Astrobits

Having more than three points of contact will almost certainly distort the mirror. If it eliminates some astigmatism then it is very lucky that the mirror was pulled in the right direction! Also, make sure the mirror is about 3mm away from the backing plate. That way it will be easy to remove it when necessary and provides a cushion against any distortion on the backing plate. It is not necessary to have three blobs on the secondary. Mine has only one small central blob and it has been that way for nearly 20 years.😀 Nigel

Yes, there are optics intended for applications other than visual observing but these are sold and described as such. I was referring to telescopes as sold for "normal" ( i.e. visual ) use. I also note in your quoted extract that they only refer to chromatic performance of their photographic optic not the spherical aberration that I referred to. They also recommend that their "visual tuned" 'scopes should be used with mirror diagonals. Regarding relative quality of prism and mirror diagonals, if they are made to the same optical standard ( i.e. all surfaces 1/10th wave ) then my comments apply. The prism will have more effect than the mirror. There are other factors affecting performance that are specific to each. Scatter from the mirror, homogeneity and strain in the prism glass and, if using an image erecting roof prism, the coatings to correct for out-of-phase condition, for example. Nigel

In microscopy the higher magnifications produced by shorter focal length objectives ( steeper cones of light ) require an adjustment to correct for the thickness of the cover slips. A cover slip, which is a plane sheet of glass, introduces spherical aberration in the converging/diverging optical path. A correction for this aberration is achieved with an adjustable ring on the higher power objectives which alters the spacing of the objective's elements. Applying this to telescopes, a prism is effectively a plane sheet of glass ( albeit folded ) in the converging beam from the objective. It must, therefore, introduce some spherical aberration to the image. This aberration will be worse with smaller F/ ratios ( steeper cone of light ) and will always show up more at higher powers. Hence the comments in the posts above. I very much doubt that any telescope manufacturer will design their objectives to correct for the use of a prism diagonal as when used without a prism there would then be spherical aberration of opposite sign present in the image. A further consideration is that a prism diagonal has three surfaces with which the light interacts giving three opportunities to add their own errors to the wavefront while a reflecting mirror only has one. The reflective surfaces of both the mirror and diagonal will add double their surface errors. The plane surfaces of the prism are transmitting the light and their individual errors affect the light much less than a reflective surface, but there are two of them. Nigel

Glad you're more or less sorted. I've been too busy the last few days to pick up on this thread but Piero has helped you. As soon as I saw the secondary mounted that way I knew it would be a ( the) problem. Nigel

Last Xmas I got a puzzle. It was the M&S round one that had the Moon on one side and some constellations on the other. The Moon was the most difficult and having done both, I decided to keep it assembled and make a round frame to hold it. The puzzle was coated with puzzle sealer ( Modpodge) to make it rigid. The frame I made from Sapele and ash woods. Nigel

There's is a valid reason coating companies will not put such material in the coaters. All organic materials are likely to out-gas preventing the vacuum becoming sufficient for a good coating. Also the out-gassed material will end up in the pump oil which again prevents a good vacuum being achieved and the oil will then have to be replaced at considerably more cost than the profit from the job.. It simply isn't worth the risk for the coating company. Nigel

Each fringe is a whole wave difference at the wavefront but as it produced in reflection then the surface error is only 1/2 wave. In use, a 1/2 wave surface error will give a 1/1 wave error on the wavefront attributable to that optical surface only. I seem to remember reading once, long ago, that one of the American manufacturers of SCT's would assemble the optical components for a telescope and then figure a selected surface to give an acceptable image. Nigel

Well it all depends on which cassegrain design you are making. Dall-Kirkham needs a spherical secondary and the primary to be under parabolised, Classical needs a hyperbolic secondary and fully parabolised primary and the Richey-Creitien needs both to be hyperbolic. IF your secondary is made with clear glass, no bubbles, no strain ( check with crossed polars) and has an optically flat back ( check against reference flat) then you can figure it with a normal Foucault test from the rear. Just remember that the measured ROC and focal length will be reduced by the refractive index of the glass. If you can't do it that way then make the reference surface correct before trying the interference test. You say the reference has a "bad edge". Which way is the "bad edge", turned up or turned down? That will give you a clue as to the overall figure on the mirror. Otherwise it looks O.K. Nigel

