Dοb 14'' διπλο!!!

[nik71]

Ωραια κατασκευη!!! και δυσκολη

 

http://www.astrosurf.com/eroyer/bino360/bino360_index_e.htm

[gregohks]

Ενδιαφέρουσα κατασκευή αλλά από ότι κατάλαβα παρατηρεί από "εμπρός",δηλαδή θα πρέπει να σηκώνεται συνεχώς για να κοιτάει μέσα από ένα telrad/finder το οποίο λογικά θα πρέπει να είναι από την απέναντι μεριά των εστιαστών.

[nik71]

Nαι θα παρατηρει απο μπροστα ως το σημειο που φτανει,και μετα θα πρεπει να βαζει καποιο σκαμνακι η μικρη σκαλα για να δει ως το ζενιθ του ουρανου αλλα το μεγαλο πλεονεκτημα που εχει αυτο το τηλεσκοπιο ειναι οτι δινει στο θεατη την διπλασια φωτοσυλλεκτικη αποδοση απο το κοινο 14αρι!!

[astrovox]

Πέρα από την πολύ ωραία κατασκευή, κρατάω από το άρθρο την ανάλυση που κάνει σχετικά με τη σύγκριση του δίδυμου τηλεσκοπίου και ενός μεγαλύτερου με δυοφθάλμιο. Μας είχε απασχολήσει σε παλαιότερη συζήτηση [ http://www.astrovox.gr/forum/viewtopic.php?t=17799 ] και για μένα είχε μείνει εκκρεμές. Κατασκευαστικά είναι πολλές οι μηχανικές προκλήσεις για ένα δίδυμο τηλεσκόπιο και που υπερτερεί σε σχέση με το μονό και ένα καλό δυοφθάλμιο; Παραθέτω το κομμάτι παρακάτω.

 

 

Why not choose a 20" (500mm) classical Dobsonian instead ?

 

That's a frequently asked question. It's true that a 14" binocular telescope has the same light gathering power than a 20" monocular telescope. So why choose the more complicated configuration of the binocular telescope ?

 

Fabrication

In my opinion, a binocular telescope is mor complicated from a mechanical point of view, but the optics are easier to make. In my case, I had to make two 14 inch mirrors. It takes more time than making a single one, but it is no more difficult. I can't say it was easy, but it's doable with some previous experience with an 8" mirror for exemple. If I had to make a 20" mirror, maybe I would still chasing the parabola. Moreover, if I wanted to get the same 1673mm focal length to keep the eyepiece low, I would have had to do a 20" f/3.3 with 33mm edge thickness (to keep the same mirror weight). This is something for very experienced opticians !

Of course the mechanical part of the telescope is more complicated, because it is necessary to provide additional adjustments for the parallelism of the OTAs, for the adjustment of inter-ocular separation, ... but I am convinced that these difficulties exist mostly because it is difficult to find information on the building of binocular telescopes (there are no books on this topic for example). It is possible to build this kind of telescopes with simple tools. I used only simple and inexpensive tools (the fanciest being a cheap drill press and a router) and easy to find materials (mostly plywood and aluminium).

 

Observing

I don't have a 20" telescope to compare the views, but a few years ago, I compared the sight through my 6" binocular and a 8" telescope with a binoviewer (both instruments have roughly the same light gathering power). In order to understand what happens, we need to study three cases.

 

At low power, the diameter of the primary mirror determines the smallest usable magnification. Let's say that the pupil size of the observer is 7 mm in the dark (this value can vary from one person to another, but that's not the point, we'll make the reasoning with a fixed observer). With 360mm, we can use a 50x magnification, whereas with the 500mm, the minimum magnification is about 70x (under that, light is wasted). So it is possible to get a wider field of view with the binocular telescope. Moreover, using a binoviewer can yeld vignetting (especially with inexpensive units and a fast focal ratio telescope). In addition, for a fixed magnification, the exit pupil is smaller with the binocular telescope, which reduces the eye imperfections. In practice, I can observe pinpoint star without my glasses at 130x (I have astigmatism). I couldn't do that with a 20" telescope and a binoviewer.

 

At medium powers, there are not a lot of differences. That's what I have found when comparing the 6" binocular and an 8" SCT. Maybe there are some subtle differences, but nothing really important.

 

At high magnification, the 20" telescope has an advantage because its resolving power is 1.4 times higher. That means it is possible to see smaller details on the moon or on planets. However, there is a particularity of the binocular telescope when the seeing is not perfect (most of the time unfortunately). In a monocular telescope with a binoviewer, both eyes receive exactly the same image : the light reaching the eye has traversed the same air layer in the atmosphere. That's not the case with a binocular telescope: each eye has its own image which has not been affected in the same way by the turbulent air. It's possible (but it's difficult to verify this) that at some times, an image is less affected by seeing efects than the other. In this case the brain can choose the details from one image or from the other. I tried this experiment several times. I watch the moon with only one eye for a few seconds. When I open the other eye, seeing seems suddenly a bit better. Is it reality or just an illusion ? hard to say for sure.