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Stargazing Best compromise for portable telescope?

  1. Aug 26, 2010 #1
    so, for best resolving power, the objective lens of a telescope needs to be as large as possible, and have the fewest optical elements. for portability, the physical size and weight of the system needs to be as small as possible. within these two disparate parameters, what do you consider the best compromise for a portable telescope setup?

    a 6 or 8" newtonian is affordable, and quite a capable instrument in good seeing conditions, but no where the equal of larger 10-14" scopes, and even a 6" newtonian is quite cumbersome to lug around and setup. a dobsonian mount is simpler but quite limiting for anything but simple viewing.

    refractors can be pretty compact and yeild nice imagery if you can live with short focal lengths, and spend many money for nice quality.

    folded optical systems offer long focal lengths in a compact system, but are expensive. a 5" cassegrain is fairly portable, but 5" is pretty minimal for quality resolution. an 8" cassegrain is quite large and heavy, though fairly common amongst avid amateurs in the field.

    so - what do you suggest for a general viewing instrument, capable of good planetary viewing as well as medium wide field viewing, capable of long exposure astro-photography, yet still portable and easy to set up for one person? thanks for any thoughts.
     
  2. jcsd
  3. Aug 27, 2010 #2

    Chronos

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    Cat's are pricey, but, portable for the aperature. The tube on my 8" SC scope is less than 2 feet long and weighs about 30 lbs. It is not difficult to manage - even for a skinny old man. That is, however, about as big as any average person would care to wrestle. The OTA on a 10" SC is much bigger and heavier - and compounded by the fact it needs a beefy [and pricey] mount to handle the load. Most SC's are f/10 - perfect for planetary viewing. A focal reducer is cheap [I paid less than $50 for mine] and works great if you want wide field views.
     
    Last edited: Aug 27, 2010
  4. Aug 27, 2010 #3

    russ_watters

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    jnorman, you understand the tradeoffs pretty well, so there isn't much I can add. Ultimately it is up to you to decide how to score/rank the pros and cons.
     
  5. Aug 27, 2010 #4

    turbo

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    jnorman, I am pretty sold on apochromatic refractors, but they get pricey fast once you get above maybe 3-4" aperture.

    Do you have a large astronomy club near you? If they organize star-parties, you should arrange to attend one or two, and get there early. That way you can get an idea of the transportation and set-up issues of a wide range of instruments. Everybody wants that "sweet spot" of manageable size and weight AND great optical performance, but everybody would rank the importance of those qualities (and others) differently as they figure out just which qualities they are willing to compromise on. (As Russ mentioned.) For instance, the introduction of more truss-tubed Dobsonian models (Orion has several available already) has resulted in many folks getting into larger-aperture Newtonians than they might have considered otherwise. Trussed Dobs' can be handled in parts, so that weight and bulk is a bit less of a problem during assembly and tear-down. They also take up a lot less room in your vehicle than solid-tubed scopes, making them an option for people with more limited transport/storage capability. Good luck, whatever you choose, but I'd strongly suggest getting to at least one or two large star-parties, so you can get a taste for what's out there. You might be surprised what qualities you will compromise once you get views through others' scopes.
     
  6. Aug 27, 2010 #5

    mheslep

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    Is it practical to perform astronomical interferometry with smallish amateur scopes? If so then perhaps it makes sense to haul a couple small, inexpensive scopes around and configure them as an interferometer to achieve the resolution of a much smaller scope? I imagine the various manufacturers would already be encouraging this if it was practical.

    http://en.wikipedia.org/wiki/Astronomical_interferometer
     
  7. Sep 6, 2010 #6
    I got tired of lugging my 10" dob around if I wanted to do any astronomy outside my yard, so right now I'm working on building a little 6" truss dob. I have designed it so besides the truss segments, everything including lenses and my binoculars will fit in a box about 10"x10"x20". Its not to hard to scale designs like this up to a bit of a bigger size and still be easy to transport. Google is littered with plans and designs for transportable telescopes. They aren't incredibly hard to build if you have access to basic woodworking tools and you'll learn a lot designing and building it. If you don't want to build one, I want to say the smallest truss dobs I've seen for sale are at least 10" so if you want something smaller than that I'm not sure if you could buy it. I might be wrong though.

    +1 on going to some star parties and local events though. Its basically guaranteed that there will be a wealth of knowledge there on all types of telescopes, home built and otherwise.
     
  8. Sep 6, 2010 #7

    russ_watters

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    I would love to do that if it is, but have never seen people doing it with amateur equipment.
     
  9. Sep 6, 2010 #8

    mheslep

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    arg, of a much LARGER scope ...
     
  10. Sep 6, 2010 #9

    turbo

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    We knew. Right now, arrays are mostly used for WASP applications, not interferometry. Projects like the LBT can effectively use interferometry to improve resolution. With adaptive optics to further reduce atmospheric interference (seeing), who know what we might be able to see from ground-based instruments?
     
  11. Dec 14, 2010 #10
    This is something my group is working on developing. We named our project “Macho Mengi” – it means “many eyes” in Swahili. Macho Mengi will be a portable optical interferometer that’s used to produce images of space objects (stars, galaxies, … etc.).

    This process is not as easy as it first sounds. That is because astronomical (optical) interferometers do not actually “see” the same way a single telescope would. They produce images by combining the actual waves of light coming from multiple pairs of telescopes. These interferometers don’t actually view colors - to produce multicolor images this process is done for each select wavelength at a time using filters. This light is focused on a sensor that records the levels of light and coverts that into data. This data is crunched using algorithms involving Fourier equations (way above my head now) and imaging software to produce an image. There are many challenges to this process like ground vibration, weather, and the air/humidity itself to name a few. However, scientist have been successful with building very large units.


    Macho Mengi is our R&D testbed for understanding interferometry in order to develop a Multi-Satellite Space telescope.

    I have read a paper about a way to do direct imaging interferometry - if it works it may simplify things for the optics, but I'm not sure if it works well for our Multi-Satellite Telescope formations.

    Do a search for “Macho Mengi” or “Seal of Valor” and you will find us.
     
    Last edited: Dec 14, 2010
  12. Dec 15, 2010 #11

    mheslep

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    Thanks for that note and welcome to PF. So given this is difficult proposition, it is important to know the advantages. I'm curious if there is a simple expression for the gain in resolution based on N images?
     
  13. Dec 16, 2010 #12
    I’m still learning about interferometry so I’m not sure if this answers your question –
    There is an expression for determining the angular resolution (R) of a single telescope and telescope array.

    Single telescope: R= λ/D

    Telescope array: R= λ/B

    λ = wavelength of the observed radiation
    D = diameter of the telescope's mirror or lens.
    B = length of separation of the telescopes in the array (baseline).

    From the expressions the resolution is the same if you use one telescope that has a diameter of 2m or a telescope array with two telescopes separated by 2m. The difference is that a single mirror 2m in diameter has more surface area to collect more light than two smaller telescopes separated by 2m – so the single telescopes image will be clearer or more crisp.
    The advantage with using two or more telescopes is say let’s try 200m. It’s very difficult to create a telescope 200m in diameter – but much easier to make smaller ones and separate a few pairs by 200m and a few at 150m and 50m to simulate the surface area of a larger mirror.
     
  14. Dec 16, 2010 #13

    mheslep

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    Yes according to Wiki. Interestingly discussing the diffraction limit is suggestive of why we do not see more attempts at amateur interferometry: amateur telescopes are limited by atmospherics well before they reach diffraction limits. I am still curious if multiple scopes could help with atmospherics, perhaps by averaging out the distortion.
     
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