Wide angle eyepieces useful?

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turboscrew
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Wide angle eyepieces useful?

#1

Post by turboscrew »


I was setting up my CC using the full moon, and what I saw through my 26 mm 70° Omegon SWAN was mostly the secondary mirror. That made me thought that one might get a nice wide view using a shorter focal length wider view eyepiece instead (smaller exit pupil), to avoid the huge secondary mirror shadow.
- Juha

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Telescope: OrionOptics XV12, Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Grab 'n go: Omegon AC 102/660 on AZ-3 mount
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 10 mm TV Delos, 6 mm Baader Classic Ortho, 5 mm TV DeLite, 4 mm and 3 mm TV Radians
Cameras: ZWO ASI 294MM Pro, Omegon veLOX 178C
OAG: TS-Optics TSOAG09, ZWO EFW 7 x 36 mm, ZWO filter sets: LRGB and Ha/OIII/SII
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Re: Wide angle eyepieces useful?

#2

Post by Don Pensack »


Yes, extremely low power eyepieces yield large exit pupils.
Unfortunately, when you look at the Moon, your pupil shrinks to daytime levels.

Let's say your exit pupil is 6mm and the secondary is a 25% width. That means the shadow of the secondary is 1.5mm wide.
When your pupil is 6mm wide, that's not a problem.
But when you look at the Moon, your pupil may shrink to a 2mm daytime size.
All of a sudden, the secondary shadow is now 75 as wide as your pupil! Mon dieu!

Use a higher power eyepiece, though, and the exit pupil and secondary shadow shrink.
But if you're looking at the Moon, 100x is about the minimum you need to see anything, and powers of 150-300x are much more revealing of lunar details.
You can simply avoid the exit pupil/secondary shadow issue by using higher power eyepieces, and if you want more Moon in the view, go to eyepieces with wider apparent fields.

That Omegon 26mm 70° has about a 31.8mm field diameter. As just one example, the field diameter on a 17mm 100° eyepiece is close to the same (29.8mm) with a more than 50% increase in magnification and a corresponding shrinkage of the secondary shadow width.
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Re: Wide angle eyepieces useful?

#3

Post by notFritzArgelander »


turboscrew wrote: Tue Sep 21, 2021 9:59 pm I was setting up my CC using the full moon, and what I saw through my 26 mm 70° Omegon SWAN was mostly the secondary mirror. That made me thought that one might get a nice wide view using a shorter focal length wider view eyepiece instead (smaller exit pupil), to avoid the huge secondary mirror shadow.
Yes, they are useful but the trick is to get an exit pupil from the eyepiece that is not larger than the pupil from the eye. The Moon in particular near Full phase can cause the eye's pupil to contract because it's so bright. IIRC your Newt is f4 so a 26mm eyepiece would give you a 6.5mm exit pupil. You might get away with that on DSOs with a dark adapted eye but not on a nearly full Moon.

I will use wide fields (when I want that) from as wide as 6mm exit pupil (about my limit) down to 1mm. It depends on the size of the object I'm framing. Mostly I'm an orthoscopic guy, though, so there's that. ;)
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: Wide angle eyepieces useful?

#4

Post by SkyHiker »


The GSO Dobs come with a cover with a hole in it for observing bright targets. If your OrionOptics has one too, you could try that, it would make your pupil react less.
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Re: Wide angle eyepieces useful?

#5

Post by turboscrew »


SkyHiker wrote: Tue Sep 21, 2021 10:29 pm The GSO Dobs come with a cover with a hole in it for observing bright targets. If your OrionOptics has one too, you could try that, it would make your pupil react less.
No such covers. Rather two shower caps. It was bought secondhand.
Also, no worries. That was just setting up the CC, not observation (it was late - I only got to sleep 5 hours).
- Juha

Senior Embedded SW Designer
Telescope: OrionOptics XV12, Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Grab 'n go: Omegon AC 102/660 on AZ-3 mount
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 10 mm TV Delos, 6 mm Baader Classic Ortho, 5 mm TV DeLite, 4 mm and 3 mm TV Radians
Cameras: ZWO ASI 294MM Pro, Omegon veLOX 178C
OAG: TS-Optics TSOAG09, ZWO EFW 7 x 36 mm, ZWO filter sets: LRGB and Ha/OIII/SII
Explore Scientific HR 2" coma corrector, Meade x3 1.25" Barlow, TV PowerMate 4x 2"
Some filters (#80A, ND-96, ND-09, Astronomik UHC)
Laptop: Acer Enduro Urban N3 semi-rugged, Windows 11
LAT 61° 28' 10.9" N, Bortle 5

I don't suffer from insanity. I'm enjoying every minute of it.

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Re: Wide angle eyepieces useful?

#6

Post by Don Pensack »


SkyHiker wrote: Tue Sep 21, 2021 10:29 pm The GSO Dobs come with a cover with a hole in it for observing bright targets. If your OrionOptics has one too, you could try that, it would make your pupil react less.
Yes, but it reduces the resolution of the instrument to the size of the hole in the cap, which is definitely not desirable when viewing the Moon.
Those small holes are primarily intended for solar viewing, 1) where a long f/ratio is desirable for most H-α solar use and 2) to reduce the size of the filter needed
for normal white light viewing and 3) to reduce the aperture for eyepiece projection of the sun's disc.
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Currently using a 12.5" dob and a 4" apo refractor
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Re: Wide angle eyepieces useful?

#7

Post by AntennaGuy »


Hmm. You could use a "moon filter" on the eyepiece.
E.g. https://www.highpointscientific.com/ape ... inch-a-mf1
Then, you just might be able to keep your pupil from shrinking too much.
* Meade 323 refractor on a manual equatorial mount.
* Celestron C6 SCT on a Twilight 1 Alt-Az mount
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