Help for a beginner grattavetro

[Astromuma]

Good evening everyone and thank you for welcoming me to your forum.
I contact you to ask you for some information and I think I will bother you even later. I was lucky enough to find it at a very low price, 4 pieces of calcium-sodium glass from 19 mm thick, deriving from the same plate; this gives me information on their contemporaneity but not on when this was produced (for the purpose of knowledge on the maturation of glass). The glass is not of quality but, to gain experience, I think it's fine.

I have already made a Dobsonian 300 f/5 e, this time, I'd like to make a Dobsonian D290mm f / 4 by doing the whole process, including rounding with pendulum roughing machine to get me experience with a short focal length.
I have some’ of preliminary doubts:

1-You think, having available a thickness of 19mm of the glass, I can manage to make a D290 f / 4, without risking damage to the disc?
(I don't want to glue discs together)

2-For the tool, I was thinking of making it using plaster cast in the negative of the primary and a mosaic of glass or portions of gress tiles (I have seen and read about amateur astronomers who have followed this path but I did not understand what they glued everything to plaster with) in your opinion it is the right way or it would be better to use something different as a support for the mosaic of cells and to adapt the tiles with glue such as the bi-component for marble?
Having to adapt the mosaic to the shape of the disc, I think with grain 80, how much should I pre-radiate and then adapt and move on to the grain 120? (I hope I have explained )

3-We come to the optical tests. for the previous work I used the Ronchi test and the Ross null test to verify the correctness of the parabola ma, ignorant, i don't know how to evaluate lambda fractions with these tests, could you give me some information on how to proceed and what to build?

Thank you for your availability and time.
Good evening to all and see you soon.
Matteo

[Giulio TiberinI]

Hi Matteo.
I answer you for what my experience is.
I like to number your questions by interspersing my answers:

D1) …. I was lucky enough to find it at a very low price, 4 pieces of calcium-sodium glass from 19 mm thick, deriving from the same plate; this gives me information on their contemporaneity but not on when this was produced (for the purpose of knowledge on the maturation of glass). The glass is not of quality but, to gain experience, I think it's fine.

R1): The important fact from the optical point of view is that there are no important local tensions in the glass structure. Although dobsons were built made with naval portholes with a thickness not limited to the current 20mm, who in their naval life have seen all the colors.
Normally these strains are rare in calcium sodium, because it comes "float" from the rolling mill, floating on a bed of molten tin, and therefore has a reasonable cooling time and “relaxation” than in the many uses for astronomical purposes (at least in the Amateur telescope Making literature) it does not appear to have given rise to the known problems instead for UNUSABLE tempered glass.

There are methods to visually check the tensions within transparent materials, by the color of the reflection of polarized light striking the surface (the internal tensions of the tempered glass are clearly visible even when wearing sunglasses with polarized lenses).

An old but always interesting approach to the subject can be found here: http://strock.pi.r2.3.14159.free.fr/Ast/Art/Tension.html

D2): I have already made a Dobsonian 300 f/5 e, this time, I'd like to make a Dobsonian D290mm f / 4 by doing the whole process, including rounding with pendulum roughing machine to get me experience with a short focal length.
I would have some preliminary doubts:
1-You think, having available a thickness of 19mm of the glass, I can manage to make a D290 f / 4, without risking damage to the disc?
(I don't want to glue discs together).

R2): If the roughing is done mechanically, with a SLOW rotating diamond bur, with any mechanical method or with the SLOW pendulum roughing method ... but always used WET (…to avoid the danger of silicosis by inhaling glass), no stress should arise to the detriment of the glass. And therefore the problem of gluing discs does not arise (always to be avoided). .
The mechanical milling of the surface avoids the important mechanical stresses of manual processing to carve the initial curve. And it makes the work much lighter.

D3): For the tool, I was thinking of making it using plaster cast in the negative of the primary and a mosaic of glass or portions of gress tiles (I have seen and read about amateur astronomers who have followed this path but I did not understand what they glued everything to plaster with) in your opinion it is the right way or it would be better to use something different as a support for the mosaic of cells and to adapt the tiles with glue such as the bi-component for marble?

