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My 14" Astrograph


gremlin
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I've always loved fast scopes. I got some amazing results from a well collimated newtonian which I owned a few years back. So earlier this year I decided I would build a custom 14" astrograph. One of the key factors I decided early on was that I was going to grind/polish/figure the mirror myself. The other design choice was a carbon fibre tube. I was consodering building this myself, but given the cost of composite materials and the possibility that I could stuff it up, I decided to just buy the tube. Well the tube is due next week and the mirror arrived back from the coating plant last week. So finally after months of hard labour, its all starting to come together.

The mirror is very fast, f3.85. I had never made a mirror this fast before. My previous was 2 mirrors around the f/6- f/8 range. I was initially scared by the daunting task of making a large(ish) fast mirror, but the practice proved a lot less scary. Yes it is certainly a LOT more work to make a mirror this fast, figuring this baby was a real learning curve too. I also had to learn how to build and use a laser interferometer to properly test. the result was a lovely figured mirror with an RMS wavefront of 1/23 wave. More than adequate for a fast astrograph.

Anyway I did write up an extensive article/tutorial to document a lot of the things I learned while making this mirror. The link is below and I hope some find it interesting/useful.

http://www.webtreatz.com/resources/Grinding_a_14inch_mirror.pdf

Ill post some piccies etc over the next week or two as the project finally reaches a climax.

Oh one last thing, the corrector. I am considering a high end wynne corrctor from ASA. Before that I'm going to give the baader mark III corrector a spin. If it works well, I can spend the difference on something nice for the scope.

Clear Skies,

Dave.

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Fantastic work on that mirror Dave!! I too ground my own 14" F5 mirror earlier this year. I am a little concerened that I may have caught the mirror making bug. Quite fancy having another go, and threads like this aren't helping, hehe.

That is a brilliant write up of the mirror making process. Some very good tips and tricks in there which will really help anyone thinking of doing their own.

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Hi Alan,

Yes it really is a bug. Congrats on your mirror. I love hearing about people going down this route. I'm sure it gives you immense satisfaction to view the heavens through an optic crafted by your own hand.

My next mirror making project is to grind an exact duplicate of a 10" f/6 mirror which I was given. Then construct a pair of 10" binoculars. I suspect the mechanics of that will be a lot more complex than making the mirror!!

CS

Dave.

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WOW!! That's extremely impressive - well done :) I would love to make my own scope but unfortunately there just aren't enough hours in the day to do what I would like and I don't have the stamina I used to have.

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Hello Dave, that's a great write up of your mirror work, I'm printing it off now 'cos I'm currently trying to figure a 12"......... heading back to a sphere... this time without such a badly turned down edge! Sometimes this hot weather has it's drawbacks.

Good luck with the build.

Keith

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Hi Keith,

Yes if you got a TDE, you need to get on top of it before you can take it any further. Is it a classic TDE with maybe only 1mm or so turned down badly or is it more of a 'rolled outer zone' where the outer zone(s) tapers down gradually? I've heard both referred to as a TDE when they are in fact two totally separate defects. This will determine how you need to treat it. If it is the latter you can sometimes shortcut the process by deepening the zone where the rolling begins (if it's not too far from spherical). If it is the former, you could consider cheating and just mask off the outer 1mm What f/ratio is your mirror?

Dave.

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Fantastic work and a very interesting read, it will be most satisfying collecting first light on this project :)

I have been thinking about mirror making whilst reading your guide and was wondering if it would be possible to "set" the parabolic shape back at the glass factory whilst the glass is in the furnace by somehow placing the glass in a rotating container. I'm assuming here that a parabolic shape is formed when a fluid is rotated (looking at my freshly stirred cup of coffee in front of me), also that the shape of the glass would not change as it was gradually cooled (whilst keeping it spinning all the time). Im assuming there are probably several technical reasons why this would be not possible, which seems a shame as this could save a lot of the initial work creating the correct shape. I would assume too that different rotation speeds would result in different focal lengths so you could get the FL exactlly right will a sufficiently accurrate motor drive.

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I have been thinking about mirror making whilst reading your guide and was wondering if it would be possible to "set" the parabolic shape back at the glass factory whilst the glass is in the furnace by somehow placing the glass in a rotating container.

Spin casting large mirrors is actually done for some of the bigger telescopes. It works. However it doesn't froduce a perfect surface due to lots of factors. It does remove a lot of the hard work, especially when making 8-m class mirrors. But final polishing and figuring is still required.. Some of the worlds largest observatories had their mirrors made this way.

Dave,

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Thanks Dave for the reply, I spent most of yesterday working on that TDE and it's virtually gone, I forgot to bring the camera in.... will post a Ronchi pic tomorrow. But I have a 10" lap and a 6" lap. the 10 is working well but last time I used the 6 it was leaving deep (well relatively) lines (not scratches) even after a lopng cold press.

Might be better in this warmer weather.

I've never had the pitch squares join up before, 32C degrees yesterday in the garage!

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Keith, my advice is to hold off until it cools a bit. 32deg is way to warm to be polishing at. The pitch is just going to close up before any real work gets done anyway. It's really just asking for trouble. Yes please post the ronchigram.

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Thanks all!!!!!! , i was polishing in the morning at 22C and stopped at 27 ish...later in the evening it had dropped to 26, I have re-cut the channels once and they are beginning to get close again.... still grooves but only 1/16th inch. Will cut back to a 1/8th again before polishing more.

Wish I'd remembered my camera!

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One of the things that can happen when pitch is too soft is that as the edge of the lap passes the edge of the mirror, the mirror sinks a bit into the lap, when the stroke reverses, the lap has to ride up on the mirror a little leading to the dreaded TDE again. All my polishing is done at around 18deg. If its gets beyond 20, I either open a few windows to cool or just leave it until its generally cooler.

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Damn, never thought I'd be cursing a 3 week heatwave! Guess I can justify buying a chiller for the garage now! This telescope's gonna cost a bomb in hidden charges!

Sorry, that must be my fault. Put a rain barrel to catch the rain from the garden shed. It hasn't got enough rain yet to get to the tap :grin:

With the pitch lap you can always make a harder lap to use during the hot weather.

Nigel

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You've got something interesting happening at the edge there - Looks like a bad TDE. Nigel will be able to give you some good advice about how to correct, but hopefully it won't require returning to fine grinding!

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Keith, in order to make a statement about your ronchigram it is necessary to know if the image was taken inside or outside if the ROC. If you knew by exactly how much, it would be possible to make even further statements on it. Looking at it as is shows you might have something going on at around the 60% zone.

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I'm sure I won't have to go back to fine grinding as I've reduced TDE significantly in 2 hrs.

The Foucault before trying to go back to spherical was posted in the 'figuring a 12" morror' thread......... I'm pretty sure I can get that smooth again.

I'll take another foucault and Ronchi tonight.

But like Gremlin said, I don't think I can do more TDE work until it cools down a bit!

Thanks all.

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With the hot weather predicted to stay for some time, why not try making up a second pitch lap with harder pitch to work during the warmer times?

The turned edge ( can't say up or down ) is too small to go back to fine grinding. Just keep on with your polishing and keep a close eye on what is happening to the edge. One of the advantages to extended polishing time is that you can be sure that all grinding pits have been removed :laugh:

If you use the knife inside/outside COC you will notice there are interference bands just ahead of the solid knife edge. You can usually see two or three. These are 1/2-1 wave apart and are a useful rough guide to the edge error.

Nigel

Edited by Astrobits
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