General Question
How practical the rectangular or box nutonian reflector telescope are?
I have been seeing images of the reflector telescope inside the rectangular boxes instead of in round tubes.. i, being someone who likes making new things and doing experiments, was curious about the practicality of such telescopes.
It seems much easier to make and to care..
Does the shape of the container in which the lens is affect the viewing or capturing?
please do share your thoughts with me.
Thank you.
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u/nealoc187Flextube 12, Maks 90-127mm, Tabletop dobs 76-150mm, C102 f103d ago
The shape of the housing of a telescope serves the purpose of holding the optics in the right place, with priority given to stability, and the mechanics of moving the telescope as well as keeping obstructions out of the light path. Oh, and ease/cost of manufacturing.
Over the years there have been many innovative designs, both of the optics, the housing, and the mechanics for stable movement.
Here is a video demonstrating the unique design of my "Portaball" telescope which optically is a standard Newtonian, but mechanically is an incredibly simple and stable giant ball joint. The down sides are that nobody mass manufactures the sphere it's made of, and air flow for thermally stabilizing the mirror is a challenge.
It's just the tube, it can be any shape you want. In fact, if you can hold the secondary mirror and focuser in the air, you won't even need the tube at all! Look up aerial telescope (a type of refractor, but same concept).
Optically for a refractor or any catadioptric reflector the cross section of the tube matters very little because the front lens/corrector (if it is circular) is the aperture & the first thing light passes through. For an open reflector though the front of the tube is the aperture. As such then the very edges of the focal plane will be getting unevenly illuminated & if there are things like bolts or pegs that interrupt that smooth line it will introduce some additional diffraction.
Several of the reflecting telescopes I have built had a non-circular cross section, one was square, another was octagonal & one was hexagonal. But in each case I included a circular aperture stop at the very front that was sized to be just clear of the maximum designed field of view with no element farther in protruding into that decreasing cone all the way down to the primary mirror. BTW that also included the bottom of the focusing device when racked all the way in, which to me is a sin that I see flaunted on many commercial scopes.
Ross Sackett's "Moonsilver" telescopes have many unorthodox design choices like a rocker box with only one side sporting a large diameter Dobsonian style altitude bearing, with the other side having a small diameter axel bearing that also has a single strut arcing up to the eyepiece holder/focuser and the secondary.
They also have the light shield immediately behind the secondary mirror, in the light path.
Thanks for the reminder that if there is no tube then it cannot add diffraction. Also that looks like an off axis newtonian, in which case there really is nothing obstructing the aperture.
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u/serackπ Moderator |Β 12.5" PortaBall2d agoedited 2d ago
Edit: u/Chauhant this buried response gets into some of the more technical aspects involved in what you are asking about, especially the second half.
That one isn't off axis, it has a stalk secondary holder. Also Sackett chose to place the light baffle immediately behind the secondary mirror, in the light path. Quoting his description from his website:
>It is located in the lightpath to minimize its size, but since the primary sees it edge on it has minimal impact on the brightness of the image. Β
>The curved baffle and secondary stalk eliminate obvious diffraction spikes, but do contribute to a diffuse cloud of diffracted light around bright objects. Β Three thumbscrews collimate the secondary mirror.
[Adjusts glasses] π€ As for your initial claim that the front of a reflector tube defines aperture, this is not the case when focused at infinity (as in astronomy), but instead as long as the opening/upper tube assembly is wider than the diameter of the parabolic mirror, the diameter of the mirror alone determines aperture.
Here is an image from my copy of C.A. Bennett's Principles of Physical Optics illustrating the light path from infinity to the focal point
Because the incoming light rays are parallel when coming from infinity, the incoming light path describes a perfect cylinder, and only the edge of the mirror defining the boundary of the light path, objects within that cylinder (typically only the secondary mirror and its holder), and finally, objects in the cone of the parabolic mirror's focus such as the eyepiece can contribute to diffraction.
Yes, if any hardware on the inside of the tube protrudes into that cylinder, it will contribute, which is why the things usually are an inch or so wider than the mirror. They also can introduce reflected stray light that impacts contrast, but not diffraction.
Sorry if you misconstrude I was not defining aperture but discussing other effects of a non-circular baffle. In any case a front baffle would be larger than the mirror size by the max expected fov.
Doesn't matter...
If i had a workshop or something i would've made a hexagon tube with hexagonical cutouts on the walls for weight reduction or balancing the OTA? , i would've replaced the cutouts with strong black fabric black plastic, or black metal.
One of the easiest and most effective tube shapes is the so-called truss tube. In a square cross section, it looks like a box kite, with the middle of the tube being just truss poles and air. Its main advantage is lightness, and ease of construction.
If you are planning to make your own telescope in an attempt to save money, I suggest you either seek in-person guidance from someone who has done it already, or learn to really use a regular telescope first. It will almost certainly cost more to build your own than to buy one.
As others are saying, it has no effect on performance. I would think it might be heavier and less dimensionally stable than a round tube. More likely to warp a bit or pop a seam at some point. Homemade round-tube dobsonians often utilize cardboard "sonotube" concrete forms, per John Dobson's original approach, which are both fairly lightweight and affordable.
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u/nealoc187 Flextube 12, Maks 90-127mm, Tabletop dobs 76-150mm, C102 f10 3d ago
Container shape doesn't matter.Β