Introduction
For deep sky and planetary astrophotography, the internal sidewalls of the various connecting tubes, adaptors, and optical components used before the camera, should be low in reflectivity. Reducing unwanted reflections from the tube sidewalls improves image contrast and helps eliminated spurious reflection artefacts, especially when the background sky is bright, or bright objects are just outside of the imaging field.
The internal sidewalls of commercial components can vary significantly in their degree of reflectivity. Usually, the walls are black anodised, often with finely pitched machined grooves (figure 1) and this texture helps reduce unwanted reflections. It does this increasing the number of reflections when light strikes it and also by creating local shadowing. Sometimes, however, the internal walls of optical tubes are smooth and ungrooved and just black anodised. This is much more reflective than grooved sidewalls. Furthermore, the type of black anodising can vary significantly, whether grooved or ungrooved – varying from satin to quite glossy. Unfortunately, this combination of ungrooved, gloss black, anodised sidewalls is not so rare and can be really quite problematic for sidewall reflections.
If you are lucky, though, the sidewalls of the new Barlow or tube extension you have just shelled out for will be grooved and will also have been painted with a matt black paint, which further reduces reflections improving the performance.
This article is for those interested in modifying their own optical components to help reduce sidewall reflectivity either by painting or adding matt black films.
Important Characteristics for Matt Black Paint
This article present the results of tests made on some commercially available matt black paints that might be suitable for applying to the internal walls of optical tubing to reduce glare and reflections.
The paints chosen are not intended to be a comprehensive list of all available matt black paints and my choices are based on paints available at the end of 2025. Since writing this article others might have come onto the market and some mentioned here might also have become unavailable.
Paints have been assessed for blackness from three different angles; face-on ie 90°, as well as from 45° viewing and shallow-angle 5° viewing. Blackness is important, but other characteristics assessed include ease of application of the paint, surface finish, robustness to handling and the effect of moisture.
Paints for Test
International Paint Matt Black
An oil-based matt black paint widely available from DIY stores
Humbrol No. 33 matt black enamel
An enamel matt black paint for model makers and widely available
Pro Acryl Coal Black acrylic Paint
A water-based acrylic paint aimed at model makers and bought online
Green Stuff World Maxx Darth acrylic paint
A water-based acrylic paint aimed at model makers and bought online
Culture Hustle Black 2.0
A water-based acrylic paint for artists by Stuart Semple, bought in 10ml bottle from eBay but available in larger volumes from Culture Hustle
Culture Hustle Black 3.0
A water-based acrylic paint for artists by Stuart Semple, bought in 10ml bottle from eBay but no longer available from Culture Hustle as it is superseded by 4.0
Culture Hustle Black 4.0
A water-based acrylic paint for artists by Stuart Semple, bought in 10ml bottle from eBay but available in larger volumes from Culture Hustle
Painting
To test the paint samples, test tiles were first prepared. These tiles were black anodised aluminium pieces approx. 53mm x 29mm in size, which were cleaned with acetone before being painted. Each paint was brushed onto two tiles with a soft brush and allowed to dry.
Assessment was made of the ease of application, how good the finish was, and how easy it was to clean up.
International
Easy to paint with soft brush but can see brush lines and clean up requires white spirit. Leave overnight to dry.
Humbrol No. 33
Easy to paint on with soft brush and a nice and even finish. Clean up requires white spirit. Leave overnight to dry.
Pro Acryl
Thin and easy to paint on with soft brush or roller and nice and even. Clean up just requires water. Takes overnight to dry and drying is significantly longer than the other water-based paints listed below. Does not take on the full matt finish until fully dry.
Maxx Darth
Quite thick and harder to apply than Pro Acryl by brush and get even finish. Can be applied with small roller to improve evenness of application. Clean up just requires water. Dry in an hour or so.
Black 2.0
Fairly thick but easy to paint on with soft brush and nice and even. Clean up just requires water. Dry in an hour or so
Black 3.0
Fairly thick and grainy rather than smooth. Hard to apply evenly and shows brush lines easily. Clean up just requires water. Dry in an hour or so. Note: see later (esp. figure 6) for difficulties in applying this material where the second coat dissolved the first coat leading to poor coating uniformity. I think Culture Hustel recommend painting 3.0 over 2.0 to make it even.
Black 4.0
Thicker than 2.0 or 3.0, but smooth and easy to paint on with soft brush and nice and even. Clean up just requires water. Dry in an hour or so.
Note on Black 3.0 and 4.0:
I am suspicious about the samples of 3.0 and 4.0 and that there might have been some mix up in the supply. This YouTube comparison suggests that 4.0 is grainy and is darker than 3.0 whereas I found it to be the other way round. This possible misidentification makes me nervous about any recommendations for Black 3.0 and 4.0.
Since writing this article I have found that others have tried these Culture Hustle paints for telescope parts and over time in damp conditions the black paint goes white – which is a bit rubbish. You can read more on Cloudy Nights.
Blackness
To assess the blackness of each paint, the arrangement shown below was used, with the different tiles being placed next to each other in a line in front of an A3 white LED light panel. Plain unpainted black anodised tiles were butted up against the far edge of each painted tile and acted as reference tiles.
Tiles were placed in order of blackness for the face-on (90°) viewing direction with tiles looking darker as you move from left to right.
