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

[_Augustus_]

Blank gets here Monday. I start classes again on Tuesday.

[KG3]

That's quite a piece of glass!  I tried grinding a 6" mirror years ago.  It got stuck to the pitch lap, I panicked and tried to pry the mirror off.  It popped off, went flying across the room and smashed on the floor and that was that.  

So how do you choose eyepieces?  I seem to recall different eye pieces work better with different focal lengths but I can't recall how or why.

Nice work though!  

[_Augustus_]

17 minutes ago, KG3 said:

That's quite a piece of glass!  I tried grinding a 6" mirror years ago.  It got stuck to the pitch lap, I panicked and tried to pry the mirror off.  It popped off, went flying across the room and smashed on the floor and that was that.  

So how do you choose eyepieces?  I seem to recall different eye pieces work better with different focal lengths but I can't recall how or why.

Nice work though!  

Yeah, if you read back my first 6" mirror got stuck to a too-soft pitch lap as well. I chipped it a bit in a vain attempt to remove it. Eventually got it off by letting the mirror/lap sit in the sun to melt the pitch, prying them apart, and then bathing them in acetone to dissolve the pitch (Acetone, like pitch, is nonpolar and thus will dissolve it). The 12" is my third mirror. Despite being quite large it's thin and thus lightweight at only 1" thick - not quite thick enough to support itself under its on weight, but thick enough to require relatively simple support.

You typically want at least 3 eyepieces - one around 20-32mm (depends on the scope) for low power (i.e. large DSOs and finding things), one from 12-18mm for medium power (most DSOs and maybe the Moon), and one from 5-10mm for high power (small DSOs, double stars, Moon, planets). Sometimes you'll want one below 5mm for really high power with a scope below f/6 or so. More eyepieces than 4 or so tend to be unnecessary and seldom used. Price usually depends on the apparent field of view, coatings, and performance (i.e. astigmatism/coma correction) below f/6. 

[_Augustus_]

12" blank is here. It's a bit heavier than I expected but still very nice.

[Kaa253]

Sorry I missed this thread last year!

Good to see a 12" is in the works

 

[_Augustus_]

Started working on the mirror today after pouring a plaster tool. Sagitta is very slight after 1 hour of #60.

[_Augustus_]

Well, I was doing things wrong (using too much grit). After just 30 minutes with #60 I've got a reasonably visible sagitta about 1/10 of the required depth.

[_Augustus_]

At nearly 1/2 the required depth now after a few more hours.

[_Augustus_]

So I've... probably reached the target sagitta. The spherometer reading is just plain wrong (I might just be reading it wrong, IDK), while the coin test says I'm probably at f/7.5?? In any case I ran out of #60 completely and my arms ache.

A tile fell off the tool somehow; it was at the edge. I might pour a new tool for fine grinding since the edge tiles are getting so thin.

Tomorrow I've got to hose down everything to prevent contamination from #60; I'll try measuring the FL using the Sun. Worst that happens is I've got to do extra grinding with #80, of which I have plenty.

[_Augustus_]

I've been busy and haven't gotten a chance to work on the mirror in nearly a week. Today or tomorrow I'm going to do some further hogging with #80 since I'm nowhere near even f/8 yet.

[_Augustus_]

I'm about to go downstairs and work on the 12". I just bought a 16" blank and an 8" homemade Dob I'll likely part out for very little money, so it appears I've found the material for my next project.

[_Augustus_]

I can fit 2 dimes in the sagitta, so I'm pretty much at the target. I did the pencil test and switched to center-over center and to my surprise the mirror and tool are in pretty good contact, even at the edge. I attribute this to my use of circular strokes like what Udjat uses instead of the regular chordal stroke.

I'm debating whether to continue with #80 or just switch straight to #120. The difference in size between #60 and #80 pits is small, and I don't see the point in working to get rid of all of the #60 pits then having to work to rid myself of #80 pits.

[_Augustus_]

My tile tool is toast. The center tiles aren't wearing down at all, there's no contact at the center, and it appears that the worn and non-worn tiles have completely different radii. In addition, the edge tiles are so thin that they might disappear after the next work session.

