Check the profile of a telescope mirror

[pixel01]

Hi all,

I know some men, who are amater astronomers. They made themseves reflective telescopes which can see the crescent Venus and many others in the sky.
They say they make the mirrors parapolic shape although the depth of the mirror is very thin (about 1 - 2 mm). The diameters are often in the ranges 150-200mm.
I am not sure how they could check the profile exactly a parapola, and quite curious about that. Do anyone know about this, please explain to me.

Thank you.

 

[russ_watters]

I'm sure they buy the mirror pre-cast and just polish it themselves.

 

[pixel01]

I'm sure they buy the mirror pre-cast and just polish it themselves.

I am sure they make the mirror themseves. They even claim that theirs are higher quality compared to some comercial telescopes.

 

[turbo]

ATMers buy blanks (flat disks of glass) and use grit with a tool (often a smaller piece of glass) to hog out material from the blank. Most of the shaping is done in this step. Then the mirror is ground with finer and finer grit abrasives to smooth the surface, and finally, it is corrected to the best figure that the maker can achieve. This part being done, the mirror is generally shipped out to be coated to make it highly reflective. Here is a site with links to LOTS of sites, many of which describe the process in great detail, and many of which give how-to tips about making the process most efficient and/or more accurate. And yes, starting with some flat disks of glass, amateurs can make telescope mirrors that are more accurately figured than those in most commercially-available scopes. It just takes time, skill, attention to detail, and more time. ;-)

http://www.atmsite.org/

 

[tony873004]

I made my own telescope. I took a telescope-making class from John Dobson. The reason for the higher quality than commercially-available mirrors is because humans are better than machines at inducing a random element into the grinding process. As you make your grinding strokes, every ~10 strokes, you rotate the mirror. After doing this 10's of thousands of times (turbo-1 did mention that it takes time), there should be no preference as to the direction the mirror was rotated, so it will average into a spherical shape. A machine, which is told how often and how far to rotate the mirror is more likely to be less random, hence there will be some preferred directions, creating stigmatisms in the mirror. Even by hand it is easy to accidently introduce a stigmatism. The back of your mirror blank will have small chips in it, and if you unwittingly line up your favorite chip with the 12 o'clock position each time it comes around, you'll end up with a preferred direction. There will be more grind strokes than average in that direction.

That said, my mirror isn't as nice as a commercially-manufactured one. My first attempt produced me a less-than-satisfactory mirror. Through it, the Moon looks awesome. Star clusters look awesome. And deep sky objects look awesome. But if I throw in a high power eyepiece to look at Jupiter or Saturn, I find it impossible to focus on it.

After finishing the grinding, you should be left with a spherical mirror. But at the focal lengths we were making, there's not a huge difference between a parabola and a sphere. So the mirror could be parabolized in the polishing process. Although some classes are taught to use a "knife-edge" method (I'm not sure exactly what this is), we were encouraged to simply eyeball it. After all, if your eye can't tell the difference, then it's as perfect as it needs to be. That's why John Dobson would laugh at all the computerized methods. And if we weren't comfortable with eye-balling it, John Dobson would eyeball it for us. He would just aim your scope at a distant ceramic insulator on a telephone pole that had the Sun gleaming off it. He would scan across the mirror making sure all points came to the same focus.

 

[turbo]

Tony, you learned from a very smart and practical guy - a folk hero in amateur astronomy. I'm jealous.

 

[pixel01]

Thanks tony and turbo. The information is just good to me bcause I am planning to make myself one telescope. Not immediately, but may be in near future.

 

[russ_watters]

Wow, I didn't realize amateurs took that much material out of the mirror blank.

 

[tony873004]

Wow, I didn't realize amateurs took that much material out of the mirror blank.

The amount of material removed is directly proportional to the size of the back seat of your car :)

The more glass removed, the shorter your focal length, and the smaller your telescope. So people knew they had to keep grinding until their telescope was small enough to fit in their car.

Typically, the scopes made by people in my class had much longer focal lengths then a commercially-bought Dob.

 

[Chronos]

My first mirror making effort resulted in a superb paper weight. The 'eyeball' method is unreliable. A perfect figure at 5x becomes grandma in spandex at 100x.