R3): Dentist plaster tools are usually made, well dried and covered with stoneware tiles glued with two-component epoxy glue; The problem arises when the long work consumes the tiles that normally do not exceed 5mm in thickness, and then the tool must be redone.
A good system, but more challenging is preparing the tool as Gordon Waite does in this video.

D4): Having to adapt the mosaic to the shape of the disc, I think with grain 80, how much should I pre-radiate and then adapt and move on to the grain 120? (I hope I have explained ).

R4): The blanks that I made myself beam, they were delivered to me at the final beam. and in the processing of surface refinement, alternating the position of tool above or mirror above, the curvature is maintained without problems.

D5): We come to the optical tests. for the previous work I used the Ronchi test and the Ross null test to verify the correctness of the parabola ma, ignorant, i don't know how to evaluate lambda fractions with these tests, could you give me some information on how to proceed and what to build?
R5): The majority of self-builders use the Ronchi to view the progress of the entire surface (which the Foucault test cannot do) ; and the complementary Foucault test, to know how, where and by how much to correct (which Ronchi cannot do) mirrors are made up to the focal ratio 5; And to get off at 4 Foucault is used first to verify a lambda / 4 surface, to continue with the Gaviola caustic test, which allows a much greater precision than Foucault, but it can only be used on short focal length mirrors.

Del Ronchi and Foucault you will find articles in this same blog, and you can practice testing your F5 mirror to practice and learn how to work well, before going into the mess of a minimal quality F4.

Hi Matteo. good job.

[Bartolomei Mirco]

Hello Astromuma, welcome to the forum…
As for the optical tests, the only thing I can add to what Giulio has already told you is that to test the quality of the entire surface of a mirror you can use both the Ronchi test, is the Ross null test. The Ronchi test as you have already had the opportunity to test is really simple to carry out and quite simple to interpret. I say quite simple because it depends on what type is the surface to be analyzed. If you have to analyze a sphere, Ronchi in my opinion manifests all his “power”, as it is very easy even to the eye to assess the straightness of the shadow lines, while the thing becomes a little complicated if you have to evaluate a parabolic surface, which will produce curved shadow lines and evaluating the right eye shape is very difficult. The Ross null test is of great help in verifying a parabolic mirror, which uses a flat convex lens that is used to exclude spherical aberration from the test, making sure that for a perfectly parabolic surface you have to see perfectly straight lines.
However, both tests are qualitative and not quantitative, in the sense that they serve to have an overall view of the appearance of the surface, but they do not allow to determine the fractions of lambda. For that we need the use of the Foucault test with Couder mask.

[Massimo Marconi]

Hi Matteo and welcome to Grattavetro.

I add to the previous considerations, that even in manual processing the thickness “thin” of a 300/19 mm does not involve particular problems in the roughing phase with short focal lengths. At least that's what I was able to see while roughing the 300 F3.8 and subsequently of 300 F2.5 where there begins to be a depth in the center greater than 7 mm.
Is’ clear that all the precautions and precautions that are normally taken for optimal processing ( support stability, tilting, support and clamping of the mirror, etc. ) they must be checked with the utmost accuracy in the case of significant depths or very thin glass.
If I remember correctly, Mirco during the processing of his 400/19 mm preferred to finish the polishing by placing the mirror directly in its cell, precisely to avoid possible deformations caused by the limited thickness.

As for the parabolization of an F4, it must be said that it is not an easy undertaking, first of all it is necessary to know how to master with the Foucault test, which is a very valid tool but has the “defect” to be very susceptible to subjective evaluation errors by the operator, especially when you don't have a fair amount of experience with this type of measurement.

Furthermore, the short throw tends to complicate things further as the classic foucault tester, as Giulio said, it is suitable for focal lengths up to F5, going further down it is preferable to use a slitless ( which makes the measures even less simple and immediate ) or adapt the classic tester with appropriate modifications, Anyhow , as has already been said by both Giulio and Mirco, the use of a quantitative tool such as Foucault is essential, that gives precise indications on where, how and how much to correct on the surface.

