A Very Low Obstruction Combo-Telescope

[DragosN]

Nytecam, I have used one small mirror, a small refractor lens, and 4 diagonals to test the position of the image. Initially the design was with only two tertiary mirror, but the test show me that the correct solution was with three.

I never count the area obstruction, only the linear obstruction, which in this case, 300 mm primary, 100 mm secondary, is 33%.

As far as I know, Dall transfer lens was never used for Newtonians, only for Mak and DK. Of course, never say never.

I realised today that my second comment was wrong; this design would not work as a substitute apochromat even if you found a way to equalise path lengths because to achieve apo performance the entrance leading to the refracting section would have to be in the middle and not off to the side.
 

My point about path difference still stands: light passing through the two sections must travel exactly the same total distance if the diffraction patterns are to merge. I notice that someone at "Overcast Evenings" pointed out the same thing.

I will think about. But if you talk about the faze of the light, this can be corrected by adjusting the positions of elements. Of course, extremely fine adjustments. VLT is working like this, and despite the light pass length is different, the images are combined. Anyway, I will look to that ''Overcast Evenings'', if I will find it.

More about this design, I'll be able to say more after I will do some tests. If the result will confirm my expectation, all will be mechanical problems (yes, a lot of it). I just hope to find time sooner, cause there are not simple.

[Soupy]

DragosN I think you need to consider what nytecam has said about secondary shadowing.

The secondary is 100mm compared to a 300mm primary this is indeed 1/3 the size but factoring in the inverse square law this gives 1/9 obstruction of light. So you need to bear this in mind with your construction.

[DragosN]

I work with some programs for designing Newtonian telescopes. They are counting the linear obstruction, mainly. And they show the area percent if you want. I usually look at telescope stores, to see what's new. Everywhere, the obstruction is given in linear percent, nowhere in area percent. Yes, the area obstruction is 11% in this case, but I didn't use it, and as far as I know, nobody does it. Almost. At last, is the same thing, the obstruction is the same, whatever is calculated in linear percent or area. Only the numbers are different. And I know enough math to do the transformation if I want and need. I still remember the square of the numbers from 2 to 20, and the square root for 2, 3, 5, and 7, with two decimal digits, even if I'm not young anymore.

[DragosN]

I  think I understood what is the source of confusion. Probably, you think that I'm interested of how much light is lost. I'm not. 

I'm interested of where this light is lost. In the center. Exactly where unobstructed system performs at their best, where abberations are minimal, Newtonians has no signal.

Maybe my idea will not work. But I know how APO telescopes, and even long achromatic telescopes are working, and that is what I intend to achieve, if it's possible.

[Ajohn]

Dall's idea wasn't really aimed at reducing light loss from 2ndry shadows. More reducing the diffraction effects that they cause so that systems with a 2ndry mirror more closely matched the performance of a refractor. Basically 2ndry mirrors cause a drop in contrast. 20% by diameter has always been considered a decent compromise and by the time it reaches 33% optical performance is similar to a 1/4 wave front error. The light loss in both cases is usually considered irrelevant.

There are some nice illustrations of the effect here

http://www.hoflink.com/~mkozma/obstruction.html

Some older boys own type astronomy books mention 3 or 4in F15 refractors and 6in F8 Newtonians as been equal. The page tries to show this after a fashion but it's a very difficult thing to tie down. A simplistic way of looking at the reasons for this is that a typical achromat has 4 surfaces and 2 thicknessses that can be used to correct aberrations. A newtonian has one surface and the shape of that is fixed.

John

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[Peter Drew]

As well as reducing the effects of a central obstruction, Horace Dall was also keen to eliminate sky flooding and achieving a terrestrially orientated image, each of these was provided by the erector transfer system. Horace demonstrated the performance to me on several occasions and the results were impressive enough for me to incorporate the design into my early commercial Maksutovs. 

[Sporadic Dobstronomer]

"Overcast Evenings"= Cloudy Nights -a joke I pinched from Lockwood Optics.

People over there are saying the same thing as I am.

[nytecam]

The unhappy conclusion is that this design, for all its ingenuity, has no advantages in resolution or light gathering compared with just having a Newtonian that is very slightly bigger.

Spot-on - 8mm extra radius or annulus of the primary mirror is of equal area of the secondary eg 316mm diameter - problem solved and easily :-)

[DragosN]

Spot-on - 8mm extra radius or annulus of the primary mirror is of equal area of the secondary eg 316mm diameter - problem solved and easily :-)

This is not about light gathering. Is about making a better telescope, in terms of contrast. Yes, is it possible that this is not the right solution, but this have to be tested to say clearly, is working or not. I have already figured some solution for testing in real life, step by step, every objection that has been raised, and hopefully, in 2 or 3 weeks I will show first results of those tests. And, if those test will show good results, I will try to make an improved version of this design, which I've already made.