Stopping down a refractor

[David Smith]

Ok, silly question time

Is stopping down a 'scope as easy as it seems, i.e just create a mask and place it in front of the objective? My plan is to place a mask of 100mm at the front end of the dew shield on my Evostar 120 but seems too easy!

Thanks

[John]

Besically thats it - a circular aperture in front of the objective stops it down. In your case you will get a 100mm aperture with a focal length of 1000mm so F/10. A bit less resolution and light capture but should show less chromatic abberation too.

[ecuador]

But it has to be a perfect circle, otherwise you will see strange star spikes etc.

[Starlight 1]

I use one on ST 120 . posts 1 & 2 are spot on.

[David Smith]

Thanks all.

[hunterknox]

Yep. Never really understood why people do it though, except in very specific circumstances (e.g. using a Daystar filter).

If you're struggling to get a perfect circle for your aperture mask there was a guy who used to make Bahrtinov masks who traded on ebay as Morris Engraving. He made me a custom Hartmann mask for a very reasonable price so I'm sure he could do an aperture mask too.

[David Smith]

Thanks for the tip hunterknox. There is a very specific reason for this which will become apparent in time