Autocostruttori

domenica, agosto 30, 2009

Yet another method for "cooling" mirrors

Amateur astronomers know that mirror telescopes often exhibit poor contrast and resolution compared to refractor telescopes. Alan Adler, in his article Thermal management in newtonian telescopes (Sky and Telescope, Jan 2002) pointed out that "the problem" is born in a thin layer in front of the mirror, which is the boundary convective layer forming on the mirror face if it is warmer than air (see also Bryan Greer, Sky and Telescope, Sep. 2000).
What happens is that mirrors store a lot of heat and, when used in cooler environments, transmit heat to the air in front of them, like the bottom of a pan. The worm air forms an unstable layer which forms columns of warm raising air interleaved with columns of descending cooler air. This structure is called "convective cells". The mirror is topped by a bad "air lens" of irregular thickness and temperature and the neat effect is a wavefront error which happens on the characteristic scale of the size of the convective cells.
One can even see the cells by defocusing a lot a star image. It is better to look at intrafocal images, because extrafocal images tend to focus on high layers in the atmosphere, and show also the effect of atmospheric seeing. An example is here. Defocused images of convective cells take on the aspect of "spider webs" sometimes according to an hexagonal arrangement. Steve Khoeler has been able to reproduce the aspects of defocused images assuming that the wavefront error spectral density peaks at the spatial frequency which is characteristic of the convective cell sizes. Here is his work. One can even judge the severity of the bad boundary layer from the pictures. Note how the in-focus images look different from the theoretical images caused by the kolmogorov spectral distribution of the atmosphere. Here there are spikes and large halos, which often are reported by observers.

In realizing that the convective boundary layer is responsible for most of the "seeing" problems of mirror telescopes, Alan Adler also found the means to cure it.
Of course, if the mirror is at the air temperature, no boundary layer is formed. Thus, letting the mirror to cool down may be a method. However, large telescopes come with thick mirrrors, whcih can be so slow to cool that they actually cannot follow the variations in the air temperature during the night and thus are never cooled. Heat exchange can be improved, for example, by means of fans blowing onto the mirror (either sides) and this is often done.
Adler however used a different approach which consists into blowing air across the mirror face to wipe the boundary layer away.
The solution presented here is somewhat similar, with some differences. The most important difference is that the air remains laminar on the mirror face. The aim is to gently "extract" the warm layer as soon as it is formed and/or to keep it at minimum thickness and a regular shape. Alan also tested this solution, but he preferred the fan blowing across the face. I think this solution cal also work, provided that a properly formed suction annulus on the mirror edge is made (see below).

The first figure (click figures to enlarge) shows a fan attached to the rear of a mirror box. Except for the fan there is no other exit from the box. The fan is attached to the box by means of velcro strips (however elastic strips could be better, should one notice vibrations). A rechargeable battery is also attached to the mirror box back, by means of velcro again.

In front of the mirror there is a diaphragm, which lets only an annular section for air intake (figure 2 and 3). Thus the final effect is that air is extracted all around the mirror edge. As the air moves towards regions with lower pressure the flow remains stable and laminar (conversely, blowing air onto the mirror face produces turbulent flow). The system is not very effective in cooling the mirror. However it does not aim at cooling the glass: it aims at controlling the convective layer. In practice I have found that it is useable from the very first minute. Turning the system on and off I can see different grades of the "spider web phenomenon" (see Steve Khoeler's article). Gradually, the mirror cools (some amount of cooling effect is obtained because the warm layer is continuously stripped away) and in one hour or two I can see further improvements in the image quality on planets. I quickly reach the limits of the atmosphere, and the scope behaves "refractor-like" (but a 400 mm refractor!).


I carried out a quick Computational Fluid Dymanic analysis, shown in the last figure. The streamlines show a rather regular flow descending on the mirror face, which "squeezes" the boundary layer and extracts it from the mirror edge. The colors represents the flow speed. Here the volume flow rate is approximately 0.04 cubic meters per second (~80 cfm) which produces a speed of the air descending in front of the mirror which approximately amounts to 0.2-0.3 m/s.
I have yet to optimize the syetem.

4 Comments:

  • Ciao Mauri, non ho la tua mail, ti scrivo qui.
    Sono Massimo, puspo sui vari forum.
    Sto realizzando il sistema di estrazione dello strato limite sul mio 16". Ho fatto due interessanti chiacchere con Massimo Boetto circa il dimensionamento e la tecnica. Trovi un topic sull'altro forum. Massimo mi diceva che tu hai abbandonato il sistema a flusso laminare, come quello che sto realizzando io, perchè ti creava astigmatismo per l'effetto "ala d'aereo". Hai voglia di parlarmene, sono curioso.
    Puoi scrivermi anche su massimo@bresciaraduno.it, come preferisci.

    Ciao

    Massimo

    By Anonymous Puspo, at 5 luglio 2010 23:16  

  • An interesting approach, though different from mine. Your method may be laminar on the mirror front. I have had great success with blowing directly at the mirror with the turbulent fan. The turbulence mixes the air in front of the mirror and homogenizes it. Thus eliminating the harmful refraction and yielding sharp images.
    Sincerely yours,
    Alan Adler

    By Blogger AlanAdler, at 16 settembre 2010 17:14  

  • Thank you for this very insightful article on how to effectively fight the boundary layer on a larger reflector telescope. I am considering precisely the approach you described in your article: place a baffle closely above the top of the primary (the baffle will also provide the lateral support hardware for my mirror), and add a fan at the bottom of my mirror cell.

    Best regards,

    Olivier

    By Blogger Olivier Biot, at 18 ottobre 2010 09:42  

  • Great information to read. Thanks for sharing. I’ve learned a lot from your blog so far. Thanks

    By Anonymous Air Duct Cleaning Coral Gables, at 23 febbraio 2013 11:47  

Posta un commento

<< Home