An Actual Eyepiece ReviewPosted By Roland Christen |
Just to add to the confusion, one of the guys here got one of those eyepieces, a 9mm. I did check it out to see how sharp it was and found it to be fairly sharp at F6 to about 60% of the field. After that you could see a small amount of astigmatism creep in, which got larger toward the edge.
However, I expected that, but what I did notice about the eyepiece was that none of the cemented surfaces had proper match coatings, so the reflections from each cement section was quite large. I feel that the optical design was quite well done, but the execution was not. Way too many internal surfaces with inadequate multi-coatings and some with no match coatings at all. If you want a really superb planetary eyepiece, I would go with a narrow field ortho or Plossl that has the minimum number of reflective surfaces and has very low scatter coatings. Less is better in the case of planetary eyepieces.
Rolando
P.S. what is a match coating? It used to be that you could cement a crown and flint element using one of the modern UV curing cements and end up with almost no reflection at the cemented surface. That is no longer the case when very high index glass is used. Cements just do not have the high index value to properly match these new designs. High index glass has to be used in order to get wide field performance (anything over about 60 degree apparent field needs high index glass, otherwise you get very unacceptable astigmatism at the edges of the field).
In order to suppress reflections at the cemented surface, now these glass surfaces must have a match coating applied before they can be cemented. That, of course, increases cost (who knew?). Also, the remaining air-glass surfaces must have individual matching multi-layer coatings, which are different for each glass. Of course doing this increases cost, so the cost shavers will put all the elements into the same coating chamber and coat them all with the same coating design. Presto, you have coated glass with lots of variation of reflectivity, some green, some blue, some orange, all higher reflectivity that they should have. You get what you pay for.
However, I expected that, but what I did notice about the eyepiece was that none of the cemented surfaces had proper match coatings, so the reflections from each cement section was quite large. I feel that the optical design was quite well done, but the execution was not. Way too many internal surfaces with inadequate multi-coatings and some with no match coatings at all. If you want a really superb planetary eyepiece, I would go with a narrow field ortho or Plossl that has the minimum number of reflective surfaces and has very low scatter coatings. Less is better in the case of planetary eyepieces.
Rolando
P.S. what is a match coating? It used to be that you could cement a crown and flint element using one of the modern UV curing cements and end up with almost no reflection at the cemented surface. That is no longer the case when very high index glass is used. Cements just do not have the high index value to properly match these new designs. High index glass has to be used in order to get wide field performance (anything over about 60 degree apparent field needs high index glass, otherwise you get very unacceptable astigmatism at the edges of the field).
In order to suppress reflections at the cemented surface, now these glass surfaces must have a match coating applied before they can be cemented. That, of course, increases cost (who knew?). Also, the remaining air-glass surfaces must have individual matching multi-layer coatings, which are different for each glass. Of course doing this increases cost, so the cost shavers will put all the elements into the same coating chamber and coat them all with the same coating design. Presto, you have coated glass with lots of variation of reflectivity, some green, some blue, some orange, all higher reflectivity that they should have. You get what you pay for.
