Eyepiece

Hi there,

I'm wondering if a greater (more than 100x) magnification gives a better
result for observing double stars.
Now I've two eyepieces 10mm and 20mm. That gives magnifications of 50x and
100x times. The aperture of my telescope is 90mm (focal length 1000mm). So
can a barlow lens 2x or an eyepiece with a very short focal length make the
difference?

Does anyone have experience with this!?

Thanks anyway

Eric

eoto wrote:
Hi there,

I'm wondering if a greater (more than 100x) magnification gives a better
result for observing double stars.
Now I've two eyepieces 10mm and 20mm. That gives magnifications of 50x
and 100x times. The aperture of my telescope is 90mm (focal length
1000mm). So can a barlow lens 2x or an eyepiece with a very short focal
length make the difference?

It is a bit unfortunate you have both 10mm and 20mm. If you had say 10mm
and 26mm (or 20 and 16 mm) then a 2x Barlow would give you 4 distinct
useful magnifications.

The nominal heuristic is that you can sometimes use up to 50x per inch
of aperture (true mainly for smaller apertures). But I would say
typically beyond 30x it is empty magnification if your eyesight has good
acuity. That is you make it bigger and fainter without making any new
detail visible.

Does anyone have experience with this!?

It might be borderline in this instance 100x is close to what I consider
usually worthwhile at 30x per inch. On double stars the extra
magnification might be worthwhile - I not really into them apart from
doing the odd resolution test.

Aperture of 90mm ~ 3.5" so you might very rarely find 180x useful. It
might be worth splitting the difference and having a 6 or 7 mm eyepiece
to get ~140 or 160x. Check the technical details for any eyepiece you
choose as the cheaper short focal length eyepieces tend to have
uncomfortably short eyerelief.

Seeing isn't quite so annoying at lower magnifications and personally I
prefer the smaller sharper brighter image that just matches eyesight.
Empty magnification is popular in department store scopes. YMMV

Regards,
Martin Brown

Hi Martin,

Thanks for your explanation about magnification! I think I should consider
this maybe when I buy another telescope.

Eric Otto


"Martin Brown" schreef in bericht
...
eoto wrote:
Hi there,

I'm wondering if a greater (more than 100x) magnification gives a better
result for observing double stars.
Now I've two eyepieces 10mm and 20mm. That gives magnifications of 50x
and 100x times. The aperture of my telescope is 90mm (focal length
1000mm). So can a barlow lens 2x or an eyepiece with a very short focal
length make the difference?

It is a bit unfortunate you have both 10mm and 20mm. If you had say 10mm
and 26mm (or 20 and 16 mm) then a 2x Barlow would give you 4 distinct
useful magnifications.

The nominal heuristic is that you can sometimes use up to 50x per inch of
aperture (true mainly for smaller apertures). But I would say typically
beyond 30x it is empty magnification if your eyesight has good
acuity. That is you make it bigger and fainter without making any new
detail visible.

Does anyone have experience with this!?

It might be borderline in this instance 100x is close to what I consider
usually worthwhile at 30x per inch. On double stars the extra
magnification might be worthwhile - I not really into them apart from
doing the odd resolution test.

Aperture of 90mm ~ 3.5" so you might very rarely find 180x useful. It
might be worth splitting the difference and having a 6 or 7 mm eyepiece to
get ~140 or 160x. Check the technical details for any eyepiece you choose
as the cheaper short focal length eyepieces tend to have uncomfortably
short eyerelief.

Seeing isn't quite so annoying at lower magnifications and personally I
prefer the smaller sharper brighter image that just matches eyesight.
Empty magnification is popular in department store scopes. YMMV

Regards,
Martin Brown