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Background darkness not uniform across field of view


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Hi all,

New to the forum. I noticed when I was out the other night that my field of view was not uniformly dark all the way across, but seemed brighter to the lower left and darker to the upper right. This configuration stayed regardless of which of my three eyepieces I was using and what direction/object I was pointed at with my telescope. It's an F/8 114mm Newtonian on an equatorial mount. Since I'm most interested in DSO hunting, this is a big problem. Any ideas what's wrong with my scope?

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Welcome aboard SGL.

Sounds like stray light is getting in, but for it to stay in the same place as you move around the sky is a bit odd.  Maybe it's opposite the focuser.

Do you have a dew shield on it at all?  If not, might be worth making a quick one out of card and duct tape to see if that keeps stray light out. If so, make a more permanent solution.

Might be worth covering the front during daylight and seeing if you can see any light through the focuser. It could be stray light getting past the primary at the bottom of the tube (depending on the design).

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May I ask what make of scope? I think there are some F8 114mm Newts which have a correcting lens fitted inside the focus tube, which seem to have a poor reputation. If it does have a lens fixed inside, I'm wondering if the variation in brightness could be caused by reflections from the lens surfaces? (I've not looked through such a scope, so could be completely wrong here!)

Cheers
Ivor
 

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It's a Bausch and Lomb 114mm Newtonian, 900mm focal length from the mid90s. Made in Japan. OTA is a steel tube, pretty sturdy and blackened on the inside. No correcting lens in the focus tube, just a shitty 0.965 inch barrel rack and pinion focuser. I managed to get some decent Antares plossls to improve the views compared to the awful eyepieces it came with. Giving a look down the barrel of the focuser just now with no eyepiece in place, the focuser barrel is also blackened, but the reflection of my eye in the secondary doesn't seem to be fully aligned so I am suspecting that is the biggest problem - collimating the secondary. 

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