Posts Made By: Michael Connelley

Hello:

The answer is yes, it should be possible for an amateur to get good distances to nearby stars. What you're looking for is trigonometric parallax, i.e. a "wobble" in the location of a star due to the changing location of the earth through the year. This is different than proper motion, which is a steady motion of the star across the sky due to the different velocities (both speed and direction) of the star versus the solar system through the galaxy. If you plot the position of a star versus time, you'll get a helical or corkscrew curve, which is the sum of the parallax and the proper motion.
You also need to take into account the aberration of star light, which looks like parallax (i.e. it causes the location of a star to wobble with a 1 year period). This is due to the fact that the speed of the earth's orbit is a non-neligable fraction of the speed of light. It's just like if you're driving through rain, the rain looks like its coming down at an angle from in front of you when it's actually falling straight down. And then there are all of the instrumental affects, like any changes in the focal length of the scope with temperature, etc. If you're interested in this, the book "Parallax" is a great read, and is very well written. The CCD has made precision astrometry available to amateurs, so a mathematically inclined amateur with the right equipment who understands the problem should be able to get a good measurement.

Cheers
Mike Connelley

Hello:

A good reference on this design, and other related designs, is "Telescope Optics: Evaluation and Design" by Rutten & van Venrooij. The Rumak design is named after Rutten. This is a very good book, and well worth getting.

Getting to your second question, the answer is yes, you can use a regular focuser rather than a moving mirror. Actually, it's technically better to use a regular focuser than a moving mirror. This is because the design of the scope is only properly corrected for spherical aberration for one value for the separation between the primary and secondary mirror (at least in the case of a regular Cassegrain, for something like a Rumak, the separation from the corrector to the primary mirror also affects the off axis image quality). A moving primary mirror allows you to move the focal plane back and forth a lot, but at the cost of introducing a little spherical aberration. The advantage of the Rumak design is that you have better control of coma. A disadvantage in my mind, versus something like a Gregory Maksutov, is that when you make a Rumak you have to test and figure the tiny secondary mirror, which will only be 2" or so in diameter. A lot of the other parts, like the baffle tube and the secondary mirror cell, get quite small. If you want to make vented lens cells, then I think that a Gregory Maksutov would be easier to make. Or how about a classical cassegrain, where there's no problem with ventilation up front!

Cheers
Mike Connelley

Hello:

Not to rain on your parade, but you're not going to see surface details on Titan. Surface details on Titan are very hard to see even with adaptive optics on the largest telescopes in the world (having done it myself), and by observing in the infrared at specific wavelengths where TItan's atmosphere isn't completely opaque. Cassini has clearly resolved surface details by observing at the apropriate wavelengths and by being very close. Surface details on the Galilean satellites is possible to resolve, however. Those are probably the only satellites where amateurs have a shot at resolving surface features.

Cheers
Mike Connelley

Hello:

I have a 10" Celestron SCT made in 1967. It's f/13 and has a central obstruction of 30%. The image quality is excellent. While your design has a 2.6" secondary, when you add the size of the secondary mirror cell and baffle, the central obstruction will end up around 3".

It's been my opinion for some time that the central obstruction issue is way overblown. People consider the usual SCT as a non-planetary instrument since they have a 33% central obstruction. I think these same people would be quite surprised to find that HST also has a central obstruction around 33%, and I haven't heard any complaints about HST's planetary images.

Cheers
Mike Connelley

Hello:

This is an inherent problem with any equatorially mounted Newtonian. I had an 8" Newtonian on a GEM for a little while, and I set it up so that the eyepiece was pointing up when the scope was pointed north and the counter weight was closest to the ground. With this configuration, the eyepiece was easy to look through whenever the scope was pointed close to the meridian, which is the best time to look at objects. There were some annoying positions, such as for targets near the celestial equator that were about an hour from the meridian. In these cases, I had to get on a stool and stand over the mount and lean over the scope to look into the eyepiece. Tube rotation rings really are the way to go, unfortunately. We refractor and Cassegrain users just cheat and rotate the diagonal around, but Newtonian users have no such joy. Bummer dude.

Cheers
Mike Connelley

Hello:

This does seem odd. My own 12.5" cassegrain has a 4" central obstruction and it uses about 3" of the secondary. Did you take all of these measurements yourself? If the primary's f/ratio was closer to 4 then things would make more sense. Have you tried looking through it to make sure that the mirror separation is set to focus for infinity? RCs tend to have large secondaries because they're fast, and to illuminate a large field of view. From your measurements, this scope is around f/13, which is rather slow for an RC. My "grabbing at straws" guesses are:

1) Some one had a 6" mirror that would work and tossed it in.
2) They were really concerned about stray light. As an example, Hubble Space Telescope only needs a 12" secondary mirror but the central obstruction is 24" to more effectively block stray light.
3) One of the measurements is wrong.

Cheers
Mike Connelley

Hello:

I've often wondered about this myself, and asked my optometrist about it a while ago (I vaguely recall that her answer was that the brain is smart enough to ignore it). The only way that your eye wouldn't have chromatic aberration is if a) the cones on your retina are in different planes so that the cones for the different colors are all in focus or b) the refractive properties of the lens in your eye is such that your eye is like a doublet lens. In the eye, most of the refraction happens at the surface of the cornea, so you (and I) basically have a single lens refractor that we're all looking through. So, you'd expect to have a lot of chromatic aberration. As mentioned above, I think the best answer is that your eye does have chromatic aberration, but that your brain can ignore it most of the time. Also, in the day time, your pupil is small enough that the chromatic aberration is minimized. There are a few occasions when I think I've noticed the chromatic aberration. For example, when I've look at blue LED lights on the front of a computer, I see them surrounded by a large halo that I have not noticed around red or green LEDs. Also, I've noticed that I have a much harder time reading blue text on a black background (even if the font is very large) whereas reading red text on a black background is quite easy. The density of red cones on the fovea is much greater than the density of blue cones, which makes sense if the image quality in your eye is best in red light.

Cheers
Mike Connelley