By Paolo Sirtoli and Danilo Rossi
february 9th, 2025
In the workshop of the good Fritz, there was truly everything: you could find a CNC milling machine, a microscope from the 1930s, electronic circuits, airplane parts, binoculars, special glues, radioactive minerals, and so on.
The good Fritz, whose real name was Dino Colatore, worked at Aermacchi, the company from Varese that produces the jets for the Frecce Tricolori air patrol team. Besides his superb mechanical skills, he had an extraordinary ingenuity and curiosity. He was interested in practically everything, and his workshop was proof of that.
Dino was also interested in astronomy and had equipped himself with a remarkable arsenal: a Celestron C11, several refractors with respectable diameters, two sturdy mounts to support the telescopes, and numerous accessories of all kinds.
Unfortunately, shortly after reaching retirement, Fritz passed away at the end of 2022, and his son, Manfred Federico (German history was another Fritz’s great passion), found himself having to “thin out” that overcrowded space.
In a closet, there was a well-kept, huge black tube: a Newton Skywatcher 250 PDS, with a 250mm diameter and a focal length of 1200mm.
The tube had a few years on it: you could tell by some dents and various scratches.
Today, a telescope like this, in the age of astronomy 2.0, which favors small, fast refractors, risks being classified as a “piece of junk”:
- The tube is very heavy (14 kg) and slightly ovalized
- It’s full of light leaks: Skywatcher mounts the same focuser on all its Newtons, which have different diameters, so on the 250, it only fits above and below, leaving a wide open space on the sides
- The components are cheap (the secondary mirror holder is a thin sheet of metal, easily deformable, with all the problems it causes for maintaining collimation and consequently image quality)
- The primary mirror cell is not shielded from stray light
- The six mirror supports generate reflections on images, creating asymmetric halos around the stars
- The interior is painted matte, but with a grayish color that doesn’t absorb enough of the scattered light
- The optical design of the Newton, with the parabolic mirror, causes stars to become distorted radially at the edges of the field of view, turning them into tiny comets (hence the defect’s name, coma)
- The focuser is Crayford style, meaning it slides due to friction between the flattened part of an aluminum tube and a steel pin. This leads to issues when a 2 kg imaging system is hung from it instead of an eyepiece.
However, it must be considered that the quality-price ratio of Newtons is unbeatable: the simple construction allows for a telescope with a fairly long focal length and a generous diameter to collect a lot of light.
All of this at a fraction of the price of a refractor with the same characteristics!
Moreover, the optical quality of Skywatcher mirrors seems good, so restoring the “piece of junk” made sense.
Our guiding light for the restoration was Valerio Giardulli, an engineer and skilled amateur astronomer of rare availability. The restoration focused on the following areas:
- Matte-fying the interior of the tube
- New secondary mirror support
- Focuser overhaul
- Coma correction with a Skywatcher 0.9x corrector
- Masking the primary mirror
- Replacing the primary mirror springs with stiffer ones
- 3D printing of a dew shield
- Reduction of light leaks
- Final collimation.
For purely aesthetic purposes, to erase the scratches, the exterior of the tube was covered with a carbon-fiber-like film.
Here’s a description of the different stages of the restoration:
Eliminating stray lights
First, the telescope was completely disassembled and cleaned. The interior was covered with adhesive black velvet. The improvement was evident, as seen in the following photo, where only a strip remains to complete the work.
A strip of velvet was also applied to the inner edge of the telescope’s aperture.
Using a 3D printer, 14 baffles were created (these are rings used to eliminate scattered light and increase image contrast: they are often found in refractors and other high-end telescopes like Ritchey-Chrétien). Each baffle was equipped with three spacers to ensure uniform spacing during installation.
The internal diameter of the baffles isn’t constant but increases linearly from the diameter of the primary mirror mask (252mm) to the diameter of the secondary spider (269.55mm). This way, there’s a slight opening angle to avoid worsening the vignetting, which is already quite pronounced in Newtonian systems. One of the baffles covers an angle of only 270 degrees because it’s installed at the focuser.
To limit reflections as much as possible they were painted with black chalkboard paint:
- The edge and rear of the secondary mirror
- The outer surface of the focuser tube, limited to the portion that enters the telescope
- The inside of the focuser tube
- The baffles and primary mirror mask
To eliminate light leaks, black silicone was applied to the base of the focuser and the closure ring of the telescope’s aperture. Unfortunately, there are many places where light can leak through, such as the circular gap between the focuser’s moving tube and the main body. Additionally, the installation of an electronic focuser had created another entry point for stray light, so a fleece neck warmer was wrapped around it to solve the problem permanently.
To shield the primary mirror cell, a black cover was used, which doesn’t interfere with thermal exchange with the environment and allows the optics to acclimate.
The first night of testing the restored telescope had to be cut short due to condensation forming on the secondary mirror.
The solution was to build a dew shield to prevent cold, humid air from reaching the inside of the telescope. This also provides additional shielding from ambient light.
The dew shield was made with a 3D printer, internally covered with black velvet, and externally coated with Reflectix, a reflective material on both sides that also acts as thermal insulation.
Improving the mirrors supports
The excellent craftsmanship of the secondary mirror support (spider) is essential for maintaining collimation and achieving perfectly symmetric spikes in astrophotography.
Despite its high cost, a CNC-machined solid support was chosen, anodized in matte black, and the three collimation screws were replaced with Bob’s Knobs for easier adjustment.
As for the primary mirror, the original cell featured six rubber anchor blocks with a metal plate to secure everything. These six small rectangles that project onto the mirror’s surface create an annoying effect in photos: the stars appear surrounded by dark rays (in the following photo, there are three, but in our case, there were six).
To eliminate this issue, a simple ring mask was applied to the primary mirror, perfectly circularizing its effective aperture. This slightly reduces the light-gathering area but greatly improves the aesthetic quality of the images.
To improve the precision and, above all, the stability of the primary mirror’s collimation, the three original springs were replaced with stiffer ones (possibly too stiff) with an elastic constant of k = 20 kg/cm.
Final results
Here are a couple of photos taken after the upgrade:
For more photos (taken with other telescopes as well), you can visit our AstroBin profile.
Before and After Comparison
The first comparison is inevitably about star performance, so what better field than the Pleiades? Here we show a comparison of two 3-minute subframes (unedited): on the left, before the modifications; on the right, after the modifications. It’s clear that the stars now have symmetrical discs, no longer affected by the six supports of the primary mirror. The spikes are longer, thinner, and symmetrical.
To further compare the telescope’s performance before and after the modifications, we combined and processed 12 5-minute exposures of the NGC 281 nebula (known as Pacman) taken on the night of January 11, 2025, and 12 more taken on February 2, 2025, after the modifications.
Actually, the comparison is affected by the different lunar phases (92% and 19%, respectively) and the different rotation angles of the camera during the two nights. However, the processing parameters in Pixinsight were the same, except for histogram transformations, which don’t affect image resolution.
The difference between before (left) and after (right) is very noticeable, especially on the stars. Even the faint nebulae show more detail, likely due to better collimation.
