly toward the 32 or 40. The thought of getting enough brightness for nebulae is very tempting even though that's pushing the Mak-Cas even further into its weak spot. It would just be something else to look at to get the kids excited.

Can you also see nebulae with the 32, or does the increased brightness of the 40 make a significant difference?


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rdandrea
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5584045 - 12/22/12 12:52 PM

68 degrees is the apparent field of view, not the true field of view.

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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: rdandrea]
      #5584068 - 12/22/12 01:09 PM

Yes, thanks!

According to the telescope simulator, the TFoV with the three eyepieces in my scope would be 78' (24/68°), 80' (32/52°), and 83' (40/43°). Are any enough to make a significant difference in real-world viewing?


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rdandrea
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5584081 - 12/22/12 01:17 PM

Not in my opinion, certainly not enough to justify the increased cost and decreased brightness of the 24. In degrees instead of minutes, you're differing only in the second decimal place of the TFOV. I think the 32mm gives you the best tradeoff.

Your mileage, of course, might vary.


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artcarter
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5584187 - 12/22/12 02:27 PM

What is the telescope simulator you speak of?

Art


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: artcarter]
      #5584193 - 12/22/12 02:30 PM

Hi Art - the telescope simulator is at http://www.telescope-simulator.com/. The telescope calculator (on the left) is a great way to approximate the effect of various eyepiece focal lengths with your telescope looking at various objects.

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*skyguy*
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Re: Reality check requested re: 32mm eyepiece new [Re: rdandrea]
      #5584385 - 12/22/12 05:24 PM

Hi John ... I use a Meade 40mm eyepiece in my ETX-125 Mak when I want to see the very faintest nebulosity this scope can deliver. It has the widest FOV available in a 1.25" eyepiece and most importantly ... the largest size exit pupil. From there, I'll usually "bump up" the magnification ... to compare the views ... and then end up going back to 40mm before moving on to another object.

That said, the difference between the views in the 40mm and 32mm .... when looking for faint nebulosity ... is going to be very subtle. For the novice observer, the views could be indistinguishable from each other. However, many beginners would probably prefer the increased magnification with the 32mm eyepiece. If you decide to buy a new eyepiece, stick with the Meade, Celestron, Vixen, or GSO plossls in the $50 price range. Personally I'd choose any of these brands and base my purchase on the best price including shipping. Have fun exploring the night sky with your children.

Jim


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GlennLeDrew
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Re: Reality check requested re: 32mm eyepiece new [Re: *skyguy*]
      #5584482 - 12/22/12 06:26 PM

The 24mm 68* would deliver essentially identical image as the 25mm 50*, except the wider field would be rather more pleasing. To me, that wide 24mm in concert with a 40mm would make a nice fit. A 32mm would be superfluous, as I'm pretty sure you'd find yourself making that more significant jump, skipping this 'orphan' more often than not.

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Jon Isaacs
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5584630 - 12/22/12 08:06 PM

Quote:

Yes, thanks!

According to the telescope simulator, the TFoV with the three eyepieces in my scope would be 78' (24/68°), 80' (32/52°), and 83' (40/43°). Are any enough to make a significant difference in real-world viewing?




John:

In reality, those differences are small and the differences seen may be the result of the inherent assumptions in the calculation.

I just noticed you are a fellow resident of "Sunny San Diego." Maybe we could get together some evening and you could see the differences between a 24mm SWA, a 32mm Plossl and a 40mm Plossl.

Jon


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markgf
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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5584709 - 12/22/12 09:10 PM

Hi John
I'm a newb so have been considering things like this too. I've read the stuff below in various places recently, including CN, but I'm not sure what's merely techo and what actually works when observing, so sorry if all I'm doing is muddying the water...

More brightness?
For a given TFoV, whether we use a 40 mm or 24 mm eyepiece, we have the same light capture, right? Doesn't the shorter EP just give more magnification, wider apparent FOV and smaller exit pupil? If our dark adapted pupil is bigger than the exit pupil, so that we take in all the light captured, I wonder how brightness can vary.

Also, best visual acuity is said to be for exit pupil around 2mm (about a 28mm EP for an F/14 telescope). Based on the above, for a 1.25" EP, a 32mm x 50 degree plossl looks OK.

Happy shopping...Mark


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5584803 - 12/22/12 10:19 PM

Jon - Thanks for the offer - I might just take you up on that! My kid's elementary school is having a star party in late January - I think it's put on with a local astronomy club. I always try to take the kids to those because they can look through some serious scopes.

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Jon Isaacs
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Re: Reality check requested re: 32mm eyepiece new [Re: markgf]
      #5584835 - 12/22/12 10:49 PM

Quote:

More brightness?
For a given TFoV, whether we use a 40 mm or 24 mm eyepiece, we have the same light capture, right? Doesn't the shorter EP just give more magnification, wider apparent FOV and smaller exit pupil? If our dark adapted pupil is bigger than the exit pupil, so that we take in all the light captured, I wonder how brightness can vary.





Mark:

Brightness is measured as photons per unit area. Increasing the magnification makes an object larger. Since the same amount of light is spread out over a greater area, it is dimmer. Or, if the magnification is lower, then the same amount of light is concentrated into a smaller area so it is brighter.

Hope this makes sense.

Jon


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markgf
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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5586024 - 12/23/12 05:49 PM

Thanks Jon. I'll have to leaven my theorising with a bit more viewing time.

We're off to get a photo of one of the kids with Santa in shorts, then on to the beach. All the best for the silly season to all at CN...Mark


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GlennLeDrew
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Re: Reality check requested re: 32mm eyepiece new [Re: markgf]
      #5586144 - 12/23/12 07:14 PM

Mark,
While it's true that magnification 'spreads out' the light, this explantation of why the image surface brightness changes with magnification is what I call a convenient fiction, because it doesn't get to the root of the matter.

Your eye has a variable iris, just like that in a camera's lens. Suppose that instead of the auto-iris nature bequeathed you, you could at will control its diameter, let's say from 7mm down to 2mm. Image surface brightness scales with the area; halve the iris aperture and the image becomes 1/4 as bright. And so the maximum change in image brightness over that extreme equals the ratio of (7 / 2)^2 = 12.25. A 2mm pupil delivers a view a bit less than 1/12 as bright as does a 7mm pupil.

When looking through a telescope, its exit pupil is located at your own pupil, thus 'replacing' the latter if it's the smaller. As far as your eye is concerned, the telescope's exit pupil is now your own pupil.

A telescope can at best only deliver a view having surface brightness approaching that delivered by the eye by itself, and only when its exit pupil is as large or larger than your iris opening. The brightness of your night sky is always seen at its brightest with no optical aid, and through a telescope could technically only equal it if the instrument transmitted 100% of the light entering it.

If you lined up a battery of telescopes, all of differing aperture and working over a wide range of exit pupil, and pointed them at the same patch of sky (say, on a sunny day), looking at the array of exit pupils from a distance where all or several could be seen at once would show that every one has the same intensity in that little blue circle, which would also be the same as when looking at that patch of sky with the naked eye. In other words, excepting the small losses in transmission, a telescope does not alter the surface brightness of an extended source of illumination.

