Minimal glass for planetary - why not for deep sky too ?

[John]

I see quite a few posts here and on another forum that some folks prefer a minimal glass approach (eg: ortho's, monocentrics, ball eyepieces etc) for planetary eyepieces.

Accepting the limitations on having a properly corrected field of view field, why not take the same approach for eyepieces for observing deep sky objects where light is, if anything, even more at a premium ?.

In the past I've seen people advocating longer focal length orthoscopics for deep sky viewing and occasionally a fan of things like long focal length Kelners saying that, in their view, they are an indispensable tool in the eyepiece case. But you don't here so much of this these days.

I's be interested in other members views are on this ?.

[E621Keith]

I thought people choose minimal glass eyepiece to reduce internal reflections, which can spoil high contrast objects such as moon and planets. This was even more important in the days before efficient anti reflection coatings.

Internal reflection is less of a problem with DSO, because their contrast and brightness is much lower. Furthermore, modern optical glass and coatings made it possible to maintain 99% transmission per air glass interface, so the actual light loss in a modern multi element eyepiece can be kept quite low. Some minimal element eyepieces uses relatively thick elements (e.g. monocentric) which can over all reduce transmission through absorption by the glass.

I'm sure wider AFOV, correction for faster optics and eye relief also play a part.

Anyway these are my thought, I can be completely wrong.

[andrew s]

John - I think a lot is history from the days before anti-reflection coatings. In addition for planets you need high and a small field of view and for deep sky normally low mag and a wide field of view. To get a wide field of view you need complex eye pieces which means more glass surfaces and ghost images (mainly in the days before anti-reflection coatings) from bright objects.

Hence simple eye pieces for planets and complex for deep sky however, I doubt you would have too many problems with top of the range eye pieces made today.

Andrew

[dobbie]

its strange you should say that, i had a meade 4.7mm 5000 uwa, and i found it really hard to get use to, i keep going back to my ortho's for dso my best lens is a 12mm volcano top ortho its better for me then the tv's i have,, its a cracker , and i even use a 8mm ortho.. on dso .. it just might be my ageing eyes that like them better..

[John]

Thanks for the interesting responses so far. I'm in the lots of glass camp myself although Tele Vue do a great job with the coatings used and the glass types to pursuade you that all those lens elements aren't there

I'd be interested in comparing the view of a low brightness DSO such as M33 for example, using my 31mm Nagler and a well made Kellner of similar focal length, just for interests sake.

[Geys1987]

My eyepiece collection is a mix of ortho's and 3 eyepieces with a wider FOV.

When I'm observing DSO's I use the low power wide field first to get a good impression of the DSO and it's surroundings (and for identifying the field). Then I switch to ortho's to get every last detail out of it. This was the way I observed with my 12" dobson and how I observe today with the Megrez (when I get the chance ).

For some reason I seem to see more with the ortho's then with any other eyepiece. Perhaps that the smaller FOV helps me to direct my attention to the DSO.

[andrew63]

There does appear from some of the threads i've been following over the last few months that a lot of people are using ortho. as a more general purpose lens rather than solely for planetary use. I think i'll have to try an or. of 12 to 15mm.

andrew

[great_bear]

It's because you want enough magnification to darken the background adequately but you still need the true-field width.

[michael.h.f.wilkinson]

It's because you want enough magnification to darken the background adequately but you still need the true-field width.

Good point. I can capture the whole of a large structure at higher magnification with a Nagler than with an ortho.

There are many psycho-visual effects associated with viewing through different field stop diameters (or in general, with visual observations). It is well-known that a wide field may make an object look smaller than a smaller field, because instinctively we "measure" relative sizes. In the same way, we "measure" relative brightness more than absolute. It is quite possible that the narrower field gives the impression of a brighter image, due to the encroaching black surroundings.

An interesting experiment would be to insert an artificially small field stop in say a Nagler, so that it matches that of an ortho. We could then see if there really is a noticeable visual difference in transmission. Measured transmission differences often suggest very small differences indeed (2-3%, or 0.02-0.03 mags), which are probably very difficult to notice. For visual observation, it is the visual difference which is important. Thus, if some psycho-visual effect means orthos give a better result, go for them. Note that these effects are very much relative: once you are used to the wide field of a Nagler, a Radian's field looks so narrow, it may show the same brightening effect as the narrower field of an ortho. There will also be inter-individual differences. For some people certain effects will be stronger than for others. I do not doubt experience also factors into this very complex equation.

