Help in polishing and figuring a 6" mirror

[Garyh]

Hi Cristian, It was a lovely night the day before so I actually got outside to do some imaging for a few hours, and put the mirror off to last night. I downloaded figure xp and I will have a play tonight with it. Looks more complicated than the other one I have but does so much more. I will give you some figures from my mask as well as my figures. I don`t know how accurate they are so will repeat them tonight again. I will need help in this last step to touch up the mirror and what stroke to apply.
I find it hard to read the outer edge as I have to keep moving my head from side to side to see the shadows and also the narrower window makes it hard as well. My measurments are the average of 3 lots, as they varied a bit on the outer zones. (need much more practice at this!!!).
Anyways let me know what you come up.
Best regards Gary

ROC = 1716mm
Dia = 154mm

Mask details:
Main mask zones
Zone R inner R outer R center


0 10.080 37.620 23.850
1 39.200 54.780 46.990
2 54.780 66.820 60.800
3 66.820 77.000 71.910

My readings are:
Zone 0 1mm
zone 1 1.9mm
zone 2 3.165mm
zone 3 3.98mm

[cristian abarca]

Hi Gary I imputed your numbers into figure XP and if you have a fixed light source like texereaux then It's looking good. Punch in your figures when you can it's not complicated and try the different buttons. Acoording to the program you have a 1/6 wave mirror give or take a little. I would be inclined to try for 1/10 . It just needs a little more tweaking and you will be ready. The outer zone is always a bugger to get. Redo the measurements a couple of times before attempting to figure the mirror again. Try 5 or seven instead of 3 also a very little bit of light (not total darkness)ay help when you are testing the outer zones. Some commercial mirrors sold as 1/4 wave don't look as good as this.Keep going. If you have any problems I'll be home after 6 tonight and online after 6.30 If you need any more help specially understanding some of the details of Figure XP.

Regards Cristian

[Garyh]

Hi again, Its another top night here so I will do a few hours of imaging of M16 and M20. Thanks for all the help so far, I shall get into that new program tomorrow and see how I go. I gave the program I have couder32 and inputted my readings and that gave me a similar result of about 1/7 wave error. With a bit of playing with the numbers in zone 2 and 3 the error would drop to 1/30 wave error. So if my readings are right if I can reduce the outer zones I will have a great mirror. But I will do another 6 readings like you said before I try to correct anything. I think I will give that ago and will be happy with 1/10 mirror!!!!.
Can I ask a silly question? with the shadows and reading them with the mask I have been moving the knife back till both windows fade exactly the same when the knife enters the path and if one window fades or lightens slightly when the knife cuts the beam then I adjust the knife carriage forwards or back again till I cannot see any difference..is this correct?..just checking..
Oh my tester is similar to texereau`s one with fixed source.
If you want the next lot of readings let me know and I will post them and go from there....
Cheers Gary

[cristian abarca]

Yes I think this is correct. don't worry about 30 or 50 wave I read somewhere that the eye can't tell the difference after 20 wave so if you end up with a mirror that is 10 wave you should be pretty happy for a first one. send me your next results. You must be living in a reasonably good area because here in melbourne it's cloud central.

Regards Cristian

[Garyh]

Hi Cristian, Still having great weather the last few days, yesterday was a killer night we would get a night this good every month or two. Last night not as good some low cloud forming about 10pm but the seeing was excellent. Got in 5 hours of imaging etc, I might post one of them and see what other members think. Spent a hour yesterday improving the tester. I managed to stop the carriage from kicking up slightly when advancing the carriage with some adjustment screws under the v cuts that run on the bar and a few drops of oil.. Ready for tonight as will stay inside and run some tests. I shall take a picture of it and post it as well. I hope that cloud lifts for you.
Regards Gary

[Garyh]

Hi Cristian, Finally done some tests tonight and put them into figure xp.
I don`t know if I did something wrong but it says I have a 1/23 mirror!!
Here is the average of the 7 lots of readings I did . I also moved everything around in between as well to change the numbers around.
Zone 0 0mm
zone 1 .825mm
zone 2 1.9mm
zone 3 2.64mm

I know zone 0 and 1 are pretty close with zone 2 varying about .4mm and zone 3 about .25mm through the tests.