The foregoing is a fundamental property of optics, and should be explored by observation if in doubt.

When the exit pupil becomes smaller than your own pupil, the image surface brightness drops as the square of the diameter. Your own maximal pupil diameter is your baseline for maximal image surface brightness. With the telescope, we can make the exit pupil smaller than the minimum our eye can achieve, all the way to a still-useful ~0.5mm. For someone whose maximum pupil is 7mm, we see that the full image surface brightness range achievable becomes a factor of (7 / 0.5)^2 = 196.

To hopefully provide further understanding, imagine two otherwise identical people, differing only in that one's pupils open to 7mm and the other's open to 3.5mm. Both are standing together, looking up at a night sky. The narrow-pupil observer will enjoy a view having half the brightness of that of the other. If their telescope is working with an exit pupil of 3.5mm, the small-pupil person will say, "This view is as bright as seem with my eyes alone." The big-pupil person will say, "Nope. It's only half as bright", and swaps the eyepiece to get a 7mm exit pupil. "Ah, now it's just as bright." Small-pupil peers in and states, "Huh? It's still the same."

The point here is that telescopic surface brightness always scales to that of the observer. The baseline brightness is set by that obtained at our own maximal pupil diameter.

In summary. Your retina gets its light through your pupil. Whether this pupil is your own, or is stopped down by a smaller telescopic exit pupil, image surface brightness is controlled identically. It's all the same; the retina cares only for the *area* of the pupil admitting the light. The telescope is always presenting a view with essentially unaltered surface brightness. The image is dimmer *only* because the exit pupil is smaller than your eye's pupil.

Magnification comes into it *only* because of it being intimately related to the exit pupil diameter. Hence the validity of the convenient fiction of magnification 'spreading out the light'. But this applies only in the restricted regime where the exit pupil is smaller than the iris. Once the exit pupil becomes the larger, image surface brightness remains constant while magnification decreases, putting the lie to the simplistic notion of the role magnification plays.

P.S. This lengthy epistle is offered in the spirit of the thread title's first two words.


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rdandrea
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5586287 - 12/23/12 09:24 PM

Glenn's the man.

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BillFerris
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5587074 - 12/24/12 11:42 AM

A nice illustration of that old truism, aperture rules.

Bill in Flag


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BillFerris
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5587107 - 12/24/12 12:00 PM

Adding a 32mm Plössl to your set would give you a useful tool for your portable Mak. Tele Vue's 32mm Plössl is excellent...flat field with excellent transmission and pinpoint stars to the edge. Whichever eyepiece you choose, bring your 90mm Mak along on the next family camping trip. Not only will it provide great astronomical views after the sun goes down but, with the 45º correct image diagonal in place, it will deliver great terrestrial views.

Bill in Flag


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5587203 - 12/24/12 01:05 PM

Yup, we have the correct-image 45 diagonal for that exact purpose - I figured the more the scope gets used, the more useful it will be! Plus, I thought it would make it easier for the kids to find things in the night sky on their own.

(Actually, now the kids do better with the reversed image than I do.)


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StarStuff1
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5587279 - 12/24/12 01:56 PM

Another way to get ° is ALT key then the numbers 248.

This works with Windows XP, not sure about MACs.


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Matt2893
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Re: Reality check requested re: 32mm eyepiece new [Re: StarStuff1]
      #5587335 - 12/24/12 02:37 PM

You'll need to use the number pad, the top row numbers do not work for this. On some laptops you might be able to use ALT + Fn then the blue numbers on the letters...

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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: Matt2893]
      #5587562 - 12/24/12 05:37 PM

Yeah, I'm on my laptop now so I can't make the degree symbol. Thought no one would notice ...

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Gert K A
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5587649 - 12/24/12 07:10 PM

@Glenn
What a clear and brilliant explanation
Thank you for that


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Pentax Syntax
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Re: Reality check requested re: 32mm eyepiece new [Re: Gert K A]
      #5587684 - 12/24/12 07:40 PM

In my quest for the widest AFOV eyepiece in the ETX-90 and ETX-125 I tried many eyepieces but the UO Konig 32 seems to be the widest. For reasons I don't fully understand, it outperforms 32 or 40 mm Plossls for sheer AFOV. If you can find one used, it might be ideal.

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rdandrea
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Re: Reality check requested re: 32mm eyepiece new [Re: Pentax Syntax]
      #5587779 - 12/24/12 09:39 PM

Koenigs are fine eyepieces in a longer focal length telescope like a Mak. No wonder you like yours.

In a shorter focus scope, not so great.

Enjoy.


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markgf
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Re: Reality check requested re: 32mm eyepiece new [Re: rdandrea]
      #5587806 - 12/24/12 10:06 PM

Thanks Glenn
I understand your descriptions of image brightness, exit pupils, and iris openings; but it remains counter-intuitive to me unless you're implying that each bit of our retina isn't sensitive to intensity of illumination, only to being illuminated at all. In that event, illuminating more retinal area would be 'seen' as witnessing a brighter 'scene'. Have I got it?

Cheers, Mark


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Starman1
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5588389 - 12/25/12 12:04 PM

John,
The Pleiades are more of a binoculars and short f/ratio scope object.
The widest field of view in your 90mm Mak is 1.24 degrees, and the Pleaides have star streams going out to over 2 degrees.

But that doesn't mean your scope won't perform excellently on smaller objects, like the Perseus Double Cluster, or the Orion Nebula.

Different types of scopes are ideal for different purposes. A 90mm Maksutov is not a wide-field, low-power telescope, but it is excellent on planets, Moon, and DSOs smaller than a degree (which is nearly all).


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Starman1
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Re: Reality check requested re: 32mm eyepiece new [Re: markgf]
      #5588393 - 12/25/12 12:07 PM

Quote:

Thanks Glenn
I understand your descriptions of image brightness, exit pupils, and iris openings; but it remains counter-intuitive to me unless you're implying that each bit of our retina isn't sensitive to intensity of illumination, only to being illuminated at all. In that event, illuminating more retinal area would be 'seen' as witnessing a brighter 'scene'. Have I got it?

Cheers, Mark



Pretty much. When the image fills the pupil, it provides the maximum amount of light possible to the retina. When the image is larger than the pupil, the iris cuts off some of the image and in the case of a telescope, actually makes the image you see dimmer. If the telescope is a refractor (no secondary shadow), sacrificing light this way isn't a big issue if the object is really bright, but the telescope has to be capable of illuminating the larger field.


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Starman1
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5588417 - 12/25/12 12:30 PM

Quote:

Yeah, I'm on my laptop now so I can't make the degree symbol. Thought no one would notice ...




In Windows:
Start Button>>All Programs>>Accessories>>System Tools>>Character map

Select the symbol you want. Copy. Then you can paste as many times as you want.

Better still, place the character map icon on your desktop or start menu or start bar and it will always be available for an instant grab of a symbol.

Makes it easy to write Plössl instead of Ploessl, König instead of Koenig and 100° instead of 100*. All the symbols are there.


