Is it okay to use a telescope to view the sun (projection method)?

Question

My telescope is a Bushnell Northstar 114mm refractor. I want to view solar features such as sunspots using the projection method where you point the scope at the sun and project an image with the eyepiece onto a blank object like a white wall or paper (I WILL NOT BE VIEWING DIRECTLY THROUGH THE EYEPIECE). I've seen some sites saying to use this method and others saying not to. I want to know if this will cause damage to the telescope at all. I know some sites talk about overheating but will viewing for only short periods of time fix this. Or is this even actually a problem?

Answer 1

Heating can be a problem but should probably be OK.  One thing you can do is make a cap with a smaller hole in it to cover the objective and stop down the aperture which will reduce the amount of light and therefore heat going through the telescope.  A solar filter for the objective is better (don't use an eyepiece with a solar filter.)  It can be a bit tricky to point the telescope at the Sun, you basically have to "shoot from the hip"  Looking at your projection screen as you move the telescope around trying to tinf and focus the image.

I've done it with a pair of 7 x 50 binoculars and it worked OK.

Answer 2

Are you sure this is a refractor (lens-based telescope)?  I mention this only because 114 mm (4-1/2 inches) is a common size for a reflector (mirror-based telescope).

If it's really a refractor, this should be just fine, although Peter's suggestion of stopping down the objective is a good one.  Make sure you use a cheap eyepiece; some eyepiece designs focus the light in the interior of the eyepiece, and this can cause damage to the eyepiece.  Also, avoid using a star diagonal while doing projection, if at all possible, as there's significant light convergence near the diagonal (closer than when being used afocally, as in visual observations).

If, on the other hand, it's a reflector, take care.  A small reflector, like one 114 mm across, may not cause problems at the secondary mirror, but a larger one definitely would run a risk.  It's better to use a full-aperture filter with a larger reflector (or compound scope).