Jump to content


32mm eyepiece on 150p.

Recommended Posts

Hey guys,

Just bought a 32mm panaview eyepiece for DSO viewing, and whilst hunting for the sun this afternoon (to no avail, t'was cloudy, funny old thing...) I noticed that I was getting the secondary mirror darkening the centre of the view.

Is this because my pupil was not dilated enough, due to the brightness (32mm on f5 gives exit pupil of 6.4mm)?

Or would this not be noticeable at night?



Link to post
Share on other sites

strangely enough I was getting a similar issue using a meade 25mm eyepiece witha skywatcher 130, looking at the moon. it was with and without moon filter. Yet my sw 25mm was fine. I would be interested in an explaination of why these dark patches should appear?

I am not trying to hijack the thread, it just seamed a similar scenario

Edited by Nillchill
Link to post
Share on other sites

Thanks for the concern, and yes, I was using a solar filter that I made with some Baader Solar Film.

I reckon I got the darkening due to my pupil being constricted in the day time (even though the views of the Sun were blocked, the thin cloud was brighter than a big bag of bright things)

Link to post
Share on other sites

Hi Dan,

From what you've said i'm not sure what effect your describing.Just a couple of thoughts:

1)In the daytime your pupil is unlikely to expand to 6.4mm.

With some eyepiece designs when the exit pupil is nearly as big or bigger than your eyes' pupil this can cause "blackouts"-where the view just blacks out until you reposition your eye slightly. not sure if this is what your experiencing?

2) I've heard it said, though never experienced it in practice, that with telescope designs with a relatively large central obstruction &when using really low powers with large exit pupils the centre of view can appear dark as the obsruction becomes significantly large as it hits the eye, so you're seeing the shadow of the secondary.

Think of it this way, if i have a 6mm exit pupil, a 6mm circle of light enters the eye.

If for example you have a 150mm mirror with a 50mm secondary that circle becomes a 6mm donut with a 2mm hole in the middle, at the eye.

So this may be what you are seeing?

Best thing to do is see what happens at night & go from there.

Hope that helps?

Link to post
Share on other sites

I think SiriusB is right on the money with both those suggestions. Blackouts and secondary shadow are all symptoms of the scope / eyepiece exit pupil exceeding that of the eyes pupil.

Things will be different at night I reckon.

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.