Making and Motorizing of Telescope

Well, I just ended my many years well served ADSL-connection and I moved ahead and have now 4G connection with integrated wlan. I like it. On the other hand, in the end of the year my telescope hobby pages Making and Motorizing of Telescope will be gone! Well, I'm trying to put the same content here in my blog..

I started my web-pages like this:

"My Amateur Telescope Making hobby started already at the beginning of 80's. As a young man I was very eager to try out everything and a trial of making a mirror by own hands did not end to the perfect paraboloidal mirror. Instead of that my 5" mirror had bad turned down edges and the polish of the surface was far from adequate! Still the telescope served many years and afterwards thinking the manual own hands made mirror left thoughts and feelings that you can if you try hard enough! And what would be the most valuable than own and share with your family and friends something which you have done by yourself!

In the beginning of the new century after 20 years of my first ATM experience I happened to find Mel Bartels' great instructions to motorize a telescope. Since then I have felt the ATM things are very interesting and sharing instructions belongs to the hobby. Many times I have found out the best instructions and solutions from the net. Let's keep it that way!
I try to show shortly on these pages how I constructed my "Motorized 5" Telescope" and the latest "Lurie-Houghton Newtonian Telescope". Making telescope optics has always been the most interesting part of ATMaking. By building grinding/polishing machine I have got the required boost to the hobby. Manual mirror grinding does not feel good anymore and it would take a lot more time too. Using of machines has also brought own kind of ATM hobby part because to learn to use machines is not always simple and straight forward. There seem to be own tricks and lessons learned things too."

There is pretty much all shortly said. I still feel that way even if I haven't done the hobby for long time. To make telescopes is very interesting but time consuming hobby. Anyway, Lurie-Houghton telescope has always been my favorite just because there is two telescopes in the same package: A refractor and reflector. At least, in principle. As soon as I figured out all the aspects of that telescope type I was inspired. I felt need to try out one! :) Well, afterwards I found myself deep in the ATM hobby but I did not want stop until I have finished it. Period. And I did! The best part was that the telescope really performed and performs still as I have read many times. It has very large field of view and almost come free optics. Very suitable for many amateur astronomic use. It's good for taking photos of sky wonders or just looking stars, galaxies with naked eye. When I got it ready I was so happy and proud of my achievement. In the end administrator of ATM Letters, a web magazine, contacted and asked me to write an article in 2006! Wow! Oh boy, I was happy. Never happened to me earlier. Well, now that my web-pages will soon go down I add here a few texts of my old stories while making LHN telescope. Actually the correct name is Lurie- Houghton Newtonian Telescope. Here we go..

"Optics of Lurie-Houghton Newtonian
(L)urie-(H)oughton (N)ewtonian telescope typically consists of a full size corrector located on the upper end of the tube, a spherical primary and a flat secondary mirror. The corrector usually is designed to have two lenses: A bi-convex(positive) front and a bi-concave(negative) rear lens. The lenses have a small air gap between each other. Also the lens glass material can be the same, ie. a good choice is, for example, Schott's BK7 or Pilkington's BCS grown glass. Actually the correct name for this kind of telescope should be "Houghton-Newtonian". I have read that people are interested in to make corrector lenses of normal plate glass. My opinion is that why to risk and spend enormously time for unsure glass material if there are am. good materials available? Of coarse a good one always costs more. That's the life. To design and calculate a LHN telescope setup a good tool is OSLO EDU or MODAS analysis software. Both can be free downloaded from the net. Also very useful web-pages are Vladimir Sacek's "Amateur Telescope Optics".

A very good, informative web-page of designing and making a LHN telescope is Rick Scott's "Lurie-Houghton Telescope Project". What are then the advantages and disadvantages of the LHN telescope? And is there any common sense to make one? A good normal Newtonian telescope performs just perfectly and is more than enough for most of us. I think now we are starting to talk about a topic which is not black and white thing. But if you try to find out pure physical facts, some of them can be listed:

Advantages:
- rather fast telescope F/4...F/5 (typically)
- large field of view
- curvature of field is almost flat
- coma free
- there is astigmatism but not a big issue
- corrector seals the tube end reducing tube turbulence
- no secondary spider vanes
- tube is "dust sealed"
- optics is easier to make, all surfaces are spherical

