Reducing aperture

[Thirdway]

Explaining aperture on a camera to wife, it suddenly dawned on me that the dust cap on my Dob has an eccentric hole for stopping down the scope. Anyone use it on bright objects and to increase contrast?

[nightfisher]

Moonshane has a huge dob, that he stops down sometimes to great effect, people do this with achro refractors when viewing lunar detail

[John]

It can be useful but its worth remembering that when you stop down a scope you are reducing its effective aperture and resolution is reduced as well. So your 8" F/6 dob becomes something like a 2.4" F/20.

[Peter Drew]

You can certainly dim objects down by using the smaller aperture but you will lose a lot of potential resolution as this is a direct function of aperture. Might be better to use filters for this purpose. Stopping down a large aperture (16"+) to a smaller but still large enough apertue can have benefits if the seeing is unsteady.

[g0ibi]

Good for a full moon and if you use suitable solar filter 'foil' you can cover the small aperture with it for solar viewing! Taking great care of coarse!!

[Ags]

I stop down my F5 ST80 to F8 for photography - it reduces chromatic abberation.

[Thirdway]

I'm going to see what the effect is. I understand that it will be a straight through scope when used with an eccentric hole because the secondary mirror doesn't interfere. Looks like the scope will be stopped down to F infinity for the next week looking at the forecast.

[umadog]

If the mirror has a good edge then stopping it down like that will only decrease contrast.

[swamp thing]

I sometimes use aperture stops if the seeing dictates it when using my Dobs.

Not really convinced that stepping the 10" down is of any benefit, as although it can help when the seeing is bad the loss of resolution seems to make detail so much harder to see that I'm better off selecting other targets for the scope.

The 16" is different as the stepped down scope is still of sufficient size to resolve a great deal of detail.

If the seeing is good there is nothing to be gained from stepping down either scope.

Regards Steve