I did a little more grinding till 11th Jan 2008 by when I was able to completely eliminate the central area on the two surfaces which was not getting abraded. Now all parts of the mirror and the tool are getting abraded. This is good news. The bad news --- the tool is still concave although not much. I have been able to measure the concavity of the tool and it is approx 1.5mm. This is much better than the 4-5mm depression that I had when I started working the first day. The #36 grit has worked really good in helping to reduce the tool concavity and in bringing in the two surfaces in contact. Another thing I have been doing over the last few times is that I have used the chordal strokes. Rather than doing the 1/3rd normal center-over-center strokes, I have been using the chordal strokes with about 1.5" offsets between the centers of the tool and the mirror. Both these factors, the #36 grit and the chordal strokes have greatly reduced the tool concavity. One problem though is that the amount of #36 grit is only about a quarter pound or so. This is likely to last just one or two sessions and I really need to make sure that the tool is at least flat, if not convex, before this grit size ends.
More things have happened. Yesterday (23rd June 2008), I was out at the meeting of the ATM club of MAS (the local astronomy club). I was able to use the 12" f/7.5 scope to look at Mars and a few other objects. the seeing was not particularly good so we used a few filters. The important part was that my 6" f/5 mirror was coated with Aluminum oxide and then an overcoat of Silicon-Oxide (SiO). The whole process took about 45mins of which nearly 30mins was spent in bringing the coating-jar to a vacuum or rather as close to a vacuum. The actual pressure was less than 1/10000th of the normal atmospheric pressure. After the coating, we did some basic testing on the mirror and things are looking good. Now comes the hard (and somewhat boring) part of building the remainder of the scope. The most important part will be the mirror cell. Most of the experienced people have told me that it is far better to buy rather than make the mirror cell. I am also thinking of buying the secondary mirror, eyepiece focusser and the eyepieces. I already have 3 eyepieces, 30mm, 20mm and 10mm. Since this mirror is a wide-field scope, I might have to buy an eyepiece of 35-40mm. That will give me a wide field with a 6-7mm exit pupil.
Finally, yesterday I have purchased a 8" mirror blank yesterday. The blank is about 1.3" thick. I also saw the 8" tile-tool but it was also concave (another concave tool) and so I straightaway rejected it. I will now have to look out for a 8" tile blank that I can use to grind this mirror. My plan for this mirror is to build something around f/8 to f/10. This would be a good scope for planetary and also for work that requires a decent magnification. It is occurring to me that the 12" mirror is a big step-up from the 6" mirror that I have been working on. It is a better idea to complete a 8" (or 10") mirror before attempting a 12" mirror. Let us see how this goes.
Thursday, January 24, 2008
Tuesday, January 8, 2008
Pre-hogging work
I have done approx 3hours of grinding on my 12" mirror. The central depression on the tool has been reduced greatly, though not eliminated completely. There is a central circular area on the mirror, approx 4" diameter, that is not in complete contact with the tool. I have been able to decipher this because of three tests. First, I put a piece of printed paper, having about 12-15 font size on a flat surface. I rest the mirror on the top with the grinding surface facing up.I try to read the material printed on the paper. I cannot read the material from anywhere except, from a central circle about 4" diameter. Secondly, this central circle also does not show any pits/scratches while the remaining mirror surface shows almost uniform sized pits. This means that the central area is barely getting ground while the remaining surface is getting abraded. Lastly, I used the sharpie test. Let me describe this test in brief.
Before doing this test, it is advisable to completely wash and dry the mirror surface. In this test, one draws a series of parallel lines 1-2" apart on the mirror using a graphite pencil. This is followed by another set of lines, but perpendicular to original set of lines. Thus, the mirror surface will resemble a series of squares. After the lines are drawn, one proceeds with the grinding. While using a less abrasive grit (such as #220 or #320) is preferable, it can also be done with the grit size you are using. I continued with the #36 grit that I have been using. I did about three wets i.e. added the powder charge and water three times and grinding the two surfaces each time till the abrasive breaks down and I have to add a new charge. After doing this, the mirror is again washed. Just allow water to flow over the mirror and try to clean the mirror surface without touching it. Gently use a very soft cloth to clear out the abrasive only if it does not wash away with the water alone. Again allow the mirror surface to dry out completely and then look for the pencil lines. The lines should either have completely disappeared or have uniformly faded. Both these scenarios indicate a proper and uniform contact between the tool and the mirror. In case of my mirror, the lines in the central 4" diameter circle were just lightly faded while lines in most other areas disappered. This just confirms my earlier observations. What this means is that, I will have to do 2-3 more sessions with #36 grit to get complete contact. Only after this will the actual hogging i.e. hollowing out the mirror can start. I am estimating that I should be able to start hogging next week.
