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Mirror making problems


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Looking good Dave. Seems like the parabolization is progressing well, looks a bit deeper in the middle than over the rest of the curve? (I may have got my ronchi-gram intrepretation wrong though!). Careful you don't dig too deep a hole in the middle -- I got a bit agressive on parabolizing recently, and ended up with a -4 hyperbola before I caught it!?!

You certainly did a very nice job getting rid of that TDE!!

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I ran into some problems and havnt been able to update with anything worthwhile. However today I got it all taken care of and have done some more parabolizing and tested using a couder mask and foucault tester. As of now I am near 1/3 wave according to my tests. I am thinking tomorrow I will have the mirror finished..

*crosses fingers*

Dave

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Dave, sorry I have not been around to comment, I have been away on holiday. Looks like you have made some progress, well done. I agree with TeaDwarf, the mirror looks a little too deep in the middle. The best course of action is to take some zonal measurements as you have suggested. If is best best to start making measurements as soon as you see the ronchi bands bowing outwards just inside the ROC. You can then ease the curve down to the required parabola by taking zonal readings every 10-15 mins. TeaDwarf is quite right, it is very easy to go hyperbolic (!). Working 'downhill' towards the desired parabola is a lot easier (and things progress more quickly) than working 'uphill' towards the parabola from a hyperbola....ie a curve that is too deep. Looking forward to hearing how you progress.

John

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22nd-900p.gifSince the last update I have had to go back to a sphere to get rid of that dang hole. It was pretty deep and took hours of work and testing. However I have been diligent on working this blasted piece of glass.

I kind of wish I would have updated more often on the steps that I took. The problem was I really wasnt exactly sure what I was doing and I did some things I am sure experienced mirror makers would scald me for =)

That being said I believe I am either EXTREMELY close or even finished now. I will be doing some more tests as the night goes on. I want to give it more time to equalize to the temperature and I will try rotating it a few times also.

I want to give a BIG Thank you to John. You helped me out when I really needed some encouragement. Your a good guy and I thought the least I could do was update you on my progress so far.

Dave (retired mirror maker) hopefully :BangHead:

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Dave, thanks for your kind comments. I love mirror making and do my best to encourage people to have a go, it can be hugely rewarding....just wait for the thrill of first light with your self made mirror! The results look pretty good, that could be regarded as done if your patience has ran out. I have to raise an eyebrow at the software that you are using...it gives a strehl ratio based on a few sample points which is quite frankly meaningless. To achieve a meaningful Strehl number you have to use interferometry which produces the required (high) number of sample points across the mirrors surface. Bit worried about your last comment...I hope you won't retire from mirror making, possibly a short break is in order.

Dave, you have done really well with this mirror, making a mirror is not the easiest thing to do, it can take you outside of your comfort zone, but the final product is surely worth while.

John

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I might try it one day - I've already been well outside my comfort zone with the pier and obsy build, so nothing new there! :) One thing at a time... :eek:

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I tried to touch it up just a little and put a zone in it GAH!

I tried to blend that zone out and made it even worse and oblate.

So today I been working on getting it back in shape again. My last test put it at 1/4 wave according to FigureXP.

It takes awhile because I am using plate glass and I have to let it sit for an hour after every figuring session.

Dave

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One of the most difficult decisions to be made when making a mirror is when to call it done! I have overcooked a few myself over the years, hope it is not too difficult for you to sort out.

Gina, if you decide to have a go at making your own mirror their is a lot of experience here on SGL...I am sure that you will not be short of support and help.

John

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Gina, if you decide to have a go at making your own mirror their is a lot of experience here on SGL...I am sure that you will not be short of support and help.

John

Yes, I know, great people here :):eek:
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Ok I have relentlously been working to get this mirror finished.

I now am 99% sure I am done lol!

Just an update and another thank you for the help. I wanted to make sure those who were following or helping out knew that I did complete the mirror.

I will admit some of this is luck because I really didnt imagine it coming out this good. I took several tests and they are all consistant and here are the results.

30th-1142aS.gif

I changed the LED on my tester and its much brighter which makes my Ronchi images hard to see with a camera now but here is about -0.300 outside focus.

SSPX1980.jpg

Thank you,

Dave

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MirrorCell.jpgYes John this will be my 1st telescope ever and I plan on making the whole thing. I decided against a few parts like the secondary mirror and focuser and eyepieces though haha they are just not worth doing.

Here is a 3D model I made up of how it will look. I have already started on a few things like the Cell and secondary cage.

I used a few programs to determine what my Cell and vertical support will be (ie. PLOP and a webpage from Nils) so thats different than in this picture I am going 6 point and no sling. Just 2 supports at 90deg.

Dave

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Just thought I would stop by and give an update.

Here is my "almost" finished secondary cage.

SSPX1994.jpg

Here is a better view of the focuser.

A guy GAVE me that focuser for free and wouldnt even let me pay for shipping. It was a guy named Steve from the Cloudy nights forums that was following my progress and noticed I needed one. He sent me an email offering to give it to me. So sweet! I wanted a crayford so bad but couldnt afford it.

SSPX1995.jpg

Dave

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