You can clean the inside of a refractor lens as well as the outside, and this is usually required every couple of years, mainly due to the “piston effect” of the focuser and dewshield drawing air into the optical tube carrying dust with it. Air circulation is a good thing to have with refractors for better stability. Please note that even a large amount of dust on a refractor lens will probably not have any visible effect on the image. That’s because the dust is at the pupil of the optical system and isn’t in focus. Therefore it is best to leave the dust on until it affects performance.
Cleaning the rear inwards facing element of a refractor is easy, and a routine maintenance job. First read this FAQ about what cleaning materials to use (generally in wards facing elements only require a dust bulb).
Here’s the disassembly procedure, to get to the rear element:
- To remove the dewshield, rotate it anticlockwise, while holding the silver bezel, and pull it off forwards.
(It helps to have the scope secured or mounted on something when you do this).
- Removing the dewshield will reveal the black metal lens cell, which is screwed into the optical tube on most refractors up to 4″ aperture. (For larger scopes, please contact Altair support). Unscrew the lens cell anticlockwise and remove it.
- Now you can invert the lens cell and clean the inner lens and outer lenses. Read the refractor lens cleaning FAQ for more info.
When the optics are clean just reverse the procedure to reassemble. There is no need to over-tighten the lens cell to the optical tube, it just needs to fit snugly up so it doesn’t rattle or move in normal use. Over tightening the lens cell can make it difficult to unscrew in future or even distort the lens cell which is made to very fine tolerances. Tip: Assuming the lens cell is correctly tightened in the first place, it helps to make reference marks with a pencil on the lens cell and the optical tube so you know how much to tighten them together again. The marks will be hidden when the dew-shield is replaced.