Simple eyepiece design benefits for DSO's ?

[John]

I've read a number of times, over the years that I've frequented astro forums, that some observers keep a simple, low power eyepiece design such as a Kellner in their eyepiece box for use on particularly faint DSO's - the idea being that the minimal lens elements and glass-to-air surfaces will have a slightly higher overall light transmission than more complex designs.

While the market in high-end multi-element wide angle eyepieces is flourishing I notice that some suppliers (eg: Telescope Service in Germany) are still marketing simpler low power eyepiece designs in the 2" format albeit with more modest fields of view:

2" Eyepiece TS RK 32 mm, 55° for telescope on eBay (end time 15-Nov-09 09:13:09 GMT)

While the edge definition in shorter focal ratio scopes will be compromised it would be interesting to know whether the coatings and optical accuracy of the latest low power, wide angle designs have now eroded the transmission advantages of the simple designs ?.

Another thought is whether there is a niche for a simple design made to really high quality standards - a sort of low power equivalent to the TMB Monocentrics ?.

John

[Takahashi]

I'm not sure if the same applies or not, but a recent purchase of mine on these forums; a 2" 32mm Erfle EP from JohnC, is proving to be just what you describe. On my current 6" f/5 Achromat; it's like a window to outer-space, and I suspect it's due to the enormous yet few elements it contains. Yes, it's very basic in design, yes, it suffers to an extent from spherical(?) aberration at the outer extremes of the FOV, but in terms of pure light-transmission and involvement of the viewer to the extent of feeling like you're gazing out of a panoramic spacecraft window, it's unbeatable. That said, I have yet to experience the more upmarket UWAs out there, but if they're anything like this, and with even better clarity from edge to centre, then I'll be saving pennies very soon.

[brianb]

I much prefer "simple" eyepieces (Plossl & Orthoscopic) to complex modern designs because the higher light transmission is noticeable to me (a visual variable star observer with an interest in faint objects). I'm quite happy to have a 45 to 50 degree field rather than a bigger one if it will get me a couple of a tenths of magnitude in light grasp, which is what my experience is.

BTW the Erfle is actually quite complex, it has 6 elements in three groups, and the light grasp will be compromised compared with a decent "simple" type. One thing the Erfle is good at is keeping a dark field background as it is refreshingly free from "ghosts" - some eyepiece designs are much worse than others in this respect; I've never known a Kellner that wasn't seriously "haunted"; however the Baader Genuine Orthoscopic (an excellent design with well polished & beautifully coated surfaces) is even better than the Erfle at "ghostbusting".

[acey]

I use Plossls for nearly all my deep-sky viewing - I'm not interested in how stars look at the edge of field, I'm interested in trying to see the little fuzzy galaxy in the centre, so light transmission and contrast are the priorities for me, not field width or flatness.

Each to his own, of course. For others the priorities may be different.

I use a wider-angle TMB planetary for high power views (x300) because it keeps objects in the dob field of view for that little bit longer. I find very little to choose between my 6mm TMB and 8mm TeleVue Plossl, but I'd say the Plossl has slightly better contrast. Hard to compare light transmission because of the differing magnifications.

I'd be tempted by a very wide angle eyepiece for use at even higher power (say x350 or x400) for the same reason of keeping objects in view longer. But high-power DSO viewing is something of a specialist activity that I've only really got into recently, now that my targets are often very distant galaxies of small angular size and relatively high surface brightness (in Abell clusters, etc), or small details in nearby DSOs (e.g. star clouds in M31). During most of my early Messier-hunting phase I found low power preferable, with the 32mm TeleVue Plossl my favourite eyepiece.

[Takahashi]

BTW the Erfle is actually quite complex, it has 6 elements in three groups, and the light grasp will be compromised compared with a decent "simple" type. One thing the Erfle is good at is keeping a dark field background as it is refresh

ingly free from "ghosts" - some eyepiece designs are much worse than others in this respect; I've never known a Kellner that wasn't seriously "haunted"; however the Baader Genuine Orthoscopic (an excellent design with well polished & beautifully coated surfaces) is even better than the Erfle at "ghostbusting".

Thanks for the info Brian. That's what I get for assuming simplicity due to the clarity and brightness of the image.

[TheMightyKong]

My experience: (nb not directly related) in an f/10 sct, I prefered the view of M13 thru a 14mm supermonocentric (3 lens elements 1 group) to a 14mm pentax XW (multi elements + groups). Haven't directly compared simple vs complex at same focal length on anything else.