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Best Low Power Eyepiece for f5 refractor?

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#1 SeptemberEquinox

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Posted 29 January 2013 - 11:32 AM

I have a 150mm aperture, 750mm focal length, f5 refractor. As a newbie I've only used this scope 3 times and the views I got with this scope was pretty amazing.

I'm studying the night sky as much as I can since I'm using cg4, so i have to manually scan the sky to find objects which is very challenging, for me at least. So I thought the best thing to get is a low power eyepiece to scan the night sky and find the object easily. But I'm not sure which one to get that is bang for your buck deal.

Currently I have Es 70* 20mm and a 25mm that came with the scope. I ordered Mead 5000 82* 24mm with a star diagonal 2", but I'm not sure when I will be receiving that order.

could you guys recommend me a low power eyepiece that is reasonable? Of course performance matter ��

Thank you in advance

#2 csrlice12

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Posted 29 January 2013 - 12:21 PM

The 24mm you have on order will give you 31X. Not certain what the lowest recommended power for your scope is. A 30mm would give you 25X, a 35m would give you 21X, and a 40mm would be 19X (probably as low as you would go). Truthfully, your 20mm 70* eyepiece gives you 38X, which is still pretty low power and wide-field and should be a pretty good finder on its own. For a fast scope though (refractor or reflector), you are going to need premium eyepieces (ES or better). The Meade UWA will work well in this scope, but is heavy, so you will have some balance problems switching out eyepieces. That scope will give you some nice wide-field views of nebulas, galaxies, etc.....

#3 SeptemberEquinox

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Posted 29 January 2013 - 02:13 PM

Can you tell a lot difference in between 21x and 31x??

#4 csrlice12

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Posted 29 January 2013 - 03:50 PM

Slightly, but negligible difference. You'll probably not notice any real differences till you get to jumps of about 20X or more

#5 Starman1

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Posted 29 January 2013 - 04:42 PM

It depends on what you view and how you use the scope.
It's unlikely you will use the scope as a daytime spotting scope, so magnifications under 30X are sort of unimportant. Real low magnifications don't provide enough separation between the stars in a lot of clusters, so binocular-type magnifications aren't very useful, really.

There are some very large objects up there (the Pleiades is an example, the Coathanger is another), not to mention hundreds of "asterisms" (chance groupings of stars with interesting shapes), for which the 6" f/5 refractor is ideal.

But let's look at some math:
Though the actual true fields might be a trace smaller, we can use the formula TF=AF/M to get an idea how big a view a particular eyepiece will give.[true field = apparent field/magnification]
The 24mm > 750/24 = 31X, or a true field of 82/31 = 2.6 degrees.
That's huge and big enough for tons of large objects that don't show up well in larger scopes with narrower fields of view.

So the 24mm eyepiece you have on order will be a GREAT low-power, widest-field eyepiece and the 2" diagonal will open up a lot of opportunities for 2" eyepieces.

Since the 6" f/5 refractor isn't really a high-power planetary instrument, I see little reason to go much past 150X in this scope. Plus, chromatic aberration is somewhat severe on a refractor this short, so its potential as a lunar/planetary scope is somewhat tenuous at best. it's a nice scope, but it can't ignore the laws of physics.

But, as a widefield, large aperture, refractor, I love this scope.
I would suggest you stick to widefield (65-72 degree) or ultrawide field(82-100 degree) eyepieces to really capitalize on what the scope can do. I used the scope with a 10mm 100 degree eyepiece, and it was really magnificent on the Orion Nebula at that power (75X) and with that size field of view (100/75=1.33 degrees).

I hope you get to use this scope under dark skies, where it really shines.

The 24mm eyepiece being a low power, a reasonable increase in magnification would be to a 10mm eyepiece (+/-), and would probably be a "favorite" in magnification. The 2mm exit pupil would be nice and sharp. The true field is still large. Widefield 9-11mm eyepieces are as common as fleas, and the field of view large enough for nearly everything.

Add a good 5-6mm eyepiece for higher power views of smaller objects (planetary nebulae, globular clusters, planets and Moon, double stars, and small star clusters), and you pretty much have all you need to view.

You may prefer to create the higher power with a Barlow lens, and I can recommend that. The Barlow will tend to reduce the aberrations inherent in eyepieces when used at f/5, and can be a versatile thing to own.
Then you could add an eyepiece in between the 24mm and 10mm so that it, when barlowed, yields a magnification in between the 10mm and the 10mm barlowed.

If you make it a 2" barlow (like the AstroTech or similar), you can use it as a lens threaded onto the 24mm to produce a 16mm, or in front of the 24mm to produce a 12mm, or in front of the star diagonal to produce an 8mm. That one Barlow could make your 24mm into 5 separate magnifications and 4 additional magnifications for each 1.25" eyepiece you buy(threaded onto the 1.25" adapter, in its own tube, threaded onto the star diagonal, and with the barlow in front of the star diagonal).

The point is, you have a great scope that is wide of field and is a lot of fun to use. The 2" diagonal and 24mm special was a great choice for the scope, and you can pretty much do what you want with other eyepieces.

