From: Mike C
Date: Sat, Mar 28, 2015 at 5:32 PM
Subject: Re: Am I close enough or should I tempt fate making my mirror more perfect
To: Richard B
Cc: Tim C, Jerry, TomT, Tom W
Latest update:I managed to get the mirror back to a sphere due to the over correction. We were about to think about parabolizing. However, at the last meeting we seemed to have a contamination problem and as a result I got one big scratch and a pit on my mirror. After that I tried to polish out the scratch I brought it over to Tom's house to pick up more cerium oxide. However, we noticed many more scratches. This Tuesday Tom and Jerry will determine if I have to go back to grinding out the scratches. This might set me back six months depending on how much work I need to do. At the very least I'll have to chuck the old pitch lap and polish, but I'm tending to think that I need to go back to 500 grade or worse. That will mean I'm back to square one which is where I was over a year ago.
On Sat, Mar 14, 2015 at 5:14 PM, Mike C wrote:
MikeIn the ;meantime I need to get serious with a more permanent way of holding the optics. My setup is a handful to keep pointed when the angles are too high and too low. That's because the board used for elevation needs to be adjustable.Thanks for getting back guys.I'll be making the Ronchi eyepiece and masks but won't be able to use them until I get back to Santa Barbara as the cloud cover is solid and might not be gone until a day or so. I might go far as to make another offset circle with a 3.5 inch circle.On Sat, Mar 14, 2015 at 11:22 AM, Richard B wrote:Hi Mike--
Have you ever done a star test on a "perfect" mirror? If you haven't, an easy way to do so is to make a circular off-axis stop about 2 inches diameter for your 10 inch. Use a magnification 1/5 as great, and run a star test. The errors over the small aperture will be small compared to the full miror, and you'll see what a "perfect" image looks like.
The effect you describe -- hard edge one side of focus with soft edge the other side -- is very common. However, the description is qualitative, and that makes it hard to know whether you can ignore what may be a 1/20-wave error or be worried about a 1/2-wave error. I used to take a Ronchi eyepiece (an eyepiece housing with a 100-line Ronchi inside) to star parties, and I looked a lot of mirrors. In general, it's best best to slip in the Ronchi, do your test, and not say anything to the guy who owns the telescope!
I suggest you make a ring-shaped stop with a diameter of 9 inches. Make sure the edge is nice and clean, such as an Xacto-blade cut edge in file-folder cardboard. Just for fun, you could make a mask that cover only 180 degrees of the mirror's rim. If you have a turned-down edge, it probably won't be as much as 1/2 wide, so you see what the star test shows with a non-turned edge.
Another useful diagnostic is to place a Ronchi screen at the focus, and view the mirror surface in the light of a star. With four or five bands, they should be perfectly straight. If the bands bow in or out, the mirror is over- or under-corrected.
The purpose of testing on stars is that "you get what you see." There is no math to interpret or figures to misinterpret. The fact that you see errors in the formation of your star images says your mirror will always perform that well and no better. The Ronchi test on a star is essentially a (not very sensitive) null test; straight bands mean good. You can also use a knife edge at focus, but you also see all the air currents.
Testing at center of curvature is good because you do it indoors under controlled conditions, but you're measuring the first derivative of the figure, not the figure itself.
On 3/14/2015 8:57 AM, Tim Cwrote:
Tom or Jerry are the experts here, but, I want to give you my take- what I would generally interpret first. Then, you can check with Tom, Jerry and Richard for more advice and most likely a clearer explanation.
Before I get there though I would like to say I use a program written by Reifke to interpret my Foucault results. I do not know what algorithm he uses in his program but I like the way it shows results. The pin stick I gave you does not use the same settings for pin placements. You will have to manually measure and change these within the program. The output of this program gives you a picture of the wavefront and where to make changes. It looks like this:
The lower horizontal line is a perfect mirror. The output is the line traced from left to right representing the mirror surface. The vertical lines represent the zones. You can visually see where the correction is needed. Both Tom and Jerry ( actually several others as well) tell me you really can't trust this because the algorithm is unknown.
In Richard's book on page 212, you see the pattern in general you describe showing an over correction. To me, that means the curve is too deep. Where I get all fouled up is visualizing if this means you have too long or too short a radius of curvature. I always seem to get it backward. But, I think it means the radiuses are shorter in an overcorrected mirror. For me, I just visually see a deeper curve or dish if you will. It means we need to flatten the curve.
What I would do, depending on where the errors are on the wavefront is to use TOT and a spherizing stroke in general.
The hard part here is you sent some images that do not represent this exact picture. I am going to search some pictures in Suiter's book a bit and see if I can't find something a little more definitive . In the meantime, yes some Ronchi pictures might really help here. Try for 4-6 lines inside or out. The other thing is, you can wait and we'll all look at this next Tuesday.T
PS- just for fun see if the pin stick placement measurements on my pin stick are the " effective radius" you see in the picture. I believe these are the measurements you would need to manually use in Reifke's program. ( if you choose to check it out)
Sent from my iPad
On Mar 13, 2015, at 9:56 PM, Mike C wrote:
MikeHi Tim:I did center over center last night as per Tom, followed by large w's to blend and got similar results. I'm not sure why things are looking exactly the same in star testing. I see a bright outer ring out of focus and then a diffused circle with bright spot inside focus. That either means that I've got over correction or turned edge. I;ve got to figure out again how to get some decent Ronchi grams to show. Again what does over correction mean with respect to the curve and what part of the curve needs working on?