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GlennLeDrew
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Re: Reality check requested re: 32mm eyepiece new [Re: Starman1]
      #5589060 - 12/25/12 11:36 PM

Don,
When you say (more than once, implying it's not a one-off 'typo'), "when the image fills the pupil", what do you mean? At the exit pupil/iris there is no image; this is an aperture through which pass innumerable bundles of parallel, image-forming light. The only image is that formed on the retina after focusing by the eye's lens.

And the cutting off of some light by an 'undersized' iris (oversized exit pupil) does not make the image dimmer. The effective aperture of the objective is reduced, of course, but image surface brightness is identical to that when the exit pupil and iris exactly match. That is, as magnification is reduced, when the exit pupil is smaller than the iris the image surface brightness increases. Once the pupils match, surface brightness is as high as it gets, equalling that of the naked eye view (less system transmission losses, of course). Making the exit pupil ever larger than this keeps surface brightness constant; it neither dims nor brightens.

Now, because an oversized exit pupil effectively reduces the objective aperture, point sources such as stars are dimmed. But extended object surface brightness is maximal and constant in the regime of the over-large exit pupil.

Mark,
Conceptually, things are made simpler to understand if you begin with the premise that in all cases the apparent field of view is always the same. For then the area of the retina illuminated is constant, thereby negating the seeming contradiction that a naive appreciation of the totality of area illuminated vs the illumination per unit area on the retina.

Not that it's complicated. A larger apparent field has no impact on the matter under discussion. Whether the eyepiece be a soda straw Huygenian or a spacewalk Ethos, at similar exit pupil diameter the illumination per unit area on the retina is identical. And that's what matters.

As to retinal sensitivity; it necessarily responds to changing levels of illumination, as it must in order to discriminate brightness variation in the scene. It does not work in a kind of binary 'on-off' fashion, bit rather sends signals whose 'strength' varies over some orders of magnitude (even discounting dark adaption, which additional gains massively extend the operating range for brightness.)


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Starman1
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5589149 - 12/26/12 01:46 AM

Quote:

Don,
When you say (more than once, implying it's not a one-off 'typo'), "when the image fills the pupil", what do you mean? At the exit pupil/iris there is no image; this is an aperture through which pass innumerable bundles of parallel, image-forming light bundles. The only image is that formed on the retina after focusing by the eye's lens.



Technically, that's right, but it's useful to think of the exit pupil as an image, one you see less of if you are either too far away from the eyepiece's eye lens or too close. What i should have said was 'when the exit pupil matches the pupil diameter of your eye'.
Quote:


And the cutting off of some light by an 'undersized' iris (oversized exit pupil) does not make the image dimmer. The effective aperture of the objective is reduced, of course, but image surface brightness is identical to that when the exit pupil and iris exactly match. That is, as magnification is reduced, when the exit pupil is smaller than the iris the image surface brightness increases. Once the pupils match, surface brightness is as high as it gets, equaling that of the naked eye view (less system transmission losses, of course). Making the exit pupil ever larger than this keeps surface brightness constant; it neither dims nor brightens.



I was referring to stars and their brightnesses and there would also be a reduction in limiting magnitude, though that is certainly not an issue at these low magnifications.
Quote:


Now, because an oversized exit pupil effectively reduces the objective aperture, point sources such as stars are dimmed. But extended object surface brightness is maximal and constant in the regime of the over-large exit pupil.



You are correct for surface brightness calculations. The main reason not to exceed the exit pupil size appropriate for your eye is the secondary shadow if the scope is a reflector, plus I try to emphasize to anyone over 40 that there is a valid reason to avoid an exit pupil the same size as your eye's pupil--astigmatism. For example, I have eyepieces that produce exit pupils of 5.4mm and 3.7mm. My dark-adapted pupil is a trace under 5mm. The star images are a lot better in the 3.7mm exit pupil. It may lead me to get rid of the low power eyepiece. I suppose I could just use a DioptRx, but the astigmatism I have has 3 cylinders at varying angles but only when the pupil is "whole eye". That, my optometrist tells me, is very common in people over 40 and even more common in people over 60 (I'm 61).

As an aside, there was another thread here on CN recently that discussed whether or not reducing the apparent field made the image dimmer (keeping focal length and exit pupil identical). I think the consensus was that the quantity of photons was reduced with the narrower apparent field, so one could say 'the image was dimmer overall' but the consensus was also that surface brightness didn't change.


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wky46
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Re: Reality check requested re: 32mm eyepiece new [Re: Starman1]
      #5589787 - 12/26/12 01:49 PM

Quote:

Quote:

Yeah, I'm on my laptop now so I can't make the degree symbol. Thought no one would notice ...




In Windows:
Start Button>>All Programs>>Accessories>>System Tools>>Character map

Select the symbol you want. Copy. Then you can paste as many times as you want.

Better still, place the character map icon on your desktop or start menu or start bar and it will always be available for an instant grab of a symbol.

Makes it easy to write Plössl instead of Ploessl, König instead of Koenig and 100° instead of 100*. All the symbols are there.


Thanks Don!

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GlennLeDrew
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Re: Reality check requested re: 32mm eyepiece new [Re: wky46]
      #5589957 - 12/26/12 04:34 PM

Don,
Rather than thinking if the exit pupil as an image, which you see less if if your eye is too far back, best to think of it as a keyhole, which in a very real sense it is. You are peering through a small hole toward a scene some distance beyond. That's how an afocal instrument works.

By suggesting to consider the exit pupil as an image, you've introduced an unnecessary impediment to understanding. These are the kinds of things I strive to remedy.

Indeed, such things as eye-induced aberrations and secondary shadow size are important factors to consider. As well as the difference in image brightness changes when the target is point-like vs extended.

My thesis here has concerned only the effect of the exit pupil/ iris diameter on extended object brightness. I've pretty arduously stressed *surface* brightness throughout, so as to differentiate from point-source considerations.

When someone asks a question 'on-topic', it's important to clearly point out in a reply those aspects which do not directly address the question. A casual lumping in of point-source brightness behavior with that of extended source behavior, without stressing the differences, can--and for the beginner especially, will--introduce unnecessary confusion.

Too often we enthusiasts get sidetracked by details which can obfuscate, or hinder understanding. It's essential to break a problem down to the minimal number of variables in order to fully appreciate the effect of each. Only when the fundamentals are understood should additional factors be introduced.

The only variable of fundamental importance as regards image surface brightness is the exit pupil/iris diameter. That's it. The additional wrinkles are optical system transmission efficiency, the relative obstruction of tthe secondary shadow when present, and variation in illumination across the field (vignetting).

Once the fundamental understanding arrived at by considering a simple, idealized system is achieved, the effects of other 'complicating' factors are then easily accommodated.


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BillFerris
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5589995 - 12/26/12 05:05 PM

Quote:

Now, because an oversized exit pupil effectively reduces the objective aperture, point sources such as stars are dimmed. But extended object surface brightness is maximal and constant in the regime of the over-large exit pupil.




While object surface brightness remains constant, object total integrated brightness is reduced. This is due to the over-sized exit pupil effectively reducing the telescope's aperture. An extended object that would, if observed at a magnification matching the observer's eye pupil, be at the threshold of visibility can be pushed beyond the threshold into invisibility, if too large an exit pupil is used.

