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Reality check requested re: 32mm eyepiece

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#1 John Kuraoka

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Posted 21 December 2012 - 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?

#2 csrlice12

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Posted 21 December 2012 - 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

#3 John Kuraoka

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Posted 21 December 2012 - 03:20 PM

Thanks!

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?

#4 csrlice12

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Posted 21 December 2012 - 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.

#5 REC

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Posted 21 December 2012 - 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.

#6 Jon Isaacs

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Posted 21 December 2012 - 04:11 PM

Thanks!

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

#7 John Kuraoka

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Posted 21 December 2012 - 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!

#8 GlennLeDrew

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Posted 21 December 2012 - 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.

#9 John Kuraoka

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Posted 21 December 2012 - 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.)

#10 Jon Isaacs

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Posted 21 December 2012 - 04:54 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.)


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

#11 izar187

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Posted 21 December 2012 - 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.

#12 REC

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Posted 21 December 2012 - 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

#13 John Kuraoka

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Posted 21 December 2012 - 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.

#14 WAVT

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Posted 22 December 2012 - 10:30 AM

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.

(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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#15 John Kuraoka

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Posted 22 December 2012 - 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?

#16 rdandrea

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Posted 22 December 2012 - 12:52 PM

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

#17 John Kuraoka

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Posted 22 December 2012 - 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?

#18 rdandrea

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Posted 22 December 2012 - 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.

#19 artcarter

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Posted 22 December 2012 - 02:27 PM

What is the telescope simulator you speak of?

Art

#20 John Kuraoka

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Posted 22 December 2012 - 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.

#21 *skyguy*

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Posted 22 December 2012 - 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

#22 GlennLeDrew

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Posted 22 December 2012 - 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.

#23 Jon Isaacs

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Posted 22 December 2012 - 08:06 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?


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

#24 markgf

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Posted 22 December 2012 - 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

#25 John Kuraoka

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Posted 22 December 2012 - 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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