EP with 72mm focal length

[yagon]

I just saw this on ebay:

http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=7610749853&rd=1&sspagename=STRK%3AMEWA%3AIT&rd=1

In my ED80, this would give a magnification of 8.3x and presumably a huge field of view.

The quality looks dubious, but I like the idea of an EP with a big focal length and huge field of view - who needs binos with a 72mm EP?! :P

Would this EP be usable/practical? Is there an issue with the size of the exit pupil?

Are there quality EPs available is such large focal lengths?

[ving]

i'd say exit pupil issues would be a problem...
russle optics arent a new company tho.... makes me wonder

[GeoffW]

just be wary of any ebay item that wants you to contact them on a specific email address other than through the ebay system.
lots of scams out there. Do some checking.

Cheers,

Geoff.

[mickoking]

Sounds dubious.

[Doug]

G'day all, while this link leads to an 85mm EP review, the article might be a good guide. It seems Russell Optics will not ship outside the US......how insular is that??
http://www.astromart.com/articles/article.asp?article_id=235

best,
Doug

[yagon]

Can anyone comment on the effect on the exit pupil?

[mickoking]

The exit pupil is the diameter of the light cone from a specified eye piece. What that means is if the exit pupil is too large a lot of the telescopes light is wasted. For example a healthy 20 year old's pupil diameter is approx 7mm. If the exit pupil of a particular eyepiece/ telescope combination is more than 7mm some of the light is wasted and the image is not as bright. To determine the exit pupil you divide the telescopes aperture by the eyepieces magnification

aperture/ magnification= exit pupil

80mm/x25 = 5mm exit pupil

or

aperture/ (telescope focal length/ ep focal length)=exit pupil

80mm/ (400mm/16mm) = 5mm

A 5mm exit pupil is good for low power eyepieces. A 7mm exit pupil is good under a very dark sky but any higher than that you will get dimminishing returns on your telescope as you start wasting light.

using the above equation with a 72mm eyepiece
80mm/ (400mm/72mm)= 14mm a twice the exit pupil of a 20 year old under dark skies the telescope effectively has an aperture of 40mm with the 72mm eyepiece.

I hope I haven't confused you :P

[AstroJunk]

Another take from televue:

http://www.televue.com/engine/page.asp?ID=141

The biggest factor it seems, may be the size of the central obstruction. That makes sense.

The following is copied from TeleVue:

Myth #1: A 7-mm exit pupil gives the lowest useful magnification.

Not so! With a refractor there is no limit on the size of the useful exit pupil. Use whatever is necessary to get the field you need to frame the subject. A reflector's low-power limit is reached when the black spot in the exit pupil (caused by the secondary obstruction) becomes obtrusive.

While a 7-mm exit pupil, by matching that of the eye, does give the brightest views of deep-sky objects, it does not necessarily give the best ones. Higher magnifications, despite their smaller exit pupils, will reveal more details, maintain contrast, show fainter stars, and help bypass defects in the eye itself.
Myth #2: Exit pupils larger than 7 mm waste light and resolution.

With refractors larger pupils do waste aperture. But the magnification is so low that the wasted aperture is of little concern: both image brightness and resolution are as great as possible at that magnification. With reflectors, however, larger pupils do waste light, but primarily because the black spot in the pupil caused by the secondary obstruction becomes larger. Both light loss and field shadowing occur with reflectors, but as with refractors there is no resolution loss because of the low power.

[Merlin66]

The televue comments ONLY apply for imaging systems ie photography.
Visually you are limited by the aperture of the eye. Yes, you can move your eye across the exit pupil but your still loose the available light. Also remember not to many CCD chips are bigger than a dark adapted eye ( the DSLR has a 18mm ? chip, so it may benefit from the lower magnification).

Long focus eyepieces can be made from old photographic lenses ie 50mm/ 80mm lens when used reversed ie looking into the front of the lens make great RFT eyepieces, well corrected and wide field! An adaptor can easily be made using the plastic lens cover ( camera body side) glued to a bit of 2" tube.

[mickoking]

Well you learn something every day. I have several old SLR lenses knocking around. I might give it a go :thumbsup: