Both models have an aperture of 127 mm and a focal length of 952 mm. In lens labeling, the new model differs only in the additionally printed model designation. Only by closer inspection you can see that the tube of the new model FCD-100 is 15mm shorter. This is a great advantage, because my intrafocal area is very short on my own model. Therefore, in my model, orthoscopic eyepieces (e.g. measuring eyepieces) cannot be used if the extension is provided with an extension sleeve. With my wide-angle eyepieces the extension sleeve always comes to commitment.

3. Objective lenses

For observation I prefer to use wide-angle eyepieces. For almost 2 years my Nagler range has also been supplemented by eyepieces in the 82° series by Explore Scientific. For the telescope comparison the Explore Scientific 2-inch eyepiece with 24 mm focal length, as well as two Explore Scientific 1 ¼ inch eyepieces, with 8.8 and 4.7 mm focal length were used.

4. Explore Scientific 0.7 focal reducer and corrector

of 64 mm (2.5 inches). To connect to Canon cameras, a special T2 adapter must be used. The adapter ring is only 1 mm thick and therefore made of steel. He has a free opening of 36.7 mm.

What about the comparison at high magnifications? Inadequate color correction of refractors is shown by blue color fringes around bright stars. To test this, the choice fell on Bellatrix, the "warrior" in Orion. This star is 1.6 mag bright, bluish giant star of the spectral class B2. At moderate magnification with the 8.8 mm eyepiece (108x), neither the CD01 nor the successor FCD-100 showed any fringing. At high magnification with the 4.7 mm eyepiece (203x), diffraction rings around Bellatrix were clearly visible, but no fringing. However, at this magnification already the atmospheric dispersion was noticeable, as Bellatrix only reaches a height of 46 ° in the meridional passage. The atmospheric dispersion is shown by blue color in the upper area and red color in the lower area of the star. However, it is an atmospheric effect and not due to the telescope optics.

In the constellation Orion there are also numerous binary stars, which can be used to test the resolution. For example, 32 Ori (STF 728) could not be resolved at a distance of about 1.2 arcseconds. For 52 Ori (STF 795), which has a spacing of only 1.0 arcseconds, both components could be detected but not completely separated. The reason for this is in different contrast: the components of 32 Ori have a brightness difference of 1.3 magnitudes, while the components of 52 Ori are equally bright. The observation of Alnitak (STF 774), the eastern belt star of Orion was also interesting. With a distance of at least 4.7 arc seconds, this double star should be easy to separate, despite its brightness difference of 1.8 magnitudes. At 203x magnification Alnitak was also clearly to separate, however the B-component could only be seen at second glance in the diffraction rings. With the 8.8 mm eyepiece Alnitak was inseparable. Of course, with this type of observation, astronomical seeing plays a crucial role. Unfortunately, lunar observation was only possible with good astronomical seeing. At all other tests the astronomical seeing was not very good. In conclusion, the difference between the two models FCD01 and FCD-100 is hardly noticeable in visual observations.

6. Photographic tests

a) Diffraction slices in the primary focus

To get a better overview of the color difference between the two models, the defocused diffraction discs were first used recorded in the primary focus at 952 mm focal length (Figure 5). For this a QHY 5L II-C color CMOS camera was used. The images were taken at a distance of -2.2, -1.1, 0, +1.1 and + 2.2 mm from the focus. The light source was Bellatrix.

Overall, my model FCD01 showed slightly more color differences between the intra- and extra-focal diffraction slices. In the intrafocal area it appears slightly greenish, while in the extrafocal area it shines reddish. The successor model FCD-100 shows no color differences between the intra- and extra-focal diffraction disks.

a) Astrophotography with the Explore Scientific 0.7x focal reducer

The photos in this review were taken with a modified Canon EOS 1100D. It can also be used for the absorption of hydrogen regions due to the lack of red filter. For the recordings, the camera was connected directly to a laptop via a USB cable and operated exclusively via the Canon "long-distance shot" menu. The focus was set on the live image in 10x zoom mode. Exposure time and sensitivity were also controlled via the laptop. The pictures could be viewed directly after the picture was taken on the screen. The pictures were taken with an exposure time of 30 seconds at 800 ASA. To reduce noise, flat and dark images were also created.

With my FCD01 model, the sharpness can be easily adjusted by the color of the diffraction disk. As soon as the color becomes minimal, or changes from a slight green cast to reddish, the perfect focus is found. This method always worked well and led to quick results. In which Successor model FCD-100, this method could not be applied due to the color purity. Therefore it took some practice to find the right focus. A self-made focusing aid has proven to be helpful. The focusing aid consisted of a ring that was placed on the dew cap. Attached to the ring was a thin ridge that ran through the center, creating a one-dimensional diffraction pattern in the image. Based on the diffraction image was the adjustment of sharpness.

The focusing aid was removed again for photography and bright stars in Orion were once again chosen as test objects. Figure 7 shows Bellatrix (spectral type B2) and Betelgeuse (spectral type M2) taken with the FCD01 and the FCD-100. Exposing time was 5 × 30 seconds and corresponding flat pictures were taken. The different colors are shown very nice in both models. Due to the red sensitivity of the camera, a correspondingly large halo appears around Betelgeuse.

Für For deep sky shots of extended objects not only the color but also the marginal image is of interest. Figure 8 therefore shows the complete picture of Betelgeuse taken with the FCD-100. The inlays show the star pictures in two opposite corners. The 0.7x Focal Reducer levels the image over the entire APS-C format of the camera. Very red stars in the border area are no longer 100% displayed due to dispersion phenomena. The strength of this effect depends on the spectral type of the star and of course on the red sensitivity of the camera and can be observed only at full image resolution (here 4273 pixels x 2848 pixels).

On the basis of the flat images, the vignetting of the two telescopes could also be determined very easily. The vignetting is shown by the edge darkening of the image field. In both models, the brightness drops to around 77% of the center brightness in the outermost corners. This corresponds to about half an aperture. Figure 6 shows another shot of the Orion Nebula taken with the FCD-100. At the time of shooting, the sky was already lit by the half moon. The colors of the stars are rendered beautifully. There was no color correction during the image process, only the tonal values have been changed. In the predecessor model FCD01, the stars appear on deep sky shots usually with a slight green tint, which is easy to correct in subsequent image editing.

7. Conclusion