Before doing this test, it is advisable to completely wash and dry the mirror surface. In this test, one draws a series of parallel lines 1-2" apart on the mirror using a graphite pencil. This is followed by another set of lines, but perpendicular to original set of lines. Thus, the mirror surface will resemble a series of squares. After the lines are drawn, one proceeds with the grinding. While using a less abrasive grit (such as #220 or #320) is preferable, it can also be done with the grit size you are using. I continued with the #36 grit that I have been using. I did about three wets i.e. added the powder charge and water three times and grinding the two surfaces each time till the abrasive breaks down and I have to add a new charge. After doing this, the mirror is again washed. Just allow water to flow over the mirror and try to clean the mirror surface without touching it. Gently use a very soft cloth to clear out the abrasive only if it does not wash away with the water alone. Again allow the mirror surface to dry out completely and then look for the pencil lines. The lines should either have completely disappeared or have uniformly faded. Both these scenarios indicate a proper and uniform contact between the tool and the mirror. In case of my mirror, the lines in the central 4" diameter circle were just lightly faded while lines in most other areas disappered. This just confirms my earlier observations. What this means is that, I will have to do 2-3 more sessions with #36 grit to get complete contact. Only after this will the actual hogging i.e. hollowing out the mirror can start. I am estimating that I should be able to start hogging next week.
Sunday, January 6, 2008
2009 Solar eclipse
On July 22nd 2009, there will be a total solar eclipse which will be in the totality stage for over 6mins in some parts of the far east, parts of China and Japan. The greatest solar eclipse i.e. the maximum duration of totality is 06mins and 38secs in parts of the Pacific Ocean, east of Japan. The path of the eclipse starts in the Arabian sea off the western coast of India, south of Rajkot (Gujrat state) and west of Mumbai (Maharashtra state). The eclipse path runs over the cities of Surat, Indore, Bhopal, Varanasi, Patna, Dibrugarh and the passes into China. The eclipse will be seen from the Chinese cities of Chengdu, Chongqing, Wuham, Hangzhou, Suzhou and Shanghai. Shanghai will see the eclipse for 05mins 56secs. The Tokararetto islands in Southern Japan will see the eclipse for about 06mins 20secs and the Iwo Jima and Kititao Jima islands are near the greatest eclipse point i.e. they will see the totality for approx 6mins 38secs. The eclipse path then passes through Marshall islands (05mins 15secs) and Gilbert islands (04mins 45secs). Western China (Wuhan and Shanghai) and Southern Japan (Kagoshima and Chichijima) seem to be the best points as they have the least amount of cloud cover for July.
This is a wonderful opportunity as the next total solar eclipse with 6mins or more is several years away. I am hoping to travel to some place in Asia to see this eclipse (hopefully). Will keep things posted as they happen.
This is a wonderful opportunity as the next total solar eclipse with 6mins or more is several years away. I am hoping to travel to some place in Asia to see this eclipse (hopefully). Will keep things posted as they happen.
Actually doing some telescope making
I worked on the 6" f/5 mirror for about 5-7mins using the W-stroke and with plenty of Cerium Oxide. I am hoping that this is enough to bring the mirror close to 1/4th wavelength from the 1/2 that it was. I have not tested the mirror after this work. BTW, before working on the mirror, I made sure that the mirror and the pitch tool were in a good contact. I placed the mirror on the pitch tool and then placed 10-12lbs weight on top of the mirror. I had the weights remain on the mirror for about 24hours and then for another 24hours I had just the mirror on the tool without any weights. Both times, I had plenty of Cerium Oxide so that the mirror and the tool do not stick together. With the dry winter weather, I had to make sure that the Cerium Oxide did not dry out. Plenty of Cerium Oxide solution (Cerium Oxide and water) had to be regularly used. I plan to test this mirror sometime soon.
On another note, I have also started working a 12" mirror. Very first day I noticed that the tool had a small but deep depression (about 4mm deep and having a diameter of approx 9"). I tried working on the mirror with #80 Silicon Carbide grit however the depression did not seem to be going away. Then I decided to take some drastic action and started using the #36 Silicon Carbide grit. This is very powerful and has a strong cutting action. The central depression on the tool is not much shallower (1mm or so) and also only a small central circle of about 5" diameter is now the problem area. This is the only part where the mirror and the tool are not in complete contact. I am hoping that 1-2 sessions of 1hour each with the #36 grit should bring both the surfaces in complete contact. Only after that can the actual process of grinding the mirror can begin.
I am hoping to do at least 3-4 hours of actual mirror grinding per week. Thats one of my New Year's resolution for 2008. I am planning to do approx 4-5 grinding sessions of 45mins each per week.
On another note, I have also started working a 12" mirror. Very first day I noticed that the tool had a small but deep depression (about 4mm deep and having a diameter of approx 9"). I tried working on the mirror with #80 Silicon Carbide grit however the depression did not seem to be going away. Then I decided to take some drastic action and started using the #36 Silicon Carbide grit. This is very powerful and has a strong cutting action. The central depression on the tool is not much shallower (1mm or so) and also only a small central circle of about 5" diameter is now the problem area. This is the only part where the mirror and the tool are not in complete contact. I am hoping that 1-2 sessions of 1hour each with the #36 grit should bring both the surfaces in complete contact. Only after that can the actual process of grinding the mirror can begin.
I am hoping to do at least 3-4 hours of actual mirror grinding per week. Thats one of my New Year's resolution for 2008. I am planning to do approx 4-5 grinding sessions of 45mins each per week.
Subscribe to:
Posts (Atom)