Bill in Flag


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GlennLeDrew
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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5590215 - 12/26/12 08:03 PM

Exactly correct, Bill. This is another side of the relationship between integrated, or total brightness vs surface brightness.

In this case of the oversized exit pupil, for *both* extended and point sources *integrated* brightness decreases as magnification is further reduced (and the exit pupil gets ever larger.)

While it *might* be worthwhile to be aware of the fact that integrated brightness of an extended object is being reduced, it really is peripheral. The important elements are that surface brightness is still maximal, and (most importantly) that the image scale is being reduced, thus reducing the amount of detail seen.

Now to the other side of the divide, where the exit pupil is the smaller...

In the regime where the exit pupil is no larger than that of the iris, as the exit pupil diameter varies, an extended object's *integrated* brightness remains constant. This is because the increase in area on the retina is exactly compensated for by the change in surface brightness.

For example, when magnification is doubled, the exit pupil is halved, image surface brightness is reduced to 1/4. The object diameter is doubled, and hence its area is quadrupled. The 4X larger area compensates for the 4X fainter surface brightness. Thus the constant total brightness.

Is the fact that total brightness for an extended object remaining constant at all exit pupils equal to or less than the observer's pupil really meaningful? From a practical standpoint, no. Of vastly greater concern is the relationship between image scale, surface brightness change and the amount of detail perceivable.


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jrbarnett
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5590270 - 12/26/12 08:38 PM

Look for a 24mm Meade Series 4000 SWA.

http://www.astromart.com/classifieds/details.asp?classified_id=799732

I've seen them sell for as little as $65 recently. Used. Made in the good old days, in Japan.

Regards,

Jim


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markgf
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Re: Reality check requested re: 32mm eyepiece new [Re: jrbarnett]
      #5590401 - 12/26/12 10:14 PM

Well Jim, I'm glad we've got that sorted.
Very droll, and assiduously on-topic...Mark.


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Starman1
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Re: Reality check requested re: 32mm eyepiece new [Re: jrbarnett]
      #5590500 - 12/26/12 11:19 PM

Quote:

Look for a 24mm Meade Series 4000 SWA.

http://www.astromart.com/classifieds/details.asp?classified_id=799732

I've seen them sell for as little as $65 recently. Used. Made in the good old days, in Japan.

Regards,

Jim



My buddy Tom, to whom I sold my 8" SCT, still has one of these 24.5mm, the 18mm, and the 13.8mm as his primary set of 3 eyepieces.
In an f/10 scope, they aren't bad.


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markgf
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Re: Reality check requested re: 32mm eyepiece new [Re: markgf]
      #5590513 - 12/26/12 11:29 PM

Back to topic:

For John's F/13.9 telescope, the exit pupil won't be over-sized. Even a 55mm plossl would only give about a 4mm dia exit pupil.

So, maximise the total FOV. (eg, for a 1.25" focuser: 24mm x 68°, or 32mm x 50°, or 40mm x 41°)

From Glenn and others above, Apparent FOV doesn't matter if the Total FOV stays the same and the exit pupil is not too big, so the higher magnification (24mm) may be the way to go as it gets you a closer view.

Now I'll put on my feather ruffling suit (heavy duty version)- There's even a situation (in my naive brain at least) that contrary to (but based on) all the learned explanation above, HIGHER mag with a NARROWER TFoV can give a BRIGHTER view; for example when there's an actual person aiming at the brightest bit of an extended object (eg, the crowded core of a glob cluster) rather than 'zoomed out' for a wider star-scape that includes lots of inter-stellar blackness).

I shall now retreat for a little while to the beer fridge.
Mark...(hic!)


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BillFerris
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5590522 - 12/26/12 11:32 PM

Well, hardly peripheral. Everything comes back to aperture.

Bill in Flag


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Jon Isaacs
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Re: Reality check requested re: 32mm eyepiece new [Re: Starman1]
      #5590758 - 12/27/12 06:29 AM

Quote:

My buddy Tom, to whom I sold my 8" SCT, still has one of these 24.5mm, the 18mm, and the 13.8mm as his primary set of 3 eyepieces.
In an f/10 scope, they aren't bad.




On the other hand, the 24mm Series 5000 is an all around better eyepiece, fast or slow...

Jon


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markgf
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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5590772 - 12/27/12 06:55 AM

sorry about that earlier post. got a bit carried away. The BBQ's over and I've put the beer fridge to bed...

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markgf
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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5590775 - 12/27/12 07:02 AM

Quote:

On the other hand, the 24mm Series 5000 is an all around better eyepiece, fast or slow...

Jon




Isn't this the same glass as the ES 24mm/68°?...Mark


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Jon Isaacs
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Re: Reality check requested re: 32mm eyepiece new [Re: markgf]
      #5590781 - 12/27/12 07:24 AM

Quote:

Quote:

On the other hand, the 24mm Series 5000 is an all around better eyepiece, fast or slow...

Jon




Isn't this the same glass as the ES 24mm/68°?...Mark




I believe it is essentially the same eyepiece. This has been a long and instructive thread. Some of the discussion is probably over the head of many less experienced observers, hopefully they have not been left by the wayside.

In any event, back to the original question, for a 90mm F/14 MAK, there are really three Widefield eyepieces that might be appropriate, a 24mm SWA, a 32mm Plossl and a 40mm Plossl. Each has it's pluses and minuses, they all provide the same true field of view but with different combinations of magnification and exit pupil/image brightness.

Since original poster John lives nearby and since I have a 24mm Meade Series 5000 SWA, a 32mm Celestron Plossl and 40 mm Celestron Plossl, I am hoping that we can get together and make a comparison of these three. I even have a 50mm Paul Rini that we could try..

It's one thing to discuss image brightness and exit pupil size, another to experience it...

Best to all

Jon Isaacs


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BillFerris
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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5590930 - 12/27/12 10:18 AM

Quote:

Since original poster John lives nearby and since I have a 24mm Meade Series 5000 SWA, a 32mm Celestron Plossl and 40 mm Celestron Plossl, I am hoping that we can get together and make a comparison of these three. I even have a 50mm Paul Rini that we could try.




Field testing is always the way to go, if that's an option. Years ago, I field tested a number of wide field eyepieces with my my old 10-inch, f/4.5 Meade Starfinder Equatorial. The long and short of that experience was I decided to buy a used Tele Vue 32mm Plössl from a fellow club member. The true field of view wasn't as wide as the other eyepieces I tested, but stars were pinpoint all the way to the edge of the field, it didn't suffer from an annoying secondary shadow and the eyepiece didn't put my scope out of balance.

I later purchased a 23mm Celestron wide field eyepiece which delivered the same true field of view as the Tele Vue but at higher magnification. The edge aberrations were annoying enough that I generally preferred the 32mm Plössl to the 23mm wide field but there were times when the Celestron delivered the more pleasing view.

Try before you buy,

Bill in Flag


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5591540 - 12/27/12 05:51 PM

I've been striving mightily to keep up with this thread, so I hope the more-knowledgeable folks will bear with my ignorance. But, having read and attempted to understand, I have three big questions looming large in my tiny brain ...

