Looking through the eyepiece..help!

Got a beginners' question? No matter how elementary, our friendly forum community and magazine writers will answer it.

Looking through the eyepiece..help!

Postby alanwah » Tue Dec 29, 2015 3:54 am

Hello all, total beginner here!
I know this may seem a dumb question but what is the best
Way to look through the eyepiece? Close up, at a distance,
I just can't seem to find a happy, comfortable medium!
I'm sure I will be on here quite a bit as I journey through
The mysteries of the telescope! Thanks.
Posts: 2
Joined: Tue Dec 29, 2015 3:42 am

Re: Looking through the eyepiece..help!

Postby cwardlaw » Tue Dec 29, 2015 12:06 pm

I asked someone a similar question and I think they said it just takes practice. It will be good to see what other advice you will maybe get.

Glad others are asking the same level of questions as me....reassuring for me but not that helpful for you perhaps.
Posts: 25
Joined: Mon Dec 14, 2015 8:08 pm

Re: Looking through the eyepiece..help!

Postby dave.b » Tue Dec 29, 2015 4:06 pm

There are at least two factors to consider: exit pupil, and eye relief.

Exit pupil is the size of the image at the exit of the eye piece. This is given as a diameter since it's round (as a result of the objective being round). The smaller the exit pupil relative to the size of your eye's pupil the harder it will be to get them aligned so that the light falls on your eye's retina. The diameter of the exit pupil is given by dividing the telescope's functional objective by the magnification. E.g. a 200mm objective at 100x will produce a 2mm diameter exit pupil. I said functional objective because you need to account for any internal baffling and field stops.

The useful workable range for exit pupils is in the the 6mm to 1mm range. (Some suggest 0.6mm, by they must have better eyes than me!) The upper end is limited by how wide you can dilate your pupils, and the lower end by the need to illuminate at least the eye's macular to see any detail.

Eye releif is a function of the EP's design. It is the distance from the EP's external lens to the eye's lens over which the eye can focus the image. Long eye relief EPs are therefore more accepting of a varying eye distance than short eye relief lenses.

Hope that helps.

Dave B.
Posts: 803
Joined: Mon Feb 13, 2012 6:35 pm
Location: Staffordshire

Re: Looking through the eyepiece..help!

Postby alanwah » Wed Dec 30, 2015 9:31 pm

Thanks for the replies, I'm joining a club, this is all a little more complicated than I imagined! Best wishes al.
Posts: 2
Joined: Tue Dec 29, 2015 3:42 am

Re: Looking through the eyepiece..help!

Postby The Man with the Corrugated Iron Roof » Mon Jan 04, 2016 11:31 pm

You can get eyepieces with long eye relief.
How can I be one with the universe when we don't know what 96% of it is.

About Me: https://www.amazon.com/Philip-Pugh/e/B0034NTCJK

My blog: http://sungazer127mak.blogspot.com/

Photos: https://www.flickr.com/photos/philippughastronomer/
The Man with the Corrugated Iron Roof
Posts: 543
Joined: Sat Nov 23, 2013 1:53 pm
Location: Wiltshire but can be just about anywhere up to 41 000 feet

Return to Ask a silly (astronomy) question

Who is online

Users browsing this forum: No registered users and 0 guests