Build a Ritchey-Chretien: Premises

Fig. 1 – Render and section of a hypothetical RC telescope

Yes, You can do this (*) by manually working the optics and you don't need special equipment or more sophisticated than what we normally use for the construction of a parabolic mirror but, Let us say right away, to embark on such an adventure you must necessarily have more working behind, to understand how to generate two hyperbolic, You must understand the construction of a parabolic surface.

At that point, Paradoxically, We realize that achieving good hyperbole ( for the primary ) may prove to be even easier than building a good parable but would be enormously difficult and extremely hard-working, try to do it without ever having addressed the normal construction of the parabola.
So from now on we will give to all necessary concepts and techniques acquired for manual work and a parabolic mirror in short focal length, avoiding to dwell on these main article as again we meet along the way, not much to tell the truth, but of fundamental importance for the understanding and success of the project.

We summarize, for the convenience of the reader, What subjects besides the fundamentals “Classics” for a parabolic mirror ( Rough grind, grinding, polishing, parabolizzazione, Ronchi test and Foucault ), are conducive to this process and see the discussion pages:

BECAUSE’ A RITCHEY-CHRETIEN

May be many valid reasons for choosing this lens construction, We're not here to list them, any fan of optics for astronomy , know the pros and cons of this configuration, but the appearance that can stimulate a grattavetro is undoubtedly that of the challenge,  the measure their skills with an important test and out of the ordinary, He did not fault tolerant, that mercilessly unforgiving   the slightest inaccuracy of construction, both the perspective that the instrument as a whole, but at the same time allows with great generosity to enter more fully into the world of mirrors,   so that we can try to understand even the most hidden secrets.

However, the extreme optical precision required for these configurations, could discourage even the most willing take a similar path so, We will try to provide some solutions, find an alternate that would make more accessible to achieve a good result and, as we shall see, the auto-builder has greatly benefited compared to a professional production of optics bound to certain construction parameters and, with a little’ of “opportunism”, We can also benefit from the mistakes that inevitably will be committed during processing, amplifying the range of tolerances for the verification of the optical system.

WHAT DIAMETERS ?

We are dealing with two mirrors, closely related in their respective resolutions, distances and deformation by certain mathematical equations and, contrary to what you may think, a small mirror is harder to work manually compared to a medium-sized, so we have to make sure that the secondary does not fall below the ten to twelve centimeters in diameter, it a process extremely complicated and uncertain outcome.

Primary schema RC 30 cm, provides a secondary between 10 and 13 cm ( Depending on the overall focal, back-focus and other parameters that we will see below ), Therefore the primary mirror of reference will be exactly 30 cm.

PRIMARY DRILLED OR UNDRILLED ?

Fig. 2 – The mirror before and after drilling.

You can opt for both solutions, the center hole does not prevent us from working the mirror with the usual techniques,  the hole allows the classic configuration and observing the back-focus, While the mirror undrilled can be used in configuring nasmyth, Obviously with the addition of a third mirror diagonal floor.

If you therefore maniacs comfort ( and Collimation ) Choose the mirror intact and the diagonal floor, otherwise you can drill the glass ASAP ( with the foresight to use the piece of glass cut as “Cap” for the hole, during the blocking-  ) or you can drill holes in the mirror once you reach the required depth,  after using the last fine grain grinding time, with regards to the “Cap” ( process that I have personally taken ).  Anyway, generous obstruction of an RC allows us to drill the hole and not to cure us of deformations in the vicinity of it given the much smaller hood hole compared to the secondary.

In these articles we will describe the processing procedures in the case of a mirror drilled which can easily be extended to a mirror intact.

THE SECONDARY

The element “new” compared to building a Newton is obviously the secondary mirror convex and hyperbolic.   also and especially for the secondary must have acquired all the usual processing techniques, to which are added the other necessary for the processing of the convex mirror, but in this case, the processing can be addressed for the first time here.  

A rule that we can define ASAP ( We will see then why ) tells us that In a Cassegrain is the secondary that is built on the primary, It means that we will have to worry about reach in case of an RC, a Hyperbola of the primary that tests the optical diagram, Although it will be slightly different   against the schema of the project, but once “fixed” the primary, It will be the secondary that you “adapt” to it, so we will always have the opportunity to touch up the secondary ( We can't afford   all errors that we want and fix them ) until it fits perfectly to the primary as a way to review one of the countless   RC optical patterns, even if it were to differ from the original project.

THE METHOD

Fig. 3 – Setup of a Cassegrain configuration possible with software “Atmos”

As we said, One advantage of auto-  constructor is to not be constrained to dimensions and tolerances concerning the host telescope mirrors, because the tool does not yet exist, will be built and tailored to the mirrors obtained.

This, as we shall see, will greatly simplify the whole process because often you find yourself to have generated some good hyperbole for the primary, sometimes even “randomly”( but for the secondary ) Whereas, however, differs from that of project and its use would result in changes to all system parameters ( distance between the mirrors. overall focal, backfocus, etc ). But this is not a problem for the DIYer , is a resource, means that the project of the telescope will be postponed until completion of mirrors and processing, If you reach a good hyperbole for the primary, I'll use that even if it were to differ ( within certain limits ) Conic constant values than budgeted.
It will mean that calculates a new configuration to test the optical scheme and will be recalculated accordingly all new parameters for the secondary.

a practical example

Anyone who has worked a parabolic mirror knows that on approach to satellite dish, a little’ for luck and a little’ by capacity, you find yourself to have generated excellent surfaces, which however differ from the parable for a small conical constant value, in practice the measures generated curve “Enter” completely in the zone of tolerance whereby, Although the trend of surface is strictly from Conic curve, the mirror is not parabolic and then cannot be used to the purpose.

From this point of view, a Hyperbola can be more tolerant of the parabola, as well as the value of K = -1 ( parabolic ), We are already in hyperbolic and field,   for a certain range of values, There is always a secondary that check the schema RC, If during processing we happen to come across a Hyperbola “perfect” with K other than the project, We don't have to do anything but make the software what secondary check the schema RC. If the resulting secondary is not “exaggerated” and there appear to be feasible, can we stop in primary processing.

Different would be having to reach with centesimal precision conical constant value of project, a long and without any guarantee of success. But with this method, What we seek is to construct a Hyperbola as even as possible, accurately measure the value of the Conic constant attained and adapt the pattern to it.

With this in mind we start from a primary “perfect mirror” for the project.

PLANNING

Then we can define a schedule for the construction of our Ritchey-Chretien this way:

  1. calculation of the optical system and of construction parameters for primary and secondary circuits with optical design software
  2. primary working towards achieving Hyperbola with the constant project conical
  3. primary testing and verification software. If the Hyperbola measured is regular with K different from project   but compatible with the schema, we stop and move on to the next step
  4. optimization of the optical system with the new Conic constant measured.
  5. construction of the secondary in function of the parameters of the optical system optimized.
  6. audits/secondary measures obtained and further check compatibility  . Optimization of the optical system with real values for both of the mirrors .
  7. definitive parameters calculation ( as a function of mirrors made ) to design the telescope structure.

AUTO-BUILD: it is’ MORE   SIMPLE

In General, optical Rc system tolerances are virtually equal to zero. The simplification that we are going to get compared to a feature “rigorous” in which both the mirrors must be made with absolute precision is therefore this project parameters  :

1 a hyperbolic primary and, If different from the project,  you “play” secondary parameters to adapt to it.

2- Similarly, the convex secondary and, If different from the design values, Yes “play” mechanical parameters (back-focus, distance between the mirrors, overall focal ) until you find the suitable configuration and functional scheme.

Like this, with a little’ of luck, it inverts the constructive approach, will the scheme to fit the mirrors made of   and not the other way around.
Then,  even if we fail to reach the estimated values, We will have the opportunity to change their configuration and find the relevant project for which mirrors are “perfect” in their real parameters, an added bonus !

in the second part we will begin to enter into the details of the prior statements,We will see how to properly design   our Ritchey-Chretien optical design software and how to get started “play” with possible configurations.

(*) These articles concerning the construction of Ritchey Chretien summed up the experience of construction took place between December 2014 and August 2016 in the implementation of the RC telescope 300 F 7.5 whose detailed chronology of the different stages can be found in the dedicated discussions: (1) (2) (3)

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