What's the difference between surface brightness, object surface brightness, and overall image brightness? As a once-fairly experienced photographer I can hazard a guess, but I want to be sure I understand the terms in context.

How do the behaviors of point source targets and extended source targets differ in real-time viewing through a telescope?

And, how much of these differences can be attributed to equipment specs vs. the nature of human vision?

I fully intend to take up Jon's generous offer of a viewing session in which to compare eyepieces, as soon as the holiday bustle quiets!


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John Kuraoka
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5591581 - 12/27/12 06:28 PM

One more question to help place things in context ...

Does a larger exit pupil make for less-critical eye placement? That is, if one is a sort of wobbly person, or has wobbly eyes, anyway, would a larger exit pupil help that person to see a more stable image vs. one that keeps blinking out?


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Starman1
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5591590 - 12/27/12 06:39 PM

Quote:

I've been striving mightily to keep up with this thread, so I hope the more-knowledgeable folks will bear with my ignorance. But, having read and attempted to understand, I have three big questions looming large in my tiny brain ...

What's the difference between surface brightness, object surface brightness, and overall image brightness? As a once-fairly experienced photographer I can hazard a guess, but I want to be sure I understand the terms in context.



I'll give this a go....
Surface brightness in a telescope image is the brightness per unit area. That area could be anything you want so long as you're consistent. It might be useful to think of a small area like one square millimeter.
Object surface brightness would refer to the brightness contribution from an extended object. If the hypothetical object were 6' x 4' rectangular in a 25' round field, that object has a brightness per unit area which doesn't change as you magnify it because the contribution from every square millimeter doesn't change. It might appear dimmer because that light subtends a larger angle in your vision field, but our brain compensates because larger details are easier to see.
Overall Image brightness would be related to the amount of your eye the image occupies (related to the size of your pupil) and the brightness of the objects in the field, and the apparent field of the eyepiece.
Example: a 50 degree eyepiece and a 100 degree eyepiece both produce an exit pupil equal to the size of your pupil. The surface brightness of each image is the same. The total number of photons is different going into the eye, but not in terms of number of photons reaching each unit area of the retina. In that regard, the brightness per unit area (the surface brightness) is the same.
Quote:


How do the behaviors of point source targets and extended source targets differ in real-time viewing through a telescope?



point source brightness is due to the photon numbers gathered by the aperture. Larger telescope = brighter point sources. Magnify the image and the apparent brightness of the background sky goes down but the star brightness does not. Hence, the faintest stars are visible at the highest powers (where the star image doesn't become large enough to be seen as an extended object).
Extended objects are like the background sky--they get apparently dimmer as the image is magnified. Yet, the magnification that produces the largest exit pupil your eye can field may not be the magnification that makes the object visible. Larger, for our eye-brain combination = more visible (at least, to a point). That is the advantage of the bigger scope. For the same exit pupil, the magnification is higher. Same brightness and bigger = more visible.
For the point source, larger = brighter = more visible.
Quote:


And, how much of these differences can be attributed to equipment specs vs. the nature of human vision?



We look through our telescopes with our eye-brain combos. the size of the optics determines what we see just as much as human vision. One good thing: human vision can be trained to see better. Telescopes can't be trained to be larger.
Quote:


I fully intend to take up Jon's generous offer of a viewing session in which to compare eyepieces, as soon as the holiday bustle quiets!



Great idea.


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Allan...
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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5591786 - 12/27/12 08:41 PM

I'm looking for ONE (can only afford one) good, low power eyepieces. Have looked at the following:
ES28mm 2" 68°
ES24mm 1.25" 68°
Astro-Tech 32mm 2" 70°
Williams Optics 33mm 2" 72°

ps: exit pupil is a consideration as I am nearing 60. Any suggestions would be appreciated; I know little about the last two brands above. thanks, Clare


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Achernar
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Re: Reality check requested re: 32mm eyepiece new [Re: Allan...]
      #5591906 - 12/27/12 10:26 PM

I would get one that gives a 5mm exit pupil, which accomodates older people better than one that yields a 7mm exit pupil. I am almost 47, and for that reason I do not use eyepieces with focal lengths more than 24 or 25mm to keep the exit pupil to about 5mm or smaller. That darkens the sky background without dimming objects excessively.

Taras


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GlennLeDrew
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Re: Reality check requested re: 32mm eyepiece new [Re: Allan...]
      #5591914 - 12/27/12 10:33 PM

Clare,
Remind us what your telescope is, as its focal ratio is necessary to know.


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Allan...
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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5591931 - 12/27/12 10:54 PM

Oh, sorry...mine is an XT8 Dob; F5.9 1200mm. I figured that the lowest I would want to go (at my age) would be 28mm which I believe is a 4.75 exit pupil with my scope. Ive not been to the optomotrist yet to accurately get my own exit pupil measurement but according to the averages for my age group, I think that its reccommended that I stick to 5 and under. Clare

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Re: Reality check requested re: 32mm eyepiece new [Re: John Kuraoka]
      #5592048 - 12/28/12 12:53 AM

Quote:

What's the difference between surface brightness, object surface brightness, and overall image brightness? As a once-fairly experienced photographer I can hazard a guess, but I want to be sure I understand the terms in context.




Don and others have done a nice job of addressing your questions. Since it can sometimes be helpful to read the same or similar information presented in slightly different terms, I'll offer the following.

Overall Image Brightness: I'll respond based on the assumption that you're referring to the brightness of a celestial object, such as a star, galaxy, nebula or some such. This is the total brightness of the object. In astronomy, we use the logarithmic magnitude scale to describe the total brightness of celestial objects. Each magnitude represents roughly a 2.5-times change in brightness. A five magnitude difference represents a 100-times change in brightness. The lower the number, the brighter the object. A 0 magnitude star is 100-times brighter than a 5th magnitude star. A 9th magnitude galaxy is 40-times brighter than a 13th magnitude galaxy.

Surface Brightness: This is, as others have explained, a brightness per unit area. Because stars are point sources, they have no surface area and, therefore, no surface brightness. Galaxies, nebulae and other celestial bodies do have surface area and, as a result, also have surface brightness. This is measured in magnitudes per square minute or second of arc. For example, M110 is an 8.1 magnitude galaxy. It is 22 arcminutes by 11 arcminutes in size. Its integrated magnitude (total brightness) is 8.1 magnitude and its surface brightness is 22.7 magnitudes per square arcsecond. To convert to arcminutes, subtract 8.9 for a surface brightness of 13.8 magnitudes per square arcminute.

Any object having measurable dimension has a surface brightness. This includes the night sky. A pristine dark sky unmarred by man made light pollution has a surface brightness of about 22 magnitudes per square arcsecond.

Quote:

How do the behaviors of point source targets and extended source targets differ in real-time viewing through a telescope?




