Last updated: 29 January 2003

ETX-90 Light Baffle mod

Subject:	Light scatter in ETX90EC baffle tube
Sent:	Tuesday, January 28, 2003 3:48:46
From: (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

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 Lewicki
Mike 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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