Dew Shield on Newt as a light baffle
Posted 25 July 2021 - 03:26 PM
Posted 26 July 2021 - 07:55 AM
I was looking at light shields for my 10" dob to help keep some stray light from street and house lights from making it into my eyepiece. I also looked at getting a dew shield for a 10" SCT to do the same thing by blocking more than light just from the one side. Has anyone done this? Pros and cons to it?
In order for stray light to make it into the eyepiece of a Newt, the light has to enter the bottom of the focuser from a shallow angle. Stand outside your tube and look into your focuser. If the angle is shallow you might consider a light shield because just one glancing reflection might be enough to hit the eyepiece field lens directly. This is especially true if your focuser is very near the upper end of your tube. You can probe the focuser with a flashlight to see how much light might potentially enter the bottom of the focuser, or you can probe it with a laser. Most focusers I am aware of have fine ribbing near the bottom opening and up the draw tube. If you shine a light on them from outside the tube, and if they appear very bright, then they are doing their job of redirecting light back out of the draw tube. Below is my design. It is relatively long to steepen the angle the light must have to enter the focuser. If the angle is steep, then the light bounces around the focuser without much chance to enter the eyepiece.
For an SCT it's a bit more complicated due to the internal baffles. But, you can do the same thing. Stand outside the tube looing down the primary baffle. If you can see deep into the primary baffle, you may consider a dew/light shield. A dew shield is almost essential, anyway. You can also probe the primary baffle with a laser to get an idea of how deep you can go before any rays can potentially make it into the diagonal and up into the eyepiece field lens. Alternatively, you can point the scope out of a daylight window with a low power eyepiece and look for bright reflection near the exit pupil.
Below are images with light shining up the visual back so I can see how far down the primary baffle I can see at a glancing angle to the secondary mirror. I cannot see all the way down, so any stray light ray should be terminated before it get's the the diagonal.
Below is a view looking off axis through the diagonal to see hw much of the meniscus/corrector I can see way off axis. Maybe some light can make it way off axis, but I do nto think it makes it into the eyepiece field lens.
Below are glare sources with the scope pointed out a brightly lit window during the day. Identify and deaden them.
Edited by Asbytec, 26 July 2021 - 08:01 AM.