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

[Moonshane]

Hi all

In another discussion about best scopes for planetary use, I agreed to run a bit of a trial between the following :

16" f4 OOUK 1/8PV Dob (with Paracorr) focal length 1840mm effective

same scope with aperture mask creating unobstructed 6.7" f11

6" f11 OOUK 1/6PV Dob on driven mount focal length 1600mm

Both scopes had been fully flocked and I have made the usual improvements to increase contrast (secondary edge blacking etc). Both scopes were (as always) carefully collimated and well cooled before starting.

I also plan at a future date to make the same comparisons with a 6" f5 scope but this was not available last night.

I did not run fans to ensure the conditions were the same and the seeing was 'OK' for a while and then deteriorated to a point where even at 70x Jupiter was a fuzzy blob and everything was heavily dew soaked. Needless to say, at that point I packed up! I observed between about 7.30pm and 11.30pm intermittently. I have a Baader Neodymium filter which I normally use on Jupiter but did not tonight to ensure that everything was even. I tried to match the magnifications as closely as possible to ensure that there were no effects of wide differences.

My targets were :

Jupiter

Moon

Delta Cygni

Epsilon Lyrae

Here are my findings for all three arrangements. I feel that the seeing had a definite impact on the observing and particularly with the 16" at full aperture. All eyepieces were Televue or Baader Genuine Ortho.

Jupiter

I used magnifications ranging from 100x-230x with the best being around 130x during the best seeing.

The seeing conditions really did not allow the full aperture to be used effectively. Detail was very blurred and the image quite (too) bright.

With the aperture mask in creating a 6.7" f11 unobstructed view, the view was completely different. The contrast was enhanced and details such as the GRS and surrounding pale areas, and various barges were readily visible. With more prolonged viewing and better conditions it should have been possible to see fine detail within the belts and possibly ovals etc. The background sky was pitch black even at lower magnifications confirming good contrast and the absense of any diffraction spikes helped too.

With the 6" f11, the same details were visible on the whole but rather surprisingly there was less contrast apparent than with the larger masked off scope. An analogy that came to me was that the view was like watching the TV in standard or high defnition. With the 6" scope, the detail was all there but with the masked 16" it was so much sharper, more contrasty and defined.

Moon

The story was much the same with the moon as it was for Jupiter. The same details available with the 6" f11 but just not as sharp and strikingly contrasty as with the 16" masked off.

Double Stars

I looked at two classic double stars; Epsilon Lyrae (Double Double) and Delta Cygni. E. Lyrae is a straight forward double to split and I consider Delta Cygni to be doable most nights but is a bit more challenging given the greater difference of the magnitudes in the primary and secondary.

I consider my 6" f11 to be superb on doubles in the right conditions and so it is, splitting these targets well and realtively easily. I could not see any colour in the elements of E Lyrae. The seeing was poor though and the splits came and went a little.

In the 16" masked and to my great surprise, I managed to get a more stable split on both double star systems and even managed to pick up a pinkish tone to one of the stars in E Lyrae.

On the whole, I have to say that I continuously preferred the view in the masked 16" over the 6" f11 and this was to my great surprise. Even though the 16" is not on a driven mount and I was using quite high powers for the doubles, the stability of the 16" Dob is superb with almost no shake and it settles instantly. The 6" is a superb scope with which I will never part, but I am quite shocked that simply putting a piece of hardboard with a hole in across the aperture of a large newt can create a view better than a purpose built scope. I will certainly be returning to this thread every now and again to update my findings. I cannot wait for one of those super seeing nights when the full aperture can be used to full extant and provide something at least close to the theoretical resolution.

Cheers

Shane

[John]

Fascinating Shane - thanks for posting these results.

I guess the next step is to think through the factors that might be causing the differences you observed. Any ideas as yet ?.

[MjrTom]

Very interesting results Shane, thanks for sharing your findings

So does that mean the f/11 dob is now redundant?

[Adrian]

I think I found it:

unobstructed 6.7" f11

Excellent post, Shane - You've got me thinking that I may want to create a mask for my 10" SCT, but 3" unobstructed seems a little small

[angusb1]

Thanks Shane, this aperture discussion has been very interesting and I've been looking forward to this test. Did your aperture mask leave a hole next to the side of the tube opposite the focuser?

[MjrTom]

I think I found it:

Excellent post, Shane - You've got me thinking that I may want to create a mask for my 10" SCT, but 3" unobstructed seems a little small

These people with HUGE scopes....

nope not a hint of green eyed monster lol

[John]

Of course you need a large aperture scope to begin with to be able to create an off-axis unobstructed 6.7" scope

I guess the factors here could be:

- lack of any obstruction

- better optical quality (the 16" has an 1/8th wave PV mirror I think)

- 12% larger aperture

Add that lot together and that might go some way to explaining the performance difference perhaps ?.

[JAO]

Shane

An excellent write up on your observational experiment. I am not suprised by the results though and would have bet upon the unobstructed arrangement being the winner. As other posters have noted you do need a good size aperture in the first intance to get a good sized unobstructed aperture.

