Too long dewshield?

After extending the dewshield on my 4" refractor, I typically add an Orion Flexishield to the end of it to get another 4 or 5 inches of dewshield protection. Paranoid about getting the objective dewed up, I guess. Have yet to get the lens wet using this approach.

Anyway, what am I sacrificing optically with this arrangement? Field of View? Brightness at edge of field? Anything at all? Little of consequence?

What if the DS sags or droops into the FOV? Will it affect the star test?

Thanks in advance for your response.
Bob

If it droops into the fov it's called vignetting and it would show up as a dent in your star test. You should be OK as long as the rigidity holds and the diameter of the shield is larger than your primary optic.

The primary optical compromise is on the loss of stability. It will increase the sail area of the scope and depending on your mount, wind, and other factors, may lead to some shaking.

Refractors with dew shields as a general rule take a while to dew up; a four-inch is pretty easily de-dewed by holding it in front of the car's heater or using a hand held 12v heater. Or use an electric dew heater on the tube itself.

regards
Greg N

Bob,

Look through the tube from the eyepiece end without an eyepiece. Be sure the end of the focuser is at the focus position. If you can see the end of the dew shield as you move your eye placement, then the shield is blocking some light. Probably only a problem with wide field eyepieces.

Dom Q.