dew shields, when to use them?

Last night I was experimenting with “collimating down” my 120 achro to reduce the amount of chromatic aberration (CA). I used a piece of cardboard to effectively reduce the objective opening of the dew shield by 1 cm. It did reduce the CA and also reduced the brightness of the view (expected). What was really apparent was how bright the view was with the dew shield removed completely. Of course, the CA on brighter objects seemed more pronounced, however, for viewing DSOs this is not an issue, and, even though viewing conditions were average, the DS objects were clearly brighter.

So I was wondering, would it be standard practice to remove the dew shield on refractors when viewing DS objects? Do any of you do this when conditions allow? Is there some other reason for leaving the shield on even when dew is not a problem? At this time of year down here in south Texas, dew is not an issue unless the instrument is out when the sun comes up in the morning. Thanks for any input/info/opinions.

Jim,

I think the usual name for the process you are using is "stopping" down. I don't know that "collimating down" is ever used to describe this.

I cannot see how removing your dew shield can make your aperture larger. And if it not doing that, I don't see how you could be getting more light to the eyepiece. (Unless the dew shield is supposed to be acting like a field stop, and cutting off the edges of your objective--and the designer would do that only if he or she felt the edges were bad.)

However, it could well be that things will appear brighter in the eyepeice because you are getting extraneous light coming in from the sides, and hitting the front element of your scope. This light is generally, though, unwanted. It reduces contrast.

In short, I don't see how removing a dew shield would increase brightness of the desired target object. (It would increase extraneous light, however).

Alex