"Poor Man's RASA project": The cheapo newtonian without secondary

[pipnina]

I had been thinking of this for a while, but until I got my 3D printer it was a bit of a non-starter.

Learning the ins and outs of printing and using FreeCAD has been quite challenging, but last night the first "finished-looking" prototype for the focuser-holder for my newtonian conversion came off of the build plate! And it fits very nicely.

 

My next step is to design the way to support it. EsunPLA+ may not be strong enough to support several kilograms of camera and focuser, so I may have to look into a hybrid approach with sheet steel vanes fixed into the PLA supports... Work ongoing!

For now, i'm happy I managed to design and produce a working holder for the focuser. While I could have bought a 2" rotating focuser as suggested by @vlaiv, which would reduce the obstruction and weight, I wanted to see what the minimum conversion cost would be for someone who already has a printer, so replacing the focuser isn't my intention at this time.

I intend to post more updates in this thread as my project progresses. And eventually find out if it's a worthwhile re-configuration for newt imagers!

So far this setup gives a central obstruction of 95~ mm diameter. Which is about 2-3mm more than the obstruction created in the RASA8's optical train. Perhaps I will work on a way of removing the focuser adjustment knob and replacing it with a printed part that can be slid on and off the shaft so it need not interfere with images...

Thoughts welcome!

[Dr_Ju_ju]

The one thing I'd recommend is not to use PLA but either ABS or, my preference, PETG ...   They will both give you a much stronger/resilient print.   

[vlaiv]

2 minutes ago, Dr_Ju_ju said:

They will both give you a much stronger/resilient print.   

When you say stronger, what exactly do you mean in this context?

ABS/PETG is more flexible so it will flex more easily under load - which is bad for holding camera

Those also have higher impact strength - but I don't see it being important in this application. They are however somewhat weaker in load bearing applications (not too much and I would not dismiss them on account of that) - and have nicer failure mode that might be of some importance for this use case - but again, if properly designed I don't see mount failing under weight.

1 hour ago, pipnina said:

EsunPLA+ may not be strong enough to support several kilograms of camera and focuser,

This is something that I consider a lot lately, and to be honest, I've been surprised by results people get when testing different material ability to bear weight.

For example - this guy does very interesting (maybe not very scientific) tests:

https://www.youtube.com/watch?v=E3WRBp-T42o

I've seen numerous videos where he is able to hold his own weight with 3d printed M8 bolt

In above video - M8 bolts printed in horizontal orientation fail at about 180-200Kg of load. I would trust such bolts to hold 20Kg for short periods of time (PLA is susceptible to creep, so might not be a good idea to put it under such load permanently).

Or how about car park / stop in 3d printed version?

https://www.youtube.com/watch?v=JSWtzMzZp9w

 

[pipnina]

1 hour ago, Dr_Ju_ju said:

The one thing I'd recommend is not to use PLA but either ABS or, my preference, PETG ...   They will both give you a much stronger/resilient print.   

52 minutes ago, vlaiv said:

When you say stronger, what exactly do you mean in this context?

ABS/PETG is more flexible so it will flex more easily under load - which is bad for holding camera

Those also have higher impact strength - but I don't see it being important in this application. They are however somewhat weaker in load bearing applications (not too much and I would not dismiss them on account of that) - and have nicer failure mode that might be of some importance for this use case - but again, if properly designed I don't see mount failing under weight.

This is something that I consider a lot lately, and to be honest, I've been surprised by results people get when testing different material ability to bear weight.

For example - this guy does very interesting (maybe not very scientific) tests:

https://www.youtube.com/watch?v=E3WRBp-T42o

I've seen numerous videos where he is able to hold his own weight with 3d printed M8 bolt

In above video - M8 bolts printed in horizontal orientation fail at about 180-200Kg of load. I would trust such bolts to hold 20Kg for short periods of time (PLA is susceptible to creep, so might not be a good idea to put it under such load permanently).

