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

[_Augustus_]

I am going to begin construction of the mechanical parts of the 10" planet killer later this week. Mirror blank will be obtained at Stellafane.

[Green Baron]

We are trying to find more planets, don't kill them :-)

You haven't given up, good news ! Are you aiming for f/10 or thereabout ?

Good success !

[_Augustus_]

5 hours ago, Green Baron said:

You haven't given up, good news ! Are you aiming for f/10 or thereabout ?

Good success !

I'm aiming for f/7-f/8. This allows for a 1.83" secondary for maximum contrast, but short enough to not really need a ladder. The OTA will use a split-tube design for portability.

[_Augustus_]

The 6" is almost separated from the lap. I might be able to finish it after all!

[_Augustus_]

The 6" is free! I will order more supplies and finish it soon.

[monophonic]

3 hours ago, _Augustus_ said:

The 6" is free! I will order more supplies and finish it soon.

Great news! I must come back to like this when my next 25 like allowance is open. 

[_Augustus_]

Ordered pitch, cerium oxide, and dental plaster so that I can finish the 6".

[_Augustus_]

I beveled the 6" mirror and then went back and ground with some 305 emery to remove some very shallow scratches. There's one scratch left near the edge but it shouldn't affect much. Polishing supplies arrive tomorrow.

Edited July 12, 2017 by _Augustus_

[YNM]

Godspeed ! That sounds extremely climatic now ! I hope you can make many telescopes...

[_Augustus_]

Pitch and cerox are here. I bought dental plaster but apparently that takes a few days to cure, and I'm not waiting for that. I will use the plaster for a sub diameter lap for my 10" later on.

[_Augustus_]

I went and bought a hot plate so that I wouldn't have to make a mess on/around the stove. The hot plate worked very well, and I probably warmed up the pitch a bit too much. But I managed to pour and press the lap without difficulty.

I trimmed the lap using the rubber grid supplied in the kit, then microfaceted it with an Xacto knife. The grid and microfacets exist to allow the cerium oxide to flow across the surface of the lap. 

I polished for around an hour. The mirror looks closer to polished out now.

The channels in the lap wore down very fast, and part of the lap appears worn (I don't have a better description). I will have to retrim the lap tomorrow. 

[_Augustus_]

Polished for another 30 minutes after retrimming the lap.

[_Augustus_]

 

[Dartguy]

How did you end up getting them separated? Sorry I missed that part.

[_Augustus_]

43 minutes ago, Dartguy said:

How did you end up getting them separated? Sorry I missed that part.

Leaving them in the sun, then pouring acetone on the mirror and tool to remove the remaining pitch.

[_Augustus_]

I may or may not make the 6" into a scope, but I am definitely getting a 10" blank next weekend. 

Edited July 14, 2017 by _Augustus_

[_Augustus_]

I set up the Ronchi tester today. I have an oblate spheroid, which is pretty close to a sphere and should return to that once polishing is done.

Apparently the focal length is actually 47", using measurements of the ROC taken from the distance between the mirror and tester. Interesting.

Edited July 18, 2017 by _Augustus_

[Zeiss Ikon]

20 hours ago, _Augustus_ said:

Apparently the focal length is actually 47", using measurements of the ROC taken from the distance between the mirror and tester. Interesting.

The focus shortens quite a bit during polishing and figuring -- far more than it seems like it should.  My 8" f/6.8 went from 70+ inches to 64 inches in those stages.  I think it's because of removing the oblateness, which makes the affected portion much flatter than the average.

[_Augustus_]

3 hours ago, Zeiss Ikon said:

The focus shortens quite a bit during polishing and figuring -- far more than it seems like it should.  My 8" f/6.8 went from 70+ inches to 64 inches in those stages.  I think it's because of removing the oblateness, which makes the affected portion much flatter than the average.

I thought it was 39" initially from measuring on the Sun, but I had a large margin of error there. I'm disappointed that I didn't grind it down further.

 

[Green Baron]

On 15.7.2017 at 1:33 AM, _Augustus_ said:

I may or may not make the 6" into a scope, but I am definitely getting a 10" blank next weekend. 

I would suggest to do it as a training and transfer results to the making of the 10". In case you meet more difficulties you could look for solutions on a sample where it doesn't hurt that much ...

Just a suggestion

[YNM]

12 minutes ago, Green Baron said:

On 7/15/2017 at 6:33 AM, _Augustus_ said:

I may or may not make the 6" into a scope, but I am definitely getting a 10" blank next weekend. 

I would suggest to do it as a training and transfer results to the making of the 10". In case you meet more difficulties you could look for solutions on a sample where it doesn't hurt that much ...

Just a suggestion

He probably means making the telescope (ie. tube etc).

@_Augustus_ Actually you could use the mirror for other things. Homemade schlieren apparatus perhaps ? Would be interesting in cloudy conditions 

[_Augustus_]

I forgot to mention that a friend who no longer has time for telescope making is sending me a whole bunch of free pitch, grit, polish, and I think some blanks. It will take a couple weeks though.

Maybe I'll build some other stuff with that. Perhaps a Gregorian?

[Zeiss Ikon]

I've wanted to make a Gregorian for literally decades.  I think making the secondary to diffraction-limited accuracy is likely to be a significant challenge, since the secondary for a 10" Gregorian would be only around 2" diameter, and it needs to be ellipsoidal, rather than paraboloidal (complicates testing a bit, in some ways).  One way to meet the challenge is to make the primary bigger, of course -- if you have a 16" primary, you could make the secondary with a standard 4" blank.  Of course, then you have to trepan, grind, polish, and figure the 16" primary...

[_Augustus_]

On 7/19/2017 at 7:21 PM, Zeiss Ikon said:

I've wanted to make a Gregorian for literally decades.  I think making the secondary to diffraction-limited accuracy is likely to be a significant challenge, since the secondary for a 10" Gregorian would be only around 2" diameter, and it needs to be ellipsoidal, rather than paraboloidal (complicates testing a bit, in some ways).  One way to meet the challenge is to make the primary bigger, of course -- if you have a 16" primary, you could make the secondary with a standard 4" blank.  Of course, then you have to trepan, grind, polish, and figure the 16" primary...

Didn't know the secondary had to be ellipsoidal. There's a nice log of the making of a 6" Greg made by this guy from the Netherlands here.

My only concern is boring the hole in the primary - I don't have the equipment to do that myself. I suppose I could do a Nasmyth design with a flat after the secondary, but that would lead to a bit of light loss and I'm not sure how one would collimate that.

EDIT: So the blanks he's sending are 4-5" plate glass blanks along with some smaller Cassegrain secondary blanks. I will pick up a 6-8" blank at Stellafane and save it for a Gregorian.

Edited July 21, 2017 by _Augustus_

[Zeiss Ikon]

On 7/19/2017 at 7:34 PM, _Augustus_ said:

A Gregorian uses two paraboloidal mirrors, both concave. There's a nice log of the making of a 6" Greg made by this guy from the Netherlands here.

Better recheck.  If your secondary is going to refocus and form a second real image without aberrations, it has to have two foci, which is part of the definition of an ellipse.  A second paraboloid would be correct if you're collimating a parallel beam to a larger or smaller diameter, but that's not what a Gregorian needs to do.

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.