Primary Mirror Mask - Notes and installation instructions

An primary mirror mask is used to reduce disturbing diffraction effects caused by mirror holders that overlap the primary mirror, but also to mask mirror defects (e.g. sloping edge). The installation is usually done on the original rubber clamps, which have only one purpose to prevent the mirror from falling out of the mirror cell.

In the following pictures you can see an image of a star taken with a Skywatcher 200PDS in its original state and on the right the image after mounting a CNC machined secondary spider and a primary mirror mask. The primary mirror mask reduces the over-illuminated area around the star (caused by a sloping edge on the primary mirror) and the secondary spider ensures perfect, razor-edge sharp spikes.

Installation instructions

In order to disassamble the main mirror cell and install the primary mirror mask on the original mirror clamps you need some tools. For most telescope types, a Phillips screwdriver should be sufficient. Since we will be shortening the original mounting clamps during assembly, a sharp cutter is also necessary.

1 ) Removing the main mirror cell

To install the primary mirror mask, it is necessary to remove the main mirror cell and place it on a clean surface. Be careful that you do not accidentally damage the primary mirror while removing the main mirror cell.

2) Removing the mounting clamps

The next step is to remove all three (six for the 10") mounting clamps from the primary mirror. To do this, unscrew the fixing screws one after the other and put them aside. Please make sure that the metal washers on the clamps do not fall onto the primary mirror after removing the last screw.

Now you can carefully check how the mirror sits in the cell. Just turn the mirror carefully. This should go easily and without noticeable resistance, and the mirror should not have any lateral gaps to the main mirror cell!

3) Shortening the mounting clamps

In the next step, we shorten the mounting clamps a little bit, since the mirror mask does not cover the full 5mm. Our aperture rings are dimensioned in such a way that the mounting clamps protrude far enough (approx. 2-3mm) onto the primary mirror after mounting the primary mirror mask to protect it from falling out. Compared to conventional mirror masks (which completely cover the original rubber clamps), your mirror has about 4-5% more effective surface (for a 6" telescope)

NOTE: This step can be skipped for some Skywatcher models! Before cutting the mounting clamps, please check whether they protrude below the primary mirror mask!!!

!!! Shortening is only necessary if the mounting clamps protrude below the primary mirror mask !!!

To determine how much needs to be cut off the mounting clamps, place the new mirror mask (flushing on the outside) on the mounting clamps and mark the overhang.

The next step is to cut off the overhang with a sharp cutter. Please be careful with your fingers! In most cases, about 2mm must be removed.

After shortening the mounting clamps, this is how it should look like from above! No overhanging rubber material should be visible anymore. If there is still some rubber sticking out, you can remove the rest with some 120 grit sandpaper.

(Please do not scratch the mirror mask, the anodized layer is only about 50-100µm thick!!!).

4) Mounting the mirror mask

The installation of the mirror mask is done in the reverse way. Due to the fact that the new aperture ring is made of EN-AW6061, a high-strength aluminum alloy, it is no longer necessary to mount the original metal washers. These are no longer needed!

First of all, place the shortened mounting clamps on the mirror as shown in the picture. Make sure that the holes in the mounting clamps are roughly aligned with the corresponding holes in the main mirror cell.

Place the mirror mask on the mounting clamps. The holes should also be aligned here.

Please make sure that the 2mm chamfer on the aperture ring points away from the primary mirror!

In the last step, loosely screw in the original screws.

Attention!!! Please do not tighten the screws!!!

5) Tighten the screws

You should carry out the next step carefully and precisely, because an only minimally too tightly tightened mounting clamp can quickly lead to mirror distortions in a primary mirror, which have a negative effects on the star image (stars become triangular).The following principle should be observed - "It is better to tighten the mounting clamps a little too loosely than too tightly!"

When screwing in the screws, it is advisable to push the mounting clamp upwards (in the direction of the aperture ring) with one hand to get a feel for how large the remaining gap is between the mirror and the mounting clamp.

If the gap is only small (like in the picture), push a piece of paper between the mirror and the mounting clamp.

Now turn both screws alternately, always a maximum of a quarter turn, and check whether the paper can still be moved.

The screws are correctly tightened when the paper can just be moved without force and without jamming and the mounting clamp still "feels" loose after removing the paper.

After successful installation of the mirror mask, the whole thing should look like this from above.

Now the main mirror cell can be reinstalled in the tube and the telescope can be collimated.

So, I hope the installation worked without any problems for you. If you have any questions regarding the mirror mask or the installation, please feel free to contact me.

Have fun with the new primary missor mask from Backyard Universe :-)

Clear skies,

Michael