Another aspect is that hardly, except having a very good experience, it will be possible to achieve a good dish for an F4 without the use of sub-diameter tools, which involve a different type of processing and where regressive experience is equally needed ( or the guidance / assistance of some expert scratch glass ) to be able to use them to the fullest.

With this I certainly do not want to discourage you, but just to say that your project has many different aspects from your previous work, some of which ( Foucault first ) it's good to start using them now, even with already made optics, to start realizing the problems that will arise and not arrive unprepared or with unclear ideas on what to do, at the time of applying these methodologies.

In our blog you will find many articles about it, in particular in the Test Foucault section you will find a lot of information from the setup to the analysis of a measurement session with both simulations and real data.

All that said, ( at the moment ), I can only wish you a good job and tell you that I am very pleased that you have decided to confront your doubts and questions with us.

[Astromuma]

Thanks a lot to you for the replies, obviously it will not be an easy job and I will have to proceed in very small steps, but I know I can count on a group of experienced and passionate people
A couple more things:

Do you think that it is possible to make this mirror with a fixed top or a rotating top is needed? I ask you because it is a while’ that I think I'll build one
For the rest, I will leave shortly.

One last thing… to maintain contact with friends who also try their hand at various types of technical constructions, I opened a channel on Youtube, on which I place and share my works divided into all phases; you think it is a problem if I ask you to analyze and comment on them? The videos are public and, this, it would make it much easier for me to understand where I'm wrong, how to correct the various steps e, Consequently, also the information I provide, especially in the phase of optical tests. The channel is called “Matteo Muccioli”.

Thanks again everyone, good job and see you soon.

Matteo

[Massimo Marconi]

Matteo, I tell you my opinion: I've seen your videos and I wouldn't have particular difficulty commenting on them, however i think i go in “house of others” to express criticism highlighting the mistakes made in a work already completed, it doesn't make much sense and would put the commentator in an awkward position “examiner” which I personally don't like and don't belong to me, especially considering that we write in a dedicated blog and it might seem that I come to your channel to dispense self-construction lessons from the top of a presumed and self-referenced authority.

It would have been different, a shared interaction during processing, in which the exchange of information and ideas aimed at achieving the best result, he would have had a more constructive approach, however leaving you free to decide how to carry out the processing and to publish the related reports.

Maybe we will be able to comment on upcoming videos in the future, as this interaction is already developing in this topic, but I'm not going to comment on previous videos to find myself writing: ” here it had to be done like this” or ” to do this you should have done this first”..

Your F5 is now complete for better or for worse, It certainly served as an experience and to understand that beyond the first horizon there is an ocean yet to be explored, if we will sail together to other constructive adventures, then it will be natural to find yourself writing both here and there…

Then, if we want to analyze your previous processing here in the forum with the help of all the grattavetro and fix the priority points on which a “renovation” of the operating modes , we can do it, but only after your explicit request.
In this way it makes more sense because we try to identify together and understand the mistakes made and to find the appropriate solutions, before facing a new and more difficult task.

[Astromuma]

Hi Massimo, I thank you for the answer, as I had previously told you, my aim was only to share a personal path of self-construction, without any other purpose…and then I didn't know your site

In my first experience I have done many things wrong and I would like to talk to you about them, not to repeat the same mistakes in this new adventure (P.S. visually, the dob works fine )

For the new project you and I ask you right now, to express your notes on all stages of processing, for me it is pure gold. in your opinion, how it would be better to proceed?
Post the videos on the channel, or place something here first?
I ask you to make it easier to comment in real time and not to risk clogging up the forum

I await straight, thanks again for your availability, See you soon.

Matteo

[Massimo Marconi]

Astromuma wrote:

Post the videos on the channel, or place something here first?
I ask you to make it easier to comment in real time and not to risk clogging up the forum

ah, if it is for this, there is no problem, at the moment we run no risk of clogging, in our forum of only and true enthusiasts, traffic is light and fast !

[Astromuma]

Ok, then I will post the videos on my channel attaching the links to them here

By the weekend I start cutting the glass and then move on to roughing; heartfelt thanks again to you and to all those who answered and who will correct me.

