Refractor dust cap

[Merkhet]

Can any one tell me why is it that largish refractors have an aperture cut out of the centre of the objective dust cap.

The hole is usually about two inches in diameter with its own little dust cap.

I wondered if it was to reduce the aperture for eyepiece projection of the sun.

I have asked a few people that own refractors but it seems to be a mystery to them.

Cheers all

[Bizibilder]

The only known reason is, as you say, to allow reduced aparture viewing.  If viewing the Sun don't forget that you still need a proper Solar filter!!

The projection method of Solar viewing is not such a good idea these days as many scopes, especially small refractors, have parts made from plastic and/or cemented lenses.  These can easily be melted/damaged by allowing direct sunlight and heat through them without a filter.

[John]

You can put some Baader Solar film behind the hole (make sure its firmly attached and fully covers the aperture) and make a useful white light solar filter. Some folks use it to stop down the aperture when viewing something bright such as the Moon but it stops down the resolution too of course. 

[RichieP]

Other given reasons are to reduce the brightness of the moon (which makes no sense to me as you are turning your large refractor into a little one complete with loss of resolution) and to aid in splitting tight doubles.

[Alkaid]

It increases the focal ratio of the scope, which is useful for reducing false colour. This also reduces resolution though too. At about f12 the blue colour fringing becomes almost unnoticeable and at f15 it's gone.

[John]

It increases the focal ratio of the scope, which is useful for reducing false colour. This also reduces resolution though too. At about f12 the blue colour fringing becomes almost unnoticeable and at f15 it's gone.

Yep, a 2" F/15 refractor is virtually completely colour free