Disadvantages:
- telescope is heavier because of the corrector
- obstruction is easily 30..33% depending on the requested FOV
- light loss because of two corrector lenses
- there is for sure a lot of work with the optics (5 surfaces)

These are the first things which come to my mind. My reasons to choose making LHN telescope was that it really is a good telescope type and I liked to practice and learn my ATM skills with lenses too. I had already made a couple of paraboloidal mirrors, so trying out lens making was evident. Optics of the LHN telescope is quite forgiving. I made at first the primary mirror trying to match the radius of curvature as close as possible to the calculated one. It doesn't matter if you do not reach the RoC exactly. The new RoC value can always be fed to OSLO file and correct the lenses' values, no problem. Maybe the final F/ratio will only change a little bit. During playing with my LHN setup in OSLO I noticed that R1 (front lens's first surface), R3 (rear lens's first surface) and R5 (primary) are the most critical ones. Ie. RoC of R1 and R3 should not only be within +/- "something" millimeters tolerance, they should also have exactly the same but opposite RoC value (R1 is convex, R3 is concave). But the good thing is that R1/R3 together can vary about +/- 5..7 mm of the calculated values. If the R1 (convex) is interference tested against polished surface of R3 the overall 3 fringe difference is about the maximum acceptable value. The primary mirror's acceptable value is app. +/- 3...4 mm if the corrector lenses are made first. So, I would say that it's better to make first the primary, adjust the values of the corrector in OSLO and then make the corrector lenses within that +/- 5...7 mm tolerance.

R2/R4 tolerances behave the same way as R1/R3. You should get both RoCs as close to the same value as possible but together they can vary and this time much more than R1/R3. For example in my setup a good value for R2/R4 is 3000 mm (R2=convex, R4=concave). OSLO Lt clearly shows that the value for RoCs could vary easily +/- 100 mm even more without noticeable destroying the final spot diagrams! If you have a spherometer at least 50% of lens diameter it should not be very difficult to achive much more accurate results than that. Actually if you grind lens pairs together checking concave's RoC is easy. Then there is that little air gap between lenses. At least what I tried to figure out with OSLO it seems that it's enough to keep it small. If between lenses is placed three pieces thin tapes or post stamp papers is enough. Then the gap on lenses edges are about 0,05...0,1 mm. Lenses thickness is once again a glad thing in LHN corrector case. Even if refractors' lenses thickness has to be quite accurate for LHN corrector lenses 1...2 mm difference (from designed) is nothing. Thiner is better but as we know a good starting point for lens disks is 1:10. Finally there is in LHN telescope setup the corrector's place compared to the other optical elements. This may start to sound monotonic but also the corrector's place on the optical axis is not so accurate. Meaning that you need not to design any specific high accurate secondary supporting system to locate the corrector exactly in the pre-calculated place! It's good to remember that I am writing these things from an ATM optics maker's point of view not from pure optics designer's view. Optics designers would try to make a perfect telescope which has a perfect spot diagrams. Yes, by giving independent values for each RoC of lenses is possible to make a little bit better LHN telescope especially Houghton-Cassegrain case. But a very good design starting point is in "Telescope Optics: Evaluation and Design" - book (page 126). The shown setup for LHN is the-all- spherical design (with R1/R3 and R2/R4 RoC pairs) and the results are more than adequate.

Making the lensesIn the first picture I am rough grinding R3 on R1. It's a nice thing in LHN corrector lenses that you can design the setup so that R1/R3 and R2/R4 surface pairs have exactly the same radius. Then you can rough grind those surface pairs on each other saving time a lot. Of coarse you can fine grind them in the same way or use a grinding machine and independent tools for each surface, as I did. In this point I have to mension "a pitch tile tool", ie. a grinding tool covered with tiles which are clued by using normal pitch. I made those tools after reading Mr. Roger Ceragioli's article "Refractor Construction Page" My opinion is that those tools work much better than normal epoxy tools. All the time the fine grinding action was good and the tools stayed in really good contact. That is a method which can be recommended honestly at least for lens making.

