Single or multi hole off-axis filters?

Jon Isaacs wrote:
So how accurately do the holes have to be cut to provide that alignment?

Each hole creates two images, each image separated by 90 degrees from the
hole, in a manner of speaking. So if the holes in a two-hole mask are
separated by 150 degrees instead of 180 degrees, you end up with two
pairs of images, each separated by 30 degrees.

Since the out-of-focus images are bloated, the alignment has to be off
quite a bit before you see four distinct images with a two-hole mask.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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On 10 Jun 2004 00:15:26 GMT, (Jon Isaacs) wrote:

So how accurately do the holes have to be cut to provide that alignment?

Not at all. You can hack a Hartman mask out of a paper plate with a pen knife.
If you make a mask with a collection of randomly located holes, you will see
that same pattern reflected in the multiple star images when out of focus. When
in focus, you will see a single star image. But note that at high magnification,
you will see a distinctly non-circular diffraction pattern around this single
star image. There may be special cases where such a mask provides good results
(such as looking at double stars of the appropriate spacing and angle), but in
general, such an aperture will not yield better results than a simple,
unobstructed circle.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

If you make a mask with a collection of randomly located holes, you will see
that same pattern reflected in the multiple star images when out of focus.
When
in focus, you will see a single star image. But note that at high
magnification, you will see a distinctly non-circular diffraction pattern
around this single star image.

Thanks Chris and Brian for explaining this too me. This explanation is
something I can grab ahold of, it makes intuitive sense to me.

So, what do you guys think about using a 4 hole mask to get rid of the
Newtonian diffraction spikes while retaining as much mirror surface and
resolution as possible?

jon

On 10 Jun 2004 01:01:39 GMT, (Jon Isaacs) wrote:

So, what do you guys think about using a 4 hole mask to get rid of the
Newtonian diffraction spikes while retaining as much mirror surface and
resolution as possible?

The obstructed zones around the holes would just be a sort of "super spider",
probably creating larger spikes than the spider they are hiding. You'll get sort
of fuzzy diffraction spikes, widened because the masked areas have edges that
are not orthogonal, and you will have a biased resolution, larger along one pair
of axes than along others.

In a way, what you are proposing is similar to a curved vane spider, but your's
would be like having at least 8 vanes to diffract the incoming light.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

In a way, what you are proposing is similar to a curved vane spider, but
your's
would be like having at least 8 vanes to diffract the incoming light.

Some people have suggested that this scheme provides a significant improvement
over the full aperture Newt as well as the signal hole aperture mask.

jon

In message , Ted Kord
writes
Martin Brown wrote in message
...
In message , Jeff Polston
writes
Just found this on Cloudy Nights Telescope Reviews:
http://www.cloudynights.com/howtos2/masks.htm

Seems like the reviewer had very positive results from a multi-hole off-axis
aperture mask.

He cannot have been operating diffraction limited then. Looking at it
another way multiple holes is like having a large weird shaped central
obstruction.


But why? Why would it be any different than having 2, 3 or 4 individual
mirror sections of equal figure and f/ratio focused on the same spot?

It isn't any different at all to that situation. The problem is
fundamental and stems from the wave nature of light and diffraction from
a finite aperture. The focussed image is related to the Fourier
transform of the aperture used. Geometrical optics is inaccurate in this
regime.

Try looking up Young's slits on the net to get a simple example.

I'm just having trouble getting my mind around this. Of course, I'm pretty
optically ignorant, so maybe I'm out in left field.

I urge you to try it out with a piece of card and two 1cm round holes
separated by about 10cm (give or take). That should allow you to clearly
see the diffraction patterns of both the small circular aperture and the
effect of adding a second one at a moderate separation.

Also if you point it at an equal brightness double you will be able to
see the variation in the fringe intensity as you rotate the mask.

Regards,
--
Martin Brown

In message , Jon Isaacs
writes
If you make a mask with a collection of randomly located holes, you will see
that same pattern reflected in the multiple star images when out of focus.

So, what do you guys think about using a 4 hole mask to get rid of the
Newtonian diffraction spikes while retaining as much mirror surface and
resolution as possible?

It really won't do what you hope for, but since you can make one so
easily from card why not try it out and report back. It is essentially
like replacing your nice thin spider supports with huge curved iron
girders.

It is useful as a focussing aid.

You will end up with an image characterised by the resolution of the
chosen small circular aperture, modulated by a finer fringe pattern
determined by the separation of their centres. For certain well chosen
parameters this might give an acceptable image, but it will still be
inferior to the original full aperture.

Regards,
--
Martin Brown

(Brian Tung) wrote in message ...
Jon Isaacs wrote:
So how accurately do the holes have to be cut to provide that alignment?

Each hole creates two images, each image separated by 90 degrees from the
hole, in a manner of speaking. So if the holes in a two-hole mask are
separated by 150 degrees instead of 180 degrees, you end up with two
pairs of images, each separated by 30 degrees.

Since the out-of-focus images are bloated, the alignment has to be off
quite a bit before you see four distinct images with a two-hole mask.

snip

Thanks for the clarification.

What if the 4-hole mask were placed directly in front of the primary, as
opposed to at the front of the tube? Would that in any way minimize
diffraction? It would be almost exactly like having four off-axis mirrors
with a single focal point. It would also minimize off-axis light hitting the
mirror through each hole. (Of course, I'm presupposing a good mirror with
equal correction in each direction, no zones, etc)

I'm asking because I actually used a two hole mask on an 8" f/6 Celestron
dob years ago, and it did seem to give a clearer image than full aperture.

Also, how detrimental would a 4 hole mask be for a f/10 SCT?

First of all, I appreciate everyone's responses and advice on this.

Although multiple holes might not yield "better results" over a single hole,
my main concern was a degradation in the image. The plus side was going to
be brighter images or shorter exposure times, but I didn't want that at the
expense of image quality.

I plan on doing some experimenting on the moon when it comes back around
(and I have clear skies). I'll see if I can actually tell a difference in
quality of the image with one hole versus three.

Jeff
http://www.mindspring.com/~jeffpo

"Chris L Peterson" wrote in message
...
On 10 Jun 2004 00:15:26 GMT, (Jon Isaacs) wrote:

So how accurately do the holes have to be cut to provide that alignment?

Not at all. You can hack a Hartman mask out of a paper plate with a pen
knife.
If you make a mask with a collection of randomly located holes, you will
see
that same pattern reflected in the multiple star images when out of focus.
When
in focus, you will see a single star image. But note that at high
magnification,
you will see a distinctly non-circular diffraction pattern around this
single
star image. There may be special cases where such a mask provides good
results
(such as looking at double stars of the appropriate spacing and angle),
but in
general, such an aperture will not yield better results than a simple,
unobstructed circle.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Although multiple holes might not yield "better results" over a single hole,
my main concern was a degradation in the image. The plus side was going to
be brighter images or shorter exposure times, but I didn't want that at the
expense of image quality.

I plan on doing some experimenting on the moon when it comes back around
(and I have clear skies). I'll see if I can actually tell a difference in
quality of the image with one hole versus three.

It seems to me that the real comparison should first be whether the mask
improves or degrades the image. A mask of any sort certainly lengthens the
exposure times and reduces the possible resolution, or so it seems.

jon