The Fickle Used Ortho Market

[Qualia]

I look forward to the review, John for as always, your knowledge and insight is invaluable. I have the usual suspects (5, 6, 7, 9mm) but just can't seem to find anything at 8mm. I was wondering, do you know of any?

[John]

....I have the usual suspects (5, 6, 7, 9mm) but just can't seem to find anything at 8mm. I was wondering, do you know of any?

I've seen an 8mm in the classic "volcano top" design but not for quite a while to be honest. 8mm does not seem to be a standard focal length in todays ortho ranges. The 8mm TV plossl would do very nicely there of course although the focal point would be a bit further out than for most orthos.

[Naemeth]

I believe there is a 7.7 UO Ortho, but these are not the BGO equivalents, they are one step below that.

[John]

Good point Jonathan. I guess you mean these:

http://www.365astronomy.com/77mm-abbe-orthoscopic-eyepiece-p-3111.html

From the reports I've read these are OK but not of the same quality as the Baader GO's, Astro Hutech's, Fujiyama's etc.

[Naemeth]

Good point Jonathan. I guess you mean these:

http://www.365astronomy.com/77mm-abbe-orthoscopic-eyepiece-p-3111.html

From the reports I've read these are OK but not of the same quality as the Baader GO's, Astro Hutech's, Fujiyama's etc.

I do, I found them here:

http://www.universityoptics.com/125inch.html

They are the top ones.

When we were in the ortho-slump (when the BGOs and Circle-Ts had both been discontinued), I looked at these as an alternative, and found many reviews that placed them behind the BGOs in terms of optical quality.

[Qualia]

Thanks, John and Naemeth. It may seem excessive but there's something about viewing Jupiter or Saturn that requires quite a run of eyepieces. Sometimes - last night for example - I just wish I had an 8mm and this has happened on more than one or two sessions. Although clear, for some reason or other seeing is not perfect for 7mm (180x) or higher but the 9mm (140x) just drops a little too much. The TV Plossls are considered cracking eyepieces but there has beeen concern raised that the 8mm is troublesome due to eye-relief. Hypothetically speaking, then, if one didn't have any problem with the eye-relief in Orthos would there still be one with the plossl?

[Naemeth]

Rob, it's not excessive - or doesn't seem excessive. Sometimes I've wanted a 9mm when using my 130P, as the seeing hasn't quite been good enough for the 7mm and 8mm.

The eye relief on the 8mm isn't that tight, but I must admit I prefer the 10.5mm TV smoothside I've got. The eye position is quite close with the 8mm, but it is usable, and probably just as easy to use as most flat-topped orthos .

[BillP]

 Hypothetically speaking, then, if one didn't have any problem with the eye-relief in Orthos would there still be one with the plossl?

From a design standpoint, the eye relief of an Abbe Ortho is generally .8 x its focal length.  So if the Abbe is a 10mm eyepiece, its theoretical eye relief would be .8 x 10mm or 8mm.  The Plossl design has shorter eye relief, generally .68 x its focal length.  If the Plossl has concave lens surfaces, like the improved TV design, then the eye relief is generally a little shorter.  Of course these "design" eye reliefs are altered when the design is altered any, so only good to use the formula as a general guideline.  Also, other things affect how the eye relief feels, like how deeply the eye lens is inset into the eyepiece, using valuable eye relief, how high above the top housing the eye guard sits, whether the housing design is flat or volcano, etc.

[Moonshane]

Thanks, John and Naemeth. It may seem excessive but there's something about viewing Jupiter or Saturn that requires quite a run of eyepieces. Sometimes - last night for example - I just wish I had an 8mm and this has happened on more than one or two sessions. Although clear, for some reason or other seeing is not perfect for 7mm (180x) or higher but the 9mm (140x) just drops a little too much. The TV Plossls are considered cracking eyepieces but there has beeen concern raised that the 8mm is troublesome due to eye-relief. Hypothetically speaking, then, if one didn't have any problem with the eye-relief in Orthos would there still be one with the plossl?

yep, I agree! http://stargazerslounge.com/topic/188794-eyepiece-groups/

[Qualia]

Thanks for the concise explanation, Bill. & lovely collection, Shane and one that makes complete sense