#6 SeptemberEquinox

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Posted 29 January 2013 - 06:10 PM

Don, I had no idea that's how you calculate the field of view. So with my ES 82* 6.4mm gives me 0.7* field of view? That's pretty awesome.

surprisingly this scope does very well on Jupiter and the moon with or without filters. I can see the details of jupiter from 30x, 37.5x, 117x and it was very clear with some fringing. And at 357x, it started to shake badly since my mount isn't superb. When I used #56 green filter, CA is completely gone and you can see Jupiter much clearer, and on the moon using same filter CA is completely gone. I do see more details when I use green filter, but things all turn green which I don't mind at all.

Don, I love this scope. And I love cloudynights community, thank you.

My next upgrade is a t shirt, Cloudynights t shirt.

#7 Starman1

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Posted 29 January 2013 - 06:37 PM

You might try a #80A blue filter on Jupiter--it enhances the contrast of the dark bands with the planet.
The green filter works on the Moon, but you would find superior contrast with a #15 yellow.
I believe there is a RA motor available for the CG-4. If you don't have one, save up for it because it's great to have tracking when you have a nice EQ mount.

#8 Aquarist

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Posted 29 January 2013 - 07:29 PM

Don provided superb advice! Congrats on your scope.

#9 SeptemberEquinox

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Posted 29 January 2013 - 08:09 PM

Don, I'll try those filters when I get my hands on them. I wanna buy the shirt first lol.

Thank you, Aquarist!

#10 Jon Isaacs

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Posted 30 January 2013 - 12:51 AM

It's unlikely you will use the scope as a daytime spotting scope, so magnifications under 30X are sort of unimportant.



From my point of view as a dedicated widefield junkie, the big, wide low power views are what a scope like this does best, that's the reason to have a scope like this and ideally one has an eyepiece that provides the widest possible field of view while maintaining a reasonable exit pupil.

To my eye, this looks like a 31mm Nagler or a 30 mm Explore Scientific 82 degree. The increased field of view, 3.2 degrees instead of 2.5 degrees, the increased brightness(66% brighter), a 6mm exit pupil versus a 4.8mm, these are significant advantages...

The difference between a 31mm and the 24mm is seeing the Veil in one field of view and only seeing a piece at a time. But more than single objects, these big bright views are about image scales and objects and features that are not normally seen, it's about zooming out and seeing the big features.

For me, magnifications below 30x are very important, particularly if the skies are dark and clear. This what a scope like this one is made for, give it's full rein and let it really do it's stuff.

Jon

#11 plyscope

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Posted 30 January 2013 - 06:24 AM

My favourite eyepieces in my Jaegers 6" f5 are the 31mm Nagler (25x) and 21mm Ethos (36x). Expensive but wonderful in this type of scope. I've also used a 17mm Nagler for 45x. I tend to use different scopes for higher powers, I think these f5 refractors are best below 10x per inch of aperture (60x). I have never tried filters.

The 24mm ES 82 degree should be very nice, especially when you're skies aren't dark enough for larger exit pupils.

#12 oldtimer

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Posted 05 February 2013 - 07:02 PM

I'm going to whip the 'dead horse' here again but I must comment on exit pupil. Your age and how dark your observing location is is going to determine how big your pupil can get. Eyepieces that generate a larger exit pupil than you can get in your eye effectively reduce your scope aperture.

If I have a 6" RFT I would think I would want to use 'all' of its aperture. So a 30mm eyepiece would produce a 6mm exit pupil and a 25mm a 5mm exit pupil.

F-5 is really tough for the less costly 5 & 6 element eyepieces and the edge distortion bothers 'some' folks. The only cure is expensive 'Nagler type' eyepieces. If you can deal with some edge distortion Agena sells a 30mm with a 80 degree FOV for less than $100.

#13 csrlice12

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Posted 05 February 2013 - 07:21 PM

You might try a #80A blue filter on Jupiter--it enhances the contrast of the dark bands with the planet.
The green filter works on the Moon, but you would find superior contrast with a #15 yellow.
I believe there is a RA motor available for the CG-4. If you don't have one, save up for it because it's great to have tracking when you have a nice EQ mount.


Actually just picked my CG4 back up today. I had it motorized. The motors cost about $120 and are for both RA and Dec. The only drawback I see is the battery pack. These motors, unlike the other Celestrons, are 6 Volt and not 12 Volt. It has a small box that holds 4 D-Cell batteries. I guess you could get a marine 6V battery and a battery charger. But, overall, I'm very happy with the CG4 and 102XLT. NOW for the learning curve of using a motorized mount......

To the OP, You're gonna love the views that 24mm gives you, it's my most used eyepiece (I have the ES Version of that same eyepiece).

#14 Jeff Morgan

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Posted 05 February 2013 - 11:44 PM

For me, magnifications below 30x are very important, particularly if the skies are dark and clear. This what a scope like this one is made for, give it's full rein and let it really do it's stuff.

Jon



I found the 35 Panoptic to be very formidable with my Jaegers f/5. It maximized my exit pupil and wasn't obscenely overweight. Under Arizona skies targets like the North American Nebula were as visible as if they had been marked out in white paint. Well, a small exaggeration ;)

At the time I did not own the 31 Nagler. I do use that eyepiece now on rare occasion in an AP Star12, but one needs to pay very careful attention to the torque on the star diagonal. I'm sure lots of folks don't think twice about it, but it makes me a bit uncomfortable. And of course, it is difficult to balance when switching to shorter eyepieces, although the Jaegers is strictly a low-power machine so that may not apply here.

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