Amateur stargazing (21_5_14)

I did some stargazing tonight after a long time. Throughout the summer, it has been cloudy and hazy. However, tonight was a clear sky with little haze. I took my 120mm diameter 1000mm focal length reflector telescope out to my office terrace, as it has the best view. I have a single plossal zoom eyepiece that I use for all my stargazing. It is a Seben zoom eyepiece, 7.5-22.5mm focal length.

I started out by aligning my finder scope to the optical tube. This is a very important step as it is quite difficult to find any celestial object without using a finder scope. To align the finder scope, I first locate the moon and get it in the telescope's field of view. This is much easier than locating point sources such as stars. I then adjust the alignment screws on the finderscope to bring the moon to the centre of the finderscope's field of view. Alignment complete.

Tonight, the moon was a beautiful waxing cresent (always reminds me of Cheshire the cat). The interface between the bright and the dark part of the moon, known as the terminator, exaggerates all relief features on the moon's surface by casting long shadows. To a person standing near the terminator on the moon, it would be sunrise or sunset. Craters, hills and cliffs look best near the terminator and make you feel like your telescope has very high resolution. It's always a delight to look at moon's craters with high magnification. I start off with 22.5 mm, centre the moon in the field of view and then zoom to the desired level. When you zoom using the seben eyepiece, you have to adjust the focuser to bring the image to a sharp focus.

Next I looked at Venus. Venus is the second brightest object in the night sky and is thus easy to spot. However, it is too bright to be viewed directly. It only appears as a very bright white disk/oval like shape. Venus is currently in the Gemini constellation. Tonight the Venus was in half phase and thus appeared as an oval. Even after zooming to 133x Venus was too bright and I could not spot any details. Next time I will use a filter to reduce the brightness when viewing Venus.

Next I observed Jupiter and its moons. Jupiter is currently is between Leo and Gemini. Jupiter is fun to observe and even with an average telescope, you can see 2 bands on jupiter and four of its moons (Io, Europa, Ganymede and Callisto). These four moons usually appear in-line because of their plane of orbit. Tonight all four were on the same side, with Io nearly on top of Jupiter. Sometimes, you can see the shadow cast by one of jupiter's moons on its surface! I could see 3 more very faint moons, but sometimes its hard to say whether I imagined it or if it was really there.

I then looked at Saturn, near the Scorpio constellation. Saturn is probably the best planet to look at. Its rings are very clear and about 4-5 times its diameter. It appears yellowish in colour. With my telescope I could see 2 of Saturn's moons. One directly in line with its rings and one perpendicular to them. Cassini's division is hard to see, but as conditions were favourable tonight I was able to see it. At high magnifications, it is hard to focus saturn as it appears blurry anyway. Also, my Seben eyepiece shows plenty of chromatic aberrations as I increase the zoom.

I spent quite some time looking for M4 globular cluster, near Scorpio. Usually it is easy to find, but since Scorpio's altitude was very low, I could not locate it.

Finally I looked at M5 globular cluster, in the Serpens constellation. Star clusters are fun to spot as they present a challenge. Light pollution in cities combined with the diffused glow of star clusters force you to use averted vision to observe them. The retina has lower concentration of rod cells(light sensing) at its center point(fovea) as compared to the surrounding region. Thus when you directly stare at a very dim object, you may not spot it, whereas staring somewhere near the dim object, suddenly brings it to light. However, this technique makes looking for dim objects very hard. But, M5 is a relatively easy star cluster to spot as it has a moderately bright star very close to it. So once you spot the star, you see the cluster using averted vision. Through my telescope, M5 appears as a greyish haze. It actually comprises of thousands of very faint stars concentrated in a small region. Thus, it is not very appealing visually, but it is very photogenic.

professional pic of M5 globular cluster

My viewing session was around 1.5 hours long. When I'm done I close the optical tube with my solar filter, to prevent dust settling on the mirrors.

Goto and tracking function will help me not only view but also take pictures of these celestial objects. I'm hoping to finish upgrading my goto mount in time before monsoon sets in.

Until next time,
Clear skies!

My attempts at spin casting a mirror

Telescopic mirrors are parabolic(almost concave) reflectors whose main aim is to gather as much light as possible. The light incident on the mirror is focused by the mirror's curvature at a single point on its axis. This is the focus of the mirror.
Telescopic mirrors are typically made of glass as it has a low coefficient of thermal expansion. This means that during temperature variations, the shape and exact curvature of the mirror would not be affected.

Telescopic mirrors need to be very precise and are thus ground to precision. This is a very lengthy and time-consuming process. Making mirrors manually, by grinding it to a finish is even more so. It took me 22 hours of grinding and 4 hours of polishing to make a small 4" mirror at IUCAA. So, I wanted to make another mirror, but better this time and also in an innovative way. The ideal shape for the primary mirror of a reflector telescope is parabolic. So I thought of spin casting the mirror.

The main idea in spin casting is to heat the substance till it melts, place it on a rotating platform and spin it as it cools slowly to give it the desired curvature.
A simplified model for this is as shown

In this simplified model when steady state is achieved, no matter which material is used(glass, aluminium, etc.) the focal point will be at g/(2w^2) in metres. Viscosity would not matter as the fluid would not be moving at the steady state.
The equation of a cross-section of the mirror would be a parabola

where w is the angular velocity in radian/sec and g is acceleration due to gravity.
For my requirement(focal length~1m) the angular velocity was 28 rpm.
I used a system of pulleys to reduce the rpm of the ac motor from 722 to 28 rpm. My uncle had a fancy 3-phase ac motor speed controller that could very accurately control the rpm of the motor by changing the frequency of the ac supply(variable frequency ac motor driver). I made an optical tachometer using infrared sensors and an arduino board to measure the exact rpm and compensate for any discrepancies. So, all that takes care of the curvature. Now for the setup:

I made a small turntable out of plywood and attached a ceramic tile on it using steel plates for insulation. Using similar bent steel plates I made a holder for the crucible/mirror cell. For the crucible I used a 2 inch section of a steel pipe of diameter 120mm with steel base welded on. It was crude but that did not matter as the mirror's surface would never come in contact with the mirror cell.

Mirror cell/crucible

Optical tachometer

What I thought was that on heating the glass, it would become very fluid and on spinning it, it would acquire the exact curvature. In fact I thought the surface finish would be so good that I probably would not have to grind it at all. I knew of a liquid mirror telescope at an observatory that uses spinning mercury(liquid) as the primary mirror. This re-enforced my faith in spin casting. However things did not work out the way I had planned...


More in a later post..