Astrobits

I hope that I am reading your post correctly. I think that Zambuto is referring to the "sweet spot" of a lap as being in the area either side of the fore/aft stroke direction encompassed by the central diagonal and either side within 80% of the diameter.

The maximum effect of a lap occurs along the fore/aft diagonal, reducing effectiveness towards the sides. If you are offsetting the 80mm lap by 30mm then that puts the maximum effect at the edge of the central hill. This would do nothing to reduce the central hill and will probably make it worse by targeting the low zone around that central hill. You need to have the centre of the lap oscillating about the centre of the zone you wish to reduce. This is one area where simple machinery fails to give us the best results as it does not vary the position of the lap enough to prevent the lap effectively digging a hole for itself.

As I said previously, I would concentrate on the outer zone first. Using the smaller lap by hand in a circular stroke, no more than about 1/2-1" in dia, over the high zone as the MOM rotates the mirror, making a flower petal pattern in the polishing medium. At present you are still trying to get to an optical sphere before going for parabolising. In parabolising you will be attacking the centre anyway and hopefully doing less and less work as you get towards the edge of the mirror.

It is a fallacy that the surface after fine grinding is a sphere to optical standards. I once made a grinding tool that used rather large pieces of glass and it so happened that there were more gaps in a particular zone than elsewhere. Despite the mirror appearing to be a smooth surface, a sphere it was not. After polishing there was a large high zone at the location of the gaps in the grinding tool. Lesson learnt.

Nigel

Perhaps you might like to think about making a Dall Null tester. You will need a plano-convex lens about 25mm dia with a focal length around 150-200mm and a bit of engineering to make up the tester.

One of the drawbacks of the Foucault test on faster mirrors is that the light areas are very light and the dark areas are very dark making it difficult to see subtle zones. With a null tester the knife edge shadows appear as that of a sphere, i.e. the parabolic mirror will grey out much more gradually than with Foucault. Any high or low zones will stand out much more.  I have used my Dall tester for my mirrors up to 500mm F/4.

Personally, I would concentrate initially on the outer areas of the mirror as the central area will be partially covered by the shadow of the secondary and it contributes little to the image anyway.

Nigel

It strikes me that there is something basically wrong with your process. All the images appear to have a similar shape indicating a recurring pattern. This could be caused by the pitch lap, the strokes in use, the mirror support or some combination of those. Looking back at your image of the mirror on the MOM there does not appear to be any soft base ( i.e. carpet ) under the mirror.

Firstly, I would change the support for the mirror. Put some thick, soft  material under the mirror. Make sure that the material does not have any sort of symmetry ( carpet is a popular choice, don't use bubble wrap). I would then switch to hand polishing the high zones using a very small, 2" dia, lap to reduce those somewhat. Be aware that small polishers like this will change the shape very quickly so use sparingly. when the zones have reduced re-press the large lap and continue with variable strokes.

Don't forget: regularity = zones, randomness = perfection.

Nigel

If you are trying to flatten a nigh zone then the centre of the lap should, on average, be over the peak of the high zone. Use quite small laps for narrow zones and use circular motion of the lap rather than straight motion as the mirror turns as this will help blend with the lower areas either side of the high zone. Don't forget to check progress as often as possible. When getting near the end I would check the figure after just one turn of the mirror with a little overlap of the start and end points and restart at a different position for another wet.

Nigel

It's quite simple really. Poor seeing is caused by a distorted wavefront and poor optics simply adds another layer of distortion to that. Hence the better the optics the less noise they add to the image.

Nigel

Firstly, vary the overlap. Fixed positioning leads to zones.

Secondly, don't rotate the mirror by ( nearly ) exact and equal amounts. Fixed angle only turns could lead to astigmatism. Rotate a little bit this time and a lot more next time then somewhere in-between the third time. Don't repeat the sequence.

Cold press often. If your lap is hard enough you can leave it on the mirror for days between polishing sessions. Just make sure it is well wetted with slurry and put it into a polythene bag to prevent it drying out.

The more random you are the better.

I once had triangular astigmatism because I thought that bubble wrap was random and I didn't need to rotate the mirror, until I looked at it afterwards and it has triangular patterning.

Nigel

As Richard ( above ) said Light overspill is considered a "statutory nuisance" Light is included in with other forms of pollution such as smells, smoke, rubbish, animals etc that can be considered a nuisance in the legislation:

https://www.legislation.gov.uk/ukpga/1990/43/part/III/crossheading/statutory-nuisances-england-and-wales

You can use this to get your local council to take action.

Nigel

You can also try:

http://www.scientificmirrors.co.uk  They are in North London & Cambridge.

My 16" was coated by them, just Al and Silica overcoat 22years ago and still in excellent condition ( despite being shipped to South Africa and back). The secret is to keep the mirror tightly closed when not in use.

Nigel

The cheaper the sheet the more gaps you will get in the internal plies. Sometimes they might be filled with any filler that was handy.

You can always fill any holes that show up in your cut pieces with a suitable filler.

Nigel

3 hours ago, skybadger said:

Has anyone made the point yet that you can test DPAC using a water bath so everyone everywhere can have a flat of the same accuracy ? 

I use it to test and setup my cassegrain scopes but use a white light led. 

