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

[_Augustus_]

I'm replacing the awful setup I had for grinding my last mirror. This time I'm using an IBM workbench (made for assembling computers) with a homemade turntable on top.

[_Augustus_]

I poured the plaster disk and epoxied the tiles on.

[_Augustus_]

I started and finished grinding the 6" f/5 with the #80 grit today. The sagitta measures to around .077" or so, making this around f/4.8.

[_Augustus_]

Worked with #120 for at least an hour. Not sure if I need to do more work with it or not. The mirror and tool have very good contact and the pits aren't bigger than the #120 grains.

[_Augustus_]

I just got started with #220 and worked for around 30 minutes with it. 

[_Augustus_]

Finished work with the #220 after another hour of work.

[_Augustus_]

I started and finished work with the #320. Tomorrow I'll use the WAO 9.25 micron grit. Then it's time for polishing!

[_Augustus_]

Today I got a lot done. I used the 15 and 9.25 micron WAO and then poured a pitch lap on top of the tiles and started polishing. The mirror is maybe 1/4-1/3 of the way to being polished out. 

[_Augustus_]

Another 90 minutes of polishing and I'm getting pretty close. Maybe 3 hours more, tops.

[_Augustus_]

It's completely polished out now. Focal length is 26" making it an f/4.3. 

Edited August 9, 2017 by _Augustus_

[_Augustus_]

Apparently the edge is a bit messed up and I have to do some more polishing. 

[_Augustus_]

Yeah, okay, it has turned-down edge. Here it is after 20 minutes of polishing with rouge to try and fix it (it's a bit better).

 

[_Augustus_]

I polished for 30 minutes with cerox. Unfortunately I can't test the mirror for another few hours (we have the maid here). I want to see if the TDE has gone away at all.

Edited August 9, 2017 by _Augustus_

[_Augustus_]

TDE is almost gone.

[_Augustus_]

It's definitely reduced but still not completely gone. There are also rings now because the lap had poor contact with the mirror (fixed that).

[_Augustus_]

TDE is gone, and so are the rings. I started parabolizing yesterday, and I'm around 1/3-1/2 of the way there. I leave for the eclipse tomorrow so I won't get to work on it again until the end of the month (today I'm tired and don't feel like working on it).

I'm going to construct the scope for star testing before I get the mirror aluminized.

[_Augustus_]

Finished parabolizing this morning. Going to start scope construction soon.

[_Augustus_]

I've also started working on a 4" finderscope for an upcoming larger Dob. Stay tuned.....

[_Augustus_]

I've started constructing the scope for the 6" f/4.3. The OTA is a Sonotube painted Royal Blue on the outside and flocked on the inside with black-painted cork. Focuser is a cheap 1.25" Chinese rack-and-pinion which I had lying around, secured to the tube with screws and epoxy. 

The boards for the mount have been cut and I've sanded and stained the ones for the rocker box. Over the next few days I'll finish sanding, staining, and assembling the mount. I still have to order a 1.83" secondary and spider (trying to find a used one to save money).

[_Augustus_]

Rocker box is done

[_Augustus_]

I had to adjust the angle brackets to get the OTA to fit but it fits snugly now. I just stained the azimuth board/bearing. Still have to order a secondary and spider (which requires a lot of $$$$).

[_Augustus_]

Scope is done, awaiting secondary mirror.

[munlander1]

On August 24, 2017 at 5:37 PM, _Augustus_ said:

I've also started working on a 4" finderscope for an upcoming larger Dob. Stay tuned.....

A reflecting finder scope?

 

[Green Baron]

Errr, i am confused. Why is the tube that long ? The focuser sits quite low ... isn't it a newton or do you consider the part above the focuser a rigid dew shield ? Or just for historic reasons,  focal length planned to be longer ?

[_Augustus_]

33 minutes ago, munlander1 said:

A reflecting finder scope?

 

Yeah. I don't know if I'll use it for the 8-incher though. Maybe a larger scope. Have put it on the backburner.

3 minutes ago, Green Baron said:

Errr, i am confused. Why is the tube that long ? The focuser sits quite low ... isn't it a newton or do you consider the part above the focuser a rigid dew shield ?

It's to block stray light, though I suppose the dew protection is nice too.