Dear friends, hope you are all well!
This is my first post here, hope you understand me. I searched the forum and, although I found some relevant information, I still don't have confidence in my final answer.
I currently have a Svbony MK105, and thinking about an upgrade, I decided to try to build my first telescope, but keeping my Sky Watcher Eq GTi mount, with a load limit of 5kg. Therefore, I don't have many design options.
Luckily, I found a very cool project on the internet, called Leavitt, available on Printables, for 3D printing that fits my case. This project uses a 203mm (8 inch) primary mirror and a 54mm diagonal.
Another contributor made a remix of this project specifically for use with his mount, which is the same as mine, with the aim of making it lightweight, using few materials and carbon fiber tubes, in addition to using hardware with metric measurements, which for me was very good. In this remix, a 200mm primary mirror and a 62.5mm diagonal were used, and the choice of the diagonal size was based on a diagonal calculator website and also on a table of diagonal size suggestions from GSO, available on the Agena Astro website .
I've already printed and assembled the entire hardware, just needing to buy the mirrors, and that's where I got stuck. So far it weighs 1830g.
My question: I think this diagonal is too big.
As a beginner in building these toys, the only thing I have in my favor is a little bit of geometric notion and a little bit of CAD knowledge. If you can, follow my reasoning:
When I use the principle of similar triangles, a diagonal of 62.5 mm causes the focal plane to be 312.5 mm from the center of the telescope tube. Considering that the outer diameter of the telescope is 218 mm, the focal plane would be located 203.5 mm from the tube, which seems like a lot to me, considering that the focuser is significantly smaller than this. In the figure below, I present a full-scale diagram of the telescope, with the pink light beam naturally converging towards its focus, but being deviated at a certain point where the cross-section of the truncated cone delimited by the light beam has a diameter of 62.5 mm, thus causing the focal plane to be far from the tube.
At the same time, according to my calculations, using the focuser model that I printed, the center of the focal path would be at a distance of 24.5 mm from the tube, which I considered a reasonable point for the focal plane (correct me if I'm wrong). From this, I was able to calculate the distance from this supposed focal plane (in green) to the center of the telescope tube (red line) and then calculate the diameter of the light cone cutoff, obtaining 26.7mm, which is much less than the colleagues mentioned calculated.
See, nothing prevents me from using the 62.5mm or 54mm diagonal, but I understand that, geometrically, in order to obtain the proper focus, I would need to position the diagonal at a certain distance from the primary mirror, which would inevitably cause a significant and unnecessary obstruction of the light beam, so optimizing these sizes would be a good idea.
Considering my inexperience in this type of project, and not having many colleagues to be able to discuss it with in person, I was sure that I would find this answer here. What is the error in the reasoning above?
During this simulation, I came across another issue - when I place a 26.7mm diagonal at the center point of the telescope, in the 3D view it is possible to notice a phenomenon that must have a name, but I don't know it, which is part of the light beam escaping from one of the edges of the diagonal, while the opposite edge of the diagonal does not receive any light, but on the contrary, it just ends up obstructing the arrival of light to the primary a little more. This occurs because when rotating the diagonal by 45ยบ, the end opposite to the focuser approaches the primary, in a region in which the cross-sectional diameter of the light cone is larger, while the end close to the focuser moves away from the primary, in a region in which the light cone has a smaller cross-sectional diameter.
The another question is: how relevant is this? The diagonal size will not be exact, and will depend on the commercial size options available and maybe a will have to buy a bigger one anyway. But, with the 3D printing tool, I could easily slightly shift the center of the spider to optimize light reflection, like this (shifted 1.3mm):
If the purpose of using the telescope is important to answer my questions, I can say that I would like a versatile telescope for visual observation, with my eyepieces and barlows and, eventually, as I evolve, perhaps start astrophotography, even for deep sky. I know that with the use of photographic equipment I may need to change some focal issues, which I do not yet have full knowledge of. I agree that an F5 is not one of the most versatile telescopes, but due to the limitations of equipment and money, I believe it will be possible to make good use of it for a while, until the next project!
I would like to thank the colleagues on the forum in this stage of my life as an ATM apprentice, and I hope to soon be able to resolve my problems and order my mirrors!
