I have been mulling over, OK lamenting, how few manufacturers make classic planetaries, and when they do that they miss the mark in many ways. So I was pondering what would be an easy to manufacture concept for a planetary eyepiece line that still retained some of the more necessary attributes. So not talking something at the level of a ZAO or XO in its execution, more relaxed so less costly, but something that is still "special" in enough ways that I would probably feel it is a "keeper" for planetary.
So what are the attributes I felt were important?
- 1mm focal length increments from 4mm to 10mm
- Focal lengths>10mm still spaced close enough for long focal length SCTs without big mag jumps
- Baffled really well internally to maximize contrast (so also includes a bottom barrel baffle like Vixen HRs)
- Multicoatings that are tune to the glass indexes again to help the contrast
- Rather than a super polish as this is way expensive, maybe a higher quality than typical multicoating application would be worth the extra cost so that contamination between layers is near zero which will again aid in contrast
- We need to keep the number of eyepiece focal lengths down so production costs are frugal, but we also want those 1mm increments, so we need to integrate a Barlow with the set to achieve this -- using a 2.5x Barlow with just 6 eyepiece focal lengths you can achieve both the 1mm increments between 4mm and 10mm, and the close spacing of focal lengths >10mm for our SCT friends
- Of course want the lens count as low as possible, but do not want terrible eye relief (which the Barlow can solve for us), and also need to be cognizant that from the manufacturing standpoint we want cost effectiveness, so the result IMO is the humble Plossl where the design is tweaked a bit (you know me, this is a big compromise )
- Optical design should be such that on-axis at f/5 the spot is "well" under 1 arcmin, which is the resolution of a good human eye; design should also have an orthoscopic FOV which is important when used for lunar observing as we don't want off-axis RD messing with the shapes of crater walls and rilles and valleys
- AFOV I am compromising to say 45 degrees mimimum; While I would like say a 52 degree AFOV, to get an easy to manufacture Plossl that is also orthoscopic have that large of an AFOV is probably impossible and do not want to compromise #8; Could move the design to a 3 element Konig or 4 element Erfle as both these have a tighter on-axis spot than a Plossl (Telescope Optics by Rutten & van Venrooij), but then we complicate the manufacture costs and simplicity
- For best eye relief the housing would be a classic Volcano that has a strong angle and the eye lens should be no more than 1mm recessed into the housing top; and again the integration of a 2.5x Barlow will help keep the ER more comfortable since for the shortest focal lengths a longer focal length will be Barlowed
- Integrated Barlow -- since the Barlow is integral to the set, it should be optimized to the eyepieces, so if it needs to flatten the field a little or help correct the off-axis for faster focal ratio scopes then incorporate that into the design, so take advantage that this would be a Barlow for just these eyepieces and not some general-use Barlow; I would also ask that the cost differential be examined for producing the Barlow to possibly a higher than typical level to make it as transparent as possible with any added scatter below the visual threshold
- Not a necessity but I like my eyepieces to not float away on me so the barrel should be chromed brass so it has some heft
So that's it. Should be easy to manufacture the optics since the only design needed is for 2 different lenses that are cemented into a doublet. Addressing #8 might be a challenge but lets see what can be done with the limited degrees of freedom of a Plossl - again, not going for a ZAO set here but a practical one that is good enough and usable across a wide range of apertures, focal ratios, and focal lengths. Realistically, I think I could compromise down to if these focal lengths were done for Plossls with the build quality of a TV Plossl if they just added a bottom baffle, changed the design so they did not need a Powermate to produce a non-vignetted view, and of course put it in a volcano top housing loosing the eye guards. So could be quite happy with that even though it compromises many places. But if that's all one can get then could settle and drop points 5 and 12, and point #8 could probably relax but it sure would make them special to have that. So what focal lengths then?
4.0 mm 10.0 mm
5.0 mm 12.5 mm
6.0 mm 15.0 mm
7.0 mm 17.5 mm
8.0 mm 20.0 mm
9.0 mm 22.5 mm
Basically this broad focal length capability can be achieved with just 6 eyepieces so not a lot to manufacture. The design is also cost effective just being 1 doublet to make for each focal length. This set will produce non-excessive magnification jumps from 150x to 50x/inch aperture for scopes from 4" to 14" with focal lengths from about 800mm to 3600mm, so it can be a single set that will work with most scopes many people might have.
Let me know what you think. Where does it not hit the mark for you for a "pragmatic" or "practical" planetary set? What aspects of a planetary specialty eyepiece can you compromise on and not compromise on? FWIW I pondered the ER issue a lot as we have all those eyeglass wearers out there. I ended up giving up on trying to incorporate that as would have to move to a positive-negative design then build 12 individual focal length eyepieces and also probably increase the overall internal design to 7-8 elements. All those things mean production costs, marketability risks, higher unit price, etc.
PS - In the discussion please DO NOT factor in pricing conjectures. This for me is about what are things folks can compromise on an not compromise on for a planetary set where the goal is to keep the manufacture costs down but also retain enough features that it would be attractive as a planetary option. Price conjectures will just side-track the discussion into no place productive. Let's stick to where our expertise is -- in determinimg what we need in terms of features from a planetary set of eyepieces that we would consider not top-shelf like ZAOs but still practically very good and very likely to want if they existed.
Edited by BillP, 30 May 2020 - 06:14 AM.