Plossl vs. Ortho

Apologies for the very basic question, but could someone(s) help clarify the relative merits of these two types? I will be viewing both night sky and daytime birds (at a distance of 20-1000 M) using 80-90 mm refractors of ~500-900 mm FL. Any additional recommendations would be appreciated.
Thanks,
Scott

Generally speaking, both have similar performance and will work well with your telescope, being the focal ratios are somewhere between f/6 and f/10. (ratio of FL to aperature). They both have 4 lens elements. The plossl will have a wider field of view, 50 degrees, versus 42 degrees or so for the ortho.

The ortho will have a slightly larger eye relief (i.e. more comfort) for a given focal length. Both orthos and plossls have small eye relief for the smaller focal lengths. For me, I don't like plossls at 11mm and below, orthos 9mm and below. Though I can stand them for a little while.

For astronomy I can state from personal experience that the Televue plossl and University Optics ortho brands are good. There are other brands of plossls around too that have good reputations. Televue is having a 20% sale right now.

There are some very premium brands of orthos around on the used market, some going to $800 each! Haven't tried those.



Mike

No apologies neccessary, we all started at the beginning. Be aware that true Plossls are not common; most such named are simply symetrical types (both lenses are identical). And some so-called orthos are in reality symetricals, too, if examined via disassembly. But well-made examples of either should provide virtually same detail in the view. And many of either type are made in the same overseas plant but with various *Brands* applied to order. True ortho examples are the no-longer produced vintage Telescopics & UO Pro series (same eyepiece with different top caps) or the old Meade professional series. The present UO orthos are same as the common orthos offered by most suppliers in days past & poccess same-curve elements (very good, but not as good as the dis-similar curves true orthos 1st mentioned above). Excellent True Plossls are the former Claves & the still-made Vernonscope Brandons. Per your questions intent, purchase well known Brands of either labeled design if buying new & realize fine results. In that context, labeled design is of little matter to viewing results (merely ad hype instead) but quality mfg is (avoid budget examples). A longer focal length eyepiece will offer greater ease of viewing & when combined with a Barlow amplification lens retains that ease at higher magnifications, surpassing the result of equivalent shorter focal length eyepieces alone. Example: a 25mm or 20mm focal length eyepiece will provide 20X~36X or 25X~45X with your 500~900mm objectives & a 2X or 3X Barlow doubles or triples those magnifications. Equivalent high powers from a single 6mm eyepiece requires the eye to be placed uncomfortably touching the eyepiece.

Scott Mellis said:

Apologies for the very basic question, but could someone(s) help clarify the relative merits of these two types? I will be viewing both night sky and daytime birds (at a distance of 20-1000 M) using 80-90 mm refractors of ~500-900 mm FL. Any additional recommendations would be appreciated.
Thanks,
Scott

Consider some eyepieces with a wider field of view.

For example the TMB Planetary series of eyepieces provides good performance at the shorter focal lengths (9mm and under) while providing a comfortable level of eye relief. In the longer focal lengths there are some nice choices too but they can get expensive.

Have you chosen your telescope yet? The possible telescopes cover a wide range from a 80mm F/11 to a 90mm F/5.6, these are quite different telescopes, particularly if they are achromats. I have several 80mm telescopes ranging in focal lengths from 500mm to 900mm, mostly achromats but there is one 80mm F/7 apochromat.

For birding, the shorter focal length is a help because it makes for a more compact package that is more easily transported and it provides a wider field of view which makes putting the bird in the eyepiece much easier. I don't consider a 900mm focal length appropriate for watching birds though obviously one can do it.

Jon

Thank you, Gentlemen, for this very helpful information.

Scott