Refractor Objective Lens Cleaning - When To?

[Ian McCallum]

What's the advice regards dust inside the lens elements of a refractor? 🤔

I cleaned the objective lens with Baader Wonder Fluid and the Baader microfibre cloth.  I used the method described below... 

https://www.skyatnightmagazine.com/advice/clean-a-refractor/

If the dust inside won't affect the image, my thoughts are to leave well alone.

[Mr Spock]

I'd advise leaving it. As long as it's not too dirty it won't affect image quality.

[Ian McCallum]

14 minutes ago, Mr Spock said:

I'd advise leaving it. As long as it's not too dirty it won't affect image quality.

Yeah, that's what my gut feelings were too.

[Franklin]

That looks very clean to me. Leave it alone. Never take an objective apart unless it's an old one you've picked up for next to nothing and only then if there was fungus growing in there. All of those tiny specs of dust you see when shining a light down there have an accumulative surface area which is basically insignificant compared to the surface area of the lens. If you used your scope as it is and then somehow miraculously got rid of the internal dust and then used it again on the same targets, same conditions. You wouldn't be able to tell the difference, 100%.

If you bought a new telescope and there was internal dust, then that is grounds for returning it. As it shows that a/ The lens was assembled in a NON dust-free enviroment, which is bad and b/ QC is out of order.

The problem with cleaning objectives too often is the accumulation of micro-scratches. A new, perfectly pollished lens seen under a magnifying glass will be smooth and clear. A lens that has been cleaned a number of times looks to the eye, smooth and clear also. But under the magnifying glass there will be many micro-scratches which cause dispersion and light scatter. Thus reducing the contrast of the image the lens is capable of producing,

To avoid having to clean lenses, maintain good avoidance tactics. Keep it capped when not in use. Don't have the scope on display (difficult for some) but keep it in its case. Give the scope time to dry out slowly before storing. Invert the scope and use a bulb blower before re-capping, EVERYTIME. By remembering to do all these little things will mean you won't have to clean your optics, which means you won't scratch them.