TUNING STEP 2: Lining a Tube with Velours

This tutorial shows how to line a tube with velours on the example of the tube of a Skywatcher Quattro 150.

What is the advantage of a velour tube liner?

Scattered light is one of the most common reasons for image errors and artifacts on astrophotographic images.

What is the reason for this?

Light has the fundamental behavior that it can only pass straight through homogeneous mediums (e.g. air or solids which are transparent to the wavelengths of light, such as glass). If light hits the interface of two different materials (e.g. air and glass), the incident light is refracted and/or reflected at the interface. We can simplify the whole thing a bit and look only at the interface between air and a solid because these are the two mediums which we have at a telescope.

Refraction: A certain amount of the incident light (red) is reflected (green) when it hits the interface between medium 1 and medium 2. The remaining light (blue) will be refracted at the interface and passes through medium 2 (glass in this example).

If medium 2 has a higher density than medium 1, the light is refracted towards the perpendicular (β < α).

If the light passes from the denser medium 2 into the thinner medium 1, the light is refracted away from the perpendicular (β > α).

NOTE: Without an antireflection coating on glass surfaces, 3.4% - 5.5% of the light will be reflected, depending on the angle of incidence (in the range of 0-50°). At an incident angle of 89° it is about 90%!!!

Specular reflection: This kind of reflection can be found on very smooth surfaces such as mirrors, but also on all other smooth surfaces, e.g. polished metals.

The incident light (red) is directly reflected (green) at the interface, without any scattering. The angle of incidence and the angle of reflection are identical in this case ( α = α').

Diffuse reflection: Non-directional reflection can be observed when the roughness of the surface is very high relative to the wavelength of the incident light. The reflected light rays (green) are reflected in all directions in this case.

For very fine surfaces with a high number of scattering sources, the scattering of light is usually no longer dependent on the angle of incidence and is perpendicular to the surface.

The first two effects shown are part of the principal of each Newtonian telescope. Without reflecting mirrors and refractive lenses, it would be very difficult to get a usable image on a CMOS sensor. Unfortunately, there are many other parts in telescopes which reflect light directionally, but also diffusely. In order to capture images without any optical disturbances, it is necessary to eliminate or minimize these sources of disturbance.

For large surfaces such as the tube, black velour is very effective for this purpose. For large surfaces such as the tube, black velour is very effective for this purpose. Velour is a generic name for a variety of velvety materials with a fine pile on a base material. The extremely small fibers of the pile partially absorb the incident light, but also ensure that the light is diffusely reflected between the fibers, which ultimately makes velour an extremely good light absorber. The Velour lining minimizes reflections (e.g. by grazing light incidence) on the primary mirror but also on other optical components. In particular, the disturbing influence of artificial light from oblique angles (street lamp, etc.) can be significantly reduced. There are also a number of colors (e.g. Black 2.0, Black 3.0, Musou Black and many more) used by hobby astronomers, but measurements have shown that especially Velour has the best anti-reflection effect and is particularly well suited for large and smooth surfaces.

Images (right - original tube Skywatcher Quattro 150P | left - same tube with velour lining)

1. Preparation

Before we can start lining the tube, it is necessary to prepare the velour for its use in the telescope. Due to the structure of velour (fine pile on base fabric), it is necessary to thoroughly remove loose fibers from the velour before gluing it into the tube. This step should definitely not be skipped, as the loose fibers will settle on the primary mirror over time and you permanently have to clean your primary mirror. If you remove loose fibers from the velour well, you won't have any problems!

Besides enough black velour, you will also need a common lint roller with a full roll of adhesive tape as well as a sharp scissors to cut the velour to the right size for your tube.

As a first step, cut the velour so that it fits to the tube of the telescope. I generally cut the velour with a small overlap of 5-10cm. This is important to create a perfect joint after sticking in the telescope.

For the tube of the Skywatcher Quattro 150P a 45cm wide sheet is quite sufficient. Since the tube is a bit longer, I will apply the sheet of velour in such a way that a small strip without velour remains in the area of the mirror cell (behind the primary mirror) and in the front area (in front of the secondary spider). However, these areas can be sticked with velour later.

