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Amateur Telescope Making (UPDATED 9/23/18) - 20" f/4 Nearly Complete


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https://imgur.com/a/rmNql

https://imgur.com/gallery/xdyAP

I long ago finished making my first telescope mirror, a 6" f/8, which you can read about from pages 1-5. It will not be used in a telescope as it has a number of chips and a fracture that reduce its usefulness anyway. I worked on it from May to July 2017, with a gap due to technical difficulties and a trip to Alaska. The mirror was made with a 1969 Edmund Scientific kit. It was intended to be f/5 but I ran out of grit and couldn't afford more at the time.

I have also finished a complete 6" f/4.5 Dobsonian. The mirror blank was given to me by a friend at Stellafane, and I managed to grind, polish, and figure it in August 2017 - see pages 6 to 7. I star tested it and looked at Saturn on October 20th to confirm a good figure. First light was on October 27th. I rebuilt it in late July 2018.

As of April 2018 I've also finished a 16" f/5 Dob. I attempted to make the mirror but that didn't turn out too well, so I got a replacement from a friend. Read about it from pages 9-10.

Currently working on a 20" f/4 - read about it from page 10 onwards.

 

Edited by _Augustus_
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Enjoy the process, and don't be so sure your "beginner skills" won't produce a better mirror than the common ones you can afford to buy.  Twenty years ago, I built my 8" f/6.8 (on a Dobson mount) completely from scratch, had never built a scope or ground a mirror -- the only finished telescope parts I bought were the diagonal flat and the focuser.  I ground the mirror starting from a miscellaneous Pyrex blank I got from Surplus Shed, and a tool made with field tile (fully vitrified tile made into a mat with silicone dots between tiles, sold to go over the drain in a shower) and cast plaster of Paris; I bought my roughing grit at a lapidary supply, and got a "grit kit" for the rest of the grades from the (Seattle) local telescope makers' club.  I wound up with better than 1/5 wave accuracy on the mirror, and an 8" Newtonian that, side by side at a star party, performs about the same (in terms of what and how well I can see) as a commercial 10" Schmidt-Cassegrain.  Mine is a little more work to point at a specific object and keep tracking, of course, but it was about 1/8 the money.

In the end, by buying a lot of my "telescope parts" at the local Lowe's Home Improvement (for instance, I used a 9" diameter furnace duct for the tube, 8" and 9" duct caps and some bits and pieces became the mirror cell, I bought guitar machine heads to tension the wire spider, which is made from .010" diameter guitar strings, and I fabricated the diagonal mount from PVC plumbing parts), I spent a total of $500 -- which, at the time, was about $100 less than a commercial 8" Dobsonian (and I spent a total of around 120 hours which, as hobby time, I don't count against the cost).

Hold yourself to the standard of "if it's not right, keep at it until it is," and you can make a telescope that will do anything a comparable size commercial scope can.  And when you're done, you can spend the rest of your life saying "I made that!"

Edit to add: I'm sure you'll find you want at least twice the amount of roughing grit in that kit you bought -- drop by a shop that sells lapidary supplies and grab a couple pounds of their 60-80 carborundum, and switch to the kit grit when you're at the required depth, before making the jump to 120 or 180.

Edited by Zeiss Ikon
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On 5/25/2017 at 10:01 PM, Starman4308 said:

I haven't done any telescopy, but I wish you the best of luck making your new telescope.

Just remember, it's Mars, not Duna (or Gratian) that you're seeing through the telescope.

I've never had luck at seeing much on Mars. The weeks around opposition last year were always cloudy, and the one day it wasn't I accidentally slept until 3 AM, by which time Mars was so low that it just appeared as a shimmery orange ball. I also had only one scope, a 4" Mak, which, excellent as it is optically, never showed more than Syrtis Major and one ice cap.

Oh, and the kit arrives tomorrow.

 

Edited by _Augustus_
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Just opened up the kit.