The visibility of a point source is dependent upon the surface brightness of the sky and the star's total brightness; its integrated magnitude. At high noon on a clear sunny day, the sky is too bright for even the brightest stars (excluding phenomena such as novae and supernovae) to be seen. But as night falls and the sky darkens, more and more stars stand out in contrast against a darkening night sky. A person with good vision can pretty easily see stars as faint as 6th magnitude under a pristine night sky. Experienced observers can detect stars as faint as 7th or even 8th magnitude.

A telescope allows you to do two very important things: collect more light from and magnify the object. For the sake of discussion, let's assume your eye pupils dilate to 6mm in diameter at night and that you're able to see 6th magnitude stars. If you bend down to look through the eyepiece of a 60mm refractor, you're looking through an instrument with 10 times the aperture of your eye. Since surface area changes as the square of the change in radius, the telescope is theoretically collecting 100-times as much light as your eye. Theoretically, you should be able to detect stars 5 magnitudes (100-times) fainter when looking through the telescope. Not only that, but the 6th magnitude star you could see with the unaided eye will have an apparent integrated magnitude of 1 when viewed through the telescope. This effect of increased aperture on the apparent total brightness of celestial objects is true for stars and galaxies, alike. Increasing aperture does make all celestial objects brighter to the eye.

Galaxies and other extended objects are a bit more complicated. These objects have dimension and, as a result, surface brightness. Surface brightness determines the contrast--the difference in brightness--between an object and the surrounding sky.

At first blush, you might be thinking, "OK, if a galaxy with surface brightness of 23 magnitudes per square arcsecond is surrounded by a sky with a surface brightness of 22 magnitudes per square arcsecond, won't the galaxy be fainter than the sky (the larger the magnitude, the fainter the object) and impossible to see?" That might be the case if not for the fact that object brightness and sky brightness are additive. As a result, every galaxy, nebulae and celestial object is always brighter than the surrounding sky. The critical issue is, how much brighter?

Let's return to our high noon, sunny daytime sky. The surface brightness of that sky might be something on the order of 3 magnitudes per square arcsecond. Now, suppose you're looking at the sky in the direction of a galaxy having a surface brightness of 23 magnitudes per square arcsecond. The difference in surface brightness between the galaxy and the surrounding sky will be 20 magnitudes. That translates to a difference of 100 million. In other words, the galaxy will appear 1/100 millionth brighter than the surrounding sky. That is much too low a contrast for the human eye, either unaided or assisted by any existing telescope on Earth, to be able to detect the galaxy at visible wavelengths.

Now, suppose we return to the same location later that night. Furthermore, suppose this spot is well-removed from city lights and the overhead sky is truly pristine. The night sky at this site has a surface brightness of 22 magnitudes per square arcsecond. The difference in brightness between the sky and the galaxy is 1 magnitude. This translates to a difference of 2.5 times so, the galaxy will appear 40% brighter than the surrounding sky. That's a fairly significant contrast (difference in brightness) to the human eye. If this galaxy is large enough, say the size of M33, it will be fairly obvious in a small aperture telescope or binoculars. This galaxy will even be visible to the naked eye.

Quote:

And, how much of these differences can be attributed to equipment specs vs. the nature of human vision?




The foundation for our understanding of the visibility of celestial objects is the physics of light and optics, and the physical nature of human vision. A wide range of factors such as sky quality, telescope optical quality, age, ability and experience of the observer will produce variability in the outcome of any experiment designed to measure the ability of a group of individuals to see celestial objects. This reality should not cause one to question our fundamental understanding of the nature of visual telescopic observation. On the contrary, we can use this knowledge to employ techniques to mitigate the negative effects of real world factors that would otherwise limit our ability to see to the theoretical limits of a given aperture.

One of the fundamental truths of human night time vision is that we are primarily contrast detectors under low light conditions. Our ability to see color in the dark is extremely limited as is our ability to resolve fine detail. But the dark adapted human eye is very good at detecting faint light sources and is still relatively good at detecting subtle differences in brightness, especially if the marginally brighter object is fairly large in apparent size. Based on this understanding, I believe--and others may disagree on this point--it critical to understand and explain the role aperture plays in our ability to see faint extended objects in terms of contrast.

This approach is complicated by the fact that a celestial object's surface brightness is always greatest to the naked eye. By definition, increasing aperture cannot present an extended object with a surface brightness greater than is apparent to the naked eye. This is what most people mean when they say increasing aperture will not make a celestial object appear brighter. Since such statements obviously run contrary to the fact that a larger aperture presents all celestial objects as having increased total brightness, I prefer to be very clear that a telescope can not present an extended object as having a surface brightness greater than is visible to the naked eye. Furthermore, the contrast (difference in surface brightness) between a galaxy and the surrounding sky will be constant in all apertures and at all magnifications to all people and telescopes at one site viewing that object in its entirety.

What does change as aperture changes, is the threshold contrast that is visible to the observer at the eyepiece. As aperture increases, threshold contrast is lowered. As aperture decreases, threshold contrast rises. In other words, looking at the night sky through a larger telescope will allow you to see lower contrast objects. If you're interested in further reading on this topic, I'll suggest you check out the online article in my signature, "Lowering the Threshold."

Best regards and clear skies,

Bill in Flag


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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5592138 - 12/28/12 04:32 AM

Bill, an excellent summation! The last paragraph could be clarified further. The naked eye can detect objects having as low a contrast as detectible through a telescope. To be pedantically accurate, the unaided eye us better because the telesco

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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5592151 - 12/28/12 04:55 AM

Bill, an excellent summation! The last paragraph could be clarified further. The naked eye can detect objects having as low a contrast as detectible through a telescope. To be pedantically accurate, the unaided eye is a bit better because the telescope is not 100% efficient. Your earlier example of the 10X60 (6mm exit pupil) has that instrument detecting an object 1/10 the size of another object seen with the unaided eye and which has otherwise identical contrast.

The larger aperture only improves contrast detection by virtue of the larger image scale. And so it's correct to state that *for a specific object*, increasing aperture improves threshold contrast, on the object.

But to categorically state that increasing aperture increases threshold contrast, one would obtain the misaprehension that increasing aperture increases contrast. As we know, changing magnification does not alter contrast in any absolute sense, but rather the *appearance* of improved contrast is due to the combination of greater total area on the retina and the improved resolution.

It bears stressing that a larger aperture at best allows to discriminate
contrast to the same levels as seen by the unaided eye. All the larger aperture does is to permit the detection of detail of a finer degree, and smaller objects

Again to the 10X60 (working at the eye's maximum pupil diameter.) Suppose we have two otherwise identical galaxies, differing only in that one is 10X larger. The larger one as seen with the unaided eye would appear as being exactly like the 1/10X galaxy seen through the instrument. There is no intrinsic increase in threshold contrast. If this were so, the smaller galaxy magnified to the size of the larger one would necessarily have to appear more detailed. But it will not, and cannot, because the instrument does not alter contrast; it only magnifies the image.

The concept of threshold contrast is only valid in the limiting case of a specific object, and how its appearance varies as the aperture or exit pupil changes. Otherwise, it can lead to the false impression that aperture alters contrast; it only alters the magnification for given exit pupil.

I know I'm repeating myself, but I can't over-stress the importance of awareness of all the relevant factors. A simple oversight can introduce untold confusion.