 

I was also going to suggest a liquid ( water, oil ) bath as the reference flat. I believe that the National Physical Laboratory in Teddington uses an oil bath as reference. I used the water method when making my reference flats ( 8" dia ) many years ago. I made the flats using the three plate method but just for the hell of it decided to include both sides of the 2nd and 3rd plates making it a 5 surface method😳. The double sided flats were polished to better than 1/4 wave yellow light ( main deviation near the edge ), put into a box and there they have remained untouched for the last 25years or so. The 5th surface was used as my main reference surface when making flats for my telescopes.

Nigel

Just measured the TAL-1 tube that i have-----------140mm

Nigel

12 hours ago, skybadger said:

I

 

What effects should I expect I'f the source and sensor are not the same distance from the mirror ? 

Spherical aberration. Although virtually undetectable if the difference is small

Nigel

17 minutes ago, Peter Drew said:

Binoscope?     🙂

Funny. That was my first thought.🤩

The early TAL's had a 32mm eyepiece dia but I believe that they changed to the 31.7mm ( 1 1/4" ) on later production. Not much difference but standard eyepieces will be very loose and not sit square in a 32mm hole. Some TAL's had a motor drive that was housed in a black box which yours does not have.

Once upon a time I inquired with the local importer of these scopes about spare eyepieces for use on other scopes. The response that came from Russia was that

" our telescopes come with eyepieces, why would you want more"

Nigel

Mirrors don't slump. I am not aware of any reports that a telescope mirror has changed shape from being in a mirror cell with three edge clips and stored sideways for years. It has been claimed that the glass in very old church windows is thicker at the base than the top due to the glass flowing under gravity ( slumping? ) over hundreds of years, but I am not aware that it has been proved to be the case. Very old glass sheet making did not produce the consistent thickness that we see today so thickness variance could well be there from day 1.

However, mounting mirrors vertically on two thin posts can induce localised stress which can show up in testing but I have not seen that particular feature during my mirror making experience despite my mirrors being mounted on two small supports.

Astigmatism can easily be seen using a simple lens ( 25mm eyepiece = 10x loupe ) by examining the return image of your source inside and outside of focus ( move the knife edge out of the way ). Both should be identical. Any elongation of the image one way compared to the other indicates astigmatism. It can also be seen in the Foucault test with the shadows not being symmetrical both parallel and normal to the knife edge. However, if the astigmatic axes are parallel/normal to the knife edge then they may be much more difficult to see in the normal course of Foucault testing.

Coated mirrors are remarkably bright compared to unsilvered ones.

Nigel

I agree with Francis.

If the camera is in the black disc on top of the circuit board then the separation between that and the source is far too large. You will need them to be as close as possible especially when you start figuring faster mirrors. Ideally the source and viewing point should be on the same axis. this can be accomplished with a beam splitter cube. When I figured my 16" F/4.7 mirror a separation of about 20mm showed significant astigmatism caused by that separation.

Nigel

I would not think that using a tripod will give you the delicate movements required. I would put the xy platform onto a flat surface so you can move it fractions of a mm at a time.

Nigel

If you want the breakdown of a product then google the product name and MSDS ( Material Safety Data Sheet).

Note that the "Zeiss lens cleaner" and "Zeiss lens cleaning spray"are different products with different ingredients.  The former is just 5% Isopropyl Alcohol in Water.

Nigel

The problem with the Foucault  test is that the focus of the side zones is different and not on the optical axis. If you draw the light beams from a parabola out on paper you will see that the focus of, say, the left side 70% zone is on the right of the optical axis (OA) and the right side zone focus is on the left of the OA. With your knife edge on the OA you are cutting the beams away from their focus points. This doesn't pose a problem with large F/ ratios but becomes increasingly obvious as the F/ ratio gets smaller. To improve your accuracy with fast mirrors you will need to measure both X and Y positions of each focal point of each zone. These will lie on the Caustic curve.

Nigel

My understanding from the OP is that a free-standing wall is required so that a 15" tool can be made on a 20" mirror. Flexible material will take on a circular shape when filled with the plaster mixture so I would go with Jim's (saac) suggestion of damp proof course. Any non flexible material will have to be circular to start with.

When making my full size plaster tools for mirrors I have used material as thin and flexible as thick plastic bags ( e.g big dog food bags ). Once taped round the perimeter of the mirror they are remarkably strong. Fortunately, I was making a number of different sized mirrors so had small blanks available for making sub-diameter tools.

You don't need to use special or fast setting plaster, any stuff will do, even concrete can be pressed into service. Just cover the back and sides of the tool with PVC tape ( prevents any of those little holes trapping gritty bits ).

Nigel

Play around with this calculator:

https://education.down2earth.eu/impact_calculator

see what it would take to eliminate that pesky security light.

Nigel

Digital angle gauge, there are apps for smartphones. I have one called "Spirit Level". You can aim the edge of the phone at the horizon and freeze the reading.

Nigel

You are correct, 30% of 12Kg = 3.6Kg

Nigel

The podcast has some extra chat after the broadcast segment.

Nigel

Next Thursday, 2nd Feb, BBC radio 4 9:00am 'In Our Time' will be on Tycho Brahe. Will be available on BBC Sounds after that.

Nigel