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

[_Augustus_]

3 hours ago, Zeiss Ikon said:

Trepanning the primary isn't too hard.  After you finish the medium grits (say, 600) and have a reasonable sphere, make a bit by mounting a piece of copper pipe concentric with a shaft that fits your drill press chuck (you can use epoxy for this, no need to solder), and use 80-120 grit applied under the edge of the pipe, with water, to drill the glass from the back side.  Stop just before you come through (1/16" remaining is about right), to finish after polishing and figuring, or if you go through, cement the plug back in with plaster of paris, to be softened by soaking after you finish polishing and figuring.  Do make sure your drill press is plugged into a GFCI for this operation -- an ordinary grounding plug won't protect, but a GFCI will.

I don't have a drill press.

I leave for Stellafane this afternoon! Can't wait to buy some blanks.

Edited July 21, 2017 by _Augustus_

[Zeiss Ikon]

10 hours ago, _Augustus_ said:

I don't have a drill press.

A drill press isn't absolutely required.  I've seen (photos of) trepanning machines built from pillow bearings, steel pipe, and plywood, powered with a sewing machine motor.  Or, if you have a heavy-ish hand held power drill, you could improvise a stand to keep it steady and perpendicular to the mirror and let it move up and down, or just buy a drill press stand for it (Harbor Freight has them cheaply, last I looked).

[_Augustus_]

Just got back from Stellafane with a couple items. There were no 10" blanks to be found but I did get TWO 6" blanks. One is fused silica and the other is a pregenerated f/5 Pyrex blank.

[_Augustus_]

I'm using the fused silica blank for a Gregorian. I will post updates when that gets started. The scope should be around f/30 or so - no need for a Barlow for planetary imaging!

EDIT: A Nasmyth is pretty much impossible due to baffling problems. Oh well.

Edited July 24, 2017 by _Augustus_

[_Augustus_]

The f/8 mirror is completely polished except for the last 1" at the edge. I will finish it today or tomorrow.

[_Augustus_]

Only about 0.5" to go until the entire mirror is polished.

[_Augustus_]

0.25" to go after another hour or so of polishing.

[_Augustus_]

It's completely polished out. Going to put it on the tester tonight and assess the shape of the mirror and what I need to do to parabolize it.

[_Augustus_]

I'm going to make the 6" f/5 mirror first. I just poured the plaster disk needed for the tool (glass tools are expensive and I only used one because I had it). 

[GDJ]

From one astronomer to another: I hope your mirrors turn out good. 

I can't offer any advise on mirror grinding/polishing since all I built were F-8 to F-10 refractors.

Edited July 26, 2017 by GDJ

[_Augustus_]

Thanks to my idiocy I poured the dental plaster disk on a non-level surface and it's slanted on the back and thus useless. I have to order more.....

[_Augustus_]

Ronchi shows that the 6" f/8 is a very nice sphere. I'm going to parabolize that thing tomorrow.

[_Augustus_]

Just ordered some more plaster and tiles from Firsthand Discovery.

I'm not going to parabolize the f/8 today. Maybe tomorrow or something.

Edited July 27, 2017 by _Augustus_

[_Augustus_]

I just opened up a HUGE box of ATM supplies given to me by a really nice guy in Oklahoma who no longer has time for making mirrors.

Here are some of the contents:

• Lots of pitch and cerium oxide
• Small amounts of #120 and #320 grit
• Hex tiles (I have extras now, I guess)
• Two 4" plate glass blanks, both of which are chipped on the back
• Two 5.75" plate glass blanks
• A 4.75" plate glass blank
• A 2" plate glass blank (can be used for the 6" Gregorian secondary)
• A green laser pointer 
• Assorted cleaning supplies
• Synthetic grease
• Another Foucault tester
• Adjustable mirror test stand
• A single-element 6" lens (useless but cool)
• A really thin 9" glass disk (under 0.5" thick and thus too thin for a mirror blank)
• A weird slanted concave 1.75" mirror that has been polished
• Dozens of Ronchi gratings
• A 3" achromatic objective

I also ordered some more grit as the amount contained in this box isn't enough.

I don't know what I'm going to do with those blanks.....

 

Edited July 27, 2017 by _Augustus_

[_Augustus_]

I was sent a nice Schiefspiegler design for the two 4" mirrors. Basically I would grind the two 4" blanks together and the "tool" would have the center 2" cored out and polished. The focal length would be so long that both mirrors could be spherical.

I think I'll make that after the 6" f/5.

[Zeiss Ikon]

On 7/26/2017 at 7:29 PM, _Augustus_ said:

Thanks to my idiocy I poured the dental plaster disk on a non-level surface and it's slanted on the back and thus useless. I have to order more.....