It's 50 years old. The newspapers protecting it are from 1968. All of the original Edmund items, including a 1" diagonal and 25mm eyepiece lens set, are in there - even the original cardboard collimation tool!

Should get started later today.

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Very important: be sure you're grinding the center of the mirror almost over the edge of the tool while hogging out the depth of the sagitta.  And don't be afraid to lean on the mirror while roughing.  Heavy pressure and lots of overhang will cut down the center fast and make best use of your coarsest grit.  Wets should be pretty short before the grit breaks down to mud.  I recall turning the mirror in my hands every couple strokes, and stepping around the work stand (a 55 gallon barrel with two bags of play sand in the bottom) the same direction every 3-4 times I turned the mirror -- and I was leaning so hard on the mirror that when the room was dim I'd see sparks through the glass (I was never sure if they were heat from fracturing carborundum grains, or triboluminescence like what happens when you bite a wintergreen Lifesaver).

Don't expect anything resembling a spherical surface during hogging out -- you're just after getting the sagitta to the right depth.  You'll convert that depth to a spherical shape in the 120 and finer grits, and it won't take long (because it doesn't have to remove much glass) -- but when you hog out for an f/5 on a 6" mirror, you'll taking out something like half a pound of glass, and the faster you get it done, the less likely you are to have to buy more roughing grit or change your plan to a longer focal length.

Edit: BTW, if you haven't already got them, it's worth trying to find copies of Richard Berry's Build Your Own Telescope and Neale Howard's Standard Handbook for Telescope Making.  Texereau's How to Make a Telescope is another standard in this field; if you can only get one, get Texereau (I seem to recall it even had an appendix on silvering your own mirror -- aluminizing service most likely isn't any easier to find than it was when I made mine).

Edited by Zeiss Ikon
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On 5/27/2017 at 7:45 PM, Zeiss Ikon said:

Very important: be sure you're grinding the center of the mirror almost over the edge of the tool while hogging out the depth of the sagitta.  And don't be afraid to lean on the mirror while roughing.  Heavy pressure and lots of overhang will cut down the center fast and make best use of your coarsest grit.  Wets should be pretty short before the grit breaks down to mud.  I recall turning the mirror in my hands every couple strokes, and stepping around the work stand (a 55 gallon barrel with two bags of play sand in the bottom) the same direction every 3-4 times I turned the mirror -- and I was leaning so hard on the mirror that when the room was dim I'd see sparks through the glass (I was never sure if they were heat from fracturing carborundum grains, or triboluminescence like what happens when you bite a wintergreen Lifesaver).

Don't expect anything resembling a spherical surface during hogging out -- you're just after getting the sagitta to the right depth.  You'll convert that depth to a spherical shape in the 120 and finer grits, and it won't take long (because it doesn't have to remove much glass) -- but when you hog out for an f/5 on a 6" mirror, you'll taking out something like half a pound of glass, and the faster you get it done, the less likely you are to have to buy more roughing grit or change your plan to a longer focal length.

Edit: BTW, if you haven't already got them, it's worth trying to find copies of Richard Berry's Build Your Own Telescope and Neale Howard's Standard Handbook for Telescope Making.  Texereau's How to Make a Telescope is another standard in this field; if you can only get one, get Texereau (I seem to recall it even had an appendix on silvering your own mirror -- aluminizing service most likely isn't any easier to find than it was when I made mine).

I have Berry and Texereau.

Edited by _Augustus_
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27 minutes ago, _Augustus_ said:

I have Berry and Texereau.

The mirror is around f/7 now - it started as a pregenerated f/8 blank.

Wow, I didn't know they sold pregenerated blanks in the 1960s.  Still, that's going to save you several hours of grinding and a lot of coarse grit (though not as much as it would if they'd provided a preground tool as well -- or did they?)

Worth noting, I recall reading about people going from flat blanks (water jet cut from thick plate glass, as I recall) to polished mirrors in a single weekend -- but those were 4" diameter, f/8 to f/10, and left spherical (which is optically adequate at those dimensions).