For larger tubes, I recommend sticking the velor along the axis of the tube. This makes it easier to create a neat joint.

How to create such a neat joint will be explained in the next step, "Gluing in the velour".

To remove loose fibers and dirt from the velour, place the velour blank on a straight surface and spread it out.

TIP: Before you start rolling the velour with the lint roller, I recommend brushing the velour with a brush. This will remove many loose fibers before you start using the lint roller and the adhesive tape lasts a little longer!

Now you should thoroughly roll the velour in several directions. At the beginning you have to renew the adhesive tape quite often because it will be covered with a lot of fine black fibers after rolling over it a few times. These are exactly the fibers we have to remove!

After about 10-15 times of changing the adhesive tape, the amount of fibers that stick to the tape should decrease significantly. If so, it should be enough then...you've done it!!!

INFO: Since rolling with the lint roller also loosens "solid" fibers, it is completely natural that fibers still stick to the adhesive even after 15 film changes!

2. Gluing in the velour

To ensure that the velour sheet sticks well on the tube, you should clean/degrease it from the inside. I usually use a lint-free cotton cloth and some alcohol for this. Since most tubes are painted on the inside with a black anti-reflective color, I do not advise using thinner or acetone for cleaning!

In order to glue the velour without wrinkles and straight to the inside of the tube, it is advisable to draw a "gluing edge" (shown in red in the picture). The best way to do this is with a pencil and about 10cm from the tube fold (see picture).

NOTE: Please do not glue the velour too close to the tube fold, because it does not adhere very well in this area (only if you press it well around the fold) and may detach if the overlap is too small!

The next step is to roll up the velor as shown above. Now remove a 2-3cm wide strip of the protective film. I always use a ruler and cut the protective film with a cutter, so that I can remove the 2-3cm wide strip cleanly.

Finally, take the velour and glue it (i.e. the 2-3cm wide section at the beginning of the velour roll where you have just removed the protective film) straight onto the previously marked gluing edge.

After you have applied the velour straight along the marked gluing edge, remove about 5-10 cm of the protective foil and glue in another piece of velour - slowly and always apply pressure along the entire length. In this way, it is possible to glue the velour into the tube without wrinkles.

At the end, the velour should overlap by about 5-10cm.

The next step explains how to create a neat joint.

Creating a neat joint without overlapping

In principle, you could overlap the velour in the tube, but the overlapping area of the velour loses adhesion over time and can suddenly come loose. Of course we want to avoid this - it would be too bad to ruin a night's shooting for this reason!

If you have to glue several velour sheets next to each other, I recommend that you always overlap the material sufficiently (about 5-10cm).

To create a neat joint from an overlap, you will need a cutting guidance (long ruler or a straight piece of wood) and a very sharp cutter!

NOTE: The cutter should be really sharp! Best you break off a piece of blade and start with a fresh one!

Place the straight edge for guidance on the overlapping material and use the cutter to cut slowly through both layers of velour.

It is important that you fix the straightedge (or whatever you use as cutting guidance) well so that you get a clean cut!

After cutting the two layers of velour, you can remove the two loose pieces (purple) and press the remaining velour in place.

In this way you will get a clean and invisible joint.

3. Removing the velour from the holes in the tube

If you have made it to this point, the most difficult part - gluing - is done....Congratulations!!!

Now all you have to do is remove the velour from the holes and openings in the tube. This works best with a small cutter or a scalpel.

TIP: If you cut the holes from the inside of the tube you can cut along the edges of the tube without detaching the previously glued velour.

FINISHED!!! Your tube is now lined with velour

I am very interested in how you liked this tutorial and if it was helpful for you! I would appreciate if you could give me a short feedback about this tutorial via my Facebook page or via email to feedback@backyard-universe.de

Thanks and clear skies,

Michael from Backyard Universe

Next Steps

COMING SOON!!! Heater for secondary mirror

Tuning Step 3

COMING SOON!!! Blacken parts

Tuning Step 4