Fascinating stuff, thanks for sharing

Adrian

[James]

I've been really enjoying the views of Jupiter through a 14" Flextube Goto.... you've got me thinking about stopping that down now... :)

James

[Peter Drew]

I've long been a fan of off-axis masks on large Newts and used a 3 hole mask on my 30" F4.1 giving a choice of 12", 10" and 8" unobstructed apertures. These nearly always gave better planetary views than the full aperture althoufh this of course always won on DSO's. It would appear that the enhanced optical quality produced by a small sample of an already high quality surface gives an edge to a conventional system. As Shane says, it is a cheap and easy mod to have a go at.

[Stu]

That is very interesting, and quite surprising. Wouldn't necessarily have expected such an increase in contrast but it sounds like a really good way of getting two scopes in one.

Slightly bizarre question......what happens if you create a mask with two 170 mm holes both between the vanes? Would you get better results almost like two unobstructed scopes working in parallel or would it create all sorts of horrible diffraction effects?

Stu

[angusb1]

I was thinking along the same lines and found this article about multiple aperture masks. I've never come across this before. Would the multiple holes provide good contrast because they are circular so not producing diffraction effects that detract from the contrast?

[Peter Drew]

Next thing to try then Shane?

[Stu]

Peter, we must have been typing at the same time :-)

Are you saying that you do exactly as I was suggesting ie two or more holes in the mask to get bigger aperture with no diffraction spikes and unobstructed optics? Does it work well?

Stu

[derekm]

Part of the improvement with the masked 16" will be due to the lack of obstruction, but also, (I assume), the masked aperture will not include the edge of the mirror, which is always the "dodgy" part when it comes to figuring.

Humps or hollows in the centre are usually in the shadow of the secondary, so can be ignored.

[michael.h.f.wilkinson]

Part of the improvement with the masked 16" will be due to the lack of obstruction, but also, (I assume), the masked aperture will not include the edge of the mirror, which is always the "dodgy" part when it comes to figuring.

Humps or hollows in the centre are usually in the shadow of the secondary, so can be ignored.

I agree in part at least with this, but there may be subtler effects at play, very much to do with the human visual system, combined with seeing effects. I am not at all surprised that an unobstructed aperture mask 12% larger than the 6" dob comes out on top. No surprises optically, especially because both scopes were F/11, and EPs work well at that speed.

The only thing that needs explaining is the lack of contrast at full aperture. Of course, at F/4 aberrations in the EPs can contribute, but I assume Shane used good ones. More likely is the edge masking effect, and effect of the size of typical seeing cells (bubbles of air of different temperature).

At around 6" you will have (roughly) just a single seeing cell at each point along the light path. Each refracts the light in the 6" wide light path in a particular way, but in small scopes almost the whole light path is bent this way or that, rather than being smeared out. Though some smearing of the point-spread-function (PSF) of the optics will occur, much of the effect is in causing wobble in the image (this may cause distortion of features, but less smearing, i.e., less loss of contrast).

As aperture increases, more seeing cells are in the (16" wide) light path at each point. In the limit to really big scopes, a huge number of cells has bent the light in different directions, creating a star image which is more-or-less the superposition of a huge number of PSFs. Because there is no preferential direction to the effect, the net effect is blur rather than displacement. This causes much more loss of contrast, but less wobble. In intermediate cases you can get multiple images combined with blur and distortion. During (fleeting) moments of stable seeing, and assuming good edge quality, the 16" could do a much better job, as is born out by planetary imagers, who rely on capturing the fleeting good moments with webcams.

Visually, the situation becomes more complex, as the human visual system is a bit better at handling motion or distortion than it is at handling blur.

[angusb1]

How would you expect the multiple circular aperture masks to perform?

[MjrTom]

How would you expect the multiple circular aperture masks to perform?

Check out the article

I was thinking along the same lines and found this article about multiple aperture masks.

[Stu]

I guess from pulling all this together, adding more holes increases the aperture without adding diffraction spikes, but ultimately also increases the sensitivity to seeing by spreading across multiple seeing cells so the performance will depend upon the seeing conditions

How do you calculate the focal ratio with multiple holes? I guess it is the same as the single aperture?

Stu

[John]

Which has more negative impact on contrast and resolution, the central obstruction or the supporting vanes ?. Or is it about equal ?.

Assuming the scope is in good collimation that is.

[angusb1]

Check out the article

I had read that before I linked to it, was just interested to see how the other posters in this thread thought the masks might work. People haven't necessarily been agreeing with what CN articles have said on these topics so I was curious as to how people thought the theory might work.

[Peter Drew]

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?

[MjrTom]

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?

The way I was thinking was that it would work similar to how curved spider vanes reduce diffraction spikes.

[John]

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?

Thats what I think too. With the single off axis aperture you have proper unobstructed aperture. With the multi-aperture approach I would have thought that you would have, in effect, a heavily obstructed scope of smaller than original aperture.

I may have to actually try it though, just to see

[spaceboy]

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?

Ditto