lenses

[1crem1]

Hi

New person here. I have some reasonably cheap plossls that I have to look very closely through which is uncomfortable over time. Is this because of their price? Where should I start? Helpful hints appreciated.

Thanks
Andrew

[janoskiss]

Hi Andrew. It is probably because they are short focal length plossls. Plossls are a scaled design, which means that the shorter focal length (ie high magnification) ones are scaled down versions of the longer f.l. ones. So at the high power end the lenses get tiny and eye relief becomes short hence no comfort. Plossl are comfortable without glasses down to 10-15mm f.l. depending on your tolerance for short eye relief.

For higher powers you are better off using a barlow with a longer f.l. plossl. A barlow is inserted in the focuser before the eyepiece and increases the magnification, most commonly by a factor of 2. You can get a capable barlow for as little as approx $40. Other option is to buy short f.l. eyepieces that are of a more complex design with longer eye relief, e.g. Burgess planetary.

What sort of telescope do you have?

[1crem1]

Hi

I have an 8" reflector, somebody's old backyard project with water pipe eq mount: very big in the 70's, but it was cheap.

Andrew

[rmcpb]

Andrew,

Welcome to IIS :)

As Steve said try using longer lenses like 25 or 15mm with 9mm as the shortest one for general use.

As the scope is of unknown pedigree I would take it to your local astro societies viewing night and try some other eyepieces to see if that is the problem or if the mirror is not up to the job. At least that way you can test it for no real outlay. Then, if its the eyepieces follow Steve's advice remembering that 150x will be your max magnification on most nights so don't get carried away with short length lenses.

Cheers

[1crem1]

thanks Rob. I started out buying cheap and got exactly that. And because it weighs a ton and my back is not good, it rarely gets out. But it works! But I'll look around CAS and try a barlow with the lower magnification lenses.

Andrew