I ordered more plaster (I'm pretty much out and don't have enough for a new tool), should get here Saturday, but I leave for Florida on Sunday and get back the following Saturday. This time it's blue instead of pink.

[_Augustus_]

Through a bizarre series of events, I managed to get a complete homemade 8" Dob with a dual-speed focuser, a bunch of testing equipment, a bunch of grit, and a 16" f/5 polished mirror for the whopping price of $150.

The 8" scope is abominably heavy (the mount is entirely constructed out of 3/4" plywood), and the primary needs recoating, so I'm parting it out because a) I don't need it and b) if I can reclaim some/all of my investment by selling the optics, why not? I am keeping the mount because while the paintjob could be better (Black mount and black tube? Really?) and it weighs about as much as a small child, it does have nice construction and even an adjustable tube cradle - and it's not worth the cost of shipping anyways. The focuser I'm keeping as well, obviously - it alone is nearly worth the price I paid for all of this.

The seller told me an interesting story. He wanted to make his own scope so he bought a kit to make his own mirror grinding machine (a mirror-o-matic kit to be exact) , and proceeded to make an 8" on it. He dropped the 8" during fine grinding and bought a GSO primary to build a scope with. Then, despite having no prior skill in even polishing a mirror, he proceeded to make the 16", and gave up during figuring because he had no idea what he was doing. Now he has an 8" Skywatcher collapsible scope and has given up on the whole ATMing thing, so he sold me everything but the machine (I don't really have the money for it nor the space).

In addition to having no prior skill in polishing/figuring, this guy used Burgundy pitch (which if you remember from when I started making mirrors is unreliable and bad) and ROUGE (messy and slow-working) to polish despite having PLENTY of cerium oxide and zirconium oxide (the latter works slower and leaves a slightly smoother surface than cerox - like rouge but without the mess). There's still rouge on the test stand he made for it that gets on my fingers whenever I touch it, and the entire side of the mirror is stained with it (though thankfully it doesn't come off).

I plopped it on the Ronchi tester (which I had to position a little over 13 feet away!), expecting either a somewhat decent sphere (assuming he just gave up on figuring without actually trying it) or a mess. I got the latter. The mirror is pretty astigmatic, as well as hyperbolic (though the astigmatism makes it hard to tell and it may not be that severely overcorrected). Just to make sure the astigmatism wasn't caused by the glass itself having issues, I tested the mirror for strain - I found none. So the messed-up surface is his fault, and I can fix it.

Despite the issues with the surface, correcting this 16" will take far less time and effort than working the 12". So once I pour a new tool and finish with #80 on the 12", at which point I'd have to clean up my work area anyway, I'm going to put everything away and focus on finishing the 16" instead. 

Does this mean I'm abandoning the 12"? Heck no! I just don't have the money to complete both at the moment and I'd rather work on the one that will take less time and has more aperture. The 12" may get redesigned to be either a faster and more portable instrument or a slower (f/7-f/8) Chief with no central obstruction, but I will still finish it. Just not right now.

 

 

[_Augustus_]

My new plaster somehow got here today, so I can vacuum the shop tonight and pour a new lap tomorrow.

[_Augustus_]

Poured the lap base. I wanted to have it match the curve, but I don't have a 12" wide wooden ring so keeping the mold rigid and having it be sub diameter wasn't possible. I ended up pouring it against the back of the 12", so it's pretty much flat. Since I'm pouring pitch on it anyway and this is an f/5 mirror, I'm not concerned.

More tomorrow.

[_Augustus_]

So my hot plate self-destructed and I had to buy a new one yesterday.

I poured the lap and carved the channels with a hacksaw. Using this thing is an annoying process as it can't overhang the mirror, but I have to be careful not to place it on the mirror too hard, otherwise the edges will chip.

I put the mirror on the tester and it's much better than before, but the astigmatism is far from gone.

