Aperture Masks

Another little project I have been working on lately is making a couple of aperture masks for one of my telescopes. 

I've been using my Celestron 120mm refractor for planetary views.  Well, actually, I purchased that telescope specifically for solar imaging but, at night, it is a decent telescope for planetary as well as lunar views so this is the telescope I currently use for planetary observing.  I'd like to eventually get a telescope with a much longer focal length for planetary astronomy but this telescope will do for now.  I'd like to eventually purchase a longer refractor telescope for planetary astronomy but I might settle for a Cassegrain telescope instead.  For now, I'll be using this Celestron 120mm refractor with a 1000mm focal length.

This 120mm refractor is an achromat so there is a lot of chromatic aberration on planetary views.  I added a Baader Contrast Booster to the front end of my diagonal and that has helped cut down most of the chromatic aberration but I decided to add a couple of aperture masks to help cut down on the remaining chromatic aberration.

Chromatic aberration appears as a purple/violet double exposure off to one side of bright, contrasty objects like planets.  Sometimes this color double exposure can be very pronounced and distracting but this particular scope isn't all that bad.  Actually, Sheila hadn't really noticed it until I pointed it out.  

The Baader Contrast Booster is known to help cut down on this chromatic aberration and it works wonders on my telescope too.  Another tool to use to cut down on this bothersome purple haze is an aperture mask.  By creating an aperture mask, I mask the outer edges of the front objective lens element.  When I mask the 120mm objective down to 90mm, this changes my f ratio to f11 (from f8.3) which then puts the f ratio into the range of meeting the Sidgwick Standard for avoid chromatic aberrarion.  

I also made another aperture mask that masks the front 120mm object down to 64mm which puts the f ratio at f15.6 putting it into the more stringent Conrady Standard for avoiding chromatic aberration.

My telescope already came with an aperture mask built into the front dust cover.  That mask cuts the 120mm front objective down to a quite small 52mm which puts the f ratio at f19 which is well within the Conrady Standard but perhaps cuts the light more than is necessary and perhaps a bit too much for my taste.  

I used some scrap 1/4" plywood to make these masks.  After making clean cuts, sanding with a drum sander and doing some hand sanding, I then painted each side of the masks with a flat black spray paint to eliminate any reflected/scattered light.  They came out nicely and fit snugly inside my front dew shield of this 120mm refractor telescope.

I now have three options for aperture masks when using this telescope.  Between the aperture masks and my Baader Contrast Booster, my lunar and planetary views should be much, much nicer!