Which low power eyepiece?

[Jaeger]

Hi guys,

I'm looking at getting a 2" low power (40mm+) eyepiece for my new 8" f/6 Dobsonian.

The candidates are:
* Guan Sheng 40mm Plossl 2"
* Guan Sheng/Bintel 40mm SP 2"
* Guan Sheng/Bintel 42mm Superview SV 2"
(The 50mm Superview falls outside the lower magnification limit of the scope.)


I've read Mark Hodson's review of the SV (thanks Mark!) but I'm not sure of the pros and cons of the other two designs - any thoughts on this?

Cheers!

[koputai (Jason)]

Hi Jaeger,
I looked for ages when getting a wide eyepiece for the same scope, 8" f6.
I bought and tried a few, and took some back. I kept two, a Japanese made Konig 40mm, and a Televue Panoptic 35mm. The Panoptic is a superb eyepiece but expensive, the Konig was much cheaper and nearly as good, but they rarely come up for sale. Basically I found none of the cheaper (sub $150) eyepieces in this range were worth having.

There's been a couple of threads about the Superview, have a look at these:
http://www.iceinspace.com.au/forum/s...ight=superview
http://www.iceinspace.com.au/forum/s...ight=superview

Cheers,
Jason.

[StarLane]

I know you mentioned 40mm +, but I have a couple of recommendations under this focal length for a number of reasons.
The Pentax 30mm XW and the TeleVue 24mm are great wide EP's from my experience (F5 scope). Do you want 60, 68, 70 or 80 degrees AFOV?, this will usually decide how much you need to spend.
From an exit pupil perspective, 40mm EP focal length will be the max limit for an F6 scope. A young adults eye usually only dialates to 7mm, and less the older you get, a 40mm EP in an F6 scope will give you 6.67mm which is still under but the conditions must be dark and perfect for the iris to dialate to this.
I would recommend the above mentioned or the Burgess/TMB 40mm Paragon (field curvature ?????) but much better than GS, or if you want to spend more money, the TV 35mm Panoptic or 31mm Nagler $$$$$$, the OZ dollar at the moment does not help.
GS eps are cheap but just useable I guess for a while, but like everything, you get what you pay for. The more you pay, the better corrected the EP will be for a semi fast scope. FC is a prob with cheap EP's, don't expect the best with the cheapies.

[Wavytone]

In an f/6 Newt the longest useful f.l. eyepiece will be 25-30mm.

With a 40mm... the exit pupil will be too big for your eye, cutting off light equivalent to losing the outer inch (25mm) of your mirror, making it work like a 6" f/8 with an oversize secondary.

it is for this reason I shelled out for a wide field 30mm, and not a 42mm.

[koputai (Jason)]

I beg to differ Wavy.
I use a Panoptic 35mm in my 8" f6, which gives an exit pupil a shade under 6mm, which is perfectly acceptable. 40mm in the same scope gives an exit pupil of 6.8mm, which is still quite useable for a lot of people. Before getting the 35, I used a 26mm (68deg also) and the 35 was a VERY worthwhile improvement.

Cheers,
Jason.

[Jaeger]

Thanks for the feedback.

At this stage, eyepieces like TeleVues are luxury items I can't really afford/justify. Maybe in the future if I get serious enough to splurge on a SCT.

Trawling the web, it appears that the GSO Superview 5-element eyepiece is an Erfle design; there were several complaints about problems focusing with some telescopes (requires more out focus than the 3-element Kellner), and the very long eye relief can cause "blackouts".

I'm pretty satisfied with the 26mm Kellner that came with the scope, so I'll give the 40mm version a go even if I end up replacing it later.

[Wavytone]

Originally Posted by koputai

I beg to differ Wavy

I'd guess you're a lot younger than I am - my exit pupil is barely 6mm at best.

I have also done some tests to find out the effect on your pupil is arising from the image you see in your scope - while you may be able to achieve much more than 6 mm fully dark adapted, within seconds of putting your eye to the scope, if there are some nice bright "diamonds" in the field of view - open clusters in particularly - your pupil will shrink in seconds. A safer bet is a 5 mm pupil IMHO in most conditions.

Secondly, the actual field of view is determined by the diameter of the field stop of the eyepiece. Between 2 eyepieces with identical field stops - say a 30mm with a 65 degree apparent FoV vs. a 42mm with say a 55 degree apparent FoV, the higher magnification of the 30mm gives a darker background and should be used in preference to the 42mm because you will see more faint detail in the 30mm, with the same actual FoV across the sky.

[Wavytone]

If the Televues are out (cost) how about the 30mm Vixen NLVW - about half the price and a 65 degree FoV - the field stop is 34.1 mm, and its very sharp edge to edge.

Lighter than it looks too, despite its size.

And BTW I love the one I have.

[Jaeger]

If the Televues are out (cost) how about the 30mm Vixen NLVW - about half the price and a 65 degree FoV - the field stop is 34.1 mm, and its very sharp edge to edge.

30mm is pretty close to the 26mm SP eyepiece I already have, but I'll keep it in mind for future upgrades.

I've ordered a Bintel 40mm SP eyepiece along with a 2x ED barlow and 12mm Plossl, which will give me the following combinations:
Normal: 40mm, 26mm, 15mm, 12mm, 9mm
1.5x Barlow: 30mm, 17.3mm (2" EPs + barlow element)
2x Barlow: 20mm, 13mm, 7.5mm, 6mm, 4.5mm

That should more than satisfy my immediate needs to learn what works and what doesn't with the scope (and objects.)

[AstroRudi]

Hi Jason Koputai (and Jaeger):
if the 40 mm you mention is this one: http://www.kasai-trading.jp/konig.htm , I entirely share your view !
Rudi

[JethroB76 (Jeff)]

Originally Posted by Wavytone

I have also done some tests to find out the effect on your pupil is arising from the image you see in your scope - while you may be able to achieve much more than 6 mm fully dark adapted, within seconds of putting your eye to the scope, if there are some nice bright "diamonds" in the field of view - open clusters in particularly - your pupil will shrink in seconds. A safer bet is a 5 mm pupil IMHO in most conditions.

Wavy, how did you test the effect of bright objects in the EP on your pupil diameter?

[AstroRudi]

Don't derive a judgement from the 40 mm to the 32 mm !
I don't know it ... and I have seen some less good opinions !
Rudi