Secondary Spider - Notes and assembly instructions

Razor-sharp and thin spikes - that' s promised by the CNC machined secondary spider from BACKYARD UNIVERSE!

Spikes at stars are a characteristic and inherent part of a Newtonian telescope. The reason for the spikes can be found in the nature of light, which is diffracted at geometrical edges. In general, diffraction is the deviation of light from straight propagation behind geometrical objects, which are not caused by reflection or refraction. Diffraction occurs at all corners and edges in the light path, regardless of their shape.

But the shape of an edge has a significant influence on the resulting diffraction pattern. The straight struts of the secondary spider are the reason for the characteristic spikes at stars, but also an focuser that protrudes into the tube can cause spikes at stars with its straight edges for the light.

The light from a star is diffracted at each edge of the secondary spider. The diffraction pattern of straight edges appears orthogonal (i.e., perpendicular) to the edge and constructively overlaps to produce the characteristic diffraction pattern on the star (spikes).

General remarks, notes

Inaccurately aligned struts (e.g. struts of sheet metal secondary spiders) become visible in the diffraction pattern (misaligned spikes)! Since the superposition of the individual diffraction patterns is no longer accurate, the spikes appear blurred and are often spread.

On the two images above you can see the image of a star made with an original Skywatcher 200PDS (left), as well as the simulation of the diffraction image of a not properly aligned secondary (right). As you can see, in both cases the spikes spread out a bit. The alignment of the sheet metal struts can be very time consuming and in most cases not permanently stable (especially if the telescope is regularly installed and dismantled), which makes a re-adjustment necessary.

This problem is solved 100% with the CNC milled spider from BACKYARD UNIVERSE. Due to the precise manufacturing on modern CNC machines, the struts are aligned absolutely precisely and the diffraction is close to the theoretical optimum! This guarantees needle-fine and razor-sharp spikes but also a significant improvement of mechanical stability.

GENERAL NOTE: Due to the precise manufacturing of the secondary spider and the exact superposition of the individual diffraction patterns, any deviations in the focus (especially with faster Newtons f/4 and faster) as well as a focuser that protrudes deep into the tube, and to which the secondary mirror spider was not properly aligned during installation, will become visible! This is not a defect or failure of the secondary spider but a proof of the extremely good manufacturing and the significantly improved imaging properties of the telescope.

In the figures below, three typical spike misrepresentations are shown.

Very long diffraction spikes on bright stars and spikes which suddenly appear in the middle of the image

Short answer: Reflection!

Especially at very bright stars the spikes, which normally consist of diffracted light, become much longer by reflection compared to what would be theoretically possible by diffraction! Sometimes "ghost spikes" may appear in the center of the image (see image on the right).

The reason for this is the light of very bright stars, which is reflected at the struts of the secondary mirror spider under grazing incidence of light. It does not matter whether the spider is made of sheet metal or machined aluminum. The more precisely the spider is made (aligned), the finer and straighter become these "reflection spikes". Most paints (except special anti-reflective paints) and anodized surfaces have a high reflectivity under grazing light!

The following images clearly show that the reflection depends on the position of the star and becomes more visible the further the star is located at the edge of the image field or if it has already left it. In the center of the image field the reflected part of the light is very weak and overlaps with the diffracted light. As long as the star is in the field of view (FOV), reflections can appear.

Elimination of reflections

Light reflections at grazing angles of incidence (Angle of incidence <2-3°) can generally only be eliminated effectively with rough surfaces. A simple but very effective method is to apply velour to the struts of the secondary mirror spider. It is sufficient to cover only the straight part of the struts (see figure).

We are currently working on a new, optionally available coating that prevents reflections as good as possible due to its special surface structure.

We recommend to apply self-adhesive velour to the spider struts in order to avoid reflections!!!

Mounting Instructions

In order to prevent the primary mirror from being damaged by falling parts during installation, we recommend to align the telescope horizontally on a mount or to place it on a workbench/table for removing the old and the installation of the new secondary spider. If both is not possible, we recommend to remove the main mirror cell!

For the removal of the original secondary spider and for the installation of the new, CNC milled secondary spider, the following tools are needed

1) Medium phillips screwdriver for removing the end ring and the secondary mirror.

