An Experiment: Newtonian to Mak-Newtonian

[ahmedsaad]

Hello, I want to share an experiment done on a reflector. It wasn't successful but the details might be interesting for people in this group.

I made a f/5 Newtonian with a 6" spherical primary in 2017. Last year during the opposition of Jupiter and Saturn I noticed it doesn't give sharp views on high powers. Upon reading I learned it's probably spherical aberration. I also learned how SCT and MKT make use of corrector plates to reduce spherical aberration.

Making a corrector plate like SCT or MKT was difficult so I thought of a cheaper and easier idea. I don't know if anyone else has tried it before.

Check the pictures. There is an acrylic disk 3mm thick, with a hole in the center and a bolt passing through it. Bolt itself goes through a spider like structure. Edge of the disk rests upon a ledge made from the same PVC pipe as the scope's body. Finally there is a nut in the bolt in disk's center and the whole structure is attached to the mouth of the telescope.

Acrylic disk has no curvature by default, but when the nut in its center is tightened, pressure induces curve in it because the edge of the disk can't move in the direction of nut's force, as the ledge is fixed on which it is placed.

Last night I was able to test it amid shifting clouds. Without this new structure in the light path, Jupiter looked glorious with four moons at 31x and 75x. But when I slid it in place, the view got distorted. I checked with and without producing curves in the disk, and keeping disk at various distances from the primary.

So the experiment failed. But hopefully I will receive helpful comments from the pros to learn more about telescope making.

Thanks

Edited May 12, 2019 by ahmedsaad
Nut, not screw

[Stu]

Sounds like an ingenious plan, but one that was not likely to be successful for a few easons. Firstly I suspect the acrylic has very different retracting properties to corrector plate glass. Also I'm sure the surface accuracy would be fairly poor so it likely to reduce the image quality even when flat.

Finally though, and most significantly, the Schmidt Corrector plate design is not just a simple curve, but is a more complex compound curve so your corrector would not have been performing the right function to remove the spherical aberration, and may well have been introducing other errors.

There is some further info here.

https://en.m.wikipedia.org/wiki/Schmidt_corrector_plate

I love your experimenting and ingenious construction methods, I'm sure you had fun putting that together. I hope you find ways of improving the scope although I suspect it may be tricky.

[fwm891]

Mak and SCT correctors are completely different. A Mak plate is normally quite thick and highly curved with a concave face to the sky. An SCT corrector as Stu says is a complex curve have effectively a convex central area and concave outer area with a neutral zone at circa 70% of it's radius.

Your acrylic sheet whilst visually transparent to the unaided eye is not homogeneous enough and effectively creates bad seeing.

If you look through the Amateur Telescope Making books (3?) there's an article on a 'Wright Telescope' which is basically a Newtonian (Ellipsoidal primary) with a weak corrector plate at the end of the tube, just in front of the diagonal mirror...

Link: https://www.telescope-optics.net/Wright.htm

 

Edited May 12, 2019 by fwm891
text added

[Cornelius Varley]

A 3mm thick sheet of acrylic placed in front of the secondary won't change your Newtonian into a SCT?Mak Newt. Both SCT and Mak Newts have carefully shaped optical glass corrector plates matched to the primary mirror. Without the acrylic sheet your telescope will perform infinitely better.

[Chriske]

You should first test the quality of that acrylic sheet. The most simple way of doing that is to place it between lightsource and mirror during Foucaulttest.  You will be surprised how bad your mirror will get performing this test. Acrylic is very transparent indeed, but it has a completely different refracting index compared to glass..!
Second acrylic is not always homogeneous. Another test you can perform to check this is to use polarising filters. Two of these filters in a 90° position will reveal all flaws, like strain.
But to be clear on the matter : plastic, any plastic at all is NOT suited to be used as a corrector..!
You could use it to seal a newt and avoid using a spider. BUT..! again that sheet should be free of strain and most important of all it must be flat at both sides. And to avoid reflections you need a coating on both surfaces.
Correctors used in SCT and Maks are made of glass. Most of the time it is optical glass. Some brands do use float-glass in their SCT's.

About that technique you're using. That is called 'Flexing'. You actually re-invented it but used it at the wrong side of your scope.
It's nothing like you did,  some bolt in the centre of the optics and start pulling.  Not at all...!
It's an old technique I used during our courses years ago. What I did was mount a device under the primary mirror and used that device to pull (very hard) with a bolt to reshape that mirror from sphere to parabola. The largest mirror I ever flexed with success was 20" f/5. During courses we flexed dozens of mirrors. In fact lots of guys are using Newt's to this day that have been flexed with such a device.
And what's more when using that device you can flex any mirror into any shape you want it to be. Say you want a parabola, ok fine, just pull that bolt until you have a perfect a paraboloid. You can actually see how far the curve is reshaped just by watching the progress in the Foucaulttest. Say you want a ellipsoid, to make a Dhall-Kirkham primary, you need to pull less hard compared to paraboloid. Steep hyperboloid, no problem, keep turning that bolt.
Needles to say it will only work with thin mirrors. To make a 10" mirror we use 19mm(3/4") floatglass.

