150 f/15.7 Refractor

[Chriske]

A while ago I got a 150mm lens from a old member of our society. A few months back the man passed away and it was his son who gave it to me for free. Build something around it 'in honour of my father' he said.
And so that's what I'm doing right now.

This is how far I got right now. As usual there's a 3D printer involved. Only the cell for that 100mm flat to draw.
The lens cell is already completed, busy printing the rest of the parts right now.
I decided to have this scope folding up because of it's rather long FL, it is a 150mm f/15.7
The flats are 45mm and 100mm diameter.
I also added a printed dual speed focuser.
The lower tube is not printed, it is a standard 160mm PVC tube.
The 480mm long upper part is printed.

 

[Chriske]

Lens cell ready.
Still to do paintwork.

 

[Chriske]

Adding a laser-unit instead of a finder-scope.

This the only location to add that laser-unit, (the rest of the refractor is 'filled' with optical path) between the lens and the secondary mirror. I did not want to mount the laser-unit externally.

The complete unit can slide in and out and is blocked by the large knurled knob(left)
The smaller knurled knob(right) is to activate the laser, by pushing the micro-endstop.
That little endstop is cannibalised out of a old microsoft-mouse. They're just a few mm across.

 

All the parts in the laserunit :
* two AA batteries(mounted in a holder)
* a green-laser unit(15mW)
* small endstop
* Collimation ring
* on-off knob
* Larger knob, at the side of the telescope, to lock the laser-unit in place
* Cover, to protect the collimation bolts while handling the unit.

In winter time a green laser will not work properly. In freezing environments it will even stop working completely. So I can remove the unit from the telescope and store it under my coat, close to my body, to keep it warm.
Starting another observing session I slide the (warm)unit in the scope and find me another object in the sky.
When done I remove it and tuck it away under my coat again.

 

To be sure that every time again the unit is correctly inserted at the exact same location in the telescope, it is pressed against a V-shape wall

[Peter Drew]

Following with interest as I've built several 6"-8" long focus folded refractors.     🙂

[Chriske]

15 minutes ago, Peter Drew said:

Following with interest as I've built several 6"-8" long focus folded refractors.     🙂

[Chriske]

there's another lens waiting to be transformed into a scope. That second is a 140mm and a whopping 3500mm FL.
Don't know yet what I will be doing with this one. it will be folded but I'm thinking of transforming it maybe into a drumscope.

[Chriske]

The inner parts of an old mimo.
I'll keep these for the endstops.

 

Laser + batteries + holder

 

Endstop + battery pack in place...

 

[Peter Drew]

A pair of 8" F25 folded refractors.     🙂

[Rusted]

25 minutes ago, Peter Drew said:

A pair of 8" F25 folded refractors.     🙂

Er.. was this taken at the equator?

[Rusted]

 

 I built a 7" f/12 folded refractor using 5" and 4" flats.

 Do NOT undersize the folding flats.
Do NOT try to make the folding angles too acute.
Both will lead to serious problems with stray light.
A narrow, folded refractor is EXTREMELY difficult to baffle effectively.

 Draw your full aperture light cone on lining paper and cut it out.
Now fold the paper cone where you want your flats to lie.
Allow a generous field diameter at focus.
 

[Chriske]

I know 'a bit' about folded scopes...;-)
In the past I've built lots of Kutter schiefs and bino-Newts.
If you do not want to build a completely closed system then you need to pay attention about cutting the oblong hole in the tube. You need to cut it very precise and add a few baffles if needed.
And btw I do not draw using lining paper anymore, I do it all on my computer these days. Far more accurate.  😉

 

[Chriske]

After this first drawing I made  a few corrections of course

[Chriske]

 

Some more corrections during filing and sanding...

 

[Chriske]

Assembling...

[Chriske]

Painting all inner surfaces...

[Rusted]

Drilling holes for lightness?

You must be a cyclist.

[Chriske]

The upper part will be printed in two separate parts. I cannot print higher then 210mm(yet).
So 2x200mm = 2x 12hours printing time.
They will be bolted together.

[Chriske]

Maybe later on this evening ; first light. Made a dummy fork to support that scope.

[Chriske]

[Chriske]

Just had a quick peek and this lens is surprisingly  good.
Need to test some more because the Moon was not to high in the sky and a few tree branches were in the line of site.

[Chriske]

Finishing this scope I have to deal with a few minor issues.
There are a few reflections from white (printed) parts. Needs to be blackened. It's known problem with folded scopes
A few baffles here and there.
A proper fork, I'm thinking of a 'Berry style' fork. The one in the picture is a 'two minute' fork so I could test right after installing the optics.
Adding some weight to shift the balance point a bit more to the centre of the optical tube. I like to keep the difference in height of the eyepiece (between high and low observing point) as small as possible. But I'm afraid I'll need lots of weight to compensate for this issue.
Finishing the mount that goes with it.

Again... first light was  👍

Edited July 12, 2022 by Chriske

[Chriske]

There's some bad and also some (very)good news...😃

Remember the story how I got this 150mm lens..?
I was very happy with it of course, finished it and this scope performs very well.
Only yesterday the manager of our observatory called me asking me to have a peek at some telescopes and parts from a deceased member. One of the items was a folded refractor that immediately drew my attention. It was a rather bulky instrument. I took it home, took it apart and the looking at the lens I thought : looks familiar...hmm...

It also was a 150mm diameter lens and the first thing I did was a quick check to find out about its focal length. Hand holding the lens and projecting an image of the sun on a sheet of paper I discovered that it's focal length was about the same as the scope I just finished. This can't be...I thought. So I removed the 150mm lens out of my own finished folded refractor and replaced it with that 'new' lens.
And yes...!!! Both lenses are exactly the same, well not exactly, I had to refocus about 2.5mm. This minor difference is allowed to make a binocular out of these two matching lenses.
Needles to say as binocular builder I'm very happy of course. It compensates more then enough  for the 'bad' news, the fact that I need to rebuild that finished telescope...😉

Intriguing question, no doubt about that :  How come two AMTbuilders, living not even close to each other, purchased the exact same lens about 30 to 40 years ago..? That was in an era btw that buying a 150mm lens was insane...!

One more thing : The owner of that last lens also had a 250(!!)mm lens (doublet air-spaced).... to observe the sun...😳😳😳😳😳

[Chriske]

So here ends the story of this refractor.
I'll be using it just a few weeks to show at our open door mid Sept.

After that I'll rebuild the lens to make me a binocular with the second lens I purchased a few weeks ago.
Question is what will I do. Will I make a folded version or not...?

Drawing is not 100% correct I know, anyway, building this version I'll be using 4 flats per scope.  Two mirrors to fold the light path and two more mirrors to bring it all together to the eyes.

The second option is building me a long binocular. Building a unfolded version I end up with tubes about 2.5m long...
I also need a very high tripod if I do that.