What's best 16" f4 masked to 6.7" f11 or 6" f11 Newt??

[Stu]

I guess that's right, but it dues have the benefit if being completely blackened to avoid any reflections, and does get rid of diffraction spikes

I guess the summary is probably that a single hole gives the most significant benefit....

[spaceboy]

I had curved vanes in my 200K and that eliminates diffraction spikes with out loosing aperture.

[John]

It's a pity that off-axis newtonains never really caught on. Orion (USA) had a small one in their range for a while but I don't think it was ever a strong seller. I believe a company called DGM in the USA makes (or possibly made) them up to 6.5" aperture.

[spaceboy]

I have seen some scopes on US sights where they place the secondary on the side of the OTA wall and do away with the spiders all together. I will have to try and find the pictures.

[MjrTom]

I have seen some scopes on US sights where they place the secondary on the side of the OTA wall and do away with the spiders all together. I will have to try and find the pictures.

I have also seen images of scopes which use a mono spider as well whereby there is just one rigid vane to link the optical tube to the secondary.

Not sure how good they would be with flexure and stuff.

I am sure it was one of these ultra compact travel dobs if I remember correctly.

[spaceboy]

This is similar to what I had seen Starizona's Telescope Basics I guess this is what John was referring to ???

[Moonshane]

wow, nice response guys thanks!

I forgot to say that my aperture mask has not been painted yet as I'm lazy. it's still hardboard finish.

reading the article and Seronik's comments I tend to agree (and said so in the report) that the detail was there with both systems, just more contrasty, sharp and pleasant in the masked 16".

this is certainly an experiment I'll repeat and hopefully some day with a 6" f5 too.

as to the reasons, I am really not sure but agree with the little extra aperture unobstructed, going a long way. the difference was really notable and also the stability and smoothness of the big dob mount really please me, having made it with my own hands.

I'll do some more thinking and testing and update the thread from time to time with the results.

as to selling the 6" f11, no way Jose! it's a really nice scope still and the views are really great but on this test, not a match for the masked 16". I have learned a lesson in the past selling gear without proper testing and it will be a long time before I do so.

cheers!

Shane

[michael.h.f.wilkinson]

I've just been looking through the article. Surely the four portions of material between the holes aught to work as a thick 4 vane spider?

Absolutely. If you do the maths, you will find that the resulting PDF has more power in the spikes. They might look better at first glance, but only because they are wider, and more diffuse.

[Adrian]

Does it have to be a circular opening or would an oval/kidney bean shape work as well?

[michael.h.f.wilkinson]

Does it have to be a circular opening or would an oval/kidney bean shape work as well?

The moment your aperture lacks rotational symmetry, so does the PSF (star image). The central peak becomes elongated (as in certain cases of pinched optics) and the side-lobes (as in radio synthesis telescopes) become asymmetrical as well. Not a very good idea, unless you have good deconvolution software (which our brain does not, in my experience).

[Adrian]

Thought there must be a reason

[great_bear]

Basically, all you're doing is spreading the muck around the image some more.

It's inescapable - and using curved spider vanes just adds *more* but spreads it further, so it's disguised. At the end of the day it's a simple choice - do you want stars with spikes across a rich black background? Or do you want spot-like stars against a less less contrasting backdrop?

For moon & planets is it visually better or worse to diffuse diffraction artifacts?

- anyone know?

[Peter Drew]

When I was a director of AstroSystems in the 1970's we experimented with all types of spiders and came to the conclusion that the best planetary performance came from a stout single radial vane, this was extended to a diametrical vane on 10" and 12" apertures in the interests of stability. The curved models eliminated diffraction spikes but reduced the contrast, when neither would do we used full aperture optical windows.

[DavidOfBanff]

So does this mean that the best possible planetary Newtonian is made from an offcut of a stonking big parabolic mirror, like a 20", and thus to a secondary that obscures no part of this offcut ? And there's no distortions or blurring or comas or diffractions spikes etc on account of the inherent long f number and absence of a spider ?

That's quite a thought.

[michael.h.f.wilkinson]

So does this mean that the best possible planetary Newtonian is made from an offcut of a stonking big parabolic mirror, like a 20", and thus to a secondary that obscures no part of this offcut ? And there's no distortions or blurring or comas or diffractions spikes etc on account of the inherent long f number and absence of a spider ?

That's quite a thought.

Essentially the idea behind various obstruction-free designs such as the Kutter or Schiefspiegler. Many larger ones have problems with astigmatism. People should not worry too much about central obstruction. Optical quality is far more important.

[RikM]

To what extent did having the Paracor in the optical train of the 16" help?

I reckon a mask on my 10" would give me a 102mm f/12. Should be worth trying.

[Moonshane]

I think it makes a difference at f4 (although to be honest I have not tried it without (yet)) and at f4.8/5 I bet it won't matter. certainly worth a try. you could even cut it out of black card first before making a more permanent one (although you might never bother if the card is thick).

[DavidOfBanff]

Essentially the idea behind various obstruction-free designs such as the Kutter or Schiefspiegler. Many larger ones have problems with astigmatism. People should not worry too much about central obstruction. Optical quality is far more important.

No Michael that's not so (if I understand this thread correctly): the offcut Newtonian/ masked Newtonian is inherently astigmatically un-compromised. Nor is it compromised by a spider, plus its long focal length negates other deficiencies. So one stonking great parabolic mirror of indifferent precision could produce five or six ideal planetary Newtonians. Is quite a thought, I propose.

But what do I know ? (see sig)

[Moonshane]

No Michael that's not so (if I understand this thread correctly): the offcut Newtonian/ masked Newtonian is inherently astigmatically un-compromised. Nor is it compromised by a spider, plus its long focal length negates other deficiencies. So one stonking great parabolic mirror of indifferent precision could produce five or six ideal planetary Newtonians. Is quite a thought, I propose.

But what do I know ? (see sig)

I honestly don't know the answer to this. Probably a stupid question, but does the mask mean that rays only go to /bounce off a small area of the mirror? This makes sense given the dimmer image. I also sense in the back of my mind that it's still an f4 mirror at the end of the day and the shape of the parabola makes the coma still present (unless corrected with e.g. a paracorr?).

[Moonshane]

In the fashion of Roy Walker, I can only say what I see. My theory knowledge is weak.

[michael.h.f.wilkinson]

There are "offset" parabola, obstruction-free Newtonian scopes. Apart from difficulties in manufacture, they do tend to show unfavorable symmetries in their off-axis performance. Besides, a 14" scope with obstruction can get more detail than a 6.7" unobstructed scope. A slow 14" offset parabolic design becomes very bulky.

[John]

Is the conclusion that an on-axis unobstructed scope would probably give the best performance then ?.

Aren't they called refractors ?

[Mr Spock]

Coma is an off-axis aberration caused light at an angle hitting the focal point in different focal planes depending on where the light comes from on the main mirror. So if you cut off the light from one side the aberration will be less

Coma gets worse the more off axis you get, and at lower f ratios as these have a steeper angle to the light path.

[Moonshane]

I agree Michael that in principle, if you wait long enough in poorer conditions or have excellent seeing conditions then even a fast 10" scope of similar optical quality could probably beat my 'contraption'. That said the seeing is rarely good enough to make this the case in the UK at least and the difference really was astonishing. I am looking forward to further trials in due course.

[DavidOfBanff]

Ok. So someone has to take a bandsaw to a stonking big mirror and produce lots of little Newtonians off it.

Can I watch?