Aperture Mask Question

[cuivenion]

I've recently run across aperture masks on this forum and they seem to be a great way of using a reflector for imaging while removing diffaction effects. I'm wondering if it's possible to have more than one hole in the aperture mask in use at the same time. For example with a 200mm reflector instead of one 40mm hole have four increasing the aperture while still avoiding diffraction spikes. Is that feasable?

[Scooot]

I don't think so. The solid bits between the holes in the mask would block the light in the same way the spider vanes would. I don't suppose the defraction spikes would be the same but the image would be affected just the same.

[cuivenion]

Oh well, thought it was worth a punt. Might try it anyway and see what happens.

[SilverAstro]

I dont think so either but,,,

One of the early forms of focus aids ( before Bahtinov et al) was a two hole (sometimes three hole) mask called variously a Hartmann (sp?) or Scheiner mask, where the two resulting images are brought together at critical focus. But I have not seen a description or example  of what the resultant combined image is. I suspect some sort of interferometer-like image. Probably knowing the separation of the holes one could measure the diameter of the (very large?) star ???

 

 

[cuivenion]

To be honest, looks as though it might work after reading up on the hartman mask. This quote from the wiki:

"Adjusting the focuser, the images can be made to overlap, forming a single bright, clear picture."

[SilverAstro]

Worth a try !

"If the end of the telescope tube is closed with a mask with two apertures, fringes are produced at the focus, showing that the light is coherent, if the two apertures are close enough together. With the Sun, or Jupiter, fringes would not appear at all because of the small coherence radius. With stars, however, the decrease in fringe visibility would be evident*, and from the separation of the apertures for zero visibility the angular diameter could be found."

from http://mysite.du.edu/~jcalvert/astro/starsiz.htm

This was a method employed in the 1920's  at Mount Wilson to first determine the size of Beetlejuice, I remember it well, not cos I was around then but my dad gave me a book ( in the 50's ) By Eddington about Einstein and the eclipse and the bending of light and other wonderous things !

Google for example ' interferometer star diameter '

* might be evident to him , but not yet to me, so I await your (our!) experiments

 

[cuivenion]

I'll try it for visual first, but unfortunately I only do planetary imaging where diffraction effects don't really matter as much.

[vlaiv]

It will work. Main reason the diffraction spikes appear is straight edges of spider. Either use curved spider or put round aperture mask. Multiple openings should not be a problem if focused correctly. It will certainly change PSF so you will loose resolution not just because of smaller aperture but also because of photon interference due to different paths (think of dual slit experiment). I guess you can expect lower contrast as well because some of the light will be directed from airy disk into rings (lower strehl). All of this should not matter if using long exposure in average seeing conditions. For high resolution work impact would be noticeable (planetary, binary stars, etc work). For high resolution work on doubles it can be beneficial because change in PSF is predictable and can be used to reconstruct image. I think this technique is called speckle interferometry.

[Moonshane]

presumably though it will also affect exposure times as the focal ratio will be longer?

[vlaiv]

3 minutes ago, Moonshane said:

presumably though it will also affect exposure times as the focal ratio will be longer?

It certainly will, same FL, same pixel size, smaller gathering area. Exposure time can be compensated by simple formula: one should extend it for factor of unobstructed aperture area / aperture mask area, to get same SNR.

[ollypenrice]

I think it might well work. I think you'd get more resloution than from a single hole becaue of the baseline separation between the holes. I can't remember the details but while taking a radio astronomy course we looked at interferometry and I seem to think that two small dishes at baseline 'x' atcually out resolved a full dish of aperture 'x' though I can't be sure about that. Beisdes there is the fact that this would be light from a single dish sampled separately, which might not be the same as having two separate mirrors.

I reckon you'll just have to try it!

Olly

[vlaiv]

2 hours ago, ollypenrice said:

I think it might well work. I think you'd get more resloution than from a single hole becaue of the baseline separation between the holes. I can't remember the details but while taking a radio astronomy course we looked at interferometry and I seem to think that two small dishes at baseline 'x' atcually out resolved a full dish of aperture 'x' though I can't be sure about that. Beisdes there is the fact that this would be light from a single dish sampled separately, which might not be the same as having two separate mirrors.

I reckon you'll just have to try it!

Olly

I don't think it would work in the case of light. Two small dishes separated work well for radio waves due to large wavelength of radio signal being observed (order of magnitude dish size, even larger). Optical spectrum has rather small wavelength compared to even smallest telescope diameter (less then 1 micrometer wavelengths). For speckle interferometry, as I understand it (but that is just a tiny bit), atmospheric distortion of wavefront combined with interference due to different light paths that each single photon can take is used to reconstruct image to a certain degree. It works because photon "traveling" thru one aperture has its path slightly differently perturbed than same photon going thru other aperture (photon travels thru both aperture openings - quantum stuff). From this perturbation difference interference pattern arises that is different then unperturbed photon going thru both openings at the same time (as in dual slit qm experiment). I think this phenomenon is used in speckle interferometry. So in order to achieve greater resolution than normal with two aperture openings, one should use sophisticated algorithms and calculation techniques.

[cuivenion]

So if I'm reading correctly it could work but I have to make sure the apertures are a certain distance from each other? I was thinking about using four apertures that way the effect on focal ratio will be reduced. I'm going to use a Skywatcher 130p for the experiment and I have a mate who'll hopefully join in and use his dslr.

I've yet to measure the size of the apertures but hopefully I'll be able to manage 4x25mm which would make a focal ratio of 6.5.

