Off axis mask

Uncle Bob says...

I made an off axis mask for my 18" just under 8". Nice images.
It really makes it possible to explore the moon with a big dob.
It cost 4.00 in materials, not including the layout compass.
Tap Plastics had a Kydex-like offering that was suitable.

I would *love* to hear more about this. Whay aspects got better?
What aspects got worse? Did you have to realign and if so, how
difficult was it?

I still don't buy that a dual, or quad off-axis mask would be equivalent
to a huge CO.

I think a secondary and spider cause image degradation because they diffract
the incoming light around them, and strike the primary in a different place.
This diffracted light hits the eyepiece in a different part of the light cone
than it should. Hence, your spikes and halo.

A dual hole mask, located directly above the primary surface, should act like
two mirrors. The only diffraction should be from the edges of the holes.
Placing it close should minimize that to it's smallest possible levels.

In that instance, it should act like two mirrors coming to a common focus.

Now, I'm sure that a mask at the and of the OTA would more seriously degrade
the image.

On 8 Oct 2004 14:43:06 -0700, (Ted Kord) wrote:

I still don't buy that a dual, or quad off-axis mask would be equivalent
to a huge CO.

I think a secondary and spider cause image degradation because they diffract
the incoming light around them, and strike the primary in a different place.
This diffracted light hits the eyepiece in a different part of the light cone
than it should. Hence, your spikes and halo.

A dual hole mask, located directly above the primary surface, should act like
two mirrors. The only diffraction should be from the edges of the holes.
Placing it close should minimize that to it's smallest possible levels.

In that instance, it should act like two mirrors coming to a common focus.

Now, I'm sure that a mask at the and of the OTA would more seriously degrade
the image.

I suggest you try it. A two hole mask will produce horrible linear diffraction
patterns. A quad mask is produces a complex diffraction pattern, like a spider
on LSD.

_________________________________________________

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

Chris L Peterson wrote in message . ..
On 8 Oct 2004 14:43:06 -0700, (Ted Kord) wrote:

I still don't buy that a dual, or quad off-axis mask would be equivalent
to a huge CO.

I think a secondary and spider cause image degradation because they diffract
the incoming light around them, and strike the primary in a different place.
This diffracted light hits the eyepiece in a different part of the light cone
than it should. Hence, your spikes and halo.

A dual hole mask, located directly above the primary surface, should act like
two mirrors. The only diffraction should be from the edges of the holes.
Placing it close should minimize that to it's smallest possible levels.

In that instance, it should act like two mirrors coming to a common focus.

Now, I'm sure that a mask at the and of the OTA would more seriously degrade
the image.

I suggest you try it. A two hole mask will produce horrible linear diffraction
patterns. A quad mask is produces a complex diffraction pattern, like a spider
on LSD.


I have, actually. A two hole mask, on the end of a 8" f/6 dob. It was cardboard,
the holes were not carefully cut to be directly opposite each other, and their
sizes may have been a bit different. The thick cardboard also had ragged edges
on each hole.

I saw no such diffraction patterns. It looked like a dimmer version of the
full aperture, minus the spikes. It did look contrastier, but I think that
might have been because I could get a more accurate focus by merging the two
images. (The Celestron Starhopper I had was a decent scope, but the focuser
was not what I'd call a precision piece of equipment, and I often found myself
hunting for focus, and never quite finding it)

Anyway, I saw no image degradation on Jupiter or Saturn (the only targets I
tried it on)

Now, I have to believe that a better made mask, without the ragged cardboard
edges, exactly matching sized holes that are exactly opposite each other,
would do even better. Even more so with a premium primary.

On 8 Oct 2004 18:54:30 -0700, (Ted Kord) wrote:

I have, actually. A two hole mask, on the end of a 8" f/6 dob. It was cardboard,
the holes were not carefully cut to be directly opposite each other, and their
sizes may have been a bit different. The thick cardboard also had ragged edges
on each hole.

I saw no such diffraction patterns. It looked like a dimmer version of the
full aperture, minus the spikes. It did look contrastier, but I think that
might have been because I could get a more accurate focus by merging the two
images.

The test is to use the double aperture as a focusing aid, and then remove it and
see if there is a difference. The diffraction effects of the double aperture can
only be worse than for a simple central obstruction, but since the effects of
diffraction are so subtle, they may be undetectable in either case.

If you don't see a distinct bipolar spike on bright stars using a double
aperture, it can only mean that your optics or the seeing are very poor. You are
working with a system that has an asymmetric resolution- something that is very
apparent in the diffraction pattern. You can use this to your advantage with
some targets- double stars, banded planets- by rotating the mask to selectively
increase the resolution on one axis and decrease it on the other.


Anyway, I saw no image degradation on Jupiter or Saturn (the only targets I
tried it on)

I wouldn't necessarily expect to see image degradation. IMO, a typical central
obstruction doesn't produce a detectable amount of degradation either. When
looking at planets, all diffraction effects simply produce tiny variations in
contrast, by tweaking the shape of the MTF. It takes excellent optics, excellent
conditions, and an excellent observer (at the same time!) to detect this.

_________________________________________________

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

Ted Kord says...

a dual, or quad off-axis mask

I was talking about a single-hole off-axis mask, but if you want to
talk about multiple holes, that's an interesting topic as well.

Well, I'll just reiterate. This was not scientifically done. The mask was
not made to exacting tolerances. Only my eye was used to test it.