Shall I just wait and star test the mirror? I still don`t have the tube and rings as yet..so will be some weeks away.
Thought I throw in a picture of my tester, cost me about $20 all up seems to work good now..
Regards Gary

[cristian abarca]

Figures look good Gary. do the star test and if that looks good then you are ready. Looks like you have yourself a pretty good mirror there. The tester looks fine too. If all looks well in the star test hen coat the mirror. well done .

Regards Cristian

[stringscope (Ian)]

This has been fascinating and most informative guys .

Thanks heaps for sharing this with us all. Can't wait for the next instalment .

Cheers,

[cristian abarca]

Here's the next installment. Today I cut a 52mm diameter hole into a 10 inch mirror so that I can make a classical cassegrain and use it as an F5 newtonian as well.

Regards Cristian

[Garyh]

Hi Cristian and everyone,
How did you cut the hole? with a buscuit cutter? Was your mirror already figured? as have heard cutting the hole can wreck the figure of the mirror. I would like to hear your progress on your project as this would be something I would like to do down the track making a cassegrain, and how you do the secondary. Very interesting!!!....

Well it has been a learning curve making this mirror and nearly there now. Making the plate glass tool saved me money as only cost less than $20 for the 10mm plate glass epoxied to some 20mm malamite which had many coats of lacquer , I cut the plate glass with a diamond wheel and a stone grinding wheel (keeping the edges well bevelled).
I found that the roughing out and the grinding are the easy parts and the making of the pitch lap was a nightmare, broke many squares using a knife etc...till I found this site.. http://www.atm-workshop.com/ws-techniques.html
Has a great way to do this. made a hot cutter from a strip cut from a bottom of a tin bent in a channel shape which I attached with 2 little bolts to my soldering iron...cut like a dream!! shall do it this way for the second mirror.

Another thing I learn`t during figuring there are lots of ways to accomplish the same thing and expect the unexpected with what supposed to happen. I found that sometimes the tool did the opposite to what the books say, but that might just be me.. being a beginer and not having the correct stroke etc. The ronchi grating really helped too show when I was close before further testing... thanks to Cristian for pointing me right there in interpreting the holes and hills correctly!..as with just the knife edge I wouldn`t have a idea!!!!. and also do heaps of cold presses at the end even after 10min polishing...Try to slowly get closer to a parabola steps at a time and not go over as found it harder to bring it back, took me about 25min in about 6 steps during one night and 1-1/2hrs pressing between, and tested everytime to see my progress, even with the mirror warm it gives you a good idea of where you are.
Hope someone finds something useful from a beginner!!!!

I would recommend anyone with a little time to spare every week to have a go!!!
Shall let you know what the star test reveals in the following weeks.
Gary

[cristian abarca]

Hi Gary Yes I used a buiscuit cutter and a bench drill at the slowest speed. It took me about 1.5 hours to cut it. It wasn't that hard just patience. Mind you I did practise by cutting 5 holes on 6mm plate glass before I summoned the courage to cut the real hole. The mirror was not figured but it was polished and spherical. It was an F6 ( One of the things I found difficult so far has been what kind of compromise to make on the telescope. A cass has so many more variables than a newtonian and finding the compromise that fitted me was difficult but fun). I started to regrind it but I had to make a new tool as the other one was worn out. one mistake I made was not making the tool the shape of the mirror so I have to rough grind a lot more. I also took a piece out the side by being in too much of a hurry but luckily it will be hidden by the arms that hold the mirror to the cell. On figuring the secondary I'll let you know how I do it but first I'll get the primary done. The way I was taught to make the pitch lap was to make a dam around a plaster tool, pour the pitch in put a grid that finish in a V (I use an old laundry basket), put the mirror on top of the grid press hard with your body weight, pull it all out and there you go. The grid is covered in dishwashing liquid first. The mirror is warmed up and covered in dishwashing liquid as well. one of the disadvantages with this though is that if you are not very carefull you may end up with scratches on your polished mirror. I can't wait till you do the star test to see how you go and to see the finished telescope. well done. Good weblink too.