I must admit some of the previous replies have gone over my head a little but I'm game. One of the thoughts I had is that this technique might be useful in taming stars like altinak when imaging them with a reflector. Eventully I want to start imaging planetary nebula's with an Altair GPCAM which would also double as a planetary cam. I may use a focal reduce with that cam on some targets to bring down the focal ratio even more. Anyway I'll get reading.

[vlaiv]

7 hours ago, cuivenion said:

So if I'm reading correctly it could work but I have to make sure the apertures are a certain distance from each other? I was thinking about using four apertures that way the effect on focal ratio will be reduced. I'm going to use a Skywatcher 130p for the experiment and I have a mate who'll hopefully join in and use his dslr.

I've yet to measure the size of the apertures but hopefully I'll be able to manage 4x25mm which would make a focal ratio of 6.5.

I must admit some of the previous replies have gone over my head a little but I'm game. One of the thoughts I had is that this technique might be useful in taming stars like altinak when imaging them with a reflector. Eventully I want to start imaging planetary nebula's with an Altair GPCAM which would also double as a planetary cam. I may use a focal reduce with that cam on some targets to bring down the focal ratio even more. Anyway I'll get reading.

I would like to encourage you to experiment, I think it is the right approach in this case, but few things to point out: 4x25mm has smaller surface than single 100mm therefore focal ratio will be smaller than 6.5 for sure. Best way to calculate your focal ratio would be: take total aperture size, calculate what is equivalent size of a single circular aperture and use it's diameter for calculating focal ratio. As for taming bright stars three things would happen in my opinion: You would loose diffraction spikes, brightness of a star would go down for single exposure but final image (target SNR) would not see any improvement in overall star brightness (as you would have to extend total integration time to get enough light), and star diameter would even increase compared to setup without aperture mask due to lower resolution. So you would not see star as tamed by any means. Only thing this is going to do for you is loose diffraction spikes at expense of having lower resolution (slightly larger stars). Also spacing between openings is irrelevant if doing long exposure. It for sure has effect in speckle interferometry, but in which way is beyond me atm.

[SilverAstro]

11 hours ago, cuivenion said:

I'm going to use a Skywatcher 130p for the experiment and I have a mate who'll hopefully join in and use his dslr.

  Excellent idea ! (as Vlaiv says ) I'll be really interested to see what you find. Here's hoping that the cloud gods co-operate soon

4 hours ago, vlaiv said:

I would like to encourage you to experiment,

You would loose diffraction spikes,

Ummm, I'm not sure about that. In the case of a single aperture that is certainly the case, but for 4 holes ? Would that not result in a really fat maltese X type of spider-equivalent ? Anyway, no need to speculate any more our Chief Experimenter and Chum are on the job and will reveal all in due course

 

[vlaiv]

7 minutes ago, SilverAstro said:

Ummm, I'm not sure about that. In the case of a single aperture that is certainly the case, but for 4 holes ? Would that not result in a really fat maltese X type of spider-equivalent ? Anyway, no need to speculate any more our Chief Experimenter and his Chum are on the job and will reveal all in due course

Well, considering theory behind formation of spikes, I think that it would not result in such pattern. When investigating possibility of creating computer simulation of telescope optics on quasi quantum level, I came across explanation that is not quite true in terms of quantum mechanics but it is based on same principals and helps to understand how and why diffraction spikes form. Consider strait edge that light passes by (spider holder). Due to uncertainty principle single photon cannot have both momentum and position well defined at the same time. If photon is passing really close to the edge it will be localized in direction perpendicular to that edge and this means that in perpendicular direction momentum will be spread over more values. Since  photon moves at constant speed (speed of light) only component of momentum that can change is direction (momentum being mass times velocity that is vector value unlike speed that is scalar one). So photon spreads its trajectory perpendicular to edge. So spikes form perpendicular to edge direction (this is why 3 stalked spider forms 6 diffraction spikes - spikes are not in direction of stalkes but rather perpendicular to each both on left and right side). With circular aperture there is also this phenomenon, but since no point on circle has it's normal parallel to other (apart from point directly opposite) there is no compounding effects but rather spread is in shape of halo around star (Airy disk and rings - rings forming due to constructive and destructive interference of photon with itself and is wavelength dependent - same thing happens with diffraction spikes if you look at the color image of diffraction spikes you might notice that it is rainbow colored in direction of spike). Based on this reasoning I don't believe that any pattern of cross would form from 4 different circular apertures.

[SilverAstro]

And all without mentioning Fraunhofer as well ! I like it !!  Yes I had thougt along similar lines ( bearing in mind that curved thin vanes are already in favour with some astronomers for those reasons, which I was going to put in my previous post but edited out on the grounds that all will be revealed soon ), but in this 4-hole case there is also a lot of symmetry along 2 axis as well for some of the diffractions to gang up ! so I'm back to Ummm-ing lol!

Good fun, and @cuivenion could be well in-line for a Nobel at this rate,,,,

[cuivenion]

I'll definitely get around to this, Just have to figure out my new EQMOD and AstroEQ setup first. 

[x935418]

i did it with my 200p reflector, making 4 circle (5cm) and i test it with moon for high power

result was horribly bad !

[cuivenion]

Bad in what way?

34 minutes ago, x935418 said:

i did it with my 200p reflector, making 4 circle (5cm) and i test it with moon for high power

result was horribly bad !

 

[x935418]

1 hour ago, cuivenion said:

Bad in what way?

 

blurring image , it was not sharp   (at 100x-150X)

[cuivenion]

Fair enough, well I'll give it a go, see how it goes.