The image with the double mask looked like a dimmer version of the image
without, minus ANY kind of spikes or halo. Cloud belts on Jupiter stood
out in better contrast. (Again, possibly due to better focus fine tuning.)
Now, this was the old style Celestron Starhopper, with the one thick stalk
for the secondary, so it never had the traditional spikes. The backround
was much blacker around Jupiter and Saturn. Saturn didn't pick up anything
that overcame the increased dimness, but did stand out better against the
backround.

As for the optics, I am no expert. I can't estimate SA by the star test.
All I can say about that is the intra and extra focal star test images were
round and concentric, and looked identical to me. I couldn't detect any
differences. Better star testers than me probably could. But, based on what I
saw I would say the scope was at least diffraction limited. It was certainly
no Royce or Zambuto.

I think the holes were about 3" in diameter. So I was going from an 8" f/6
system to about a 4.24", or more correctly two 3" f/16 mirrors.

It was enough of a success that I was going to try a 4 hole mirror, produced
to exact tolerances, but I bough a Nexstar11GPS, and sold the Starhopper.
(And since my wife informs me that I am a one scope guy, there will be no newts
to test it on here until I sell this one)


I'd like to hear what someone like Roland Christen has to say about this.
Or maybe Tom Back or even Valery D. Does multiple off axis holes equal
a bigger spider vane?

In message , Ted Kord
writes

I think the holes were about 3" in diameter. So I was going from an 8" f/6
system to about a 4.24", or more correctly two 3" f/16 mirrors.

I'd like to hear what someone like Roland Christen has to say about this.
Or maybe Tom Back or even Valery D. Does multiple off axis holes equal
a bigger spider vane?

Yes. There is a direct mathematical correspondence between the shape of
the aperture used and the shape of the resulting diffraction limited
point spread function. It is fundamental and due to the wave nature of
light and there is no way around it.

Regards,
--
Martin Brown

In message , Ted Kord
writes
Chris L Peterson wrote in message
...
On 8 Oct 2004 14:43:06 -0700, (Ted Kord) wrote:

I still don't buy that a dual, or quad off-axis mask would be equivalent
to a huge CO.

I think a secondary and spider cause image degradation because they diffract
the incoming light around them, and strike the primary in a different place.
This diffracted light hits the eyepiece in a different part of the
light cone
than it should. Hence, your spikes and halo.

A dual hole mask, located directly above the primary surface, should
act like
two mirrors. The only diffraction should be from the edges of the holes.
Placing it close should minimize that to it's smallest possible levels.

In that instance, it should act like two mirrors coming to a common focus.

Now, I'm sure that a mask at the and of the OTA would more seriously
degrade
the image.

I suggest you try it. A two hole mask will produce horrible linear
diffraction
patterns. A quad mask is produces a complex diffraction pattern, like
a spider
on LSD.


I have, actually. A two hole mask, on the end of a 8" f/6 dob. It was
cardboard,
the holes were not carefully cut to be directly opposite each other, and their
sizes may have been a bit different. The thick cardboard also had ragged edges
on each hole.

I saw no such diffraction patterns. It looked like a dimmer version of the
full aperture, minus the spikes. It did look contrastier, but I think that
might have been because I could get a more accurate focus by merging the two
images.

You should see an image which looks like the characteristic image
through a scope of aperture equal to the size of the holes used crossed
by finer dark fringes determined by the separation of the centres of the
two holes.

If you did not see this then either the seeing was inadequate, you were
not using sufficient magnification, or you were not looking carefully
enough.

Try smaller holes in proportion to their separation to make the
resulting diffraction effects larger, less prone to seeing and more
obvious.

Now, I have to believe that a better made mask, without the ragged cardboard
edges, exactly matching sized holes that are exactly opposite each other,
would do even better. Even more so with a premium primary.

Unequal sized holes will to some extent mask the appearance of the
diffraction fringes so this might explain why you noticed nothing at
all. Masking down the aperture should also produce an obvious loss in
resolution - if it does not then there are other things wrong with your
setup.

It is much easier to see these diffraction effects clearly on a
moderately bright unresolved point object like a star. You have no
chance of judging these relatively subtle effects on a complex extended
object like a planet.

Regards,
--
Martin Brown

Martin Brown says...

You should see an image which looks like the characteristic image
through a scope of aperture equal to the size of the holes used crossed
by finer dark fringes determined by the separation of the centres of the
two holes.

If you did not see this then either the seeing was inadequate, you were
not using sufficient magnification, or you were not looking carefully
enough.

Try smaller holes in proportion to their separation to make the
resulting diffraction effects larger, less prone to seeing and more
obvious.

Unequal sized holes will to some extent mask the appearance of the
diffraction fringes so this might explain why you noticed nothing at
all. Masking down the aperture should also produce an obvious loss in
resolution - if it does not then there are other things wrong with your
setup.

It is much easier to see these diffraction effects clearly on a
moderately bright unresolved point object like a star. You have no
chance of judging these relatively subtle effects on a complex extended
object like a planet.

Assuming that I already have a big newtonian reflector set up, I can
see a use for a single-hole off-axis mask as an essentially zero-cost
alternative to setting up a smaller OA reflector next to the big scope.
My question is, is there any point at all to making a multi-hole mask?
Does it do anything for me that the single hole or the no-mask setups
don't already cover?