Dull 9mm Plossl

[bigrbuk]

I bought a cheap second hand Celestron 9mm plossl for my heritage 130P to replace the out of the box 10mm that came with it, but I'm really struggling to get a sharp focus and if anything, the stars look duller than with the 10mm EP. I know the mag factor will affect the brightness, but here's the weird bit, when I barlow up the 9mm, the issues seem to disappear, brightness seems to increase as does clarity.

Is there any reason for this or are my eyes deceiving me?!

[faulksy]

doesnt make any sence, when you barlow it the image gets brighter. is there a lens missing from the e.p. were did you get it from ?

[bigrbuk]

doesnt make any sence, when you barlow it the image gets brighter. is there a lens missing from the e.p. were did you get it from ?

Second hand on astrobuysell

[hwsoderlund]

Maybe you are just not reaching focus without the barlow? If that is the case you probably need an extension tube.

[michael.h.f.wilkinson]

Maybe you are just not reaching focus without the barlow? If that is the case you probably need an extension tube.

That makes sense. You could try loosening the thumbscrew that holds the EP in place, and pulling it out a bit, if it reaches focus, you know what the problem is.

[bigrbuk]

I seem to get to what should be focus and its quite clear when I'm either side of it, but the EP does wobble in the EP thread a little - I need to put some PTFE tape in their to make this more snug, but that's a side issue - or at least I thought it was - thinking out loud, when I barlow up, the size and weight of the pieces combined may hold the EP more steady in the the holder, so perhaps it could just be the slackness of the piece in place causing issues?

[Capricorn]

I would suggest that someone has taken it apart and replaced the 2 lens incorrectly - facing the wong way, but that cannot easily explain whay it is better with the barlow unless somehow the new image position and the incorrect eyepiece just manage to work.

[michael.h.f.wilkinson]

I would suggest that someone has taken it apart and replaced the 2 lens incorrectly - facing the wong way, but that cannot easily explain whay it is better with the barlow unless somehow the new image position and the incorrect eyepiece just manage to work.

That is a good point. The slower focal ratio using the barlow would mean the aberrations of the "new design" plossl with e.g. planar sides facing each other rather than the convex sides, do not show up as much.

[bigrbuk]

Thanks chaps, I might have to investigate this.

(http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&nav=0&kbarticleid=2230)

[michael.h.f.wilkinson]

That looks like sensible info

[Naemeth]

I wouldn't want to have to put any eyepiece lenses back together, but imagine putting a Nagler back together .

[michael.h.f.wilkinson]

Plossls are easy. The older symmetric even easier. Make sure not to tighten the retaining rings too much, that results in pinched optics (not good for image quality).