Focal length question

[Doc]

Say I made an aperture mask for my dob what would the Focal length change to?

My scope is:

400mm Aperture

F4.5

FL 1829mm

If I had....

Aperture mask off 145mm

So 1829mm divided by 145mm = F12.6

But does the focal length now change or is it still 1829mm

[Dave Smith]

The focal length will remain unchanged. The only way to effectively change focal length is to us an additional lens, e.g. a focal reducer.

Dave

[Doc]

The focal length will remain unchanged. The only way to effectively change focal length is to us an additional lens, e.g. a focal reducer.

Dave

Thanks Dave I thought that was the case but just wanted to check incase it wasn't.

[yeti monster]

That's what I thought too, as when I use place aperture masks over the end of the scopes the image remains the same size and focus, it just becomes dimmer (thus gaining contrast on a full moon).

[Doc]

That's what I thought too, as when I use place aperture masks over the end of the scopes the image remains the same size and focus, it just becomes dimmer (thus gaining contrast on a full moon).

It may become dimmer but on planets wouldn't the contrast be much better, hence more detail will emerge. Remember it's cutting out the secondary so it should improve.

[yeti monster]

I suppose that would depend on the percentage decrease in aperture, too much and you'll not be getting anything like enough light down the scope for decent planetary work. For (half-full) lunar viewing I recuce the effective aperture to about 1" on both the 84mm and 114mm newts, which works really well.

Having tried planetary on my pals 70mm frac, I'm convinced that more light is the route for gaining any sort of decent image on planets (unless they suddenly become REALLY close)