Visible Central Obstruction

I have a Celestron 130 mm f/5 Newtonian that works pretty well as a light, quick look telescope from my garage. I notice though that the secondary mirror is visible in the plane of focus when using long focal length eyepieces (40 and 32 mm). This is especially annoying in daylight. 1) What is causing this, and 2) aside from looking around this obstacle to see what I want is there anything else that can be done to deal with this that is not costly.

Michael Tencza said:

I have a Celestron 130 mm f/5 Newtonian that works pretty well as a light, quick look telescope from my garage. I notice though that the secondary mirror is visible in the plane of focus when using long focal length eyepieces (40 and 32 mm). This is especially annoying in daylight. 1) What is causing this, and 2) aside from looking around this obstacle to see what I want is there anything else that can be done to deal with this that is not costly.

Given the large exit pupil delivered by these eyepieces and the fact that you are using it in the daytime, yeah, it's normal. What can you do about it? 1) Use shorter focal length eyepiece. 2)Ignore it.

8)

Michael Tencza said:

I have a Celestron 130 mm f/5 Newtonian that works pretty well as a light, quick look telescope from my garage. I notice though that the secondary mirror is visible in the plane of focus when using long focal length eyepieces (40 and 32 mm). This is especially annoying in daylight. 1) What is causing this, and 2) aside from looking around this obstacle to see what I want is there anything else that can be done to deal with this that is not costly.

In the daytime, your eye is only open up to 2 or 3mm, so when you use a large exit pupil, you are stopping down the telescope. For example, the 40mm gives you an 8mm exit pupil. If your eye is only open to 3mm, you are only using 49mm of your aperture, and the central obstruction takes up a large percentage of that and becomes visible.

Clear skies, Alan

Michael Tencza said:

I have a Celestron 130 mm f/5 Newtonian that works pretty well as a light, quick look telescope from my garage. I notice though that the secondary mirror is visible in the plane of focus when using long focal length eyepieces (40 and 32 mm). This is especially annoying in daylight. 1) What is causing this, and 2) aside from looking around this obstacle to see what I want is there anything else that can be done to deal with this that is not costly.

Michael:

Others have answered this more than adequately but I am going to put in my two cents. I happen to have an Orion 130mm F/5... here's the deal:

As Alan says, during the day, your eye is open 2-3mm, not much. Now these scopes are F/5 and have a 30% or so central obstruction. That means that the exit pupil, the beam leaving the eyepiece will have a hole in it that is about 30% of the diameter of the exit pupil. If you step back and look at the eyepiece, you will see the bright spot with that dark hole.

This is OK at night because your eye is open to 5mm or more. But during the day it can be a real problem. One computes the exit pupil by dividing the focal length of the eyepiece by the focal ratio of the telescope, with the 40mm this is an 8mm exit pupil, with the 32mm this is a 6.4mm. Now if you take 30% of those exit pupils you will find that the diameters of the holes are about 2.4mm and 1.9mm respectively.

So, if your eye is only open to say 2mm, it is easy to see that if you center your eye on the exit pupil, about all you will see is that central hole. This also happens when viewing the moon at low powers as it is bright enough to cause the pupil to contract to daylight levels.

Jon