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Stargazing Planning to buy a first telescope?

  1. Jun 27, 2017 #81


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    How "nearly black"? Can you see anything through them? A solar filter is so dark you literally can't see anything but the sun or a bare filament on a clear light bulb. Anything more and it isn't safe for solar viewing.

    Also, the filters are almost always placed over the objective, where they intercept all of the light spread out instead of focused.
  2. Jun 27, 2017 #82

    jim hardy

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    good point, i tend to forget details.
    Think about the energy collected by the objective lens. Ever burn paper with a magnifying glass? Galileo wrecked his eyes.

    You don't want all the light gathered by the objective to get absorbed as heat in your little eyepiece, it'll likely crack.

    I've seen metal covers with just a little hole in the center to go over an objective lens , blocking probably more than 99% of the light.
  3. Jun 27, 2017 #83


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    It is pretty important to have the filter over the objective of a large diameter reflector (an expensive solution, unfortunately) If you try to get way with a high density filter at the eyepiece end, the power from the sun on the secondary reflector can overheat it. There are cheapish solar filter mylar films ( less than £20 for 200mm diameter) which are easy to mount on a cardboard support. You could do what I did and used a cheap round baking tin with the bottom cut out which fits over the end of the tube of an 8" Newtonian.
    But solar observations can be a big disappointment unless you use extremely expensive narrow band etalon filters. The are what you have to use if you want to see those very impressive pictures of solar features that people publish. But sunspots are quite impressive and well worth looking at on a big sharp image of the Sun.

    I have to give the statutory warning against ever ever trying to look directly at the Sun through a telescope. 10W of light, focussed on your retina will totally fry the nerves. So any filter must be fixed well and the observations have to be well supervised by a competent adult!!!
  4. Jun 27, 2017 #84


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    The resolution is poor then - back to the pinhole camera problem. The objective cover on Newtonians often has a 40mm (approx) hole for lunar viewing but I think the resolution could suffer a bit.
  5. Jun 29, 2017 #85
    So dark, that the only way I can see anything is to point it at the sun. I did it on a cloudy day. The only thing I could see was a circle of clouds, barely larger than the sun, moving past the circle. Once a cloud passed the bright sun, it was no longer visible. Looking at a light across the room did not allow me to see the light, but then, we use low wattage bulbs here. I did not try getting close to a lightbulb to test it.

    The whole idea of the eyepiece heating up makes complete sense to me. However, this is a small telescope, having a lens just about 2-1/2 inches in diameter. Still, you have me concerned, so next clear night when I am awake, and the moon is out, I will point it sky-ward, and see if this wasn't intended to be filtered for moonlight, rather than solar. Can I assume that if these eyepieces are lunar filtered that I will be able to see the moon well, and if they are not lunar filtered, that the moon will not be easy to view? I really don't know a better way to test them.

    Thank you for helping me care for my eyes - It would kinda suck to damage my (or anyone else's) vision, just because we wanted to check out the eclipse, up closer.
  6. Jun 29, 2017 #86


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    Standard practice for smallish solar scopes is to use what they call a Solar Wedge. It consists of a mirror at 45degrees with a dichroic reflecting surface which just reflects visible wavelengths into the eyepiece. The rest passes through and heats up a large heat sink, avoiding any localised high temperatures. A filter upstream of the eyepiece would be in a position where a fairly small diffuse image of the Sun would heat up a small area of it. It's only at the objective end that the power is easy to deal with.
    You can set fire to paper with 70mm lens!
  7. Jun 29, 2017 #87
    So if I understand you correctly....

    You think that the dark lenses I have, and that came with this (cheap, probably bought at K-Mart 20 or more years ago) telescope are most likely made for viewing the sun... or did I misunderstand?

    Some further description:

    My eyepieces fit into a palm-sized piece that makes a right angle between the scope and the viewer. (...Which fits into a barrel marked "2X", which then fits into the telescope...) The mirror inside looks like it bisects that angle, making two 135 degree angles where the edges of the tiny mirror are attached to the inside of the body of the angled piece. I can not see anything else remarkable about the piece, other than 2 tiny screws on the outside, on what looks like a removable panel, presumably designed for allowing access to service the mirror... I have not attempted to open it, yet, but I may have to, in order to get the mirror clean, as it has collected a lot of large dust particles.

    (Did I describe that well enough?...)

    ~ Thanks
  8. Jun 29, 2017 #88


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    As you cannot see anything else through them, I guess they must be, lol.
    It would be interesting to see if the filter gets warm. It may not matter for an inexpensive scope but thermal effects can really spoil the high image quality in a high quality scope. People say you should leave a telescope for some while when you take it outside on a cold night so that it can equalise the temperature all over and get back to good collimation. Heating it up from the Sun could spoil the picture - try experimenting and see if it gets worse after ten minutes or so.
    Your right angle viewer is useful, particularly for looking high in the sky. If you are not very careful with the mirror, you can easily scratch the weak reflecting surface and leave it worse than it is now. A bit of dust will only decrease the contrast but a scratch can leave every star with identifiable lines going across it (like windscreen wiper tracks and street lamps). Many people say you should stay well away from such tinkering. But, of course, care and skill are all you need to do any of those jobs. Google "Cleaning Telescope Mirrors" and there are several interesting videos.
    That 2X barrel sound like a Barlow lens, which doubles the magnification of your scope. Try with and without. I have a cheap one and I'm not sure that the bigger image actually improves matters - for a start, the image is dimmer. It all depends
  9. Jun 29, 2017 #89
    That post was chock full of good, useful information.

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