Made lots, including the mirrors, up to 20".🙃

Me too! In fact, any old plaster. Nigel

Oooh! Look what I've got: From a Tal-1 in poor condition for disposal that nobody wanted ( advertised on ABS---no response). Yours FOC. Just need to collect from Somerset just off the M5. There are some other bits if you want them. The mirrors I am using in another project. PM me if interested. Nigel

If it's only lasting 1 1/2 hours then the lap seems to be a bit soft to me. I always used quite hard laps which would last throughout polishing and figuring ( and do for the next mirror!). The pitch needs to be thick enough to conform to the mirror and if it gets too thin then this is hampered and can lead to difficulty in figuring. Perhaps the scratch was caused by the soft pitch thinning and exposing a hard particle? I don't know if you have tried to feel the mirror surface immediately after a polishing session. It can feel quite warm from the friction of polishing. That won't help a soft lap stay in shape. I would continue with the strokes you are using, maybe reducing the W a little, if at all at this stage. Your centre looks to be on the right profile, you just need to take it down slowly and the zone should slowly disappear. Nigel

In my experience those simple hand trigger sprayers in your post have very variable sprays which are NOT adjustable other than by varying the finger pressure applied to the trigger. I have looked on the web for ones similar to those sold by Angelguilding ( which I assume have got adjustable nozzles ) and have so far come up blank. This is not to say that the simple sprayers won't work, They are cheap enough to try them, it might be necessary to purchase more than two to select the best pair. Nigel

I will be interested in using two separate bottles, one for each hand. I can foresee that the two hands approach might give some variation in quantity of material dispensed and possible inhomogeneity to the coating but worthwhile to try that. Certainly the cheapest way. The recipe in ATM book 2 includes glucose and silvers the mirror suspended face down in the final mix ( obviously not practical for 800mm mirrors ). Also the Material Safety Data Sheet ( MSDS ) for the Angelguilding reducing mix ( buried on p9 ) shows that it contains an unidentified ingredient which may well be glucose or something to do the same job: Nigel

Your post mentioned "the bundle" and I assumed that you were referring to the whole kit and not just the chemicals. When (if) you purchase the chemicals you will have far more than needed for one mirror so you will be able to experiment on window glass to get the concentrations for one coat. My suggestion of starting with the lowest concentrations was simply to minimise the waste incurred in any trials that fail. Clearly, the American ATM's are using the product as supplied by Angel Guilding who have done these trials and determined the best concentrations for one coat. By starting with the lowest concentrations you will be able to get an idea of the concentrations that should provide a one coat solution. If you need to spray twice then doubling up the concentration should get you near there, if you need three coats then three times as much etc. I'm interested in what equipment you choose for this project as I might want to go the same route. Nigel

On their "How to" video they use multiple coats if necessary. Therefore I would go for the low concentrations and expect to need to do multiple passes to get the thickness. However, you will need accurate scales to measure gram amounts. I don't see the kit for $130. The two spray bottles and wand nozzles alone are priced at $208? Nigel

I saw that Angel Guilding article in Sky & Telescope January 2020 and have considered doing it with my mirrors ( when I get round to making them, or my current scope needs re-coating). I don't know if the spray bottles are available here or will need to be obtained from the States. The raw chemicals are common and should be available fairly easily although I have not tried to purchase any yet but should be cheap enough to do some experimental silvering to get the desired effect. Nigel

Yes, immersion procedure. This was way back in the late 60's/early 70's and spray silvering hadn't been developed. I don't remember using Nitric acid in the procedure and don't think it is necessary. I suspect that it was used as it was readily available then ( as a teenager I used to buy concentrated acids, Sulphuric, Nitric and Hydrochloric, from my local chemists!!! ). The Fulminate normally appears when the mixed solutions are left standing for a period of time. I'm not sure of the exact chemical reactions that generate it. See this link: https://www.compoundchem.com/2017/09/06/silver-mirror/ Nigel

Yes. I have silvered a few small mirrors many years ago. I am fortunate to be an industrial research chemist so had all the chemicals needed readily available. It is fairly easy to do but can go spectacularly wrong if you are not careful and don't follow the rules. There are ( if they haven't been taken down) some recipes on the web. The main hazard is that the recipes can also produce silver fulminate which is explosive and incredibly sensitive and can detonate by itself (so no good for the usual uses of explosives). The ATM books have instructions and recipes and Stellafane.org has a page with some links. Nigel

