It will soon be 100 years since George W. Ritchey and Henri Chrétien built their first telescope in the configuration later named after them. For a long time, Ritchey-Chrétien telescopes were available in large sizes and used mainly by professionals, later by wealthy amateurs. The Hubble Telescope, the VLT in Chile, and many other modern telescopes used in observatories are of this type. And about 10 years ago smaller and more affordable constructions became available, even as small as 6 inches.
The RC telescope consists of two hyperbolic mirrors placed at the proper distance from each other. Such a system eliminates a large part of the optical aberrations – we do not have chromatic aberration, we do not have coma, and keeping the appropriate distance between the mirrors (is essential!) eliminates spherical aberration. Some field curvature remains and slight astigmatism. The curvature of the field is not that big, and in practice, the smallest 6-inch RC can give a flat image on a 533 sensor camera, 8-inch will support the popular ASI1600 or QHY163 with a flat image, 10-inch will already give a nice field on the APSC size sensor, and 12-inch should give a satisfactory image on full-frame camera format. The disadvantages of this design include quite a large central obstruction, which lowers the resolution to some extent, and a bit of unusual collimation – but more on that later.
As I have never had a telescope in this system, I chose the 8-inch model TS-Optics 8 “Ritchey-Chrétien Pro RC Telescope 203/1624 at the beginning. This is of course the GSO construction branded by a whole lot of sellers. I decided to save 500EUR and I have chosen a model with a metal tube 🙂 There are good reasons to have larger telescopes of this type – central obstruction is less annoying, the flat image circle is larger, but the price rises quickly, and the large Ritchey-Chrétien telescopes become bulky. 10 inches instrument weights already 16kg.
The collimation of RC telescopes is not trivial, and many have failed in this process. On the CN or SGL forums, you can find a lot of threads describing various types of problems that could not be solved with laser collimators for several hundred euros. The source of many of these failures is the assumption that collimation on the telescope’s mechanical elements is identical to the optical collimation, and this is not always the case. Generally, the Cheshire eyepiece is enough to coarse the collimation of the RC telescope, and of course, we make the final collimation with the stars. It should also be noticed that the telescopes of this type became available for amateurs not so long time ago, and it took some time to get familiar with this construction and how to handle it.
I have done a lot of readings before the telescope finally arrived in my observing shed. There are a few important general aspects that I noticed:
- the distance between the mirrors is important. Until we know how to set it correctly, we base it on the distance given by the factory and do not move the central secondary mirror screw, and also do not move one selected pair of primary mirror collimation screws (e.g. the one under the focuser), so as not to change this distance
- in budget RC telescopes from GSO, the focuser is integrated with the main mirror cell. When collimating the main mirror, we also move the focuser tilt and this operation success is based on the assumption that the axis of the focuser in the factory was aligned with the axis of the primary mirror. This can be checked with a laser by removing the entire primary mirror cell from the tube. This is described, for example, here http://www.hnsky.org/RC_collimation.htm – Step 1, and if someone is not afraid of such an operation, it is definitely worth checking at the beginning.
- collimate carefully, no more than 1/4 turn of the screw at a time, preferably 1/8, and make the final corrections very small. RC optics are very prone to collimation and a quarter-turn of the screw can completely ruin the image
The collimation itself is described in many different ways on many pages. I liked the method described by Tommy Nawratil at http://interferometer-tests.blogspot.com/2013/06/2542000-10-ritchey-chretien-gso.html. First, we set the secondary mirror in line with the focuser tube. Then we set the main mirror looking at the secondary mirror spiders and their reflection. The whole process is iterative because changing the position of one mirror usually makes it necessary to correct the other. But the process converges quickly and after 2-3 rounds we have a pre-collimated RC telescope on the desk, which then has to be adjusted with the stars.
This is what the picture from the desk looks like – the center of the secondary mirror is in the middle of the focuser tube, and the spider’s arms are aligned with their reflection:
Then we do the final adjustments with the defocused star image. We adjust only the primary mirror during this step and we must keep the star image in the frame center.
After these steps, we should have star images in the frame corners either also pinpoint (if we have a small sensor in the camera), or symmetrically distorted by the field curvature. If there is no symmetry and we are confident for the secondary mirror setting, then our focuser tube may be not aligned with the primary mirror optical axis. Some RC telescopes (like GSO truss models) have independent focuser adjustment screws. If we do not have that, we may need to add a tilt adapter between the optical tube and focuser or replace the focuser with the one that may be adjusted itself (like Baader Steeltrack or Diamondtrack).
If we want to use a larger camera and get rid of the field curvature, we have to get a corrector or a corrector with a reducer, or a larger RC telescope, that will have smaller field curvature 🙂 There are several dedicated correctors for the Ritchey-Chretien telescopes, but there are also many reports on the use of field flatteners dedicated for the refractors. I tested my 0.8x adjustable flattener and reducer designed for 70-100mm aperture triplets, and the results on the APSC size sensor were very good.
The image above shows the “Hall of mirrors” effect that can be a sensitive indicator of both mirrors’ parallel positions.
As I mentioned before the distance between the mirrors is also important. The correct distance minimizes the spherical aberration of the Ritchey-Chretien telescope. It also affects the field curvature and focal length. The 1mm change in the distance between mirrors can alter the telescope focal length by about 10mm. So it is a good idea to check the factory default distance by plate solving the star field image and calculating the exact focal length. For my RC8 telescope, it was 1619mm – so very close to the 1624mm value in telescope specifications.
If we discover the spherical aberration in our RC telescope (either with the star test, or using the Ronchi eyepiece), then the distance between mirrors may require adjustments.
Two images above were captured using an ASI290MM camera with the IR pass 685nm filter at the native 1620mm focal length when the Moon was about 30 degrees above the horizon. Despite this low altitude the amount of detail is quite good.
The image below is a 30 x 1 minute stack of the M27 planetary nebula imaged with a QHY268M camera with an APS-C size sensor. The field curvature is visible in the image corners. I estimate the flat image circle to be 22-25mm in diameter for that telescope.
Another image is M3 globular cluster. That is again a 30×1 minute stack captured with a QHY268M camera – this time it is a crop resized to 50% of the original size. The stars are resolved quite well, but the native 1600mm focal length is probably too much for a 3.8um pixel size camera, so I plan to adapt some focal reducer with a flattener for imaging with this telescope.
And the last first lights are the M57 Ring nebula and M51 Whirlpool galaxy. M57 was imaged with LRGB filters and QHY268M camera – that is 90 minutes of total exposure time with 1-minute subframes. Little bit cropped, so the most elongated stars in the corners were cut out. M51 is 70 minutes of luminance in 1-minute subframes.
I am quite happy with this telescope’s performance so far. The collimation takes some time, but the image quality is as good as I expected. The good news is also, that you do not need a sophisticated and expensive laser-pattern collimator to achieve good collimation. A little bit of patience with a systematic approach will lead you directly to proper adjustments.
Clear skies!
Some sources I found useful:
http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf
http://www.hnsky.org/RC_collimation.htm
https://deepspaceplace.com/gso8rccollimate.php
http://interferometer-tests.blogspot.com/2013/06/2542000-10-ritchey-chretien-gso.html
https://www.astronomics.com/wp/wp-content/uploads/2017/04/astro-techastro-tech-at8rc-manual-pdf.pdf