2) 4 mm Allen key for mounting the new secondary spider in the tube and for the installation of the secondary mirror with the included M5 countersunk screw

3) 3mm Allen key for the collimation of the telescope with the included stud screws

4) Caliper gauge

1 ) Removing the end ring

In order to be able to install the new secondary spider, the end ring must be removed. In the case of the Skywacher 200PDS, the end ring is fixed with six M5 screws (cross head) to the tube.

On the smaller Skywatcher models like the 150PDS or 130PDS, the end ring is fixed with only four screws.

Remove all six (four) screws and pull off the end ring.

2) Removing the secondary

Before we remove the old sheet metal secondary spider, we first remove the secondary mirror. In principle, this step can be skipped and the secondary mirror can be removed from the old secondary spider after removal! But due to the fact that the old sheet metal spider is not very stable and there is a high risk that the secondary will be damaged, we recommend to remove it first.

NOTE: Removing the secondary mirror first can be difficult with the smaller telescope models (especially the 130PDS)! In this case, the old spider may have to be removed together with the secondary!

3) Removing the old secondary

Removing the old secondary spider is also very easy. All you have to do is completely unscrew the four knurled nuts and remove the spider from the front.

Once you have removed the sheet metal secondary spider, you might notice how unstable such a spider actually is!

4) Installation of the CNC machined secondary spider

For practical reasons we would like to mention that it might be useful, especially for the smaller Skywatcher models (150PDS/Quattro, 130PDS), to mount the secondary mirror on the new secondary spider first, before mounting the new secondary spider in the tube!

How to mount the secondary mirror or the new secondary spider is described in Section 5: Mounting the secondary mirror

GENERAL NOTE: If you have lined the tube with velour or maybe coated the inside with an anti-reflective paint, it might be difficult to get the secondary spider into the tube. Our secondary spiders are manufactured so that they are not too loose even in the original tubes. Usually, the secondary spider can be installed without much force, even if the tubes are lined with velours! However, since the inner diameter of the tubes may vary slightly due to manufacturing tolerances, it becomes may be necessary to remove the velour in some cases!

Please do not use a hammer or similar to knock the spider into the tube! If you have to knock after all, make sure to put a piece of wood underneath and knock only in the area of the outer ring! Never on the struts!!!

4.1 Alignment of the spider in the tube

When installing the secondary spider, please make sure that it is correctly aligned in the tube and that the front and rear side are not mixed up.

4) After placing the new secondary spider into the tube and aligning it as described above, the mounting holes in the secondary spider should roughly be centered in the mounting holes (of the old spider) in the tube.

Now loosely screw in the four mounting screws. Do not tighten them! Before you tighten the screws, you should align the spider parallel to the end of the tube.

NOTE: Due to tolerances in the manufacturing of such tubes, it is possible that the holes of the spider are not centered in all holes of the tube! Do not worry about this!

5) Mounting the secondary mirror

To mount the secondary mirror again, you have to instert the included screw (M5x30mm) into the central hole of the secondary spider. After that, slide the compression spring over the mounting screw from behind.

At this point, you should also screw in the stud screws for collimation later.

The next step is to mount the secondary mirror holder to the secondary spider using the fixing screw. Please make sure that the compression spring does not slip off the screw and is between the secondary spider and the secondary mirror holder.

NOTE: If you mount the secondary mirror before inserting the secondary spider into the tube, please pay attention to the alignment of the secondary mirror relativ to the spider (see Section 4.1: Alignment of the spider in the tube).

OPTIONAL: The included stainless steel washer can optionally be glued to the secondary mirror holder with a thin layer of silicone. This has the effect that the adjustment screws for collimation no longer damage the soft aluminum material of the secondary mirror holder, which improves the collimation properties permanently.

For better adhesion, the secondary mirror holder should be roughened slightly and any bumps should be smoothed out before gluing the washer with silicon on it. We also recommended to degrease the surface and the washer with some alcohol before gluing!

YOU made it!!! Now you can collimate your telescope as usual...

I hope that you had no problems with the installation of the CNC machined secondary spider. If you have any questions about the Secondary Spider or the installation, please feel free to contact me.

Have fun with your new secondary spider from Backyard Universe:-)

Clear skies,

Michael