Building a Cassegrain

[geminimac]

Hello guys, my name is Luke and I too
decided to try to build a personal telescope, I have already started to observe the sky with a mak 90/130 mm

I have read the various 3d regarding the construction of a telescope and from here I get some questions that I hope those more experienced among you will be able to explain…

I begin first of all to stilarvi the size of the telescope that would like to achieve:

Technically I'd like to build a 410 mm cassegrein type but with the Nasmyth variant that includes a 45º glass to deflect the beam and avoid puncturing the primary glass…
For the concave primary mirror 41 cm in diameter and 3 cm thick, estimated arrow 0,85 mm, focal ratio F3
convex hyperbolic secondary mirror of 13 cm in diameter and 1 cm thick
To achieve the primary would have thought to use a thick stoneware tile 2 cm as a tool (instead of the second glass)recovered charge from my workplace…it seems to me that apparently has the same hardness of glass…What do you think about?! The idea can go?!

As for the mirror polishing I would need to understand what the last phase includes…I read that some use cerium oxide and other pitch
And for the aluminizing?What should I do?!

I await your response….
Thanks in advance to those who will help me

[Giulio TiberinI]

Hello geminimac.
Not knowing you and not knowing your degree of skill in DIY, I jumped on the chair when I read that as your first creation would scratch a Cassegrain F3..e well 400mm diameter.

The leap is because it is a realization enormously difficult, that is impossible for a beginner, and the reasons are:

1) the difficulties of realization esponenzialmnte rise with the increase of the diameter of the mirror and with the shortening of its focal length. In other words, realize a mirror from 200 or 300mm is feasible for a novice if its focal length does not descend below F6…For example a parabolic mirror with a focal ratio F8 is very easy to realize in practice because it is simply spherical, or it owns a small piece of parable for its width and shallow depth, It does not differ from the sphere of a diameter of the mirror of 200 or 300mm

2) realize a parabolic mirror diameter 400 with F3 focal ratio involves digging the primary arrow to a depth of 3.66mm and still parabolizzarlo 1.16 microns and are very demanding jobs for experienced professionals.

3) the secondary mirror CONVEX and hyperbolic is even more difficult to achieve than that of the primary mirror.

4) the stoneware tile as tool may go well if it is thicker at least 15 o 20mm. to below the thickness could (PERHAPS) only be useful to build a parabolic mirror with focal ratio F7 or F8, where for a 300mm excavation of the arrow would only be respectively 2.7 or 2.3mm.
This is because the tool must accompany your realization from beginning to end keeping in that way the complementary convex curvature perfectly identical to that of the concave mirror. If you beginner you had to do the tool in the final stages of your work, you would have a great chance of not being able to get on top of your mirror.

You say “I would need to understand the last phase includes ... What I read that some use cerium oxide and other pitch”
And for the aluminizing?What should I do?!5)

Let me answer that, as you know, in the processing of parabolic mirrors is interposed abrasive powder and few drops of water between two glass panes, rubbing the center of the glass that will be the mirror (which therefore become concave), above the edge of the glass which will be the tool (and therefore it becomes convex so exactly complementary). You start the job with very coarse grained abrasive and then we proceed with progressively finer grits, to obtain the desired spherical curve and very finely satin. Then polishing ALWAYS necessary that the tool is coated with pitch squares because the fine abrasive grains of cerium oxide, no longer need to roll between the two glass panes, but they must be planted in pitch and act as latches. Only then, polishing.

This your doubt, however, makes me realize that it is absolutely essential that you clarify good ideas on what it means to also build just an easy parabolic mirror 200F6.

then I highly recommend you read the following brief presentation, and watch the movie for about an hour contained in it. Only after seeing from top to bottom what, you can judge for yourself if it may affect you or not.

Tutorial video: Construction of a parabolic mirror 200F6

As for the aluminizing, it is deposited by sending the finished mirror, in development at companies specializing in metal deposits.

( Finally, I think you should open a new thread your staff by introducing yourself, because this was open from maurizio63).

[Bartolomei Mirco]

Hello geminimac,
Welcome to the forum I too…
I agree fully with the suggestions you gave Giulio, the construction of a 400 / F3 is an extremely difficult, even for those with expertise in the field, and almost impossible for a novice…
Obviously if you dare to undertake this project, we will be here to help, but it would not then that the objective difficulties of implementation will bring to abandon everything in half work.
You just read the posts written by Massimo on the realization of its primary and secondary 300mm hyperbolic to understand the difficulties involved (and it considers that Massimo is certainly not the first mirror and has now already reached a certain level in the working hand of the optical).
I personally would recommend you start with something more manageable, kind 250, maximum 300 mm f6.

