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Fix for Strong Light Interference From Moon And Solar Observing

by: Clay Sherrod

The Need to Block Stray Light Entering the ETX and other Telescope OTAs

Recently, there was an observer who was plagued with excessive stray light and "light flares emanating from around the image of the planet Jupiter, double stars and other bright objects.  Soon after,  I received another inquiry from an Meade ETX user expressing the same concern when observing the sun.

This is not a new problem nor one that hasn't been addressed, but this is something that can cause problems even when NOT viewing near something so bright as the moon.  Indeed, our first oserver complained of stray arcs of light entering the field of view surround the bright double star CASTOR!

After hearing his woes, I attempted to duplicate his problem (large arcs of light with the bright image positioned just outside the field of view) and was "very" successful; placing my fingers across all three points quickly showed that this was the source of unwanted light and glare.

01 sherrod lightleakphoto

It can get better or worse as the angle of your telescope changes relative to a bright nearby object; the angle of something such as Jupiter, Venus, the moon or even a bright star to the CAMERA MOUNTING PLATE seen in the above figure is critical to letting extraneous light enter the optical tube THROUGH the housing for the small flat flip mirror at the eyepiece end.  Unfortunately there are three (3) sources of light leaks at this point, but a very simple remedy for them:

    1) Two (2) 1/4-20 threaded holes that are drilled into the mounting plate to attach a threaded receiver knob from a standard camera tripod; these "open up" just under the OTA and allow light to scatter into the chamber holding the optical flat mirror; this is particularly bad when using an axial (i.e., prime focus, or eyepiece projection/CCD) type of shot through the back of the telescope.

    2) There is a gap that measures about 3/8" on the ETX 90 and 5/16" on the ETX 125 that is at the FRONT of the camera tripod plate - the worse possible place. Anything that the telescope is aimed at that is bright, will also shine directly into this opening; the gap is clearly visible in the photograph.

To rectify the light from the 1/4-20 holes, a standard 1/4-20 bolt by less than 1/2" inserted and secured into these holes is adequate; you can see in this photo of the underside of my ETX 125 that I have "dressed it up" a bit by using some finished black "thumb knobs" securely firmly into place.

Regarding the front gap between the camera tripod plate and the curved blue OTA, you can see that I have merely inserted far out of the way a firm piece of black foam that is securely in place and cannot slip.

Unless you take the steps described above, you are prone to very strong light interference when observing/photographing the bright objects, even the sun in daylight. I have noticed severe detrimental effects of such light while scanning the terminator of the moon's surface, particularly when a Barlow lens is inserting into the light path of the small flat right angle mirror.

NOTE that although this "light leak" reference is specific to the Meade ETX telescope, all observers should examing their equipment carefully for any avenue through which light can enter the OTA other than the light being focused by the optical system.  Any light that enters the OTA that is not welcome needs to be blocked.

This can be as simple as adding a long dew shield (RULE - shield should be at least 2x longer than the diameter of the telescope), or painting some bright and shiny surface in or around the telescope a flat, non-reflective black paint.

For little or nothing, and just a moment's time, you can improve the overall contrast and performance (not to mention steadying your nerves) tremendously.

Clay Sherrod
Arkansas Sky Observatory
Conway / Petit Jean Mountain
Arkansas
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