Message posted by Gary Sellani on September 20, 2000 at 01:14:46 EST:

I agree that morning light is best, but I'm a wimp.;-) I rather not stay on the peak all night. Maybe sleeping in the SUV at the base would be a good compromise as it would save an hours drive.

I've been talking to a few telescope addicts about terrestrial observation. One of these addicts is an eyepiece vendor who is going to make a slight modification to one of his eyepieces to get it to fit my projection set up. He said you really shouldn't use a reflector much larger than the ETX90 (i.e. 3.5 inches) for terrestrial work. The explanation is complicated, but it has to do with the "column of air" that the scope has to see through. When looking at the horizon, you have the worst case condition of turbulent air. A larger mirror will gather more of this turbulent air, while the atmosphere will prevent the image from getting any sharper. [Of course if conditions are good, the larger reflector telescope will be better.] He said that the next step above a 3.5 inch reflector is a 5 inch or so refractor. These tend to be heavy because they have lenses instead of mirrors, and big because the optical path is not folded. I ran this past some astronomers I know, one who has an 8 inch Celestron SCT, and the other who is selling his 12 inch Meade reflector and going with a Takahashi refractor instead. Both agree the ultimate solution for this kind of "work" would be a good refractor.

Now this whole reflector/refractor nonsense may sound like arguing over white wine and red wine, which both get you drunk. However, those who want to study nebulas need light gathering capability because nebulas are dim, so they go for the large reflectors. Those who want to study planets go for the refractors because planets are relatively bright. A refractor gives a sharper image for a given size, but you have to be able to have enough light to see the image. Planets are bright compared to nebulas, so there is the difference. The hole in the middle of the front mirror on a CST (reflector) makes the image less sharp. I was told a 5 inch refractor and an eight inch CST have about the same resolution. Note that reflectors have no color abberation problems because the image does not pass through the glass but rather is only reflected. It took some modern lens making techniques to make a good refractors, which is why they are now a hot item in the scope business today.

Regarding my slow exposure time, it is due to the large magnification. I may get a simple 2x barlow, which would make the image brighter because you get less magnification than eyepiece projection. A 2x barlow does not fill the frame without vingetting, but you could turn the camera around so the image is vertical and let the parts where the light falls off go to non-critical parts of the image that you would probably chop off anyway. I think you have to get the exposure time down to at least 1/8 of a second so that you only get one thermal "flutter" in your exposure. These flutters always seem to be a few cycles per second at the most. Of course, the faster the exposure the less likely the wind will disturb the setup.

All that said, I think the next step may not be better optics, but rather a CCD imager. CCD's are very sensitive to infrared and much more sensitive to light than film. A CCD with an infrared filter has the advantage that it can see through the haze and you can monitor the results immediately on a notebook computer. Of course, all this is expensive, but with technology the price of CCD imaging will come down.

In Reply to: Re: Perfecting your telescope technique posted by Groom lake watcher on September 19, 2000 at 22:26:07 EST:

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