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Telescopes: what can you see without CCD?

  1. Oct 14, 2007 #1
    My question concerns what deep field objects you can see by eye using a quality 8" or 10" Cassegrainian scope? I understand that you can get some nice photographs with long exposures, but what about just looking through the scope?

    Planets are a given, but what about spiral galaxies, as an example. Can you make out the spiral arms? Will the galaxy be tiny, or can you magnify enough such that it will occupy a reasonable portion of the field of view? I realize it depends on which galaxy you are viewing, but assume it is one of the easier ones to see.

    As an aside, I once built a 6" reflecting scope with wooden equatorial mount and was able to see the rings of Saturn, but this was in my youth and I didn't stick with it long enough to answer the questions above.

    Thanks for any help you can provide.
  2. jcsd
  3. Oct 14, 2007 #2
    depends some on the location
    in or even near a city NO and light polution has gotten worse lately
    also on weather clear and cold still air is best
    thats why the real big stuff is on mt peaks
    also a real good steady mount helps with higher power eye pieces
    and a clock drive so the object does not zip out of the limited view
    or learn to keep a dob light bucket on target
    as a 18'' dob = cost of a 10'' Cassegrainian but is a witch to move and track with
  4. Oct 15, 2007 #3
    Thank you for the reply, Ray. I completely understand that good conditions, a steady mount, and magnification are important, but it leaves my question unanswered.

    So let's assume that per my initial post, I use a high quality scope. Also that I have an excellent mount with a top-notch clock drive, and it is a clear night.

    The perfect answer to my question would be something like:

    "The best observable spiral galaxy at maximum practical magnification in an 8" scope would appear about the size of a 50-cent piece within a field the size of a compact disc. One would be able to see clearly the spiral arms".

    Or perhaps, this:

    "The best you could do without astrophotography would be to resolve a spiral as a blurry object the size of a dime within a field of view similar to the size of a compact disc".

    An answer such as one of these would give me a good picture of what to expect on a good night without too much light pollution with quality equipment.

    Thanks in advance to anyone who can help with this in an unambiguous way. I think it would be of help to anyone considering buying a scope.
  5. Oct 15, 2007 #4


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    The most relevant answer to your question is exemplified by sketches done at the eyepiece. This web-site shows the visual appearance of deep-sky objects in a small telescope.

    http://www.skyrover.net/ds/ [Broken]
    Last edited by a moderator: May 3, 2017
  6. Oct 15, 2007 #5
    Thank you Turbo, that is an excellent resource and pictures are much better than a verbal description. My only remaining confusion is about the type of scope he used, since the only description I see is a rather cryptic "telescope: n 250/1000". I googled it and was unable to identify it as a type of scope so I am left to wonder if it is a 3" refractor or a 14" Dobsonian, or something in-between. Any thoughts on this?
    Last edited by a moderator: May 3, 2017
  7. Oct 15, 2007 #6


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    I believe the "n" is a newtonian and the 250/1000 are aperture and focal length in mm.
  8. Oct 15, 2007 #7
    Turbo, you are the man! That makes perfect sense, and as a bonus places it right at the aperture range I was considering. Thanks for the great info.
  9. Oct 15, 2007 #8
    Good day dilletante,

    I think I understand your question because at one time I asked the same form of question.

    So, rather that relate to optical mathematics and formulation, I'll tell you what I can personally do.
    First I have a 90mm refractor, a 3 inch Newtonian, a 10 inch Newtonian and access to a 17 inch newton
    If we discuss DSOs (Deep Sky Objects) the 90mm and 3 inch will not be addressed.

    With the 10" (f5) in a low light pollution location on a excellent viewing night I can get most Messier object that are within my line of view at that time.
    The will not show detail as would be the case using a CCD or long exposure photography.
    As an example the Andromeda galaxy will appear as a bright nebulous object with very little definition. However, if the seeing conditions are excellent and I use averted vision I can just get a wispy hint of a dust lane.

    I can get an excellent view of the Ring Nebula.
    I can view the brighter stars in the Herclus cluster rather than just a foggy patch.
    However, as a rule nothing looks the same as a CCD image.

    Using the 17" is much like the 10 with the exception with the exception I can go to higher magnification and still have a half decent observation.
    Also using the 17" on a very good night I have no problem viewing dust lanes in Andromeda. Also going after dimmer NGC objects.

    I have the opportunity to once in a while view through a members 25" Newtonian.
    Needless to say there is no problem with dust lanes or detail in many of the objects.

    So, needless to say if one wants to observe deep sky stuff, size does count.
    The more aperture to work with, the better light grasping, separation and detail can be had.

    But light pollution, quality of mirror / lens, sky transperiency and f number also contrubute to a point.

    Hope is of some help.

  10. Oct 15, 2007 #9


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    You're welcome. I like that web site in part because of the "inverse" function that lets you go from black-on-white to white-on-black for a more realistic approximation of what you might see.
    Last edited: Oct 15, 2007
  11. Oct 15, 2007 #10
    Hi Ron,

    Thanks for the info, it makes me want to get a 25" ! Unfortunately I would like it to be somewhat portable so that doesn't sound practical. I do have a remote cabin about 8500 feet up in the mountains in Utah, so I should get some good viewing conditions. Unfortunately there is no astronomy club in the vicinity and my home town (Las Vegas) is about as bad as you can get for viewing.
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