Images were taken of the tiles illuminated in this way with the camera face-on to the tiles (ie 90°), at 45° to the tiles and at a shallow 5° to the tiles. Tile images taken at the different angles are shown below:
Blackness results are shown in the table below. Values given are the amount of light reflected from the black surface as a percentage of the light reflected from the white area of paper identification strip, sitting below the tiles. In both cases the brightness of the white strip and the brightness of the tile are measured from images taken at the same angle (90°, 45° or 5°). As the angle decreases the white paper becomes less bright so comparing results for one paint at different angles is not really meaningful. For example, many of the 45° samples have lower reflectivity values than at 90° but all are in fact more reflective at 45° than at 90° – my values are comparitive to the paper rather than absolute measurements of reflectivity.
Paint robustness and water-wipe resistance
Ideally, manually handling of the black surface would have no impact on the reflectivity of that surface. In reality, handling the surface can polish or bruise it and leave contaminants behind, all of which have an adverse impact on the blackness and leave it looking spoilt and uneven.
As a crude assessment of the film robustness and impact of handling, each sample was rubbed 3x down the centreline in the same direction using the firm pressure of the pad of a dry index finger. This mechanically polished the surface, affecting its reflectivity
A second test checked the impact of water on the dried paint surface by wiping the bottom edge of each painted tile with a damp cloth 3x in the same cross direction. After this, an assessment was made of the ease at which the paint had been wiped off the aluminium surface and how black the wiping cloth appeared. This test was done after a day of drying and again on a different area after 2 weeks of drying.
After these two tests, the samples were rephotographed from the 45° direction. The resulting images are presented below and show the clearly detrimental impact of the finger rubbing as well as the loss of paint from the wet cloth wipe test.
A subjective assessment of the results of the test are shown below where 0 is no impact and 5 severe impact:
Tubing Painting
As a final test of the paints two 2” diameter black anodised tubes were painted in sections with three of the paints, Pro Acryl, Max Darth and Black 3.0. A section of the tubing was left unpainted. One tube was coarsely grooved whilst another was mainly plain walled. The tubes were photographed with light flooding in from the rear with the same white LED panel that was seen in figure 2.
From left to right: Unpainted, Pro Acryl strip, Maxx Darth strip, then Black 3.0 strip
Light-Absorbing Films
After exploring different paints reported above. I decided to test some black light-absorbing films that were commercially available, and which might be used for this same application.
Aktar Velvet Metal
This is a black material deposited on an aluminium foil which is available with or without an adhesive coating on the back. You can buy the film in various forms from Edmund Optics
Aktar Lambertian Black
This is a black material deposited on stainless steel foil which is available with or without an adhesive coating on the back. You can buy the film from Edmund Optics
Black flocked self-adhesive PVC velour film
This ‘sticky-backed plastic’ material is available from many different suppliers from Amazon. My sample was an old Fablon brand. The film is made of numerous fine black hairs which are stuck vertically onto a backing film and is a very good light-trapping surface as a result, at all angles. Overall thickness was about 0.5mm.
Blackness of Film Samples
A similar set-up to that used for the black paints was used for the black films. Photos below show the three films on the right-hand side against a selection of the painted surfaces as reference.
As before, and using the white paper as reference again, I was able to measure the brightness of the film samples from the images taken. In the table below the top set is the same data as before just presented for reference with the new data on the film samples below.
As you can see, the flocked black velour is by far the best of the films. The Velvet Metal is good at 90°, but tails off significantly at shallow viewing angles. The black flocked material if fact is lower reflectivity than all the materials, paints and films, at all angles.
Film Robustness and water resistance
As for the paints, the same 3x finger rub was used to test the film robustness.
The photo below shows results after the rub test and show minimal effect for the Lambertian Black and the Metal Velvet both being better than any of the paints. The flocked material showed no effect at all. It is known, however, that the black flocked material can shed black fibres and some samples are worse than others. If you are going to use this material in a critical application, do vacuum clean the surface carefully before you apply it
Unsurprisingly, none of the film samples was affected by a water rub.
Recommedations
With no one material ticking all the boxes, the choice of which paint or film to use depends on the application and will often be a compromise of several factors. Despite this, I can make the following general recommendations.
Non-grooved tubing really does needs some sort of matt black finish and the best material for this is use well vacuum cleaned black flocked velour material. This should be measured to fit and cut to size before being carefully and progressively applied to the inner face of the tubing using the self-adhesive backing. Peel back just the first part of the liner and cut that off first, to allow careful positioning of the starting section, before the rest of the liner is removed. An alternative method is to make a sleeving tube out of thin plastic card that is black flocked on one side and which you can insert into the optical tube. This might be less fiddly to make than applying the film directly to the tubing. If you really want to paint non-grooved tubing then follow the recommendations below, erring towards the blacker Maxx Darth, unless it is going to see the damp.
For grooved tubing use Pro-Acryl coal black paint as this is easy to apply and reasonably black and is less prone to moisture deterioration than other paints. If you want something more permanent then choose the Humbrol 33. For finely grooved tubing, the Pro-Acryl might not be so dark so then plump for the Maxx Darth except in environments where damp might be an issue, in which case you should use the Pro-Acryl or the Humbrol 33.
Warnings
My recommendations are given in good faith based on my experience and my studies here. It is unlikely there will be problems if you are careful, but it is up to you to assure yourself that no harm will come to your optics.
For any painted components make sure the coating is completely dry before assembling optical components, especially if the walls are in a trapped volume. I would recommend that even water-based paints are dried in good ventilated conditions for several days if intended to be used in a closed component.
Be aware of the polishing by handling or rubbing of painted parts and also the dissolution by water in their maintenance. The only maintenance you should need to do will be blowing down with compressed air or using a brush to remove dust which should cause no issues.