Basically you are increasing the focal ratio of your scope by reducing the aperture. Usually this helps when skies cant support larger apertures because you are trying to see through a larger area of a turbulent sky. More area under mediocre seeing conditions usually doesn't help. This is just one of many reasons why sometimes you hear of people boasting how their smaller scopes seem to out perform a larger scope.
Super planetary telescopes are generally high focal ratio scopes. You would get darker back ground skies, and sharper optics. Its not night and day difference but it all helps. The Classic refractor group can talk circles about this because we love the high focal ratio scopes.
This is an APO myth which comes from conditions which favor smaller apertures due to turbulent or mediocre conditions, not really because of optics. Another reason could be because the larger scope takes longer to acclimate to outside conditions then smaller scopes so quick grab and go sessions usually favors smaller scopes.
Back to your question:
Focal ratio (Focal number) = Focal length / diameter of your aperture
so in your case:
1650mm / 355mm aperture = F4.6
so that's your focal ratio.
If you make an aperture "mask" which I think is the same as your description of a cap and make a cover so you have 100mm of opening, (Aperture) your focal ratio would be:
1650 / 100mm = F16.5
In the refractor world, people would ogle over an F16.5 100mm telescope.
You would have to make an offset hole, so you block out the secondary mirror, and even position the hole between the spider vanes.
It may sharpen things up though you will loose a large amount of light and some resolution. Experiment with different sizes, just make it out of cardboard and place it over the opening of your scope. You can tape it down so it holds itself in place.
I use aperture masks to tone down the light when looking at the moon instead of using Neutral density filters.
The masks help noticeably when splitting double stars when you try and push the magnification and your stars could get a little puffy. So it could help sharpen up your planetary views.
I've made aperture masks for my 6" APO, 1200mm FL. I made a 100mm mask (F/12) and an 80mm mask (F/15) in less then ideal seeing conditions. The results are subtle but it helps and its fun also. Its noticeable on puffy stars cleaning them up and making them sharper in turbulent conditions.
I hope this helps.
Edited by aa6ww, 11 September 2019 - 04:51 PM.