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

[Spaceception]

Congrats on getting pinned

This project is pretty cool, more pics please?

[_Augustus_]

Started and finished #400 grit today.

There's some confusion on what the focal length is. Today it measured at ~45". Should be cleared up with Foucault testing.

EDIT 7/27/17: Yep, it was cleared up with Foucault testing. The FL is actually 47-48".

Thanks for pinning my thread!

Edited July 27, 2017 by _Augustus_

[Zeiss Ikon]

23 hours ago, _Augustus_ said:

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

It usually isn't the tool that has this problem; you're grinding that convex anyway, so the edge has an obtuse angle.  The concave surface of the mirrror, where it meets the edge, is the potential problem.  It doesn't take a large bevel to prevent trouble, just enough to take the "sharp" off the ground edge.

Edited June 3, 2017 by Zeiss Ikon
typo

[_Augustus_]

Started and finished #600 grit today. I'm pretty much done with grinding unless it turns out I'm supposed to use the emery supplied in the kit for something.

As Zeiss Ikon was saying, the mirror blank and tool getting stuck together was a real problem at #600. But I suppose that means that the mirror is a good sphere, because otherwise that wouldn't happen.

It's likely that I will buy quite a few mirror blanks in various sizes at Stellafane, because everything at the Stellafane swap tables is soooo cheap (Seriously, I would recommend going to Stellafane just for the swap tables).

Edited July 27, 2017 by _Augustus_

[Zeiss Ikon]

3 hours ago, _Augustus_ said:

I'll probably pick up a secondary mirror for this scope at Stellafane, a large star party in Vermont. It's likely that I will buy quite a few mirror blanks in various sizes, because everything at the Stellafane swap tables is soooo cheap (Seriously, I would recommend going to Stellafane just for the swap tables).

I'd forgotten Stellafane -- when I made my 8" Dob I lived in Seattle, and my first star party was in New Mexico (three days drive from Seattle, and another three back, but it was worth every mile -- Enchanted Skies had a dark site where I could see the Trifid and Lagoon nebulae with the naked eye).  Now, I'm in North Carolina, so roughly twelve hours' drive from the Stellafane site.  When is the annual gathering there?

[_Augustus_]

Just now, Zeiss Ikon said:

I'd forgotten Stellafane -- when I made my 8" Dob I lived in Seattle, and my first star party was in New Mexico (three days drive from Seattle, and another three back, but it was worth every mile -- Enchanted Skies had a dark site where I could see the Trifid and Lagoon nebulae with the naked eye).  Now, I'm in North Carolina, so roughly twelve hours' drive from the Stellafane site.  When is the annual gathering there?

July 20-23. 

[_Augustus_]

Polishing starts today! I just have to get my hands on a red laser pointer and heat up the pitch. Might make a video.

[_Augustus_]

I have started warming up the pitch!

[_Augustus_]

Have started polishing. Will post videos shortly.

[_Augustus_]

I spent ~30 minutes polishing. It requires so much force that my grinding board moves when I push on the mirror, so I need to work on a better system.

[_Augustus_]

Here's a video of me melting the pitch and polishing.

 

[_Augustus_]

In my video I forgot to press the mirror and tool together, which is why it was so hard to do each stroke. It's much easier now.

Spent another hour polishing this morning.

Edited June 9, 2017 by _Augustus_

[_Augustus_]

Just did another hour of polishing. Shining a laser on it shows progress. Foucault tester arrives in the next few days.

[_Augustus_]

 

[_Augustus_]

Spent another hour polishing today. Definitely made some progress.

[ProtoJeb21]

On 6/7/2017 at 3:15 PM, _Augustus_ said:

Started and finished #600 grit today. I'm pretty much done with grinding unless it turns out I'm supposed to use the emery supplied in the kit for something.

As Zeiss Ikon was saying, the mirror blank and tool getting stuck together was a real problem at #600. But I suppose that means that the mirror is a good sphere, because otherwise that wouldn't happen.

I've also started thinking about what I'm going to do after this project is done. The tool can be used again to grind another few 6" mirrors, and then be ground into a thin meniscus-shaped mirror, but there's no real point in that unless I make them to sell.

I'll probably pick up a secondary mirror for this scope at Stellafane, a large star party in Vermont. It's likely that I will buy quite a few mirror blanks in various sizes, because everything at the Stellafane swap tables is soooo cheap (Seriously, I would recommend going to Stellafane just for the swap tables).

 

On 6/7/2017 at 7:03 PM, _Augustus_ said:

July 20-23. 

Wait....a nearby star party? In the middle of summer? When I have nothing else to do? Oh I am SO going to (try to ) go to that!

[_Augustus_]

56 minutes ago, ProtoJeb21 said:

 

Wait....a nearby star party? In the middle of summer? When I have nothing else to do? Oh I am SO going to (try to ) go to that!

You'll have to be able to convince a parent to camp there for a couple days - otherwise you won't get much observing time in.

[YNM]

Now that I see pictures it's interesting ! Good luck !

 

Is it hard to get all those tools you need to have ? I mean, well I'm not where most of this forum's users live but , and getting telescopes here are very hard ! Problem is, it's probably going to be even harder to make one as some of the steps would probably be hard to find and be done...