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Re: Reality check requested re: 32mm eyepiece new [Re: Allan...]
      #5592161 - 12/28/12 05:41 AM

Clare

I would not be so concerned with the exit pupil but would rather try to maximize the field of view while maintaining a reasonable exit pupil. At f/6 eyepieces like the 31mm Nagler and the more affordable ES 30mm 82 degree are generally a good choice for this, providing a 5mm exit pupil. A some oversized exit pupil is reasonable, just don't try a 55mm Plossl.

Your optomotrist will dilate your eye with medication so that is not a good measure of your dilated eye. There are other techniques but it is a difficult measurement.

Jon


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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5592177 - 12/28/12 06:18 AM

A couple of comments regarding brightness:

- the visual brightness of an object listed in books and computer programs is the total light added up as if it were a point source. It's not a good indicator of visibility.

- the surface brightness is a measure of the intensity of the light of an extended object. For bright objects like Jupiter it is uniform. For nebulae and galaxies it can greatly vary over the surface of an object. That's why we can see the core of m31 from an urban setting but require dark skies to see the entire galaxy.

- the exit pupil squared is a measure of the relative surface brightness. Since the effective exit pupil can never larger than your dilated eye, an objects surface brightness can never exceed that of the naked eye.

- The eye detects faint objects more easily if they are larger. This is why a larger aperture allows an observer detect small, faint galaxies and nebulae more easily, a larger aperture scope provides greater magnification at the same level of brightness, the object can cover more of the retina while maintaining sufficient brightness.

- For large faint objects, a small telescope can be the right tool. An object like the California Nebula which is about 3 degrees in length will fill the field of view of a larger telescope and so it all looks the same, there is no contrast. This is why I always use a large scope and a small, short focal length scope when the skies are dark and clear.

- This stuff is good to understand because it can help guide an observer in choosing objects suitable for the conditions and equipment, in choosing the right scope and eyepiece for an object. But it is an intellectual understanding and at first it does not seem to agree with what one sees at the eyepiece.

Think about it, but don't think too much. Instead, get out there and look as much as you can with whatever you have. Someday it all might make sense but tonight just enjoy the views.

Clear skies ahead..

Jon Isaacs


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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5592462 - 12/28/12 11:04 AM

Thank you Don, Bill, Glenn, and Jon for that! It was very helpful to benefit from different explanations of the same phenomena - I found myself bouncing back and forth among them. I'm printing this out so I can have it to re-read, along with Bill's article.

Quote:

Think about it, but don't think too much. Instead, get out there and look as much as you can with whatever you have. Someday it all might make sense but tonight just enjoy the views.




Sound advice! Thanks!


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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5592648 - 12/28/12 12:55 PM

Quote:

The larger aperture only improves contrast detection by virtue of the larger image scale. And so it's correct to state that *for a specific object*, increasing aperture improves threshold contrast, on the object.




Actually, image scale need not increase to detect a lower contrast object. Imagine a garden variety galaxy cluster, Abell 1656, the Coma Cluster, for example. It's populated by a host of galaxies ranging in brightness from NGC's 4874 and 4889--both visible in moderate aperture--to scores of tiny 15th and 16th magnitude stellar gothams. Many of these faint galaxies are similar in size. Here's a simple experiment folks can perform to confirm that image scale need not increase to detect lower contrast objects: The next time you're out with a group of observers on a clear, late spring night, recruit three friends with large Dobs of different aperture to aim at the Coma Cluster. Ask them to use eyepieces generating the same magnification view in all three scopes. Starting with the smaller of the three scopes, moving next to the middle and finishing with the largest aperture instrument, compare the views. You'll see more and fainter galaxies with each increase in aperture. Image scale will remain constant in the three scopes but lower contrast galaxies will emerge to the eye as aperture increases.

The reality is that, when it comes to faint arcminute diameter galaxies, a moderate aperture telescope is more than capable of generating enough magnification to present these tiny targets with enough image scale to be seen. The problem is, a moderate aperture doesn't collect enough light from the galaxy to stimulate the rod cells and produce a detection. Increasing aperture delivers more light (more information) to the eye, enhancing our ability to detect low contrast objects. Don't get me wrong, image scale is important. The dark adapted eye's limited resolution requires that faint galaxies appear several degrees in size to be seen as extended. But even for a tiny, arcminute diameter galaxy, this can be accomplished with a magnification of 200-300x. That is is well-within the capabilities of a 6- to 8-inch scope. The critical need is to collect enough light from the object to stimulate the eye's ability to detect low contrast objects. This can be done by increasing aperture to lower threshold contrast.

Quote:

But to categorically state that increasing aperture increases threshold contrast, one would obtain the misaprehension that increasing aperture increases contrast. As we know, changing magnification does not alter contrast in any absolute sense, but rather the *appearance* of improved contrast is due to the combination of greater total area on the retina and the improved resolution.




I'm sure you meant to write, "increasing aperture reduces or lowers threshold contrast," as that is the relationship between the two. Correctly stated, I doubt too many folks will conclude that increasing aperture reduces or lowers an object's contrast versus the sky.

Properly understood, the concept of threshold contrast is a rather elegant solution to the challenge of framing the role aperture plays in the observation of faint, extended objects. One needn't alter the contrast of an object versus the sky. Rather, increasing aperture to lower the threshold contrast at which objects become observable effectively raises fixed-contrast objects to or above the threshold of visibility. QED.

Bill in Flag


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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5592725 - 12/28/12 01:49 PM

Bill, you're forgetting in your example that if magnification is kept constant, the bigger scope will have a larger exit pupil, producing a brighter image.

Not that I disagree with the point--the larger scope raises the contrast threshold. But it's difficult to demonstrate: if the magnification is kept the same the bigger scope has a larger exit pupil = better visibility of the object. If the exit pupil is held constant, the bigger scope produces a bigger image = better visibility of the object. Does the bigger scope produce better visibility of faint objects because of these factors only, or is there something else?

A star produces a better example. Hold exit pupil the same and increased magnification will have no effect on the size of the object. YET, the star will be more visible in the larger scope. In this case, it's a simple matter of the number of photons gathered into the point. Though the two examples in my previous paragraph certainly hold true, I can also envisage every "point" in an extended object receiving more photons in the larger scope. After all, even an extended object is made up of a finite number (though large) of Airy discs for each point in the image.


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Re: Reality check requested re: 32mm eyepiece new [Re: Starman1]
      #5592787 - 12/28/12 02:31 PM

Bill,
Thanks for pointing out my mistake; I'm in bed with the flu, and am a bit foggy.

Your points are very true, but the lowering of threshold contrast with increasing aperture is more relevant when discussing a particular object. And so when gazing at a galaxy cluster, more aperture brings more fuzzies into view. But this is so axiomatic as to hardly bear mentioning; of course more aperture allows to see fainter and in better detail.

What I'm stressing is this. More aperture does not allow to detect lower contrast. Be it a 2" or a 20", at the low light levels of DSO observation, each will allow to discriminate down to the same ~6%.