Regards Cristian

[Garyh]

Hi again, Well I managed to get some knife edge shots and here are the best ones. After doing lots of tests I think the mirror will be about 1/12 PV. I can manage to read the outer zones well now and they all fall very close in readings, I have more problems getting the centre zone nulled as there seems much more deviation in the centre. Also I seem to have a slight hole in the middle which is more obvious by eye. Worth trying to fix or wait to star test? I have heard MOT normal stroke will reduce it.. Zone 0 to zone 1 has the largest error being a bit overcorrected while from zone 1 to 3 are real close.
Let me know what you think??
Gary

[cristian abarca]

Hi Gary. Do the star test first. If you are happy with the result then call it done. If not then try to remedy the problem. it's a bit hard to tell from the images. Also what size is the hole in the centre and is it bigger or smaller than the secondary. If it's smaller then don't worry. remember that the .707 mark on the mirror is actually the half point of the mirror so getting that zone correct is more important than the centre zone. That's not to say that the inner zone is of no value. When you do the star test to see the image of the star put the ronchi screen in front of the focuser as well and see if it gives you straight lines like when spherical. One of the members of our local ATM group did this to test the mirrors of some telescopes one star party I attended. It will tell you if the mirror is the correct shape.

Regards Cristian

[Garyh]

Hello again Cristian,
I managed to put the scope together roughly during all this bad weather. Came Staurday night it was a good night so I was able to star test the mirror. I still havn`t made the rings as yet so I proped it up against a chair. It took a half hour to cool down enough to get any descent views which I am not used to having refractors...Nice and sharp with 20mm eyepiece but slightly astigmatic so put in a nice televue 13mm eyepiece now without any astigmatism showing (20mm is a cheap synta possl) Image still nice and sharp then added 3x barlow as well to give over 200x focus now a bit soft and there is some minor flaring but could be due to the non painted tube (seemed to be on the same side as the focuser so maybe due to the slight penetration of the focuser barrel but still I was impressed as was nearly as sharp as my televue. I had a long look a Jupiter which showed me the bands clearly as well as some faint details within the bands (just), not bad for a uncoated mirror I thought...
Defraction rings inside focus were easy to see with the outer ring slightly brighter as well as the inner one. Outside focus they all seemed very much the same in brightness. The seeing was not perfect as the rings rippled a lot but would have given it as a 6-7/10.
I also took a picture of some trees a 1km away as well as the OTA, as you can see I have a 50mm secondary so I don`t think that hole will bother me. Its that big so I have a large illuminated field of view for the camera but I guess thats not so good for the planets etc.... maybe I should have a smaller one to swap for visual use too. Let me know what you think..
Gary

[cristian abarca]

Congratulations Gary. It feels good doesn't it. Before you go and buy another secondary for visual use get the mirror coated and have a proper look through it. Keeping the size of the secondary small will give you more detail but what you'll gain with a 6 inch mirror I don't know. I know that keeping the secondary obstruction to 20% is ideal. It can be hard to come to this ideal measurements because there a few variables involved. Like the distance of the focuser to the secondary, (low profile focuser as opposed to normal one) and also the sizes of secondaries available. The sizes available may not be exactly the size that you need and you might have to go up to the nearest one. Once the mirror is coated you will get a better idea of what you will need. Where are you going to get your mirror coated? I inquired at a place in Sydney about a year ago (I think it was quin ching or something, they advertise in Sky and Telescope) and for my 10 inch it was about $180 or so plus posting so it may pay to shop around. Again well done. As for me I've almost finished polishing my 10 inch mirror only a few more hours fix some errors and figure it. I'll post pictures and some results of my measurements in another post I started, in another week or so.

Regards cristian

[Garyh]

Thanks Cristian, yeah its great to make something like a mirror, couldn`t wait to sneek a peek!! has driven the wife quiet mad the last few weeks!! and still a bit of work to finish it off. Thanks for all your help and Love figure xp its a great program and know how to use it quiet well now...
Actually sending it of to the same guy, Chi Qin Co I think? to get it coated, has good prices and he gives you the quartz overcoat.
I shall put up a pic of the finished scope with a photo through it as well in the next few weeks..can`t wait to get a view with it coated!!!
I shall follow your new post through as what you are making is what I would like to do for a second project in the future....
Thanks for all your help..
Regards Gary