Posts Made By: Marcin Klapczynski

Hi Guys,
I am new in the field and I just have started gathering materials and reading about ATM.

Although vast majority of people say that a parabolic mirror is necessary for good quality pictures in newtonian telescope (no spherical abberation), I found some suggestions about grinding a spherical mirror. They say that making proper parabolic shape is "extremely difficult to even an experienced mirror maker". Moreover they argue that they have never seen blurred picture using spherical mirror...
But then you have to follow Maksutov equation (lamda/4) and 8'' mirror (that I am planning to make) would have to have focal length = 72 inches. That would make focal ratio f/9 ! It is kind of awkward... :S (or it isn't?)

So my question is: is it really so hard to make perfect parabolic 8'' mirror for a guy with lots of patience and excellent manual skills like me? 8)

Thanks for any advice!

Hi guys!
I am finnally about to start grinding 203mm (8'') f/6 mirror. I got all equipment, built Focault knife and I am ready for hard work! According to Jean Texerau I have made 45 deg / 2mm chamfer on mirror edge.
My question is: should I take the chamfer into consideration while measuring sagita depth? This would reduce the mirror diameter to 199 mm (2mm chamfer on both edges) and change sagita depth required for f/6. But will the chamfer dissapear during grinding anyway?
Oh, and btw - what is the most convenient and precise way to measure sagita depth?

Hi guys.
I just started grinding 203mm f/6 mirror. After over two hours I reached sagita depth of 1.194 mm (target is 2.1 mm)- measured by using feeler gauge and metal ruler.

When I look at the mirror surface I can see 'grinded circle' - it starts around 1 inch from mirror edge. (attached picture) Is it correct? I mean, isn't it too far from the edge? Moreover this 'grinded circle' is moved a little bit towards one edge (it's center is placed correctly though). What I am doing wrong?

thanks

I am back doing my 8'' f/6 mirror after 2-month break. ROC is perfect, I did sharpie test for checking T/M contact. I started smoothing with WAO already and moving at decent pace.

I would like to ask you guys about your preferences for building a polishing lap.

1) Is it better to apply pitch squares individually or pour pitch in a cast and carve channels in it?

2) What is the best way to fasten individual squares? Beeswax or turpentine? Where can I buy those?

3) Finally do you have some tips & tricks about polishing? I know general rules from Texerau's book and some web pages, but it is always good to know other mirror makers' opinion

Thanks!

I am after 13 hrs of polishing my 8" f/6 mirror. Today I have had a chance to play with a Foucault knife for the first time. It was really exciting, I took lots of pictures and got familiar with the tester.
Could you guys take a look at the mirror in the attached series of pictures? Does it have any serious errors at the first sight? I think that I dug too deep hole in the middle, but I may be wrong...
Please ignore my friend's fingerprints on center-down-right part - we couldn't stop looking at heat 'waves' expanding from different parts of our bodies while looking at the mirror Also, what's the line on 2 o'clock, that looks like a crack? I can't see it neither with magnifying glass nor with laser... :S could it be something inside the mirror?

So is it time to quantify it?

Oh, this was a LED moving light source and the knife was moving from the left hand side.

I called local abrasive supplier asking about cerium oxide. They asked me if I need #1 or #2 or #3 :S I didn't know they come in different types. Previously I bought CEO in the kit and it wasn't indicated which number is it.
I said I needed the finest one. Do you know the difference among them?

I started a new thread, cause the old one ( http://www.astromart.com/forums/viewpost.asp?forum_post_id=387189&poll_id=&news_id=&page=2 ) became monstrous.
I did additional 10x 5min stroke / 5 min press and retested the mirror. I attached some pics - I think I am getting better at a camera setup, but I got still some misalignment.

I didn't notice any radical changes neither in a shape nor in a surface smoothness. :S I am a little bit concerned about my new CEO. The old one I was working with was pinkish, this one is rather white with light 'touch of pink' :S Does it tell something about the actual size or compounds? I also noticed that this one can be suspended better in water while the old one quite quickly settled down.

Ok I am back with numbers and I kinda sense a major screw up. So I need some experts' advice and hand holding. Just to remind you - I finally decided to start parabolizing from an undercorrected oblate spheroid I got. These are some of last figures I showed you before:

https://www.astromart.com/forums/viewpost.asp?forum_post_id=392558&poll_id=&news_id=&page=

https://www.astromart.com/forums/viewpost.asp?forum_post_id=392957&poll_id=&news_id=&page=

So I was working for a while with no visible progress and got so annoyed that abandoned project for two weeks or so. After some time I quit whining and came back to work and was excited about project again. So finally I obtained a figure sequence suggesting overcorrection (parabola/hyperbola/somethingelse) without getting a sphere 'on the way'.

So I set up a Couder mask and took readings and got some scary numbers. Wtf? Why these numbers are so huge? And notice that this is with moving light source, so with fixed one they would be even higher. I fed FigureXP with this data - the surface looks like some kind of volcano with an error of 2.64 waves (See attached picture)

Ok, what should I do? Attack zone 3 with localized pressure or try to get a sphere and re-parabolize? If second option is preferable, how can I get back to sphere from this weirdo I got?

Thanks

If you guys have other software that you trust, and feel like checking my mirror, you can use these data:
ALL IN MILIMETERS!

Mirror diameter: 203
ROC: 2425.7
MOVING KE
Mask Radius: inner / outer (effective):
Z1: 18.75 / 37.5 (28.886)
Z2: 51.75 / 65 (58.562)
Z3: 75.25 / 84.5 (79.942)
Z4: 93.5 / 101.5 (97.541)

Knife readings: #1 #2 #3 (ideal value)
Z1: 0.00 0.00 0.00 (0.00)
Z2: 2.12 2.61 2.58 (0.535)
Z3: 5.50 5.62 5.42 (1.145)
Z4: 6.14 6.14 6.18 (1.789)

Hi everyone!
Look at the attached figure. After 17 figuring steps I have 1/7.16 lambda error at zone 4 and slightly raised center. I don't really see it during qualitative test, but numbers don't lie.
I have learned a lot, tried many tricks and finally ended up switching c-o-c 1/3 overhang stroke with parabolization with pressure on edges. I REALLY tried to turn the edge down but my pitch lap is apparently too hard and the edge always stayed up.
The numbers looks kinda nice everywhere except zone 4. I was thinking I could use TOT and use only tool's center to press and do the small zig-zag only on zone 4 blending it with zone 3. Do you have an experience with this kind of strategy of correcting the edge?

Or... should I just forget about further corrections and take the mirror for aluminization? I am really tired and starting to loose my enthusiasm.

Here are the numbers from 3 readings:
Z1: 0.08 0.02 0.12
Z2: 0.48 0.50 0.51
Z3: 1.12 1.10 1.13
Z4: 1.58 1.57 1.55

Mirror dia: 203 mm
ROC: 2390.8
MOVING KE
Mask Radius: inner / outer (effective):
Z1: 18.75 / 37.5 (28.886)
Z2: 51.75 / 65 (58.562)
Z3: 75.25 / 84.5 (79.942)
Z4: 93.5 / 101.5 (97.541)

Hi guys!
I finished my 8" mirror with 1/12 lambda surface error and am about to coat it.
On the coating company web page I saw an option to coat a mirror with 'Beral':

"Beral™ is a hard metalic coating for front surface mirrors. Its reflective properties are quite similar to those of Aluminum. The average reflectance is 91% in the visible spectrum. Beral™ is much harder than pure Aluminum and can be easily cleaned and maintained. Beral™ was developed exclusively by H. L. Clausing Inc. for use on astronomical telescope mirrors. "

Have you ever heard about it?