Message posted by lone wolf on January 30, 2005 at 12:43:41 PST:

I'm not a fan of reflector telescopes for terrestrial use. They have a few disadvantages. First, I find the focus tends to drift as the ambient temperature changes. Remember, you are photographing the base at first light, but at first light the temperature is changing rapidly. Second, reflectors tend to be both darker and not as well focused at the edges. Since you are making a panoramic, the quality of the image at the edges is important. Of course, you can crop more to make up for this. You can also buy "coma" correctors for reflector telescopes to make the edges sharper, but nothing can get rid of the light fall off. [You can fix the light fall off if you know what you are doing in photoshop.] Third, the central obstruction (mirror) reduces contrast, and contrast is very important when photographing over long distances. The 26 miles of atmosphere steals contrast. If you look at all the gear sold for "birders", most are using refractors. [Leica, Kowa, Swarovski, Brandon, etc. ]Birding is the classic terrestrial application, though in a sense we are birding too.

I haven't used one myself, but many people like the Orion ED80. To get better than ED type glass, you need to go for an APO, which costs at least double of not tripple. There is nothing like fluorite in your optics.

The reflectors have the advantage of being light and compact due to the folded optics. You will need to "barlow" more with a refractor telescope, that is, the focal length of reflectors is generally above 1000mm, while refractors that could be carried up Tikaboo will be more like 500mm to yabe 650mm. Incidentally, I've done some tests and have found that eyepeice projection (using an orthoscopic eyepiece) is more cost effective (in terms of bang for your buck) than using a barlow. For a small refractor telescope, I'd say 18mm to maybe 24mm orthoscopic eyepieces would do the job. You get quite a bit of magnification with eyepiece projection if you keep the distance from the eyepiece to the focal plane large.

There is a school of thought that under "poor seeing" conditions, the smaller aperture telescopes work better than the larger aperture scopes. However, you can only go so low on aperture without the light hitting the focal plane being too low for decent shutter speeds. That said, a refractor in the range of 3 to 4 inches would be optimal for Tikaboo. If money is no object, the Takahashi Sky 90 or Atrophysics Traveller would capture all there is to see from Tikaboo, I see Takahashi is running a sale on the FS-78 again, slightly smaller aperture, but probably just as good. [Of course, there is a waiting list again.] At a 1.5 arc second resolution for the scope, the atmosphere is probably the determining factor. At 1.5 arcseconds, you would get about 1ft resolution to Tikaboo under ideal conditions. The best Tikaboo panoramics just barely capture the telephone poles which are certainly bigger than a foot wide, so probably the atmosphere is the limit. Thus a 3 inch refractor will do the trick. [Remember, refractors have no central obstruction, so you really do get 3 inchs of aperture. For reflectors, you need to subtract the light loss due to the central obstruction to get the true aperture. The obstruction on the C5 is 1.75 inches, so the aperture is really more like 4.15 inches, and this is strictly related to light gathering. I'm not sure how the airy disc is degraded with the central obstruction.)

If you do go the reflector route, you should probably get a 1.4x teleconverter for your camera. This is because the "pipe" coming off the secondary mirror will not be large enough to fill the 35mm focal plane from a 5 inch reflector. Lomo makes reflectors for use in photography, but the sharpness suffers due to the larger obstruction. You would probably need an 8 inch reflector to fill the 35mm frame if used prime.

If you go the refractor route, you need to make sure the telescope can accept 2 inch eyepieces if you want to use it "prime", i.e. without a barlow or eyepiece projection. The 1.25 inch visual back will be too small to fill the 35mm frame. Again, you could use a 1.4x teleconverter to get around this, but it is better to have a telescope that will accept a 2 inch eyepiece. With a refractor, you will also need focuser extension tubes to get the system to focus at infinity. This is because the manufacturer assumes you will be using a diagonal plus eyepiece for viewing, and the diagonal has some length to it. The focuser extensions make up the length normally used in the diagonal.

The refractor set up would be something like:
telescope
extension tube(s)
2 inch visual back
prime focus adapter (plus filter)
3x or 4x barlow
T ring
camera

or
telescope
extension tube(s)
eyepiece projection fixutre
orthoscopic eyepiece (approx 20mm) plus filter
t ring
camera

This is a slightly cheaper solution than getting a good Televue powermate barlow. University Optics makes some reasonable quality orthoscopics. Takahashi makes two eyepieces just for use in eyepiece projection. You probably need one less extension tube since the eyepiece projection fixture adds to the optical length. I think eyepiece projection through an orthoscopic eyepiece gives you a flatter field than using the barlow.

The link shows the ED80 on a Bogen 410 geared tripod head. You need this head at the very least. Simple friction heads will slip under the weight of the telescope.

In Reply to: Re: How do Orion Telescopes measure up posted by Chuck Clark on January 30, 2005 at 8:00:45 PST:

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