Last updated: 29 January 2003
Subject: Light scatter in ETX90EC baffle tube Sent: Tuesday, January 28, 2003 3:48:46 From: firstname.lastname@example.org (Martin Lewicki) When I aim the ETX90EC into the night sky (no moon) and look through the focus-tube without an eyepiece there is a considerable sky-light glancing off the inside of the baffle tube. This produces a pale, grayish concentric circle around the reflection of the aperture of the scope. I suspect that this stray light must be slightly fogging deep sky views reducing image contrast. I also figure this happens because the baffle tube extends considerably out in front of the mirror and is exposed to off-axis skyglow finding its way down there. It is worse in heavily light-polluted sky. Solution: Make a dewcap from card about 20cm long so that it fits over the OTA, and so that it can be slided in and out. Paint the interior of the dewcap matt-black. By sliding the dewcap along the OTA you change the effective length protruding at the top end of the scope. Road test on deep sky object like Orion Nebula (with eyepiece) . As you slide the dewcap out (effectively increasing its length) you will notice contrast increasing slightly with the background sky darkening and stars just that little more brighter. The dewcap shields the baffle tube from stray skyglow. Again, take out the eyepiece and you can see the proof of this technique. When you slide the dewcap out the grayish reflection on the inside of the (concentric) baffle tube diminishes to almost invisibility while the sky in the aperture remains the same brightness. Martin LewickiMike here: Also, light can enter the OTA via the photographic tripod mounting hole on the bottom of the tube between the fork mounts. Covering this hole will eliminate a source of stray light.
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