Good day everyone and see you soon

Matteo

[Astromuma]

thanks a lot, Giulio and Mirco for their valuable advice, I will keep you updated step by step…hoping not to do too much damage

Have a good day and see you soon

Matteo

[Astromuma]

Goodmorning everyone,
I have just completed the glass cutting tool; I post the photo and then ask you some information

[Astromuma]

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[Astromuma]

In the construction I was very careful but, at a first test on a multilayer surface I measured a difference in the cutting height of about 2mm between two extremes, of the series, the cutter turns out to be a little’ inclined (photo 2 e 3) my question is, being the wood a little’ rubber band, it's possible, being very careful, in the first phase of cutting, contact the first part of the cutter by squeezing the mixture of water and carborundum 120 e, as soon as the rotation begins, keep squeezing until the whole cut is in contact or should I redo / trim the cutting edge?

thanks a lot, See you soon

[Astromuma]

I try to link the images again

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[Giulio TiberinI]

Hello Astromuma.
To tell you mine, I have to refer to company maintenance memories of the 70s that, albeit technologically similar, however, they had nothing to do with astronomical optics.
The "do-it-yourself" obtaining of glass discs with a diameter beyond what is available on the market for diamond core drills, it's not an everyday thing.
And chips are the number one enemy of this kind of glass activity.
To reduce them to a minimum, there are some precautions to be implemented. But there is no guarantee that blind luck or bad luck that sees very well will prevail, who are occult escorts.

The construction of a hole cutter like yours, for the grit abrasive 120, has the handicap of having a large flexible wood surface, than in pressing the center for advancement, flexes to the edge.

1) The problem you describe related to the 2mm contact difference you experience on the abrasive circumference of your cutter, it is already an aspect that may or may not affect. But since chipping is always the number one danger of this kind of work, it would be better to “smooth out” the edge of the cutting edge in advance, before starting to mill the glass, simply by turning it wet on a base with abrasive or on sheets of waterproof abrasive paper (coarse-grained sepia paper, or finer but to be soaped from time to time so as not to clog) fixed on the flat base of the drill press, or the machine you will use to do the job. It will take patience and time because you cannot press too much to improve the removal. But even a lathe would have a hard time "facing" your milling cutter 2 mm with pin on wood, and with those cuts already made, which interrupt the circumference to be turned.

If you do not bring the cutter level, nothing may happen that is not an extension of the work. But it depends on “hurry to finish – bad luck and / or luck”.

2) Beware that chipping is guaranteed if not “sheaths” the glass, by gluing your blank glass plate above and below, two thick protection plates 3 o 4mm, through which to start cutting (tool entry), and finish (tool output) cutting.
For processing gluing, optically, rosin and beeswax-based glue is used, called “Arcanson” or “Ciment d’opticien”, of which you will find the recipes by googling those two names. It is normally done with 2 parts of rosin and a part of beeswax. But the quantities (as you will see on the net) they are in customized and different proportions depending on the optician.

The detachment of the glued parts takes place easily with cooling in the refrigerator, and by tapping on the cold pieces. But in doses for stronger bonding (that is, with a prevalence of rosin compared to beeswax), the sacred texts read that it may be necessary to soak the object in acetone to dissolve the glue.

Someone (Sergio Bertorello) I have read that he also uses the simple and more available sealing wax. But with any type of these HOT adhesives, it is ALWAYS necessary to pre-heat all the glasses at least just above the melting temperature of the glue mixture created.
Otherwise, in addition to the risk of thermal fracture of the glass, there is the one that upon contact the glue instantly solidifies and creates thickness, making the bonding much less robust and especially the glued surfaces that are not flat, and therefore subject to those impacts that would prematurely detach the bond during work.

3) Very important splinterproof (I almost forgot about it) and equally intuitive, is to make a chamfer of all the "teeth" in the direction of rotation of your cutter. In other words, the teeth must NEVER have a 90 ° sharp edge on the side according to the direction of rotation, first encounters the material to be cut.

As you can see, the problems are not few, but interesting things are like that!!

[Author]

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