Nothing useless there.  All you have to do is shim up the tool so it's more or less level on the front.  Grinding technique will govern how the shape is generated; a little off level won't cause trouble.  If you're feeling OCD about it, you could grind the back of the tool on a sidewalk or other rough-but-even concrete to take out most of the wedge.  Save the plaster you ordered for the next one.

[_Augustus_]

14 hours ago, Zeiss Ikon said:

Nothing useless there.  All you have to do is shim up the tool so it's more or less level on the front.  Grinding technique will govern how the shape is generated; a little off level won't cause trouble.  If you're feeling OCD about it, you could grind the back of the tool on a sidewalk or other rough-but-even concrete to take out most of the wedge.  Save the plaster you ordered for the next one.

It would've been way too thin though. It was really slanted. I got rid of it.

Edited July 29, 2017 by _Augustus_

[YNM]

12 hours ago, _Augustus_ said:

I was sent a nice Schiefspiegler design ...

I didn't even know they existed until you mention it !

Edited July 29, 2017 by YNM

[_Augustus_]

10 hours ago, YNM said:

I didn't even know they existed until you mention it !

A larger Schief would be difficult due to the secondary having to be a convex hyperboloid like in a Cassegrain, but with a 4" f/28 you can just cut the middle 2" out of the glass tool and use that as the secondary.

[Zeiss Ikon]

8 hours ago, _Augustus_ said:

A larger Schief would be difficult due to the secondary having to be a convex hyperboloid like in a Cassegrain, but with a 4" f/28 you can just cut the middle 2" out of the glass tool and use that as the secondary.

You could do the same with a larger aperture, as long as you keep the focal ratio similarly long -- in other words, as long as it's f/28, it doesn't matter if it's 4" or 12" (though it may be impractically long in the latter case).  Should give some nice planetary views with that long focus, though.

[_Augustus_]

1 hour ago, Zeiss Ikon said:

You could do the same with a larger aperture, as long as you keep the focal ratio similarly long -- in other words, as long as it's f/28, it doesn't matter if it's 4" or 12" (though it may be impractically long in the latter case).  Should give some nice planetary views with that long focus, though.

I was going to say that Foucault testing such a long focal length mirror would be an issue, but one doesn't technically need that for a long-focus sphere. Mounting requirements would be the only problem. A 4-5" Schief is already pushing a CG-5.

[Green Baron]

Nice idea with the Schiefspiegler (wasn't aware that this found its way into English language). Cock-eyed reflector :-) ? But they have their fan base. If someone asks himself how to pronounce it: like sheefshpeegler.

Could be used for nice pieces of "artwork" in photography due to its enormous aberrations, but i assume that is not why you want to build it.

Good success !

[GDJ]

On 2017-07-27 at 11:57 AM, _Augustus_ said:

 

• A 3" achromatic objective

 

Depending on the F ratio that would make a wonderful finder scope, especially if it's in the F5 to F6 range.
I built a 3" F5 from parts from Surplus Shed and it works very well. Good enough light gathering to make out Magnitude 8 stars with ease, Magnitude 12 if you're in a Dark Site. Hang on to that.

Edited July 31, 2017 by GDJ

[_Augustus_]

4 hours ago, Green Baron said:

Nice idea with the Schiefspiegler (wasn't aware that this found its way into English language). Cock-eyed reflector :-) ? But they have their fan base. If someone asks himself how to pronounce it: like sheefshpeegler.

Could be used for nice pieces of "artwork" in photography due to its enormous aberrations, but i assume that is not why you want to build it.

Good success !

The focal ratio is so long that there will be little/no aberrations. And I can leave both mirrors spherical.

http://www.clearskyobserver.com/index.php/20-telescopemaking/21-4-schiefspiegler

2 hours ago, GDJ said:

Depending on the F ratio that would make a wonderful finder scope, especially if it's in the F5 to F6 range.
I built a 3" F5 from parts from Surplus Shed and it works very well. Good enough light gathering to make out Magnitude 8 stars with ease, Magnitude 12 if you're in a Dark Site. Hang on to that.

I measured it and it's around f/4 or so. I will probably use it as a finder for a large scope.

[GDJ]

2 hours ago, _Augustus_ said:

I measured it and it's around f/4 or so. I will probably use it as a finder for a large scope.

At F4 there might be some aberration in the imaging around the edges, but it would make a very wide field scope. If its slightly higher than F4 then it would probably image well.

As for eyepieces for this short of a focal length, I've had decent results with a 20mm Kellner. Your mileage may vary of course.