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6 minutes ago, Zeiss Ikon said:

Wow, I didn't know they sold pregenerated blanks in the 1960s.  Still, that's going to save you several hours of grinding and a lot of coarse grit (though not as much as it would if they'd provided a preground tool as well -- or did they?)

Tool is not pre-ground, which is annoying.

Still at f/7 for some reason.

How long will it take for me to get down to f/5?

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You do, however, need to get the tool ground all the way to the center.  If your sagitta is already deep enough (f/6 is close, it'll come in some as you fine grind and polish, then a bit more as you figure), you can switch to a W stroke, with no more than 25% overhang at the ends of the center stroke and much shorter strokes toward the sides (which should also not exceed 25% overhang).  This will keep the mirror from getting too much deeper as you get the tool fully ground and will bring both surfaces closer to spherical, which they'll need to be before you finish the 220, at the finest.

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Completed rough grinding using the #120 alundum.  No large pits on the mirror. Curve extends all the way to the edge.

Will post more pictures soon. Tomorrow I will start fine grinding.

Here's my grinding stand before I got the wise idea of covering it with trash bags and switching them between grits.

 

Edited by _Augustus_
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10 hours ago, Aperture Science said:

What is this, Isaac Newton Simulator 1668?

Actually, Newton and Herschel made their mirrors out of metal that had very low reflectivity, was difficult to figure, and tarnished quickly - aluminizing machines and even chemical silvering hadn't been invented yet. Newton himself never even made a parabolic mirror - he just made a tiny one with a spherical mirror that was close enough to a parabola at that size.

 

Edited by _Augustus_
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You're getting to the point where you have to watch for the mirror and tool sticking together if you let them stand still in contact (especially near the end of a wet) for even a minute or two.  Evaporation of water from the abrasive mud will create a partial vacuum between the two pieces, and make them almost impossible to separate (a tile tool like the one I used doesn't do this, but a glass tool surely will).  If this should occur, gently plunk the whole thing into a tub of room temperature water, wait a few minutes or longer as needed, and then slide the mirror off the tool (or vice versa).  Don't try to wedge them apart (you'll chip the now-sharp edge of the mirror); don't try to force them to slide without soaking (you might succeed -- and propel one piece or the other across the shop to its doom), and most especially don't strike either piece with anything -- you'll injure your hand, and anything sturdier may injure one or both pieces of glass.

Speaking of now-sharp mirror edge; this is about the point, also, where for the first time you might want to take a carborundum sharpening stone (grinding wheel, wet-dry sandpaper, etc.) and grind a bevel at the edge of the mirror.  Don't worry about losing light gathering area; most amateur made mirrors (especially first mirrors) gain image quality by having the outer edge (rule of thumb is 10% of radius) masked off, even though they gather a bit less light that way (polishing tends to "turn down" that edge, and it's quite hard to prevent that if you do all your polishing with a pitch lap).  But putting a bevel out there reduces the likelihood of having a conchoidal chip knocked out of the edge of the mirror, taking a much deeper bite out of the reflecting surface to be.

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5 minutes ago, Zeiss Ikon said:

Speaking of now-sharp mirror edge; this is about the point, also, where for the first time you might want to take a carborundum sharpening stone (grinding wheel, wet-dry sandpaper, etc.) and grind a bevel at the edge of the mirror.  Don't worry about losing light gathering area; most amateur made mirrors (especially first mirrors) gain image quality by having the outer edge (rule of thumb is 10% of radius) masked off, even though they gather a bit less light that way (polishing tends to "turn down" that edge, and it's quite hard to prevent that if you do all your polishing with a pitch lap).  But putting a bevel out there reduces the likelihood of having a conchoidal chip knocked out of the edge of the mirror, taking a much deeper bite out of the reflecting surface to be.

The tool was beveled and seems to have generated a small bevel on the edge. Not concerned.

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