Edited February 3, 2018 by _Augustus_

[KG3]

Are you in school for astronomy or engineering?  Are you planning on doing any astrophotography with any of the scopes you are making?   I'm amazed at how amateur astronomers team up with professionals these days to do real science.    

Sorry to hear about your hot plate.    

[_Augustus_]

7 hours ago, KG3 said:

Are you in school for astronomy or engineering?  Are you planning on doing any astrophotography with any of the scopes you are making?   I'm amazed at how amateur astronomers team up with professionals these days to do real science.    

Sorry to hear about your hot plate.    

1. Nope.

2. I've done some afocal Moon shots with a phone but that's about it (I might be able to get some decent planetary shots, too, but still nothing amazing). Serious planetary AP requires a fancy webcam with a tiny chip and tiny field of view, which without tracking means the target drifts out of the field after a few seconds. Long exposures? Forget it.......

3. No worries, bought an identical new one for $15.

[_Augustus_]

Okay, so it's been a while! I was in Florida to watch the FH launch and got back a few days ago and neglected this thread.

I got back to refiguring on the 11th and noticed that my wooden turntable didn't rotate evenly and wasn't staying level, which I thought was odd. Turns out that it has buckled into a saddle-like shape, probably because it was stored in the garage for a while before I used it. I did get around half an hour or so in before I realized this. I have a new turntable, but I haven't bothered working on the mirror yet since.

I did some further Ronchi/Foucault (I sort of understand how to do the latter now) after my work session, and I found the following:

• The astigmatism is nearly gone. It's still noticeable and distorts the bands a little, but I can now actually read the Ronchi bands and they're not curvy. I rotated the mirror just to make sure it wasn't some sort of coincidence of the astigmatic axis being dead horizontal or vertical, and still saw the same thing.
• The mirror is slightly overcorrected. I don't know by how much, but doesn't differ a whole lot from the "ideal" patterns for a 16" f/4.9 (I remeasured the ROC to be around 157") on Mel Bartels' Ronchi page, so I'm not too concerned. Why it's not spherical is anyone's guess, but I'm not complaining......
• There's a 3" diameter "hole" (depressed zone) at the center of the mirror. I'm not sure how it got there, and I'm not sure if it was there before and I just didn't notice, but since it'll be behind the secondary mirror I don't particularly care.

Since the mirror is probably capable of throwing up a good enough image that I can determine the focal point, I've started working on the scope itself. I currently have the mirror box (not the cell or bearings, just the box) finished and am about to move on to the mirror cell.

 

[_Augustus_]

Secondary mirror is here, and the trusses should get here Tuesday. I ordered a spider as well, but that's going to be a while - it's possible I won't have it until mid-March. That sucks, because I need the spider to determine the OTA balance point......

[_Augustus_]

Mirror cell is done.

[_Augustus_]

So the biggest local astronomy club has their annual Messier Marathon on March 16th this year. I could bring the 6" to that, but there would probably be a fair amount of M's that would be either invisible and show no detail, either. To that end, I'm now targeting a date of March 7th or so to have the scope done (so I can allow a week for coating), giving me the possibility of bringing it to the Marathon.

Because of the date requirement I'll make my own curved spider - the secondary is small enough that I think it will do. Later today I will go to Home Depot and get my FRP, foam wrapping for the trusses, and more screws, and the local wood shop to get my UTA rings and alt bearings cut (my %&@* router's sub-base does not detach and thus it's useless for cutting circles as it cannot attach to a jig). Tonight or Friday I'll finish constructing the entire scope, and prepare for star testing (looks like I can do that Tuesday).

[_Augustus_]

Sorry for the lack of updates. I went to HD yesterday and got my own circle jig for my router, genuine Formica, screws/bolts, etc. Also started attaching the trusses to the mirror box and added ventilation holes to the back.

[_Augustus_]

OTA is (mostly) complete (I'm 6' 1" for scale).

The wood blocks mounting the trusses to the UTA need to be re-done because the bolts are angled and thus won't fit through the trusses, but other than that the only work left is the mount (50% done already) and curved spider. And there's some painting/staining work that I'll do after I test this with the uncoated mirror.