P.S: precise that the arrow would be the primary 410mm f3, It would be well 8.54mm !!!

[geminimac]

Hi guys,
First thanks for the replies…
I already knew the company to realize this mirror entailed huge effort and experience…why I still decided to go ahead…

@Giulio
That video you posted I've already watched several times…precisely why I decided to start the company although it will be very difficult…especially for the arrow 8,5 mm to dig😫😫😫
As for the tile so I can feel comfortable…just drop me at work to procure immediately!

I am confident of the entire board support…
As soon as possible to update the situation by opening a new post!

See you soon and thanks for the advice
Luca

[Massimo Marconi]

Hello and welcome Luca !
I moved the post to a new discussion, to give way to you to have all the space you want in a dedicated discussion and to not interfere with a different topic in the discussion of Mauritius.

Maximum

[Giulio TiberinI]

Luke well! The important thing is the awareness of actions and the positive determination I see you possess!
Here we will certainly try to help is based on the experience coming from the situations that each of us has already experienced; both strange situations in which you are confronted by reasoning, and as usually continues experimenting.

[geminimac]

@Maximum
Thanks for the support!
I wanted also to tell you that I'm also following your post which by the way I thought it was a godsend…is just for my case!
I saw that in your project, please refer to the constant tapered both mirrors…I'll do it too?Is essential?
I also tried to calculate a focal might be able to get a project finished…3600 circa!
It might come as the length of the tube arrives at about 85 cm?!
As soon as I can I'll post also my project

@Giulio
I trust in you all!!

[Massimo Marconi]

hi Luca, thank you for your interest in the site and discussions.

You'll have to excuse me but… I have to say: I can only share in the middle of what you have written and Mirco Giulio.

Anyhow, to answer your question, and ! the conical constant will serve absolutely, rather it is likely to become your best friend ( or worst enemy ) and you too will dream the night…

With regard to the measures / focal / overall dimensions, etc.… can “fun” to configure various optical patterns with any optical design software, ( you can find them even in our download section )

most of the answers that you are from there, from trying to configure an optical diagram in a functional way, but you must first take confidence with the Cassegrain and its variants, understand how systems work with two mirrors and then you can decide if a focal length is best for your project over another.

[geminimac]

Everything ok Massimo no need to apologize…The rookie is me!!
I fully what they say Giulio and Mirco…for me it will be almost impossible to complete the company but I want to try the same….I just hope not to disappoint!!

As soon as I start the work will update….

See you soon and thanks for the info!!!

[geminimac]

Hi guys,
I update you on the situation…I finally found the glass to be processed (41 cm x 3)
I would say that next week I should be operational, I start the first scratch ,we hope well

I tried to use ATMOS ricavarmi for various values ​​of the telescope…

choosing the Cassegrain setup I find some unclear rumors

The first speaks of “Effective focal length” It means the definitive focal you want to achieve?
The last entry “Df in mm” what does it represent?
I also want to load some jpg project I must use of external sites?!

thank you!

[Massimo Marconi]

Hello Lucas, in the world are the 3 cm ? :good:

for your questions:
1 – Yes
2 – It indicates the extension in mm of the full light field (CPL), then the amplitude of the image on the focal plane and presumably fully illuminated usable because by correct aberrations. for example if you need to use a photo sensor to 16 mm diagonal, You will need a CPL of that extension or higher. In a pure Cassegrain 16 mm is too, a Newton could have, Rc can do more, etcetera.. Also according to the value of the CPL is determined diameter and the bell of the secondary lens is required. Read more about this talk because it is important, not so much for the construction of the optical, but because it leads to understanding patterns and typical optical aberrations.
3 – Yes, such that: https://postimages.org/
:Goodbye:

[geminimac]

Hi Massimo,

I must say that I only had a blow of c..o
I found a guy on eBay who sold two 90 Euros and I have cornered me one
based on the information you gave me I will try to set the values ​​then update for some advice , I had already tried some rough calculations with ATMOS but do not coincide with my project in fact the secondary mirror would be completely different in size…

thanks for now I update

[geminimac]

Here is my piece of glass

[Massimo Marconi]

bello, congratulations ! But you put the image size Preview, He sees little, the next time you do that:
– upload files PostImage
– copy the link “direct link”
– back to the post you're writing, and click on “img” on the top toolbar.
– paste the link

[geminimac]

So I had already tried as you say….In fact, I had loaded the direct connection, however, there must be a problem as visualized only the title dell'img….then try again

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