Edited June 11, 2017 by YNM

[_Augustus_]

10 hours ago, YNM said:

Is it hard to get all those tools you need to have ? I mean, well I'm not where most of this forum's users live but getting telescopes here are very hard ! Problem is, it's probably going to be even harder to make one as some of the steps would probably be hard to find and be done...

Carborundum - probably available at at an auto shop or hardware store, if not, you can always try screening your own sand like John Dobson himself did

Mirror blank and tool - portholes, etc. could be used. If you're short on glass, make a plaster/tile tool instead of using a glass one.

Pitch - can be made from pine sap, IDK where you could buy it

Rouge - Probably available at an eyeglasses shop?

For aluminizing the primary mirror, you'd probably have to ship it somewhere (maybe Australia/NZ??). The secondary mirror and eyepieces would have to be ordered online. The focuser, mirror cells, OTA, and mount can be made with plywood, PVC, and a few other things.

Here in the US you can just order kits and parts from various astronomy shops. I bought mine "used" (it had just been sitting in a box) on an astronomy forum for a slightly lower price than a new one.

Edited June 11, 2017 by _Augustus_

[_Augustus_]

Did a bit more polishing. The mirror is significantly more transparent.

[YNM]

3 hours ago, _Augustus_ said:

Carborundum - probably available at at an auto shop or hardware store, if not, you can always try screening your own sand like John Dobson himself did

Mirror blank and tool - portholes, etc. could be used. If you're short on glass, make a plaster/tile tool instead of using a glass one.

Pitch - can be made from pine sap, IDK where you could buy it

Rouge - Probably available at an eyeglasses shop?

For aluminizing the primary mirror, you'd probably have to ship it somewhere (maybe Australia/NZ??). The secondary mirror and eyepieces would have to be ordered online. The focuser, mirror cells, OTA, and mount can be made with plywood, PVC, and a few other things.

Considering I own a telescope myself already, hypothetically :

- Carborundum : Not really sure on this one. Most sites here will point up at sandpaper instead when searched on, so it's not going to be very easy. EDIT : Actually found it from local sources/importers, along with the sizes, but still online.

- Mirrors, tools (lap etc) : Well let's be honest I never hold any glass or anything optical of the size I saw you using for your mirror now...  here we do have a lot of ceramics and marble stones instead so could we just make one mirror out of that instead ?

-  Pitch : does it really need pine sap ? We don't have pines... Although I remember once using something very similar, it starts as flakes, then after heated up until it melts it'll settle and hardens like plastics, ends up used like a polymer glue. Presumably not from pines, completely forgot the name.

- the rouge (polish) : I have completely no idea what's that actually is, but sound hard to find one...

- Aluminizing : this. I have completely no idea again, only saw chroming so far (and even that's on metals, not glass !).

- OTA etc. : well it's not that bad really but there's no telescope shop to always run into here. Only one physical shop in the entire country that I know fully sells telescopes here, the rest falls into being sold at home appliances shop and so handling isn't their expertise (and all of those are concentrated to the capital city area).

Yeah, sending things from overseas seems to be the only way out now. I presume the tax and shipping won't end up completely different, but not sure on that one. Are any of your tool worth more than fifty to a few hundred dollars in one piece ?

Edited June 11, 2017 by YNM

[_Augustus_]

9 minutes ago, YNM said:

- Mirrors, tools (lap etc) : Well let's be honest I never hold any glass or anything optical of the size I saw you using for your mirror now...  here we do have a lot of ceramics and marble stones instead so could we just make one mirror out of that instead ?

-  Pitch : does it really need pine sap ? We don't have pines... Although I remember once using something very similar, it starts as flakes, then after heated up until it melts it'll settle and hardens like plastics, ends up used like a polymer glue. Presumably not from pines, completely forgot the name.

- the rouge (polish) : I have completely no idea what's that actually is, but sound hard to find one...

- Aluminizing : this. I have completely no idea again, only saw chroming so far (and even that's on metals, not glass !).

You have to use glass for the mirror.

There are alternatives to pitch but I don't know how good they are. 

Red optical rouge is used for eyeglasses sometimes but I don't know where you'd get it.

For aluminizing, there are probably people in Australia/NZ that can do it for you for a pretty decent price. Or you could try chemical silvering, but you'd have to resilver your mirror every couple of months.

Edited June 11, 2017 by _Augustus_

[Adstri]

Interesting... I am currently trying to build one with an aperture length between 8-12 inches to do some deep sky work. I am currently making the blueprints. Hopefully my building process goes as good as good as this one!

[_Augustus_]

2 minutes ago, Adstriduum said:

Interesting... I am currently trying to build one with an aperture length between 8-12 inches to do some deep sky work. I am currently making the blueprints. Hopefully my building process goes as good as good as this one!

You mean aperture?

I would go for a 10" as a complement to the 4SE. Only reason I did a 6" is because budget constraints. A 12" is too big for a first-timer to grind.

 

[Adstri]

2 minutes ago, _Augustus_ said:

You mean aperture?

I would go for a 10" as a complement to the 4SE. Only reason I did a 6" is because budget constraints. A 12" is too big for a first-timer to grind.

 

Did I say aperture length? (－‸ლ)

 

I have a 4" and a 6". I WOULD go for a 12" IF my budget wasn't so tight. A 10" would be best. My head is just in the clouds