Over about the past ten years I have utilised a variety of eyepiece pairs with my William Optics e-bino-p binoviewer. I finally reduced this to eight pairs.
I use these predominantly with three particular scopes: 127 mm SW Maksutov-Cassegrain, 102 mm Altair Starwave (ED doublet) and my 80ED Evostar. Although it’s worth pointing out that the BV was originally purchased for use with my 235 mm SCT. The refractors require the 2x Barlow BV nosepiece to achieve focus, effectively halving the focal length of the eyepieces deployed.
Overall these give me an exit pupil range of between around 2.5 mm to 0.5 mm. Useful enough for mostly lunar/planetary observing with a possibility of some DSO’s. In my experience any telescopes used in conjunction with prismatic binoviewers require focal ratios of between f/12 to f/15. Whether this is achieved with Barlows/glass path correctors or not is irrelevant. Any instrument with a focal ratio much faster than about f/10 to f/12 may have difficulty focusing with certain focal length eyepieces. Also, as many regular stereo viewers discover, lower magnifications than usual are often preferable. Apparently the objective size in millimetres is the ideal magnification limit. No greater detail will be observed at higher magnifications than this figure (1 mm exit pupil).
For example; my 80mm Evostar will not produce any better resolution at magnifications above 80x. It may be easier to discern detail at higher magnifications, but the resolution of 1.45 arc seconds (Dawes’ Limit) will not be better; as I understand it.
BV Eyepiece Pairs:
1/ 32 mm BCO
2/ 25 mm Ohi (Astro Hutech) orthoscopic
3/ 20 mm WO SWAN
4/ 15 mm TV Plossl
5/ 12 mm GSO Plossl
6/ 10 mm SvBony aspheric
7/ 8 mm TV Plossl
8/ 6 mm SvBony WA
I had some concerns about the 12mm GSO Plossls after recently witnessing some odd bright and distracting diffraction rings with them when observing Saturn in the vicinity of a bright Moon. There’s nothing fundamentally wrong with the GSO’s but I’ve seen this effect before with them and similar with some cheap Japanese Plossls (the rubbish Starbase ‘orthoscopic’ Plossls manufactured by the equally rubbish *Kubota Corporation). Not all Plossls are equal and many actually have slightly differently designed elements. Even the appellation ‘Plossl’ is arbitrary. My 15mm Tele Vue Plossls didn’t suffer the same aberration as the others. I also possess a pair of SvBony 10mm 60° aspherics. These are pretty decent, but having plastic housings, and also deploying a polymer element, I wondered about their longevity and eventual replacement.
The aspherics can also often display some chromatic aberration. As I already have a 10mm BCO, essentially all I needed was a second BCO for a 10 mm replacement eyepiece pair. In the end I decided to split the difference and acquire a second 11 mm TV Plossl, effectively replacing both the aspherics and the GSO Plossls. The exit pupils and magnifications are all incredibly similar and I can simplify my collection down to seven bino pairs.
Unfortunately I do not get on with TV eyeguards. When folded down the eyecap will not fit back over them. I don’t generally use the 11 mm’s with the eyeguards folded up in their default position. As I have a spacko right hand that is not easy for me to manipulate in the dark (particularly when I’m stressed out and swear a lot lol). The answer is to pull the guards off, as I don’t need them. The problem then is that the eyecap will be too big to fit back on the eye lens end. In a fit of creative genius (lol) I realised the TV end cap will fit perfectly as an eye lens cap. The TV field lens end caps are difficult for me to remove ‘in field’ as they tend to stick in the cold or damp.
So, I replaced them with slightly looser generic end caps. Problem solved. Compared to the 12 mm GSO’s the 9.1 mm wide 11 mm TV field stop seems slightly bigger. Both Plossl makes have an 8 mm eye relief. I don’t miss the eyeguards at all. Daylight trials at 109x (80ED Evostar) with the 11 mm TV’s are extremely promising, with a wide field stop, excellent colour separation, acuity, and contrast. The EP’s are easily merged and compare well with the 12 mm GSO’s. The barrels fit smoothly into the dioptres with no problems. It was worth paying four times as much as the GSO’s (probably). OK, I’ll readily admit I’m a bit of a TV fanboi. I’m looking forward to using these babes in the night.
*Although, to be fair to the Kubota Corporation, they mainly manufacture tractors, combine harvesters, rice transplanters and vending machines. I don’t know whether they are also rubbish products or not (I personally wouldn’t trust them).