This would not be my choice for a spider design. Every edge produces diffraction. If you ever do a Foucault test you can see the diffraction just adjacent to the ( single ) knife edge when it is inside or outside focus. A single vane, having two edges, produces two edge diffraction patterns plus a combined diffraction where the two edges interfere. These are all superimposed on one another and cannot be seen separately so appear as one. Now double that up with the two parallel vanes and they can also interfere to produce further diffractions. You now have TEN diffractions ( each edge interferes with all the other edges as well as producing a diffraction pattern all by itself ) all superimposed on each other so will not be seen. However they will all be taking light from the centre and spreading it into the surrounding areas potentially reducing the ultimate resolution of the scope. ( Don't forget that the Airy disc and rings are formed by the edges of the primary objective and the further apart they are ( the bigger the aperture ) the finer the resolution. Conversely, the closer they are the lower the resolution and the two vanes are quite close. ) I would expect that for imaging the loss of resolution will not be apparent but attempts to split the closest doubles for the diameter of the scope might well result in a lack of success. Nigel

You could attend one of the optics days at an RSPB site where you can try out some of the scopes they sell. There's some dates in November and December in West Yorkshire. Alternatively look up FOCUS OPTICS for their offerings. A bit further to travel but a bigger range on offer. You could take your current kit to compare with whatever is on demo. Nigel

Yes, it is not easy to get your hands comfortable on a 6" mirror. Try a 4" mirror, there you are down to fingertips🤪. If you are doing nice random strokes and rotating everything randomly then it it irrelevant where the channels are cut. If you are not randomising everything then it is irrelevant where the channels are cut, you will get zones. ( yes, the location of the channels is irrelevant whatever you are doing ). This is where doing it by hand is better than a machine. Machines tend to be repetitious in their action ( by their very nature ) and can produce zones much more easily than you can by hand. When doing the Foucault test you should see that very close to the knife edge there are some diffraction lines, akin to the airy disc and diffraction pattern of a star. These can help you to determine the extent of any turned edge as they are 1 wavelength apart. These should be visible with the Ronchi screen as well but as I haven't ever used a Ronchi screen I cannot confirm that. ( A Ronchi screen is simply a series of knife edges and sliding a simple knife edge across the beams gives me the same info). When you are happy with the edge you will need to lengthen the stroke and widen the "W" to parabolise. ( Doing a 10" f/5 I needed to push the centre of the mirror to the edge of the lap to get the parabola deep enough ). I would try 1/2 dia strokes to start and see how the centre deepens then go further as needed. Nigel

Firstly, fix the edge as that will be the more difficult of the two options. Just a thought, are you curling your fingers over the edge of the mirror? If so you might be heating up and thus expanding the edge area which then gets polished off and shrinks when cooled to leave a turned edge. Nigel

What if you hit a nice parabola while the edge is still incompletely polished? To parabolise a 6" f4.5 from a sphere you need to remove approx 2 microns of glass from the centre and none from the edge. Finish polishing before you even attempt to parabolise. Nigel

As Phyllis says-- re-cut and press. You don't need to press very hard, if at all. Also, you don't need to use long strokes. In grinding that is used to deepen the centre of the mirror but you don't want to do that now until figuring. Thus the shorter the stroke the better in polishing. I worked my mirrors on a pedestal/barrel and rotated the mirror and moved a step round every stroke in an almost continuous movement. I was told, many years ago, that the polishing action occurs at the edged of the blocks not on the faces. I don't know if that is true but without those channels the polishing slurry will not get to the centre of the mirror. Using a net produces more edges so that might be the reason for that technique. I always checked the whole surface of my mirrors with a 10x loupe and placing the mirror on to a light box ( easily made- does not need to be evenly illuminated ) with a black card which had holes cut out. This gave an obliquely illuminated mirror surface with a black background when viewed near the edges of the cut-outs. As you have a reasonable polish I would suggest that you check the mirror now using a simple pinhole and 10x loupe at the centre of radius. You will get a fairly accurate measure of the FL and by looking at the discs of light inside and outside of focus you can check for any astigmatism. Nigel