Or how about car park / stop in 3d printed version?

https://www.youtube.com/watch?v=JSWtzMzZp9w

 

 

For now, I think I can only print with PLA anyhow as ABS requires a warmed containment for the printer to avoid warping, and PETG/Nylon etc filaments require hotends that run much hotter than my Ender3 can handle (above 235c I think is unsafe as the hotend has PTFE tubing running all the way up to the nozzle, which fumes up and can even burn above 240c)

PLA+ seems "stronger" than normal PLA, but only because normal PLA is very brittle, while PLA+ tends to accept a lot more bend. I think tests do show normal PLA to take more force before total failure however.

I expect creep to be the big problem here. A test part I made for a catflap cover has warped and all I did was leave a plate on top of it. I am thinking that steel spiders might be a necessity, but the holder for the focuser seems rock solid even with this PLA+ so I don't expect to need to change that material.

I am planning on making the spiders out of plastic first, and attaching them to the focuser holder and seeing what weight they handle. I may put some weight on them and wait a few days, see if they move... I also plan on making them quite tall, as I think they can be any depth without impacting the image... Not 100% sure about that though. I always assumed only the thickness impacted diffraction. I guess eventually i'll find out!

[vlaiv]

5 minutes ago, pipnina said:

I also plan on making them quite tall, as I think they can be any depth without impacting the image... Not 100% sure about that though. I always assumed only the thickness impacted diffraction. I guess eventually i'll find out!

My guess is that you'll create different spikes depending where star sits in the field of view (it might be hardly noticeable and won't detract from the image).

Thick spider will present very thin profile when the star is at optical axis - but as soon as you star moving away from optical axis - it will "widen" in its profile.

Difference between thick and thin spider is in level of dispersion. If you've noticed, spikes tend to have rainbow effect on them in RGB images. This is because these are actually little diffraction gratings (spider support I mean) and diffraction grating will diffract light at different angle depending on wavelength. Width of spider support creates different type of grating - which results in different diffraction.

I think that narrow spider supports tend to spread rainbow more than thick ones.

[andrew s]

You will need a robust spider design to resist the torque changes as the Newt points to different altitudes.  Maybe a truss of some kind like on the Hale 200" (but smaller).

I seem to recall a schmidt camera design where focusing was done my moving the mirror with three fine lead screws.

Regards Andrew 

[Peter Drew]

I've made several Schmidt cameras and was halfway through making a 18" F3.5 photographic telescope with just a Newtonian primary similar to what you are proposing.  I have to ask you the same question as I eventually asked myself before I abandoned the project, "What was I hoping to achieve?"  My initial thought was to avoid the cost of a secondary and also to increase the light throughput.  Further research showed that, for a photographic instrument. the light gain was not enough to warrant the mechanical complexity.  Regardless of this, a single Newtonian mirror will still have the optical characteristics of a Newtonian, nothing like the corrected RASA or its variations.

If you do decide to continue I would recommend a primary mirror adjustment for focusing.  This can be achieved by fitting a 2" plug to the rear of a backplate of the mirror cell and fitting it to your focuser.  The focuser itself would be fitted in a fixed position on the telescope backplate.  Compression springs between the cell

and the backplate would take up the weight of the primary mirror and produce a light focusing action.    🙂 

   

[pipnina]

6 minutes ago, Peter Drew said:

I've made several Schmidt cameras and was halfway through making a 18" F3.5 photographic telescope with just a Newtonian primary similar to what you are proposing.  I have to ask you the same question as I eventually asked myself before I abandoned the project, "What was I hoping to achieve?"  My initial thought was to avoid the cost of a secondary and also to increase the light throughput.  Further research showed that, for a photographic instrument. the light gain was not enough to warrant the mechanical complexity.  Regardless of this, a single Newtonian mirror will still have the optical characteristics of a Newtonian, nothing like the corrected RASA or its variations.

