eyepiece query

KG8 · #1 02-05-2007, 05:27 AM

I have a little celestron mak 90mm, fl=1200 f/13.3 that I have been using here in the inner city to look at the planets and some brighter binary stars. Anyway a celestron 43mm plossel I use gives crisp pinpoint stars but when I drop my newly purchased widescan III 13mm into it the stars get a little 'airy' type diffraction ring around them and lose all the crispness. The brighter the star the worse the effect. I focus and focus but it's there to stay. The planets look ok, I can get plenty of detail out of them, even Jupiter's moons look ok. But bright stars?Is this a mismatch of eyepiece and scope perhaps? Would worn or defective coatings on the EP cause this?Thank you for your opinions.

CoombellKid · #2 02-05-2007, 07:52 AM

Eyepieces aside, your currently in an area with a jetstream. This could be
part of the problem at present. I live just a lil south of the border and under
the same jetstream. I have been taking in the views of the planets, to me
they look a lil mushy Antares looks like a fir ball.

regards,CS

rmcpb (Rob) · #3 02-05-2007, 08:35 AM

Does this happen with this eyepiece all the time? If it has just started lately then it is probably seeing. This eyepiece gives about 100x magnification which is well within the theoritical limit of your scope but seeing conditions are probably the cause.

Keep looking up.............

janoskiss (Steve H) · #4 02-05-2007, 10:09 AM

At 1mm exit pupil, which is about what your 13mm EP gives you, you will indeed start to be able to see the Airy disk (diffraction image of a point source), and the first diffraction ring will be visible around the brighter stars also. otoh I don't know about "loss of crispness"... not sure what you mean exactly...

KG8 · #5 02-05-2007, 12:04 PM

Ahhh! jetstream, I would never have guessed. And yes it did seem to give better views a week or so ago when I picked it up. When I said crispness I guess I meant sharpness, like no furball or the boofy diffraction ring pattern I saw last night. EP is brand new and by all reviews a good one so I was a bit confused? Now I will wait a bit for the weather to use it on the binary stars. Plenty of reading to do anyway. Thank you for the detailed information. PS. Is their a link to a page that has these details affecting seeing in SE qld?

erick (Eric) · #6 02-05-2007, 12:07 PM

Search in the "Resources" link on the left side of the IIS page.

KG8 · #7 02-05-2007, 06:24 PM

too easy! Thanks erick