My latest crazy project AR150/2250 (F15)

[pupak]

The sun is out of sight again, so I set up a new AR150/F15 refracted telescope on it.
Just a skeleton so far, but even that counts.

[Bob Yoesle]

Lack of machine shop capabilities is the root of all solar telescope evil.

[christian viladrich]

What a nice piece of mechanics. I like it !

[Ivan]

Wow! All of our own production, including a tire iron! Focal length of more than 2 meters and compact design. You have golden hands.

Ivan

[marktownley]

Way cool!!!!

[Rusted]

Great project! Your machining and design skills are amazing.

The difficulty of baffling a folded refractor is exacerbated by bringing the parallel sections of the light path too close together.
I spent ages folding a full sized, paper light cone to achieve the best optical layout with the flat mirrors I had: 125mm and 100mm.

Then there is the problem of collimation. I wished I had provided remote control rods for the optics of my own folded refractor.
An iStar 180/12 R35 with two folding flats of 1/20th wave on Zerodur. Shown here on my home made equatorial mounting on its test stand out in the garden in 2016. I [eventually] preferred the simplicity of [collimation] using a [long] straight tube.

folded refractor rsz 600.jpg (200.78 KiB) Viewed 2594 times

[Martin_S]

That's an amazing project Pupak. I will be eagerly awaiting a first light report.

[pupak]

Great project! Your machining and design skills are amazing.

The difficulty of baffling a folded refractor is exacerbated by bringing the parallel sections of the light path too close together.
I spent ages folding a full sized, paper light cone to achieve the best optical layout with the flat mirrors I had: 125mm and 100mm.
Then there is the problem of collimation. I wished I had provided remote control rods for the optics of my own folded refractor.
An iStar 180/12 R35 with two folding flats of 1/20th wave on Zerodur. Shown here on my home made equatorial mounting on its test stand out in the garden in 2016. I [eventually] preferred the simplicity of [collimation] using a [long] straight tube.

A very beautiful refracted telescope.
Too bad you don't use it. I have big problems finding straight mirrors, so for now I'll use what I have and hopefully find some better ones. I envy yours.
I know I have a lot of problems ahead of me. I base everything on precision machining to avoid an unstable construction as much as possible. 2250mm is really too much for my EQ-HD mount, so I have to shorten the telescope. The weight should not exceed 14 kg, which is OK. The sun is behind the clouds, it's snowing, so I have to do something.

[Rusted]

Thank you pupak. I wasn't trying to hijack your build thread.
Merely pointing out the difficulties I discovered back in 2016.
My own build was inspired by Dave Trott's folded refractor.

The optical flat mirrors came from Steve Dodds of Nova in the USA.
He had a lot of ex-government flats to dispose of.
Now I can't find his website any more.

My own folded refractor weighed as much as the straight tube model.
The objective alone, in its cell, weighed 10lbs.
So this became an issue when lifting the whole OTA.
Onto the mounting, on a pier, well above my head.

I built a bayonet mounting [with keyholes] onto two plates.
With hex socket head screws locating the objective plate.
This allowed me to fit the objective after the OTA was mounted.
Except that it was still a terrifying lift from a stepladder!
It needed three hands and I have only two.

[pupak]

On the contrary. I am very glad that you provided a picture of your refracting telescope. Any advice and inspiration is welcome.

[marktownley]

I do like a telescope engineering thread!

[pupak]

First picture.

[Rusted]

How far away is the target?

[pupak]

Cca 40 m.

[pupak]

It didn't hurt too much either and it's done. Somehow I forgot that I should collimate it before I start testing, but surprisingly it is made so precisely that I made a small adjustment to the mirrors and lens and it was done in 10 minutes. But what is absolutely essential are the apertures. When I tried to remove them, the contrast almost disappeared. I still have to strengthen the bar because it vibrates a bit, but otherwise it's waiting for the Sun and then the champagne will pop, or my head.
Even with full equipment and 180mm DERF it weighs 13.2 kg, which is only 30 dkg more than the 150/F8 weighed and it is about 30 cm shorter. So far I'm happy with it, so maybe it's not too early.