I had a little bit experience of trepaning mirrors, so making the lenses center holes for connecting the secondary went quite nicely. As I had read about crowns and flints that they are much more breakable than Pyrex or Duran50 glass, it seemed to be true. I was able to use a drill press which had a centering base. Then you can center a biscuit cutter exactly to the center of a lens. If you do not touch to the settings you can drill 3..4 mm from one side and turn over the lens and drill now until you reach the first side's pre-drilling. That way you can quite safely drill through a lens without a fear that you will break pieces off the edge of the center hole. A crown glass really is easily breakable!

I think that it's better to trepan lenses after you have ended the 220 carbo stage or so. That's the same no matter are you making a lens or a mirror. The reason is that after 220 carbo you have already grinded off the material which is needed to reach the RoC (especially in concave case). If you now make the center holes the required bevels in the edge of the hole and the core will last to the end of the fine grinding. Without a fear of taking the core off, re-beveling and plastering the core back...Oh boy what a mess and work! Okay, this is a quite small thing but still...please believe me. :)

My lenses are Pilkington's BSC crown glass which is equivalent to well-known BK7. I got the glass as a 15 mm thick molded glass slab (400x240 mm). Because of my work I got two 150 mm round glass disks water jet cutted. The cutted edge surfaces were quite smooth and both disks were in the same size enough to start grinding. I thought a lot how to edge lenses with my home tools. Then I figured out that the only way is to connect them to my grinding machine's shaft through the lens holes with a bolt after the lenses were trepaned. So, just after 220 carbo stage both lenses got their exact diameters. I bolted both lenses together to the shaft with a bolt and a large steel and rubber plate. I adjusted the lenses until they rotated centerly as possible. Then I used a diamond wet stone with a stand and grinded the lens edges to the same diameter. It took about 10 minutes. Finally I used a piece of steel plate and 20 um al-oxide to fine grind the edges.

After treppaning and edging the cores have to be clued back with Plaster of Paris. I think that stuff is well-known all over the world. Here is one method which I have used: Take a piece of plastic contact sheet and clue it to the backside of the lens or mirror. Now lay down the core from other side of the lens and quide it through the hole with three pieces of wood or plastic sticks. The tichness of the sticks should be the same as the gap between the core and center hole. The core should stick to the backside plastic tape rather well. Now you can pour thin, watery Plaster of Paris around the core. Normally I have layed down the lens on it's pair lens, only the plastic contact sheet between them. If you now make the plastering you can be quite sure that the core is almost in the same height as the lens itself. Finally after the plaster is dryed you have to take out a little bit of the plaster from both sides of the lens. Let's say app. 3 mm. The gaps can be filled carefully with candle vax or equal. If you don't do that there is a big risk that the core get loose. Candle vax is maybe not the best possible material. I tried also sanitate silicone. It was good stuff but making the seal was quite difficult. Anyway, the purpose is to make the plaster waterproof.

In the beginning of the carbo 220 stage at the latest you have to start to measure the wedge of the lenses. I made a wedge tester just from left- over plywood. In Texereau's "How to Make a Telescope" - book on page 200 is described well how de-wedging should be done. If you have done by manual method a mirror you learn de-wedging once you have tried it. The question is just manually grind mirror without rotating the upper disk in your hands. The thickest edge is placed towards you. Just little bit harder pressing during grinding. After 5 minutes you have to make 15 minutes normal grinding to ensure the surfaces are spherical enough before wedge testings. This way it has to be continued with both lenses until in the finest grades the wedge is about 0.025 mm. I left the both lenses to value 0.03 mm. In LHN corrector case it seems that the wedge can be much bigger than 0.025 mm. Anyway, Mr. Ceragioli gives in his article good values for wedge which can be and is realistic to achieve in different grades. So, I made de-wedging allways in the beginning of the new grinding stage and then continued until the surface marks were gone. Of coarse in the end I checked the wedge value before moving finer stage. I faced a surface quality problem with 2 surfaces, so I had to come back to coarse stages. With machine it seemed to be so that the wedge did not change even if I took 3 the finest grades again.

