I'd like to hear your experiences with best practices for avoiding internal condensation in refractors.
Allow me out explain my question by outlining this scenario:
- A grab-and-go APO is brought outside from room temperature to colder air for an observing session.
- The air inside the refractor slowly cools down below the dew point...
- ... and moisture in the air condenses on internal surfaces, including the inside of the main objective lens.
- At the end of an observing session, the dew cap is put back on the dew shield to prevent fogging up the objective when the telescope is brought inside again.
- When the telescope is brought inside again after an observing session, the main objective lens is protected against fogging by the dew cap, and the cool, moist air inside the tube warms up again to bring the temperature above the dew point.
- Any dew inside the tube evaporates, but a small amount of dew residue (air contaminants?) possibly remains on the rear surface of the main objective lens.
- This is repeated for most observing session for a while, and the objective after a while seems slightly hazy on the inside.
Over the years I have cleaned the rear lens surface on several of my refractors due to finding the objective to be somewhat hazy on the rear element. Cleaning it removes the haze, but I'd prefer avoing having to do, so I'd appreciate if you would share your own experiences with this.
Have you experienced the same problem? How do you avoid it? I have heard that some observers prefer to uncap the telescope focuser and pointing the OTA down for a few minutes prior to each observing session to replace the warm air with cooler outside air, but I have until now been under the impression that this practice was mainly driven by the desire to cool down the telescope quicker, and wasn't motivated by concerns about dew. But I can imagine that replacing the warm, moist air with cooler air might be helpful for controlling internal condensation, too.
Any thoughts on this? For instance, how would this work with an APO triplet/quadruplet with widely spaced lenses keeping a large volume of air to be trapped inside the tube?
I recently acquired CQ 1.7 extender module for my Tak FC-76 DCU, turning it to an extremely nice, flat field quadruplet, but I expect the addition of the extra lenses also increases the potential for trapping moist air between optical surfaces.
I'd appreciate your thoughts!