[Rusted]

Your constructions are certainly very beautiful.
Larger optical flats allow considerably more shortening of the OTA.
One must not try to use all of the folding optical flats' [mirrors] full aperture.
The edges may be turned. So it is best to play safe and use slightly less of the available diameter.
A retaining lip on the mirror cells will help here.

40m is very short for testing a long focus refractor.
I have the good fortune to have a small wood four-five hundred meters way for testing.
I could easily see flies moving in the trees at this distance in the folded 180mm.

A tubular extension, forward of the focuser, proved vital to increase contrast on mine.
I fitted a baffle at the opening to avoid grazing reflections inside this [blackened[ tube.

The folded form of refractor cannot perform well without careful baffling. Any more than a straight tube can.
I used the bright, folded beam of the sun to confirm there were no errors of position and aperture of my baffles.
Without this simple test the baffles can easily block a lot of light. Or completely fail to do their job.
Tubes carrying baffles, must not themselves, impede the full [folded] light cone. You'll need a tracking mount to perform this test.
Place a Herschel wedge in the focuser to avoid setting fire to your surroundings or observatory. Ask me how I know this?

Those who have clever software to design their folded refractors may find my simple [white light] test unnecessary.

[pupak]

I don't have that much space, but you must understand the terrible need to see if I can at least see the end.
In addition, I forgot the collimation, so the photo was taken in a non-collimated state. I have an optical simulator which is a big help. Without him, I wouldn't even have started building a telescope with mirrors.
The Vixen 90/F15 showed me the magic of long focal lengths, so I decided to give the 150/F15 a try. If it works, I'm done with the experiments. He already has a decent collection of binoculars and I don't want to be a collector.
I leave a 2mm border for the mirrors. Hopefully it will be enough. If not I close the curtains.
I love the job. Design, implementation and then the first look at the Sun or the stars. When it works, it's a joy. In today's world, a person has to find a way to not go crazy with it all, and so far I'm doing pretty well.

[pupak]

Todays morning test.

[Rusted]

That is a great improvement. The texture of the tree bark is very evident in the second image.

[pupak]

I was just thinking, what kind of demented person am I to go take pictures without collimation. The silicone in the mirror sockets is still working, so I still have to adjust the collimation. I hope it settles down after a few days.

[pupak]

In order not to test the quivering leaves, I attached a cardboard packaging from the glue to the stick. Distance cca 35m. Font size is approx. 3 - 12 mm. As it is constantly raining and snowing, I am taking photos from the open workshop door, and the warm air escaping outside creates strong air ripples.
In doing so, I came across an interesting paradox. At F15, the picture flies and I can hardly choose one or two decent pictures from 20 sequences. With Barlow 2x (F30), like a miracle, the image calms down many times and there are many more good pictures. I changed F15/F30 several times and the phenomenon persisted. I tried different cameras and BINNING 1-4. At F15 the wilderness is significantly calmer at F30. I have no explanation for this. I mainly tested the size and placement of the aperture in front of the eyepiece. This is absolutely key. Attached are two samples on F30 with IMX432 BIN1 and BIN2.

[pupak]

One surprising image on F30 with Altair 183M in BIN2.

[Rusted]

Thermals aside, stopping down may be kinder to the image simply due to your very short target range.
The full aperture of the objective may be struggling if it is working beyond its [infinity] design parameters.
Stopping down may increase diffraction effects but will provide a deeper sharp field of view.

I did a lot of visual testing on multi-coloured text targets from packaging. To prove that a certain spotting scope was not worth its asking price.
My ancient Vixen 90 f/11 was always the same, or superior, at every distance from 10m up to 100m.
The spotting scope had the same aperture. Focal length unknown but claiming to be an APO. It was returned for a full refund.