To have this device working perfectly all parts must be made/glued at the correct spot.  For instance, you must be prepared to glue a large piece of neoprene at the back of your mirror. Second you need to find someone, who is prepared to coat that mirror having that layer glued at it's back.
I have my own coating bell-jar, so no problem for me. It can actually hold mirrors up to 20"(500mm)

What you need to do is to glue a layer of neoprene (about) 80% of the diameter of you mirror in the very centre on the backside of that mirror. When done you need to glue a sheet of 6mm glass (same diameter) onto that neoprene. And on top of that a bolt(any bolt) must be glued again in the very centre of that sheet of glass.  There's no room for error at all..!!!!!
We've been testing this device for years before we introduced in our courses.

To be clear on the matter, this technique is not ours but is invented by a man called Adler. He even posted a software tool to calculate the diameter of the puller and the pusher of that flexing-device.
These days we do not use it anymore because a few of our course members did not executed it perfectly and had to remove the neoprene layer and parabolised the old fashion way. Again all must be mounted absolutely perfectly centred ..! When this is not done properly you get severe astigmatism..!

This is how we made dozens of flexing-units.
From left to the right.

The puller parts :
1- Mirror
2- Neoprene
3- Sheet of float (6mm)
4- Small aludisk (glued onto the float and to hold the threaded rod

The pusher parts :
1- Neoprene ring
2- Plastic ring(spacer)
3- Piece of wood
4- Washer/wingnut

Notice that it is a very large area that is pulled on, not just a spot in the middle of the mirror.  In the very beginning we did test that flexing technique by pulling just with single bolt glued at the back of the mirror. At the Foucaulttest we got a very 'pointy' shaped parabolo-ish 'thing'.
 

 

[Astrobits]

I have made Acrylic sheet. I worked in Perspex research. The sheet is cast against ordinary glass sheets and will therefore AT BEST take up the surface morphology of the glass. If you test ordinary glass sheet you will find that it is not optically flat but covered with hills and valleys of at least a few wavelengths high and low. These undulations will be present on BOTH sides of an Acrylic sheet. As mentioned in above posts the bulk of the material may, or may not, be homogeneous. All these defects will degrade any image in addition to your spherical primary.

Chriske mentions that float glass is used in some SCT's but that will have been selected for homogeneity and optically polished both sides to produce the complex curve SCT's require.

Maks require more simple curves but of very short radius and the thickness of the corrector is important. Go check the formula for a Mak.

Nigel

[Chriske]

2 hours ago, Astrobits said:

Chriske mentions that float glass is used in some SCT's but that will have been selected for homogeneity and optically polished both sides to produce the complex curve SCT's require.

Nigel

Correct Nigel, still I find it strange that float is used to make these correctors in the first place. But on the other hand it is 'only' a corrector and not like refractor's primary lenses a set of components to 'produce' images.

Another thing I've tested in the past is whether plain float can be used as a diagonal in a Newt.  Answer Yes/no.
We always buy our float glass (to make our Newt-telescopes) in a company nearby (Glastec). They do cut their glass with a water-jet. I've seen that machine. Seen it while it was cutting glass. The least I can say, that is a im-pres-sive machine..!

Anyway, in one of my orders (to start a new course a few years back) I added a file in my mail and asked them to cut me diagonal mirrors. These diagonals were suppose to be used for 130mm long focus Newt's. The idea was to make these scope as cheap as possible, so I thought : why not..? I anticipated lots of bad  flats, so I ordered 100 flats 25mm diameter. Negligible cost.
I checked all these flats with an interferometer. And guess what... If you take 1/4 lambda as a standard acceptable flat, well nearly 30% could be used.
To be clear these flats were ok but only because they were so small. I doubt if I would get that same amount usable mirrors if I would have ordered say 40 or 50mm diameters flats.
After been cut we also check for strain and striae. Not once we received a bad blank during all the years we bought from that company.

[johninderby]

SCT correctors are placed on a shaped plate and pulled against the plate by vacuum to distort the glass and then polished flat. Then when the glass is removed it assumes the correct shape. Simple idea but machining the plate isn’t easy and they are extremely expensive to make. However makes automated production of correctors relatively straight forward.

More than you probably want to know about SCT design. 🤔

https://www.telescope-optics.net/SCT.htm

Edited May 12, 2019 by johninderby

[Stu]

Clever idea!