is it me?

[philhas]

Just had a chance to properly try out my 25mm BST Starguider and I have to say it just doesn't work for me. Unless an object is dead centre it descends into out of focus mush. The 12mm seems better although I am seeing some odd reflections within the eyepiece. I never had this with the 32mm Revelation (wish I hadn't of sold it now) or 25mm Skywatcher. Anyone else come across this? Maybe I need to go Televue and stop mucking about. 

[andrew63]

No it's not you ! I've owned a few of them - and the 25mm is the weakest performer -  a lot of field curvature so edge performance is not great. The 12mm is much better but i noticed some reflection on jupiter. A  Humble plossl does a good job though.

andrew

[Moonshane]

you can get used TV plossls for about the same price as a new BST...............enough said as far as I am concerned. the BSTs are not bad value but unless you need long eye relief, there's no contest between these brands.

[LeeB]

I like the 18mm bst but the 25 is not as good, I like the old 5000 meade (or an ortho) at 25mm, , in fact my eyepiece set is not a set at all, I've odd ones and twos and choose each focal length on merit, eyepieces are very a personal thing and scope specific.

[alan potts]

I agree with Moonshane.

Alan

[AstroAdam]

I like my 8mm, but have always heard similar things about the 25mm. My ep collection is most definitely a mishmash. No two the same!

[philhas]

Thanks for the advice guys, thought I was going nuts. Think I will keep the 12mm and get some TV plossls. 

[Stu]

Which scope are you using it with Phil? I've found field curvature a problem with short focal length doublets and Widefield TV eyepieces too, namely a TV76 and 21mm Ethos! I've taken to using a field flattener when using this combination.

I guess my point, or question is whether the curvature issue is with the scope or eyepiece? The 25mm would naturally show this up more than the 12mm because of the wider field.

I'm sure a 32mm TV Plossl would be better, but this is in part to do with the narrower afov anyway.

I'm not pretending to know the answers, just throwing it out there as a discussion.

Cheers,

Stu

[philhas]

Hi Stu,

Its just a 90mm Opticstar Acromat F5.5 nothing fancy. 

Best Regards

Phil

[Stu]

I suspect it's the type of scope as much as the ep then. This link has some interesting info on it. Short focal length refractors have the most curved fields.

http://starizona.com/acb/basics/equip_optics101_curvature.aspx

Not sure if TV Plossls specifically correct for this, probably they do but others can confirm I expect.

One thing, if you pick up a second hand TV Plossl you would be able to sell it on with little or no loss if you choose not to keep it, but I don't expect you would.

Stu

[philhas]

Thanks for the link Stu some interesting stuff, I am still convinced its the eyepieces as the 25mm Skywatcher seems much better (and I know the field of view is less however the actual view was much better) than the BST 25mm and the 32mm Revelation was excellence before I sold it :-( 

Like you say I can always sell on a TV Plossl, we both know that probably won't happen!

Cheers

[Stu]

Hi Phil,

Yes, I agree that it will be the eyepieces ie how well they are handling the field curvature of the scope.

Hope you find a good used TV Plossl :-)

Stu

[Pig]

Stu, I find this condition with my 31T and the Pronto albeit slight but it is there. However it does not happen with the 17.3mm Delos

[Stu]

Stu, I find this condition with my 31T and the Pronto albeit slight but it is there. However it does not happen with the 17.3mm Delos

Yes, I think it is at its worst with wide afov ep's so I am sure the 21e is worse than the 31 Nag although I've not done a comparison between then.

The Delos is shorter focal length and narrower afov so as you say won't show the same problem.

With the 21e it was at a level of being not worth using, so I picked up a secondhand TS field flattener (supposed to be imaging only) and that works very well once you have the spacing roughly right, doesn't need to be exact.

Stu

[Pig]

Mind you it does give a view of 5.5 degrees and it is only slight, maybe its because the Ethos is 100 deg as apposed to the 82 of the Nagler.

[Stu]

Mind you it does give a view of 5.5 degrees and it is only slight, maybe its because the Ethos is 100 deg as apposed to the 82 of the Nagler.

Agreed, I think it us the wider apparent field of view which makes it worse

Stu

[jetstream]

If we define field curvature as the inability to maintain focus across the FOV simultaneously,then most all my eyepieces exhibit this,including the21mm Ethos 17mmEthos,8mm Delos,6mmDelos.The 21mm Ethos does exhibit more F.C. than the others,but at f 4.8 this can be expected, as total field curvature is the sum of EP curv. and telescope curv.(+ or -).Eyepiece distortion control is a give and take proposition,with the individuals perception of this distortion a major factor.A friend gave me this.It is interesting. http://www.holgermerlitz.de/globe/distortion.html