Apodizing Masks

[Laurie61]

I know I am not the only stargazer who likes to fettle their scopes; many try different techniques such as flocking and fitting fans in an attempt to eek out a little more performance. Another method which has been tried is to use an Apodizing mask/screen. Link-

http://www.astronomy...om/apodize.html

This entails the fitting of a screen/mesh arrangement to the front of the scope with the aim of improving resolution. Given the positive comments by other amateur astronomers, who've tried this, I thought it would be worth making my own and giving it a try.

The first problem I could see was that the screen design caused a strange diffraction effect in the outer part of the field of view leaving only a relatively small central hole to view through. This is caused by the screen material and can’t be designed out if using mesh.

Another way you can achieve the same effect of progressively blocking light from the outer part of the aperture is to make something which a friend of mine calls a Doily . What this does is eliminate the mesh effect (scattered colours/ moiré) while providing the, theoretical, beneficial increase in resolution. The mask I have ended up with was generated by my brother who happened to have a piece of software on his lap-top which allows you to simulate the effect of various designs on the Point spread function and is a compromise between maintaining image brightness whilst still improving resolution. After viewing with and without the filter we now leave it in as we prefer the view.

Below is the design we came up with, if you want to try it download the image and print it off at an appropriate size for your scope. It will work best on scopes with smaller or no central obstructions.

This is my 152mm Mak with the Apodizing mask fitted-

and the view through the diagonal with eyepiece removed.

[ronin]

What is the measured airy disk reduction with the filter ?

[Laurie61]

All the calculations were done on my brothers laptop several years ago and I didn't make a note of the numbers unfortunately. The effect of this type of filter will be subtle and seems to provide the biggest benefit to the double star folks.