That brings me to the final components in the optical train: your eyes and brain. Experienced observers see more, not necessarily because their kit is better, but because they have trained themselves to see better in the dark. I only started buying new, more expensive EPs when I started finding shortcomings in my old.

BTW, my favourite DSO EP in my old 6" F/8 with its 0.965" EPs, was a Circle V (Vixen) Ortho 25mm. Really nice EP.

[John]

Somebody did a fairly comprehensive survey of the light transmission of many well know eyepieces across a number of selected wavelengths a couple of years back. I can recall there were some real surprises in the results but not the details, unfortunately. I'll see if I can find it again.

I think light transmission and the way that scattered light from brighter objects is controlled are vital contributors to eyepiece performance.

[michael.h.f.wilkinson]

Somebody did a fairly comprehensive survey of the light transmission of many well know eyepieces across a number of selected wavelengths a couple of years back. I can recall there were some real surprises in the results but not the details, unfortunately. I'll see if I can find it again.

I think light transmission and the way that scattered light from brighter objects is controlled are vital contributors to eyepiece performance.

Would be very interesting. Light scatter is very important of course. Coatings have been instrumental in getting us this far. In fact, blackening the edges of lenses was not that common when I started (1977), now it is commonplace. One reason may be that improvements in coating have reduced glare from internal reflections so much that the contribution from lens edges became more noticeable.

[great_bear]

I reckon spectral differences in transmission make total transmission differences appear quite different to what they really are... I suspect "cooler" tone is perceived as "brighter"...

[michael.h.f.wilkinson]

I reckon spectral differences in transmission make total transmission differences appear quite different to what they really are... I suspect "cooler" tone is perceived as "brighter"...

Quite likely.

[Doug]

1.Is this the sort of thing you are all on about Baader Genuine Orthoscopic ?

2.or is there a lot better out there ?

[John]

1.Is this the sort of thing you are all on about Baader Genuine Orthoscopic ?

2.or is there a lot better out there ?

1. Yes, sort of.

2. No.

[newman]

To be perfectly honest, I think the "less glass in the optical chain" thing is taken a bit too far these days. With modern coatings found on quality eyepieces the light throughput is high enough for differences to be hardly noticeable. I personally prefer a nice wide view through an Ethos, as it gets out of the way and it does feel like you're out there. An ortho has less glass, delivers very nice and sharp views, but feels like squinting through a keyhole. I know I'll probably get flamed by glass minimalists for saying this, but honestly I can't say I see any dramatic differences in terms of brightness and contrast between an ortho and a premium wide field of a similar focal length. The good thing about BGO's is that they deliver premium quality views for a comparatively low price. But they're quite confined inside that very narrow AFOV. In addition, shorter focal length orthos start having a short enough eye relief to bother people who don't wear glasses. To each his own, but I'd do a side by side comparison between a premium wide field and a corresponding ortho before deciding that less glass is always necessarily better.

[great_bear]

The minimal glass thing is indeed nonsense when used in the general sense.

Sure it makes a difference - and of questionable value - if you're dealing in optics at the limit of technical excellence - i.e. Zeiss Orthos vs. Tele Vue Ethos, but on lower-level eyepieces, the choice of glass type, its purity, the quality of polishing, and the sophistication of the coating technology all make a much greater difference to the quality of view than mere "millimetres of glass used".

It's like arguing over the length of loundspeaker cables without taking into account the quality of cable used.

[newman]

The minimal glass thing is indeed nonsense when used in the general sense.

Sure it makes a difference - and of questionable value - if you're dealing in optics at the limit of technical excellence - i.e. Zeiss Orthos vs. Tele Vue Ethos, but on lower-level eyepieces, the choice of glass type, its purity, the quality of polishing, and the sophistication of the coating technology all make a much greater difference to the quality of view than mere "millimetres of glass used".

It's like arguing over the length of loundspeaker cables without taking into account the quality of cable used.

Thank you, that was exactly my point. Well said.

[michael.h.f.wilkinson]

...

It's like arguing over the length of loundspeaker cables without taking into account the quality of cable used.

In cases of LP it's like arguing over the best sound system whilst living in a poorly insulated house 10 yards from the M25 during rush hour

[johninderby]

I did read a head to head comparison between Zeiss Orthos and Ethos eyepieces for planetary observing a little while ago. Think it was on Cloudynights but not entirely sure. It was declared a virtual tie, but they gave the slightest edge to the Ethos for brightness and contrast.

In a related subject we could always start a new argument. Is the old classic definition of a true APO now out-dated due to advances in glass and coatings.

John