If you do decide to continue I would recommend a primary mirror adjustment for focusing.  This can be achieved by fitting a 2" plug to the rear of a backplate of the mirror cell and fitting it to your focuser.  The focuser itself would be fitted in a fixed position on the telescope backplate.  Compression springs between the cell

and the backplate would take up the weight of the primary mirror and produce a light focusing action.    🙂 

   

I found it rather hard to design a newt in the online newt calculator tools that had good illumination across a good FOV, plus by removing the 90 degree light path turn you make it easier to baffle the tube and remove stray light from entering from the side of the focuser.

It's also just something interesting to work on regardless haha. A fair few correctors work with f3 mirrors which would be quite a good setup, something like a 10" or 12" F3 mirror and camera style focuser might be a bit better regardless of not exactly being a RASA.

But if I find like you did that it's not all that, I won't have spent much money to find out!

[Dr_Ju_ju]

In answer to Vlaiv, here are some studies comparing PLA/ABS/PETG:

https://3dinsider.com/abs-vs-pla/   https://www.3dsourced.com/guides/pla-vs-abs-filament/

https://3dsolved.com/pla-vs-petg/  https://www.3dprintingspot.com/post/petg-vs-pla-main-pros-cons-of-both

Personally I've found that while PLA prints easily, and is great for initial test printing of parts for Astro/outside items etc, over time it will warp, shrink and/or 'break' down, especially if the sun can get at it ...

I've regularly printed using ABS, but it does require a closed room, not necessarily an enclosure, and an application of Elmers/Koores glue stick, to the bed (mine are all PEI sheets) to ensure first layer adhesion. While the the bed temperature required is higher (85c as opposed to 60c), after the first 5/10 layers are completed the bed temperature can be reduced... 

I used Voron Designs recommendation to use ABS, when I printed the parts for my Voron2 machine, all printed on an OLD Prusa i3 MK2 ...

Hence my recommendation to use use PETG, to mainly get around the limitations of PLA/ABS.

Yes there are additional steps required, mainly in the slicer to get a 'cleaner' print etc.  (After trying many others, I settled on using PrusaSlicer...) overall I'm more than satisfied with PETG, and I used it to re-print the Z carriage/Extruder parts for my Prusa i3 Mk2, following many years of service, but an unfortunate accident cased some damage, and I wanted to upgrade some parts... 

 

 

[vlaiv]

35 minutes ago, Dr_Ju_ju said:

In answer to Vlaiv, here are some studies comparing PLA/ABS/PETG:

I'm not sure that answers my question though.

My original question was this.

14 hours ago, vlaiv said:

When you say stronger, what exactly do you mean in this context?

I do respect the amount of expertise you have with 3d printing and above is genuine question.

We often (at least I do because I lack any training as mechanical engineer) confuse different terms like strength and toughness.

ABS and PETG are tougher but less strong than PLA. They are also more elastic. That is, at least, what I figured from looking at videos like these: https://www.youtube.com/watch?v=ycGDR752fT0

40 minutes ago, Dr_Ju_ju said:

https://3dinsider.com/abs-vs-pla/   https://www.3dsourced.com/guides/pla-vs-abs-filament/

https://3dsolved.com/pla-vs-petg/  https://www.3dprintingspot.com/post/petg-vs-pla-main-pros-cons-of-both

I'm inclined not to overly trust sources like that because to me it looks like it has been generated by AI rather than human that knows what they are talking about. Take for example this part:

Quote

In addition, PLA is known for its brittleness, especially if kept in an open setting for a few months. PLA also fares badly if kept in an open setting over time, becoming increasingly brittle and causing filament bubbling, whereas ABS is more ductile.

When was the last time that you've seen same information in two consecutive sentences just phrased differently when reading the genuine article written by a human (without emphasis on something important - ie - "let's reiterate that once more ...").

[pipnina]

Personally I've based my opinions around printing materials on this guy's tests:

He tends to do a variety of tests that show tensile, impact, and other kinds of strength. So in general I feel it's good enough for us amateurs!

[kbrown]

I was thinking the same as @Peter Drew above. What would you gain with the conversion? Surely you're getting a bigger obstruction than a secondary mirror? Also how would you collimate this?

Edited January 16, 2023 by kbrown