SW 150PDS - fitting a primary mirror mask

[Padraic M]

For future reference for anyone doing this mod. I searched and I couldn't find any definitive instructions, although it's a very straight-forward procedure.

The mask is designed to improve images in two ways:
1. The edge of the primary mirror reflects light irregularly across the image plane, depending on its design or its imperfections. This can cause star halos/bloat, and can reduce contrast generally in the image.
2. The primary mirror is held in place by three mirror clips. These clips protrude over the edge of the mirror by ~5mm and can cause secondary refraction spikes on stars - or in fact, they show as interruptions in a star halo.

Both of these issues can be reduced or removed by masking off the outside edge of the mirror by a few millimeters, at the expense of those few millimeters of aperture. You can cut a mask from a variety of non-reflective materials; 3d-print your own (patterns available online) or you can buy one ready-made from the astro retailers. I bought a Wega primary mirror mask from teleskop-service. I'm sure Wega are the big name in mirror masks 🙂

1. Remove the primary mirror cell
On the 150PDS, the primary mirror cell is held in place by four Philips-head screws on the side of the OTA - not the collimation screws on the bottom. Once these are removed, the cell lifts out easily. Be careful not to drop the cell - I stood the OTA upright on the floor, mirror-end up.
Stand the mirror cell on a good surface, mirror facing up.

2. Remove the three mirror clips - there are two ~1" Philips-head screws holding each clip. The clips themselves consist of a large rubber-type block, with a metal face laid on top. Try not to drop the metal face on to the mirror surface.

3. Don't remove or move the mirror. Fit the mask in place over the mirror - I started by loosely fastening down one clip with the mask attached; then insert the other two clips between the mask and the mirror carefully. Screw them down incrementally until the six screws are all hand-tight. You might notice that the mask deforms slightly as you tighten the screws in turn so try to keep it circular. Not sure that it makes a huge difference to the image but you might as well be neat.

Here's the finished article. I took an air blower to any loose dust on the mirror but otherwise didn't do any deeper cleaning.

 

Here's an idea of how clear the aperture is with the mask fitted. It doesn't cover the clips 100% but it should make a big difference. I suspect it will also help block some of the light leaking in from the back.

Now that it's finished you will need to re-collimate. I found that it was only slightly out but needed some adjustment. I took care to fit the mirror cell back into the OTA in the same orientation as it was previously.