Polishing the lenses is quite the same thing compared to mirror polishing. I had polished a few mirrors with my machine so the concave surfaces R3/R4 were quite easy job. Only it's good to remember that thin lenses have to be supported well with, for example, a concrete base. Between the support base and the lens I used 2 mm rubber plate. A couple of turns of plastic tape around base/lens is adequate to keep them together. R1/R2 of the front lens are convex. I have used my Mirror-O-Matic style machine for some time and typical for this kind of machine is to use sub-diameter tools for grinding and polishing. 75% tool is good value and it always works well for a concave surface. Now I had a good situation to try out would a small tool work also for a convex surface. Actually I used full-sized-tool for R1 and 75% tool for R2. Both styles worked well. I achieved rather good spherical figures and app. that 3 fringes match for concave pairs. Of coarse it's good to remember that all surfaces have to be polished about half way so that you can see through a lens while making interference testing. And if a correction need is detected a half-way-polished surface is much easier to correct. I have had always little problems to get a perfectly polished optical surface without pits. Ie. many times I have had to come back a few stages and fine grind again. In these corrector lens cases I happily noticed that 2 surfaces came out at first trial very nicely. Either the crown optical glass is easier to grind or the new tile-pitch-tool worked so much better. I would say the tool was the biggest reason. At least I like to recommend it's use.

Finally after 5 months I got the whole telescope optics ready for testing. Now (08-Jan-2005) the primary is in Helsinki for aluminaizing but before sending it I tested the telescope twice. In star testing from 5 to 10 rings visible intra and extra images were very much equal. The outer most ring's texture, thickness and brightness looked good in my eyes. I am not an expert, can only compare to my normal Newtonian telescopes. Also if I came very close to focus point the donut was round indicating no astigmatism. The subjective "snap" test also worked as it should do. Ie. a star or the surface of the Moon snapped right in one point in the best focus. Actually I was a little bit suprised, so well the telescope performed! Well, as I have read the zero power corrector seems to work properly producing just opposite direction spherical aberrations for the spherical primary. No lateral colour as in a refractor, just pure nice and sharp images.
With Parks Erfle 20 mm eyepiece the telescope produces magnification 32x. I looked the full moon and estimated FOV was 3.5 full moons. The secondary size is 1.83" (46.5 mm) and the obstruction then 32.7 %. I look forward to continue my testings after the primary mirror comes back from aluminaizing.

Well, the Lurie-Houghton Newtonian telescope seems to perform as well as I have read. If consider the work amount which is huge compared to an ordinary Newtonian LHN telescope splits amateurs' thoughts I'm sure. If you are willing to spend a lot of your time and also money for optics making it's a very good choice. If observing is the main point and hobby, buying or making a good ordinary Newtonian is the right way to go. But if your hobby is making telescope optics as mine is, do not fear to try out LHN optics. In principle there is combined two telescopes with their good features in one package: a refractor and a reflector!"

The Moon through my Houghton-Newtonian Telescope

Some close-ups through the telescope.

Ready-made optics of Houghton-Newtonian on a table.

Final prove of the optics performance.

Dimension drawing of the telescope.

The final OTA (Optical Tube Assemble).

Nowadays the optics is in safe place waiting for my inspiration to make a fiber tube OTA.

And finally my self-made M-o-M (Mirror-o-Matic) grinding/polishing frequency inverter controlled machine. In the photo there is a 8" Classical Cassegrain primary on the table but that's another story.

When do I have time to start that all over again! Very good and interesting hobby. It only needs your time a lot..

A Start of My Blog

OcularBiker - My friend suggested for me that name. Why not because it maybe describes my two hobbies. I have made telescopes for years and nowadays my free time has filled with long missed motorbikes. Actually 1980 Honda CX500 Standard. I have made a few improvements especially on the ignition side. I guess I will tell about both hobbies. But I'll start with the CX..

Well, I bought my CX500 last autumn and right away I found very good and informative web-pages cx500 forum. I didn't stay away from the site reading as much I could all possible aspects of that great bike! Little by little I found both the mechanical and electronic aspects what should I consider while repairing my bike. The am. forum is one of the bests I have ever found! Forum members are giving willingly good advice for novice ppl like me. Perfect. You are not pushed away no matter what you ask. It's like to be at home. :)

Very soon I realized that CX500 bikes are still very common and sought-after. Even if all they start to be over 30 years old bikes. 496cc water cooled V-engine gives nice rides and surprisingly well and easily you will find spare parts! I think I do not waste anybody's time starting telling the cx's the most common mechanical weak points cause you can easily jump on the cx500 forum and just start reading about the bike.. Well, maybe it's better to summarize those things though:

- cam chain and it's tensioner
- water pump mechanical seal
- stator

These are maybe the most common reasons you have to stop riding your cx. These three things are normally changed in the same time, so the name "triple bypass". All these things need the engine to be dropped. Period. Okay, the water pump mech. seal can be changed the engine in the frame. It's called "Shep's method" according to one the most experienced cx riders and method inventor. Of course, other things can fail in your cx, for sure. It's a mechanical device. CX500 is also known to be rather sensitive to produce tapping and ticking noises. Rattling noise is also very common for this bike. Owners recode videos for others to listening. I cannot blame. CX does have push rods and that way many gaps to be adjusted right. Not speaking of wears and tears all over the engine. Well, I have listened those odd sounds many times too. :) In spite of all those mechanical aspects Honda CX500 is really a nice bike to ride and the engine has reached in many cases many, many miles.

Well, after studying cx's mechanical points and got the bike working well you might face problems on the ignition side. Yes, there is numerous cases that hobbyists have all sort of problems with bike's electrics. Old fuses are kaput or there is a bad connection in one of the many connectors. All this is typical with a bike in this age. Maybe the best you can do your cx ignition is to replace the both ignition coils and spark plug caps. They are very cheap and at least take away a few malfunction points of the bike. Spark plugs are commonly available, no problem. Well, in the end you may face the situation your old ac-cdi unit under the seat is the problem. The AC-CDI unit gets it's name from the words Alternating Current - Capacitor Discharge Ignition. The original stator inside the back cover of the cx's engine has two extra coils which produce about 115 VAC voltage to power up the cdi unit. AC-CDI unit could also be called as analog cdi unit because inside the box is just active and passive electronic components. No microprocessors as in many cases nowadays. What the ac-cdi unit do then? Of course, it finally produce a spark to the spark plugs as a final product but before that it has to handle quite many tasks. Here is my explanation for the different parts of the ac-cdi unit and the auxiliary devices:

"Both cylinders get sparks constantly 15 degrees before TDC from idle to about 2000 rpm. After 2000 rpm advance pulses amplitude rise and cross over the triggering level of the thyristors and begin to be dominant. When engine speed rise more (>2000 rpm) the front edge of the advance pulses cross over the triggering level earlier causing ignition advances. Now the advance angle rises more or less linearly with a rising slope to 37 degrees before TDC at the engine speed 5500..6000 rpm. At the point of 37 degrees BTDC also the pick-up coils negative pulses mix causing the advance angle won't rise anymore. So, the max. advance angle stays at 37 degrees up to the red line area. On the red line area the left side small thyristor (SCRP) cause the advance pulses go to the ground. (Note idle positive pulses still trigger the thyristor discharging the capacitor but advance is not right on the left side.) No more higher engine speed with only the right side cylinder's correct advance. If revs drops under that the left cylinder comes back in the game because advance pulses can trigger again.
Two thermistors keep the advance curve the same regardless of ambient temperature. Otherwise the advance curve would change it's starting point according to temperature. No good!"

I copied the text from my Flickr-site with the photo:

Well, what actually is inside the box? Here is a photo of my ac-cdi unit the cover removed.

As we see there is just capacitors, resistors, thyristors and thermistors. All those components have their specific duties and in this point I think it's worth of mention that originally Honda has made fantastic job. In the end of 70's with those time electronic components they made an analog ignition unit which really works fine. No matter is it hot or cold climate the unit can produce steady advance curve for different revolutions of the cx's engine! I think now we are in the heart, the soul of the bike!! In order to make the unit work really well is not easy. Well, in that point of view, at least, I appreciate their work. But eventually you come to the point that nothing is ever lasting. We have to remember that an electronic unit which has worked well over 30 years is awesome work.

Well, after buying my bike I became aware of that some day I might face a situation the old ac-cdi unit won't work anymore. So, I started to figure out would there be a possibility to build own replaceable ac-cdi unit? After digging out many, really many writings and forum threads I came a conclusion it really is possibly. There simply is nothing special or invented-thing in my unit! If you have designed and made your own electronic boards you will find out what it is all about. Just try and you will find it! :) Okay, I add here a video of my testings of the cdi unit.

Well, that's all now..more to come..