And to show how more aperture can be detrimental, consider a sinusoidal target (like a blurry bar code) having contrast as low as can be reliably detected, and DSO-dim. A small scope will not resolve the pattern, and so it will be a featureless blur. A moderate aperture just nicely resolves to the point that the variations are seen. A large aperture makes the pattern even more readily visible. But a very large aperture magnifies so much that the expanded pattern actually becomes more difficult to discern (kind of like Jon's earlier example with the huge California nebula.)


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Re: Reality check requested re: 32mm eyepiece new [Re: Starman1]
      #5592844 - 12/28/12 03:16 PM

A few thoughts:

This discussion is beyond what is appropriate for the beginners forum... It should be kept simple and understandable by a beginner.

- In Bill's example there are three important factors: Contrast, magnification and image brightness. In his example, the magnification is the same, the contrast is the same, what changes is that the larger aperture provides a brighter image so fainter objects are visible in the larger ecopes, it's just that simple.

If the example were changed so that the image brightness/exit pupil were held constant, the larger scopes would have provided greater magnification and more objects would have been visible.

Contrast, brightness, and image size/magnification are the three important factors. Contrast cannot be increased without a filter, it is fixed. Brightness and size can be changed. Tor detecting a particular object, one can optimize the magnification and brightness to so that it is best matched to the eye.

Last night it was the full moon. But the skies were clear and otherwise dark and I was out observing with my 80mm apo and my 12.5 inch Dobsonian. In the early evening I pointed the Dob at the Veil Nebula and its was faintly visible with an O-III filter. The filter had improved the contrast. The Veil was no brighter but the background sky was much dimmer.

Later I viewed M76 with both scopes. With the 80mm, I had to crank up the magnification to see M76, the larger size allowed my eye to detect it. The contrast had not changed, the image was dimmer but it was larger and more easily seen. Later in the evening I pointed the 12.5 inch at m76. At the same brightness/exit pupil where it could not be seen in the 80mm, it was easily seen because it was much larger. Increasing the magnification showed more detail.

Brightness is important, brighter objects are more easily seen. Contrast, the ratio between the object brightness and the background sky, is important but a telescope does not increase the contrast.

And magnification to increase the size of the object is important. At the limit, this is what a telescope does. It is difficult to believe but the surfaces of the coma clusters galaxies can be no brighter in a 25 inch telescope than they are naked eye. They can be no more contrasty than they are naked eye. The reason they are visible in the big scope and not naked eye is that they are larger, the telescope provides a larger image that covers enough of the retina that the eye and mind
can perceive it.

Jon Isaacs


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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5592940 - 12/28/12 04:26 PM

Glenn,

I hope you're feeling better, soon. Battling a flu bug is a tough way to end the year.

Bill in Flag


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Re: Reality check requested re: 32mm eyepiece new [Re: Starman1]
      #5592955 - 12/28/12 04:42 PM

Hi Don, I didn't forget. In fact, I discussed the fact that a larger aperture collects and delivers more light to the eye. This includes both celestial objects and the sky. Regarding your point that a larger exit pupil will present a celestial object as having a higher surface brightness, the sky will also have a higher surface brightness. The net result is that the contrast of a galaxy versus the night sky remains constant in all apertures using magnifications presenting the object in its entirety...another point already mentioned in this thread.

Bill in Flag


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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5592996 - 12/28/12 05:27 PM

Quote:

Hi Don, I didn't forget. In fact, I discussed the fact that a larger aperture collects and delivers more light to the eye. This includes both celestial objects and the sky. Regarding your point that a larger exit pupil will present a celestial object as having a higher surface brightness, the sky will also have a higher surface brightness. The net result is that the contrast of a galaxy versus the night sky remains constant in all apertures using magnifications presenting the object in its entirety...another point already mentioned in this thread.

Bill in Flag




It is true that the contrast is constant but the brightness is not and one cannot ignore brightness. If one thinks of the eye as a sensor, then a brighter image raises the image above the noise floor of the eye which increases the signal to noise ratio.

It's about contrast, surface brightness and image size.

Jon


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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5593041 - 12/28/12 05:55 PM

Bill,
Thanks for the well wishes. It's been about 36 hours so far, and I'm definitely improving.

Jon,
Indeed, we've been covering territory considered rather beyond the beginner's level of treatment. But I prefer to see the audience as being capable of grasping more complex material, if motivated to learn.

And it's wise to catch the beginner early on, so that misperceptions and their promulgation can be circumvented.


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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5593094 - 12/28/12 06:32 PM

Surface brightness, integrated brightness, image size, magnification, exit pupil, aperture & threshold contrast...
They're like a swarm of midges darting about. Easy to mistake one for another, or blend bits of them together, such as with magnification and image size.

Thank you for chiselling the ideas down to a fairly orderly scrum for us noobs.
Cheers...Mark


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Re: Reality check requested re: 32mm eyepiece new [Re: GlennLeDrew]
      #5594062 - 12/29/12 10:15 AM

Quote:

Bill,
Thanks for the well wishes. It's been about 36 hours so far, and I'm definitely improving.

Jon,
Indeed, we've been covering territory considered rather beyond the beginner's level of treatment. But I prefer to see the audience as being capable of grasping more complex material, if motivated to learn.

And it's wise to catch the beginner early on, so that misperceptions and
their promulgation can be circumvented.




Glenn:

I hope you get well soon.

I agree that it is important to address issues like exit pupil, surface brightness, and contrast as soon as is reasonable because there are misconceptions and misperceptions that are very common...

But I also think care must be taken to provide the necessary foundations with clear explanations and definitions. It is all too easy for knowledgeable observers to assume something like contrast is well understood and what can result is a discussion of our own particular formulations that are essentially identical but stated differently.

That's my concern.

Jon


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Re: Reality check requested re: 32mm eyepiece new [Re: Jon Isaacs]
      #5594374 - 12/29/12 01:10 PM

Quote:

Quote:

Hi Don, I didn't forget. In fact, I discussed the fact that a larger aperture collects and delivers more light to the eye. This includes both celestial objects and the sky. Regarding your point that a larger exit pupil will present a celestial object as having a higher surface brightness, the sky will also have a higher surface brightness. The net result is that the contrast of a galaxy versus the night sky remains constant in all apertures using magnifications presenting the object in its entirety...another point already mentioned in this thread.

Bill in Flag




It is true that the contrast is constant but the brightness is not and one cannot ignore brightness. If one thinks of the eye as a sensor, then a brighter image raises the image above the noise floor of the eye which increases the signal to noise ratio.

It's about contrast, surface brightness and image size.

Jon




Jon, that's a nice way of understanding why an increase in aperture lowers threshold contrast. Object contrast versus the sky remains constant but the increase in aperture delivers more light (more information) from the object to the eye. The larger light packet improves signal to noise ratio with a net result of being able to discern lower contrast objects.

Bill in Flag


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Re: Reality check requested re: 32mm eyepiece new [Re: BillFerris]
      #5658570 - 02/02/13 05:34 PM

I got together with Jon, and tried out his 24/68° and 32mm and 40mm Ploessls in my 90mm Mak-Cas (thank you Jon!). What I learned (so far) is here:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5658544/page...


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