"Floaters"

[John]

Why is it that when I use a plossl and a barlow lens I can often see the "floaters" in my eye transposed onto the image but when I'm using a Nagler eyepiece, which incorporates a barlow-type element in it, I don't have that issue - is the visability or otherwise of "floaters" determined by eye relief or some other factors ?.

John

[FLO]

The visibility of floaters depends on the eyepieces exit-pupil diameter. The narrower the beam of light as it enters and travels through your eye, the better it is at picking out debris floating across its path. Similar to light coming through a slit in your bedroom curtains - the more narrow the slit, the more dust is highlighted.

[James]

Floater visibility is aa function of exit pupil I think. When the exit pupil gets below about 1mm for me I start to see floaters. I think its why an exit pupil of 0.5mm is regarded as the absolute minimum due to this problem.

Apologies - I can't for the life of me remember how to work out the exit pupil for a given eyepiece/scope combo...

James

[James]

Oops - Steve got there first. Better explanation too!

James

[FLO]

I type faster

exit pupil = telescope aperture / magnification

[John]

Steve, James - both very helpful replies - thanks.

Still not quite sure why a x2 barlowed 11mm TV plossl shows them while a 5mm Nagler used on it's own does not

Are the exit pupils of these combinations, used on the same scope of course, not almost the same ?.

John

[FLO]

Yes, they are almost the same. Perhaps it is connected to how sharp and well defined the exit-pupil (beam of light) is ... I am guessing.

[John]

Perhaps I'll e.mail Al Nagler at Tele Vue and ask him - after all he designed 2/3 of the glass in this equation (the barlow is a Celestron)

JOhn

[FLO]

He'll probably say you should be using a Powermate

[John]

He'll probably say you should be using a Powermate

Well if he would like to send me one gratis I'd be prepared to give it a go I suppose

John

[PJF]

The visibility of floaters depends on the eyepieces exit-pupil diameter.

:shock:

I was reading "Astronomy" magazine naked eye in broad daylight on Sunday, and was alarmed by the interfering presence of the floating barstools.

[Fraser mode] I'm doomed, Cap'n Mainwaring; doooomed. [/Fraser mode]

[FLO]

You often see floaters in bright light because that is when your eye-pupil is at its narrowest.

If you suddenly notice an increase in floaters, that's the time to visit your optician - quickly!

[PJF]

Fortunately, these blighters have been floating with me for many years. I moaned about them last time I was at the optician but he wasn't concerned when he looked in there. "Tough luck for becoming an old git" was what he said in a slightly more polite manner.

Thanks for the warning tho...